Electric Motors [Troubleshooting and Repairing Commercial Electrical Equipment]

It is commonly held that a large, 3-horsepower motor is worth rebuilding, whereas a small, fractional horsepower motor is not worth fixing and, if it quits running, should be discarded without further ado. This is generally true, but there are exceptions.

Large commercial and industrial facilities typically have many fans for air circulation in interior areas, to cool transformer rooms, in conjunction with refrigeration equipment, and so on. These fans are usually powered by fractional horsepower, single-phase, 120- or 240-volt induction motors. Quite often workers will inform a maintenance electrician that a fan is not working. It is true that these small motors are not worth rewinding, but there are often simple repairs that provide new life.

Easy Motor Repairs

Frequently, a motor will fail to start when powered up after an idle period. It may or may not make an audible hum, or it can trip the overcurrent device. To begin, check to see if there is power and if the guard or a fan blade has become bent so that the fan cannot turn.

(Watch your fingers!) Remember that when the blades are forcing air forward, the fan is pushed back (if there is endplay), so it might be necessary to loosen the setscrew and shift the blade a slight amount. Then, with the power disconnected, try turning the blade. It should be freewheeling, so that if you give it a spin, it will coast awhile before coming to a stop. If it shows resistance, the bearings are probably dry. This often happens with motors that are only about a year old because the bearings are still tight and particularly prone to seizure. The remedy is simple. Spray a little penetrating oil around the bearing where the shaft exits and spin the fan by hand until it is freewheeling. See if there is endplay. If so, work the shaft in and out as this will help distribute the lubricant.

Penetrating oil is good for freeing up seized bearings, but it does not last, so it is best to follow it up with some high-quality machine oil or a little automotive oil: 30-weight for hot environments, 20-weight or multi-viscosity if the motor will have to start when the ambient temperature is expected to drop below 60°F. It is usually held that sealed bearings cannot be lubricated. This is not entirely true. There are specialized syringes for injecting oil. In addition, it is sometimes possible to spray penetrating oil around the seal. Some of it will seep in and free up the bearing.

Do not over-oil. Excess oil can spread around inside the motor and deteriorate sensitive parts such as brushes and insulation, and encourage dirt accumulation. Often the front bearing can be lubricated from the outside, but to access the rear bearing will require motor disassembly. However, the front bearing has to work harder, and it is often sufficient just to lubricate that one. It is amazing how often this simple repair works for a motor that was reported "burnt out." Put the motor back in service and add another drop of oil after the first day's use. Older motors have grease fittings or oil cups. Recommended lubrication interval is given on the nameplate and should be observed.

Another easy fix applies to motors that power woodworking and other equipment in dusty environments, where mechanical parts such as switches and the like may become clogged and fail to operate. Clean them out and you should be good to go. It may be possible to make a shield that will prevent recurrence, but beware of doing anything that will impede airflow.

A word of caution: Certain outside and inside areas are classified according to type and degree of hazard. You should not work on a motor or any other part of the electrical installation unless you have specific training and experience in these matters. One spark now or in the future could cause an explosion or flash fire with tragic consequences.

Before covering motor troubleshooting and repair in detail, we will look at various types of motors as well as their close relatives-generators and alternators.

Motors are generally understood to mean devices that convert electrical energy into rotary motion that can be coupled to a load to perform useful work. Linear actuators such as the common automotive starter solenoid are also motors, and they may fail as well. All motors are subject to troubleshooting and repair, with the exception of repairing some submersible pump motors and the like, where windings are encapsulated, as well as hermetically sealed refrigeration motor-compressors, yet even these have some external wiring that can be checked and repaired.

Two Parts of All Motors

Rotary electric motors are made up of stators, consisting of permanent magnets or windings that are mounted on the inside of the case, and rotors, consisting of windings, permanent magnets, or magnetically permeable soft iron. These elements are attached to a shaft that is mounted in bearings so that it is free to turn. Extending to the outside through an end bell housing is an output shaft for attaching a pulley, gear, saw blade, grinding wheel, or other useful tool. Some motors have output shafts at each end, while some have no output shaft if the work to be done is internal, as in a rotary inverter. Some motors are integral with the driven equipment, such as a refrigerant motor-compressor or a submersible pump. They may be separable.

What makes a motor turn? It is the magnetic interaction of stator and rotor. The current to one or the other has to be switched continually so that the rotor is always chasing after the stator. Therefore, the switching has to be coordinated with the turning, or the turning has to be coordinated with the switching. This switching action may take place inside the motor or outside as in a synchronous motor, driven or regulated by the ac line power with its changing polarity, or it may be a stepper motor, driven and regulated by pulses created for the purpose within the controller.

The basis for all this is the interaction of electricity and magnetism. Where there is electricity there is magnetism and where there is magnetism and a conductor in place, there is the potential for electricity. To have current flow, there has to be relative motion between the conductor and the magnetic field. This relative motion can consist of the conductor and the magnet moving, or of the magnetic field fluctuating.

The relevant formulas together with our still incomplete understanding of magnetism are complex, but everything we need to know in order to troubleshoot and repair electrical motors is straightforward. In the first place, we must see that electrical current and magnetic flux both go in circuits, which in many ways are similar. (Either of them may be an open circuit.) These are the similarities:

• Magnetomotive force (mmf) in a magnetic circuit resembles electromotive force (emf) in an electrical circuit.

• Flux in a magnetic circuit resembles current in an electrical circuit.

• Reluctance in a magnetic circuit resembles resistance in an electrical circuit.

• Permeance, the reciprocal of reluctance in a magnetic circuit, resembles conductance, the reciprocal of resistance in an electrical circuit.

• Flux is established between the north pole and the south pole in a magnetic circuit, just as current flows from the negative pole to the positive pole in an electrical circuit. (Current is usually thought of as starting and ending at a common source whereas flux exists between two poles that are physically separate.)

• In a magnetic circuit, energy is needed to establish the flux, but not to maintain it. In an electrical circuit, continuous new energy is needed to maintain the flow of current whether it is ac or dc.

The unit of MMF is the ampere-turn, which is created by 1 ampere of electrical current flowing in a single turn of wire. Accordingly, the strength of a magnetic field can be increased by winding multiple turns of the conductor, forming a coil around a magnetically permeable core composed of a material such as soft iron.

We are familiar with Ohm's Law for electrical circuits:

E = IR

where E = volts (electromagnetic force)

I = amperes (intensity)

R = ohms (resistance)

The similar formula for magnetic circuits is Hopkinson's Law:

F = FR

where F = magnetomotive force

F = magnetic flux

R = magnetic reluctance

In an electrical circuit, electrons flow through the conductor, which, because it has some slight resistance, dissipates energy in the form of heat. In a magnetic circuit, nothing flows and nothing is dissipated. Magnetic circuits are not linear. Unlike resistance, which is constant, reluctance varies depending upon the strength of the magnetic field and the amount of flux.

We have mentioned that both the rotor and the stator must have magnetic properties. In the case of the stator, this is simple-just wire the electrical supply to permanently mounted coils, which may be connected in parallel or in series. As for the rotor, it is more of a problem. If the rotor were to be wired directly, the wires would quickly twist and break off.

Transfer of power to the rotor may be achieved by having brushes, which contact the turning commutator. It, in turn, is wired to the rotor-mounted coils, which have soft iron cores. The brushes are both electrically and mechanically active, so they are subject to failure.

Fortunately, they are inexpensive and easy to change. For portable electric saws, handheld electric drills, and similar tools and appliances, they are often accessible from the outside and the only tool needed is a screwdriver. They are held firmly (not tightly) against the commutator by spring pressure. If this spring loses its tension, it may be stretched a little.

An ohmmeter should show continuity from one side of the attachment plug, through the switch (turned on), in one brush, through the commutator and rotor circuit, out the other brush, and back to the other side of the attachment plug. Sometimes, in an electric drill, the switch is double-pole and both sides have to be working. This simple test checks the complete circuit of the hand tool and many other similar items. Leaving the ohmmeter hooked up, you can flex the cord, wiggle the switch sideways, put pressure on the brushes, etc. to bring out and identify intermittent faults (see Fig. 2-1).

There are brushless strategies for getting power into the rotor or not getting power into it but having it active magnetically. Keep that in mind as a primary issue in motor design and repair. If you want to troubleshoot a piece of electrical equipment, especially a motor, you have to know the current flow.

Figure 2-1 To find a fault inside a tool, connect your ohmmeter to the plug. Then try the switch, rotate the armature, move wires around inside, and watch the meter.

The principal types of motors are as follows.

The dc motor was the first motor ever built, long before Tesla and Westinghouse conceived of alternating current and built a generation and distribution system to make possible its use. Even today, there are many dc motors. They were used in large power applications such as elevators and ski lifts, where smooth operation, variable speed, and reversibility are important, before the advent of the variable speed drive (VSD). They are still common in low-power applications including automotive starters, ink jet printers, children's battery-operated toys, and tools and appliances for off-grid users that have dc power.

DC is supplied to the stator coils, creating a stationary magnetic field. Current is also fed to the rotor. Unlike the stator, this field cannot be stationary or else the motor would rotate at best a part turn, finding the place of least reluctance. To make the dc motor work, the rotor current and resultant magnetic field have to be switched regularly, and this is done by the commutator, which performs its function in time with the turning of the rotor, so that speed may be varied merely by adjusting the supply voltage, and direction may be changed by reversing polarity. DC may be supplied from a battery, dc generator, solar PV system, or utility power that is rectified.

There are two broad categories of ac motors-synchronous and induction. By their very nature, neither of these will work on dc.

A synchronous motor works on the principle of reversing magnetic fields, but the switching comes from outside. It is inherent in the nature of the power supply, whether utility, on-site ac generator, or dc changed to ac by means of an inverter. Rotation is dependent upon line frequency, generally 60 Hz in North America. The period of rotation is equal to some multiple of line frequency, based on the number of magnetic poles. Speed is precisely keyed to the frequency and will not be altered by adjusting the voltage.

Because of this frequency dependence, synchronous motors will not start, except for very small motors where the inertia of rest is easily overcome. Large synchronous motors have separate dc excitation, or they are brought up to speed by a small auxiliary motor.

Large synchronous motors are noted for their superior efficiency, and with a favorable power factor, they help to balance out equipment with a lagging power factor significantly reducing energy costs for a large industrial occupancy.

The National Electrical Code requires emergency lights to be regularly inspected and a record kept. The time interval is not specified. In a large facility, maintenance electricians are entrusted with this important job. Place the results in an Excel file and compare results to see which units are eating batteries. They will need new circuit boards.

Induction motors are much more common, and the method for getting energy into the rotor is a brushless and overall quite elegant solution. They are called "asynchronous" because the speed is not directly synchronized to line frequency, although it is dependent upon it. They require ac to operate. Induction motors are also called "squirrel cage" motors.

The principle is that the power gets into the rotor by means of electromagnetic induction. In effect, the stator is the primary and the rotor is the secondary of a power transformer, so no brushes are required.

Slip in an Induction Motor

In an induction motor, the rotor speed is not synchronized with respect to the stator's rotating magnetic field. If it were, there would be no relative motion, no induction, and no power transfer. The rotor speed is a certain percentage less than the stator's rotating magnetic field. This difference is called "slip" and it is what makes an induction motor possible. It is important to realize that in an induction motor, the rotor is not merely out of phase with the stator's rotating magnetic field, but it is actually turning more slowly.

Increasing the frequency will increase the speed, but there is no direct synchronization.

The rotor windings are closed loops. Since power is transferred by magnetic induction instead of brushes, induction motors are fairly maintenance free.

The typical amount of slip for a small induction motor is around 6 percent. For a large induction motor, it will be about 2 percent. If, due to overloading, the slip increases beyond 20 percent, the motor will stall and if power is not disconnected soon, the motor will overheat, the windings will become heating elements, and the motor will be destroyed.

A stepper motor is a brushless dc motor that, unlike other motors, is able to turn in small increments, fractions of a rotation, rather than continuously. The amount of each increment is called the step angle, which varies depending upon the stepper motor model. It can be less than 1 degree or as much as 90 degrees, or a quarter turn. The stepper motor turns one step, then stops whether or not voltage is removed from the circuit that feeds the motor. If current is maintained through the winding, the motor remains at rest but maintains a definite holding torque-a very useful function in many applications (see Fig. 2-2).

If you apply a steady dc voltage to one pair of wires, the motor may or may not turn one step, depending on the polarity of the voltage, which pair of wires the voltage is applied to, and the position of the rotor. If it does turn, it will turn only one step and then stop, holding that position with available torque. It will remain motionless until another pulse is applied.

Figure 2-2 Stepper motors from ink-jet printers.

Accordingly, it may be said that the commutation takes place outside the motor, and is the province of an outside controller. Depending upon the characteristics of the motor, an audio frequency may be applied and the motor will really spin, albeit rpm levels are generally less than for other motor types.

Since all commutation takes place outside, stepper motors have no brushes or moving parts to wear out, other than the spinning rotor. For this reason, all stepper motors are extraordinarily long lasting and reliable. Failure modes are bearing fatigue, which is infrequent because of low rpm and duty cycle when operated in stepping mode together with typically light loading, and overvoltage on the field coils, which can be mitigated by installing overcurrent protection and protective diodes to prevent inductive voltage spikes.

To understand how stepper motors work, it is instructive to consider the various forms they have taken.

The variable reluctance stepper motor is remarkably simple. It has two parts-stator and rotor. The rotor is mounted by means of front and rear bearings so that it is able to rotate and perform useful work by virtue of its output shaft and an attached gear.

A variable reluctance stepper motor is characterized by a distinctive feature-it has a soft iron notched-tooth disc. Soft iron is highly permeable to magnetic flux; that is, it has low reluctance relative to air. That is the reason it is called a variable reluctance motor. The stator is equipped with field windings, usually three, four, or five. This would imply six, eight, or ten wires, but often there is a single common conductor with the connection made inside, so there will be one wire for each coil plus one common wire. These are easy to ring out with an ohmmeter and to label. The common wire is generally connected to the dc power supply positive terminal. The controller applies power to each of the windings in sequence. A series of pulses is what is needed. Like all stepper motors, the variable reluctance version will respond to a wide range of voltages. In many cases, you can manually pulse and step variable reluctance units with a 9-volt battery.

Even without seeing what is inside, it is possible to identify a variable reluctance stepper by the fact that, with no power applied, it will turn relatively freely by hand whereas the permanent-magnet version will exhibit a more pronounced cogging.

In contrast to the variable reluctance stepper motor, the permanent magnet stepper relies upon attraction and repulsion between a rotor-mounted permanent magnet and stator-mounted windings. Both structures have sets of teeth, offset with respect to one another, so that when timed voltage pulses are applied to the stator windings, the rotor will turn in an incremental fashion. The greater the number of teeth, the smaller the step angle, making for a finer resolution.

Permanent magnet stepper motors may be unipolar or bipolar. The bipolar stepper simply has a single winding per phase on the stator. The unipolar stepper is similar, but each winding has a center tap. The unipolar may be used as a bipolar stepper by disregarding the center taps and leaving them disconnected. The bipolar unit is a simpler design, but control becomes more complex. For the unipolar stepper, the magnetic field direction is reversed by switching on each section of windings, thereby reversing direction.

A microcontroller will turn on the drive transistors in the correct order. Unipolar steppers are easier and less expensive to implement for this reason.

As noted, bipolar stepper motors have a single winding for each phase. It is necessary to reverse current direction to reverse magnetic poles, making the drive circuit more complex.

An H-bridge is required.

Hybrid steppers combine characteristics of variable reluctance and permanent magnet types. They provide superior speed, torque, and step resolution, but at a greater cost. Step angles may be as low as 0.9 degrees, which translates to an incredible 400 steps per revolution. The rotor has numerous teeth and a concentric shaft-mounted, permanent, axially magnetized magnet. The teeth on the rotor mean increased holding and dynamic torque in contrast with variable reluctance and permanent magnet versions.

If it is possible to use the permanent magnet stepper, the cost savings are significant.

For prototyping and experimental purposes, stepper motors may be easily obtained by disassembling inkjet printers. These typically contain two steppers and one 2-wire dc motor, valuable components for robotics projects. Ink-jet printers are usually quite inexpensive because the manufacturers provide them at low cost and then make their money by putting a high price on replacement ink cartridges. Moreover, a new computer frequently comes with a new printer, so ink-jet printers are everywhere and may be had for the asking.

Similarly, fax machines, dot-matrix printers, and disc drives have stepper motors. These should be perfectly good long after the overall machines are worn out or obsolete.

Another excellent resource is an old, obsolete, large satellite dish, also freely available. These had larger stepper motors because, unlike modern small satellite TV and Internet access dishes, a directional motor was required to aim at different satellite locations (see Fig. 2-3).

When harvesting stepper motors from obsolete or surplus equipment, it is desirable to save full-length leads with connectors, as well as mounting screws and hardware and any peripheral electronics such as power supplies, LEDs, protective fuses, diodes, drive transistors, and the like.

Small motors that are integral parts of factory-made equipment may not have nameplates.

On stepper motors, markings may be incomplete or nonexistent. If there is a name or number stamped on the stepper motor, it may be possible to do an Internet search and print out a datasheet. Another approach is to plug in the printer and take voltage readings.

Figure 2-3 Old-style satellite dish with stepper motor.

Stepper motor applications include:

• 3-D printing

• Telescope clock drives

• Solar array trackers

• Computer peripheral and business machines

• Robotics

In terms of robotics, a number of considerations arise. Steppers must have an appropriate resolution, based on step angle, for the function that is contemplated. There must be sufficient power for the mass involved. A smaller stepper is better in terms of onboard space required and power consumption. For robot mobility, too much torque can mean a tendency to spin or slip, whereas too little torque can result in stalling or inability to climb out of a hole or negotiate an uphill slope. Either of these limiting cases can spell loss of mobility.

A stepper motor is commutated and controlled from outside. To work, it has to be supplied with a brain.

For robotics projects, a great way to implement these functions is by means of an Arduino, which is an open-source electronics prototyping platform based on flexible, easy-to-use hardware and software. An assembled board costs approximately $30. It is a programmable controller with numerous and diverse features. It is cross-platform, which means that you can connect to your Mac or PC using a USB cable. Drivers and software are available free of charge at www.arduino.cc. The site also includes reference material with each Arduino command and relevant code snippets. The Arduino design is open source, so the hardware is available (assembled or in kit form) from various manufacturers.

A rather complete and well-written book on Arduino use including programming and interfacing with robotic projects is Arduino Robotics, by John-David Warren et al. This work assumes little prior knowledge, yet takes you far into the world of electronics for robotics.

When upgrading to a larger service, the old branch circuit wiring must be removed from the entrance panel so that a new breaker box can be installed. Do not pull the stripped cables out through the connectors-that is sure to nick the conductors causing shorts later. Instead, remove locknuts from the inside of the box and leave the connectors in place on the cable jackets. Reattach in the new box. That way is easier and less hazardous.

Returning to motors in general, in troubleshooting motorized electrical equipment, there are two issues to consider: whether the motor or its control circuitry is at fault and, if so, what is the nature of that fault?

A Common Fault

There are instances when the motor fails to turn when the machine is powered up, and yet the motor is good. An example is a large commercial dryer, as found in the laundry of a big hotel. One of these units consists of a large drum capable of holding sheets, tablecloths, towels, clothes, and other items to be dried straight out of the washer. The drum capacity is many times greater than that of a residential-type dryer. These machines require a large motor to turn the drum in order to tumble dry the contents, and a means to inject hot, dry air into the drum and expel the cooler moist air, usually vented to the outdoors.

The heat may be electric, like most residential dryers, gas, or steam, the latter heating air by means of a heat exchanger.

There is a door that is opened to add and remove contents and, like a residential model, there is an interlock switch with a relay to interrupt power to the motor when the door is not firmly closed. This simple switch can be checked with an ohmmeter or by operating it by hand. The switch may be defective (open) or the mechanical linkage that operates the switch in response to the door position may become bent or broken so that the motor will not run.

It is to be emphasized that a broken interlock switch should not be electrically shunted or otherwise defeated just to get the machine running, even on a temporary basis, as severe injury to a worker could occur.

This interlock switch is one of several devices or systems that are in series to control the motor.

Front panel controls include a main power switch and pilot light, provisions for setting the temperature so that it is appropriate for the material to be dried, and a timer calibrated in minutes to regulate the time of the drying cycle and a shorter cooling-down cycle. As mentioned, the door interlock switch cuts power to the motor. It also shuts down the heat, if electric, but if you open the door and quickly look inside, you may be able to see light from the heating element before it fades entirely.

In older models, these controls operate relays to switch the power to the motor, heating elements, and blower. Newer units invariably have a printed circuit board that performs the same functions efficiently and reliably.

If you verify that you have power to the motor and find that it is entirely unresponsive, or if there is no power to the motor because it has tripped the overcurrent device, it is probable that the fault lies within the motor. However, you need to verify that the driven load is not locked up or turning abnormally hard. On moderately sized equipment, it may be possible (with power disconnected and locked out) to turn the driven load by hand. You should be able to judge, depending upon the nature of the equipment, if it has the proper drag. The other thing to do is disconnect the load. If power transmission is via a driveshaft with a universal joint or semiflexible coupling with rubber bushings (a Lovejoy coupling), some wrench work will have it apart. However, when you power up the motor, take care that there is nothing that can whip around or otherwise create a hazardous situation. If the motor still will not run and you have verified continuous correct voltage to the terminals with no tripping out, you have to conclude that the motor is defective. You can go a little further. At times, there is a broken or burnt wire inside the case, and if the break is not too close to the winding, it could be amenable to soldering or to a crimp connector if there is room inside. When making repairs of this sort, it is essential to ensure that there is nothing live that can ground out against the enclosure given that there will be vibration and temperature changes when the motor is running.

Now we come to the important topic of brushes and commutators. They are subject to wear and damage from overheating, but the motor is constructed so that they can easily be replaced. Of course, not all motors have brushes. Brushless (permanent magnet) dc motors, stepper motors, and induction motors are all unencumbered by brushes, but where there are brushes (conventional dc motors, synchronous motors, shaded-pole motors, and universal motors), they are the first things to check.

Early dc motors conveyed electrical current through the commutator and into the armature by means of actual braided copper brushes that were connected to the outside dc power source-hence, the name. Because of their high conductivity and inaccurate contact with the commutator, they have been replaced by graphite, which transitions more smoothly from one commutator bar to the next.

As with many things electrical, heat is the enemy. Regarding brushes, this is definitely true, and the primary source of heat is sparking. Graphite against metal is an excellent lubricant. (If you want powdered graphite to use as a lubricant, grind up some discarded brushes.) As for sparking, in a new motor it is minimal, but eventually the commutator or brushes may become pitted or distorted, and there will be a discernible sparking. Of course, the heat that is generated accelerates this deterioration.

Many motors, such as in portable drills and circular saws, have openings (for ventilation) in the housing, so that it is possible to watch the brushes ride over the turning commutator while the motor is running. If you observe a number of these motors that you encounter, you will quickly get a sense of how much sparking is normal and when it becomes excessive.

Once sparking at the brushes becomes excessive, it rapidly gets worse until it has the appearance of a gas flame. The result will be a damaged commutator if not a burnt up tool or even an electrical fire. Therefore, at the first sign of hot sparking, the brushes should be replaced if new brushes are available. If they are not available, the alternative is to fix the existing brushes if they have not become too short. You can wrap a piece of sandpaper, abrasive side out, around the commutator, press the brushes one at a time against the sandpaper, and rotate the commutator to grind down the brushes a slight amount. This will smooth the mating surface and make it conform better to the curvature of the commutator. Afterward, clean the brushes and commutator with isopropyl alcohol to ensure that no abrasive particles remain.

The lengths of the brushes and pressure supplied by the springs should be uniform. Then, if you try running the motor, you should find the sparking greatly diminished.

If replacement brushes are not available, some resourceful individuals have attempted to make new ones by cutting down larger brushes that are on hand, and grinding the correct curvature. This procedure may not provide a long-lasting solution because brush material and the heat treatment ("sintering") that is used to harden it are carefully designed by engineers to match the electrical and mechanical characteristics of the motor.

Going Deeper

As for the commutator, it should be inspected and serviced when necessary, perhaps every second time the brushes are changed. Anything that is not right with the commutator will translate into rapid brush wear, further damage to the commutator, and poor motor performance. The commutator should not be deeply grooved where the brushes ride. The pathway should be polished, but not ground away, and it should not be rough or pitted.

Measured with a micrometer, the commutator should not be out-of-round or irregular.

Specifications are found in motor manufacturers' service manuals.

The insulating gap between commutator segments must be recessed a slight amount.

After the commutator has been cut down on a lathe, it is necessary to deepen the grooves between segments. There is a special tool for doing this, or a hacksaw blade will work.

Overheating, moisture, exposure to corrosive materials, or excessive voltage will damage motor windings. Repairs must be performed by a motor rebuilding shop, and should not be attempted unless you are prepared to invest in the equipment and acquire the expertise that is involved. Such shops have large ovens in which motors are placed before and after repairs are completed. Over a period of hours, moisture is baked out of the windings in order to prevent internal current leakage and other harmful effects.

Other than brushes, the major replacement items in electrical motors are the bearings.

These may be sealed or equipped with grease fittings or oil cups so that they can be lubricated. The trend today in motors as well as all kinds of machinery is toward sealed bearings. There is a palpable savings in labor and materials over the life of the machine. In addition, the risk of introducing dirt or abrasive material is eliminated. Nevertheless, many technicians prefer bearings that can be greased, feeling that these are more trustworthy. However, it is a fact that improper lubrication will result in bearing failure, so it is necessary to follow these guidelines:

• Do not neglect the specified lubrication interval. In high ambient temperatures or when heavily loaded, when the load is pulley driven as opposed to shaft driven, the interval should be shortened. Side loading puts more pressure on the bearing surfaces and tends to squeeze out the lubricant.

• Do not introduce contamination. Before fastening the grease nozzle onto the fitting, remove any dirt accumulation in the area, and then wipe off the fitting with a clean cloth. Avoid setting the grease gun down in an area where it will pick up dirt and, if necessary, wipe off the nozzle before greasing. Any abrasive material that finds its way into the bearing will wear the metal surfaces and cause premature bearing failure.

• Do not overlubricate. Too much grease will burst the grease seals, eventually allowing moisture and contamination and wasting grease. Overlubrication is also harmful, especially in high-speed applications, because when the bearing is overfilled, the moving parts have to work harder, eventually making for overheating. It is better to do more frequent light lubrications.

• Keep records. At the location of a large, expensive motor, a log should be posted with the date and description of any maintenance operations including lubrication.

• Use the correct grease. For low-speed operation at moderate temperatures, a multipurpose lubricant is appropriate. Check the operator's manual if in doubt.

Temperature measurements, taken at regular intervals and entered into the maintenance log, are helpful in keeping track of bearing wear so that bearings may be changed at a time when downtime will not interrupt the work flow. A number of different instruments will measure temperature even at a distance. They are easy to use and accurate.

Motors can be tested using a variety of meters. The tests are made both in and out of circuit. Assuming a motor has been in use and continues to perform in a satisfactory manner, begin a preventive maintenance program. The results should be written up in reports and analyzed on a regular basis so that there will not be an unexpected outage.

To begin, take the motor out of circuit. This can be done without disrupting terminations, cutting wires, or introducing unnecessary splices if there is a proper disconnecting means or overload protection. Many motors can be isolated at the controller.

An example is a properly installed submersible well pump. Most makers use the motor and control box manufactured by Franklin Electric, bolting their proprietary submersible pumps to it. Some, like Goulds Pumps, put their own name on the box, but most retain the Franklin branding (see Fig. 2-4).

In a three-wire system, a red wire provides 240-volt starting current, black is for the run winding, yellow is common, and there is no neutral. A bare or green wire is the equipment ground. (It is not a conventional three-wire, two-voltage system as in a common single phase service.) The electronic components, capacitor, microchip, relay, etc. are mounted on the cover, so that when it is removed, there is access to these items. Moreover, the pump motor, underground wiring, and conductors going down inside the well casing are isolated from the power source. Often a failed system can be restored merely by replacing the relatively inexpensive cover. The meter readings are documented inside the box. These specifications vary depending upon the horsepower of the motor.

Figure 2-4 All the electronics for a submersible well pump are in this control box.

With the cover removed, there is access to input and output terminals. The input terminals are still live with 240-volts from the branch circuit.

Using the multimeter volts function, check the input. There should be 240 volts between the line terminals and 120 volts between each of them and the bare or green equipment ground. Next, switch to the ohms function and take readings at the output. Compare these values to the manufacturer's specifications. Like any motor, the resistance of windings to ground has to be high-in the megohm range. If it is low, the motor windings are grounded, and the motor must be replaced. (A submersible water pump motor is sealed with epoxy encapsulated windings and cannot be repaired.) Alternatively, there is a fault in the wiring between the control box and the pump. Very often, a wire chafes against the inside of the well casing, or a fault develops in the underground wiring.

The next test also involves ohms readings at the control box output. Check the resistance individually between red and yellow, and between black and yellow. These readings have to be in the low ohms range, as specified in the box, varying with the horsepower. If the resistance is too low, near zero, there is a short. If it is high, you have an open circuit, either in the wiring or in the motor.

Sometimes these faults are found to be in the wiring right at the wellhead, where there are typically wire nut connections, and that is an easy fix.

These resistance tests are the way to get started, but they are not definitive. Information that is more complete is gained by taking current readings with the cover replaced and the motor running.

These current readings are best taken using a clamp-on ammeter. As mentioned previously, the reading has to be taken on only one conductor. If the hot and return wires are both enclosed in the jaws of the clamp-on ammeter, the two currents will cancel because they are flowing in opposite directions, and there will be a reading of zero.

Therefore, you have to find a place where the wires are separate in order to take this measurement. If the wires coming out of the enclosure are in raceway (as they should be), that means it is necessary to look elsewhere. Some possibilities are inside the entrance panel or load center, or under the well cap. The current readings must conform to the specifications shown inside the control box, in the owner's manual, or posted on the Internet for the system to work. Of course, if the system is cutting out, either tripping the breaker in the entrance panel or load center, or cycling at the control box, these readings cannot be taken. In addition, if there is an open circuit so that the motor is not running, there will be no current flow. Otherwise, the ampere readings will reveal the condition of the motor.

Submersible pump motors present unique challenges in terms of troubleshooting because access to the motor requires pulling the pump/motor assembly up from the depths.

The test procedure for other types of motors is similar, but usually they are right out in the open. Start with the multimeter volts function to check supply voltages, then take ohms readings with the motor disconnected from the power source and any control wiring, and finally do a dynamic test by taking clamp-on ammeter current readings with the motor running, if possible. This final test is useful when a motor trips out after running under load for a period of time, sometimes 15 minutes, sometimes an hour or more (see Fig. 2-5). High current draw can mean that the motor is getting tired, that is, it is in need of rebuilding or replacement, although a thorough cleaning and a check of all splices and connections will sometimes solve the problem. Rarely, a breaker or overload protection trips at an abnormally low current level. The clamp-on ammeter reading will clarify the situation.

Other Motor Test Procedures

There are other, more advanced, tests that can be made. A spectrum analyzer or oscilloscope will reveal power quality problems--clipping or harmful harmonics--that cause poor motor performance and perhaps overheating of the conductors. These problems can occur when certain other equipment is simultaneously running, an example of the load influencing circuit parameters.

Then we come to the important subject of the megohmmeter, or megger (named after one of the companies that manufactures it; Romex and Amprobe are also trade names that have become generic) as it is called. This valuable instrument can be bought for between $150 and $500. Battery-powered and hand-crank (magneto) versions are available. Since high voltages are used, there are important safety considerations. One hazardous aspect of electric shock is that the muscles involuntarily contract, so that it is possible that the affected individual, grasping an energized object, will be unable to release it. The victim will be exposed to the harmful voltage on a continuous basis, unlike the usual scenario when one recoils and receives only a brief jolt. The skin, normally a partial insulator when it is dry, begins to break down and blister, becoming more conductive and increasing the hazard.

This danger to the megger operator is lessened when the hand-crank model is used, as presumably the hand cranking will stop immediately.

Figure 2-5 A clamp-on ammeter will reveal the condition of many types of motors.

Accompanying the megger is an extensive technology that is at once very revealing and a war of nerves because if it is overdone, the object being tested will have its insulation degraded.

The megger is used to test the insulation integrity of many types of electrical equipment, as well as power and data cable before or after installation. One of the underlying ideas is that very high amounts of resistance, such as many megohms, cannot be tested with a conventional ohmmeter because the low test-voltage is insufficient to create a current that is measurable. The other underlying idea is that many materials will present a high resistance to the flow of electrons unless a sufficiently high voltage is applied for a length of time, whereupon an ionized path is established and the electricity blasts through.

A prominent example is lightning. A static charge, due to migration of charged particles from one area to another, builds in intensity until there is a sudden ionization, or breakdown in the insulation that is the air between the two regions. An electrical circuit is established and there is an enormous flow of current for a brief period of time.

When there is a question regarding the insulation integrity of an electrical motor, megger tests are commonly conducted. In research laboratories and for product development, the test can be prolonged to a point where the motor or other equipment is destroyed in the interest of gathering information concerning the underlying design.

However, in our maintenance and repair environment, especially when it is a large, expensive motor, we want to make sure that the test remains nondestructive, which means placing a limit on the voltage level and duration of the test.

As time passes, electrical insulation will deteriorate, usually very slowly, but rapidly if there is exposure to moisture in the air or due to flooding, nearby leaking pipes, corrosive vapors, or liquids or physical trauma. Temperature extremes, vibration, or other conditions may be factors. Harmful situations will compromise winding insulation inside a motor, and the megger test can reveal a developing situation before it becomes catastrophic, by which we mean hazardous to productivity, property, or human life.

The megger is a little more complex than a conventional ohmmeter. Besides the internal generator or power source, it consists of both voltage and current coils, so that variations in the amount of applied voltage do not invalidate the resistance readings. The amount of applied voltage is usually between 500 and 1000 volts. (Higher voltages are used in a high pot tester, which requires specialized training and familiarity with what is called medium voltage work.) There are several types of megger tests. The most frequently performed is the short time or spot reading. The meter is connected across the insulation to be tested and the voltage is applied for 1 minute. For a motor, an acceptable resistance reading is considered 1 megohm for each 1000 rated volts. For any voltage under 1000, the reading should be at least 1 megohm. This amount is subject to variation, however, because of changes in ambient temperature and humidity. In addition, a new motor will exhibit a much better reading, especially if it has always been stored in a dry location. (Some large, expensive motors have provision for a continuous low-level voltage to be applied to the windings when the motor is not being used, the current resulting in just enough heat to drive out the moisture, thereby preserving the insulation integrity.) By taking and recording regular readings at periods of uniform temperature and humidity, it is possible to spot trouble on the horizon and deal with it during scheduled downtime as opposed to experiencing disruption to production or worse in the event of catastrophic failure.

The time-resistance test consists of taking successive readings at specified intervals of 5 to 10 minutes. Good winding insulation will exhibit an increased resistance at each reading, while bad winding insulation will show a drop in resistance at each reading. For this test, the focus is not on the exact values, but rather the relative change.

Some meggers have multi-voltage capability, and if you have one of these instruments, you can do a stepped-voltage test. It involves testing winding insulation resistance--first at a lower voltage, and then after discharging any voltage that is stored capacitively, at a higher voltage. If there is a significant drop, you will have reason to believe that the integrity of the winding insulation is compromised. It is possible to apply to the motor moderate heat in a dry location for an extended period of time. There are instances where this procedure will give new life to the motor, improving megger readings and motor performance. More likely, the unit will have to go to a motor rebuilding shop for an extensive overhaul.

It is to be emphasized that megger tests should be performed on a motor only when it is disconnected from the power source to ensure that hazardous voltage is not backfed to wiring, circuits, or equipment elsewhere, invalidating the test results as well as creating outside damage or danger to persons.

Three-phase motors are less expensive and simpler to install, maintain, and service than their single-phase counterparts. They will start on their own; hence, there are no separate start windings, circuitry, wiring, or switch mechanism with which to be concerned.

Moreover, it is not practical to build a single-phase motor over 5 horsepower, so they are just about all three phase. Small, fractional horsepower motors also may be three phase, and they are common in industrial locations. Why, then, are not all motors three phase? The answer is that three-phase power is not available at all buildings, either because of the service or because of the utility distribution system.

Driving throughout suburban neighborhoods, it is instructive to examine (visually, from a safe distance only) the distribution lines. Three-phase power is characterized by three hot legs at the highest level, with a grounded neutral a few feet lower. Well below these power lines are telephone, CATV, broadband cables, and so on. Single phase, in contrast, has one hot conductor at the top, with a grounded neutral at a lower level and "low-voltage" communication and data cables below. The principle is that there is to be sufficient separation so that in an aerial bucket workers can service the lower-voltage conductors without coming near the high-tension lines. In addition, sufficient elevation ensures that trucks and equipment will not snag on the lines when passing underneath, and children will not reach them by climbing the poles. Ground clearances and conductor separation specifications are all spelled out in the National Electrical Safety Code, which covers, among other things, utility wiring (excluded from the National Electrical Code).

You will notice where there is three-phase power a single-phase service conductor is connected to one of the three-phase utility lines for each individual single-phase step-down distribution transformer. This is done on a rotating basis in order to keep the loads balanced insofar as possible.

You will see that the three-phase lines follow roads in more populated and industrialized areas, while single-phase lines extend further into more remote areas where electrical usage is lighter.

The key concept is that single-phase service can be derived from a three-phase line by means of simple electrical connections, but three-phase cannot be derived from single-phase power except by means of specialized equipment-a rotary or electronic phase converter.

Single phase is derived from three phase at the power pole or inside the building. It is a frequent layout to have three-phase power to a high-rise building, broken down into single-phase circuits emanating from load centers on each floor, again with a view to balancing the loads. No transformers or substations are necessary, just double-pole breakers within three-phase boxes. Three-phase panels have three busbars, so that either type of power is available.

Three-phase motors are installed, maintained, diagnosed, and repaired in much the same way as single-phase motors, with the exception that phase order becomes very important.

Three-phase motor direction is changed by reversing the connections of any two of the three legs. A single-phase motor, with the exception of a universal motor, cannot have shaft rotation reversed merely by switching wires, for there is no polarity distinction in alternating current. A single-phase motor with reverse rotation capability has separate clockwise and counterclockwise windings, with four wires in theory, but actually three because one is common. The three wires are brought to an outside box where the operator controls rotation and start-stop action. A three-phase motor can be controlled in a similar way, although frequently it is intended to run in one direction only. This is done as part of the permanent installation. The conductors may be changed at the motor, a junction box, a load center, or the service-entrance panel, as long as other motors are not on the circuit. So now the question is, how do you ascertain the wiring connections for the desired rotation of a three-phase motor? It is impossible for information on the nameplate or in termination markings (unless they are added after the installation) to provide the answer because there is no defined phase order in the three-phase circuit, not to mention the utility supply.

The easy answer is by trial and error. However, beware that certain motor/pump units can be damaged if run the wrong way, with the seals being instantly destroyed. For many installations where the load will not be negatively affected, it is a simple matter of observing the action. Some pumps and fans will move liquid or air in the correct direction for either motor rotation, but one way produces more output than the other does due to the cup shape of the blades or impellers. This is true of submersible water pumps.

Another method for ascertaining motor rotation is by means of a three-phase motor rotation tester, available for a little over $100.

If a telephone has no dial tone and appears "dead," many times it is just the line cord, receiver cord, or receiver in that order of probability.

There is another phase connection issue for three-phase motors. The fact is that not all three legs may have precisely the same voltage, due to local single-phase loading and line impedance between transformer and load. At the same time, the motor windings may exhibit slight differences in impedance. Phase imbalance will be either increased or decreased depending on how the lines are matched to the motor windings. The objective is to get uniform voltage with the motor running. To this end, it is possible to change the connections to get all three possible combinations without reversing the motor direction.

This is done by moving A to B, B to C, and C to A. The procedure is called "rolling" the connections, and performing this tune-up will often improve motor performance and reduce operating temperature.

Where three-phase motors are in use, reliability and efficiency are prime concerns; and for large, expensive motors that play a key role in the workflow, it is essential to have a good preventive maintenance program in place. The basic elements are temperature monitoring and lubrication-with a written log posted nearby-as well as cleanliness, so that dirt and debris will not accumulate and impede air circulation necessary for cooling.

Additional Considerations

Do not neglect the load. It may not be part of the electrical installation per se, but it impacts motor performance greatly. Any binding or mismatching can cause the motor to be overworked, making for expensive operation and short motor life. Vibration, moisture, and high ambient temperature are harmful for three-phase motors. The two primary concerns, as with single-phase motors, are insulation integrity and bearing life.

Years ago, there were more dc motors, and speed control could be accomplished simply by varying the supply voltage. This is not possible for an ac motor. Some ac motors operate at two or more speeds, but this is accomplished by having separate windings for each speed, with appropriate switching.

Rotary power in industrial settings has been vastly enhanced by the development of the variable frequency drive (VFD). This is an umbrella term for an ac motor that receives its power from a main drive controller in conjunction with an operator interface. The usual ac motors controlled by VFDs are three-phase synchronous and induction machines. While synchronous three-phase motors are sometimes preferable, the three-phase induction motor pairs up nicely with a VFD and has the advantages of lower cost, simplicity, and ease of maintenance. The combination is very workable and it means you can get a continuous speed range out of a single-speed motor, as required in many applications including high-horsepower installations.

Most VFDs conform to a single design, although some are built with numerous bells and whistles for enhanced operation and self-diagnostic capabilities. Typically, a three phase 460-volt ac input comes into the enclosure via three conductors of the proper ampacity based on the drive's nameplate rating.

A heavily heat-sinked three-phase full-wave diode bridge provides dc to large capacitors. The dc goes into the inverter section, which produces three-phase ac power for the motor. The inverter contains transistors, diodes, and other electronics that create this waveform at the desired frequency to determine the speed of the three-phase synchronous or induction motor.

Troubleshooting the system is greatly facilitated by having on hand the manufacturer's manual with schematic and parts list.

A VFD presents diagnostic challenges but an orderly approach and understanding of the fundamentals will work wonders. As always, begin with a visual inspection. VFDs often operate in a harsh environment, with a lot of vibration, heat, and dust. Moisture is always a possibility. Thoroughly clean the inside of the enclosure and surrounding area after locking out power and discharging all capacitors. A vacuum cleaner is the tool of choice. It should have a plastic nozzle so that nothing can be shorted. Heat sinks must be thoroughly cleaned, as dirt will impede heat transfer. Make sure that there are no foreign objects including manuals or paperwork that could block airflow or present a fire hazard. If there is a cooling fan, verify that it is working. It will not hurt to lubricate the bearings.

Check all wiring connections and retorque as needed. Loose input and output connections are a major cause of failure in this equipment.

Check input and output voltages. Input voltage should not exceed 5 percent deviation, and the three legs should be uniform.

The VFD troubleshooting protocol assumes that the electrical supply and motor have been checked out. Do not neglect the load and power transmission (shaft, belts, etc.) between motor and load. If these components are okay but the motor is not running or it is tripping out, then you have made the right choice in going into the VFD.

The following tests may be made with a multimeter.

Verify that there is no power on the dc bus. With the multimeter switched to the diode check function, place the negative lead on the positive dc bus. Place the positive lead on each of the input conductors that are connected to the rectifier diodes. They are now reverse biased, and you will read a voltage drop on each input terminal. Then place the positive lead on the negative dc bus. Put the negative lead on each of the incoming lines and see if there is a forward diode drop. From these measurements, you can determine if the diode bridge is shorted, open, or good.

If your finding is that the input rectifier diodes are not defective, you will want to check the VFD output section. Place the positive meter lead on the negative dc bus and connect the meter's negative lead to each of the three output terminals that are connected to the motor.

Look for a forward-biased voltage drop. You may see that the output circuitry is either open or shorted, that is, not functioning in good semiconductor fashion. Alternately, the dc bus fuse could be blown. This may be the sole problem, or it may be indicative of a fault elsewhere.

Throughout, carefully inspect input and output devices for any sign of failure such as a burnt appearance or physical distortion.

The next step is to use the multimeter to check all capacitors for open or short. This is a likely failure mode, and is frequently apparent upon visual inspection. An oscilloscope will reveal a harmful ripple after the capacitor.

You can make saddle, offset, or simple bends in PVC conduit (RNC) by heating it uniformly and forming the bend by eye. Do not use a propane torch; you will burn the outside before the inside is soft enough to bend. An electric blanket PVC conduit bender works very well but is expensive and prone to burn out. Various tools that use your vehicle's exhaust are on the market. Use your ingenuity and see what you can invent. I had good results holding the conduit over a charcoal grill and turning it slowly.

These are the principal diagnostic procedures for the main current path within a VFD. However, much more is involved in that operating parameters need to be programmed into the unit, and there are always the possibilities that the procedure was improperly performed or that some errant electrons caused a glitch to develop. Many technicians have problems in this area. The VFD functions in accordance with commands from the user interface, and these inputs have to find their way through the internal electronics in such a manner that the VFD will do what you want. It is a very smart machine, but despite what you may think when it is indulging in erratic behavior, it does not have a mind of its own.

(We are still a few years away from machine consciousness.) Most VFDs have onboard diagnostics as part of the user interface, and they are depicted on the alphanumeric display in the form of error codes. For the most part, the error codes are listed in the manual, and clear explanations and corrective actions are given. Most of this equipment is user-friendly and, as you expand your troubleshooting skills, you should be able to diagnose and repair these machines. Here is a suggestion: Install a telephone jack adjacent to the VFD (or use a cordless phone) and call the manufacturer. A technician will talk you through the necessary procedures.

A final word on VFD work: Lethal voltages are present throughout the drive, from power supply lines straight through to the motor windings. Even with power disconnected and locked out, the capacitors will hold a charge for a long time because they have high capacitance. You need a safe way to connect the proper load across each capacitor in order to discharge it. I suggest large insulated alligator clips, connected while wearing insulated lineman's gloves. These gloves are tested regularly by inflating them with air. A small pinhole or crack in the insulating material can expose you to hazardous electrical energy.

A further word of caution: When putting a VFD back in service, it is important not to overspeed the load, or you could send the contents of a conveyer right through a brick wall.

Furthermore, you have to realize that the motor and load will be used on a daily basis, sometimes around the clock, and a malfunction down the road, built into the system and latent like a terrorist sleeper cell, could emerge at some time in the future to jeopardize the safety of workers.

Thus far, we have not had much to say about the National Electrical Code (NEC). It plays a prominent role in the professional life of electricians in areas where it has jurisdiction, although for the electronic technician the impact is somewhat smaller. One area where it becomes crucial is in the design of motor installations, especially in the workplace.

Motors have to be set up correctly at the outset, and the electrical supply wiring configured in the right way, or the installation could be hazardous or fail to work at all. Setup errors can make for a very short motor life as well. High-horsepower motors, including installation costs, can run many thousands of dollars, constituting one of the major investments in an industrial location.

NEC and the Motor NEC is vital in all of this. Nevertheless, motor design is somewhat counterintuitive, especially when it comes to overcurrent protection and ampacity calculations. As an electrician, you have to set aside temporarily what you know about "ordinary" wiring when undertaking a large motor installation. The principal guide and source of knowledge in this matter is the Code, so we will do a survey of the relevant material.

The basic fact is that motors draw vastly more electrical current during start-up than they do once the load is brought up to operating speed. If you were to provide overcurrent protection at the source of the branch circuit based on the full-load current of the motor in question, it would cut out every time you tried to start it. Therefore, it is necessary to approach the matter differently. A large motor branch circuit is protected at its source at a much higher (less sensitive) level. This overcurrent protection is valid only for branch circuit short-circuit and ground-fault protection, not for overload at the motor.

To see how this is done, we will back up and examine in some detail NEC Article 430, Motors, Motor Circuits, and Controllers. First, we will put the venerable NEC in historical context in order to provide perspective.

If a small electric motor like in a fan will not start when energized, see if it turns hard by hand. If you give it a spin, it should coast awhile, not stop right away. If it does not turn freely, oil the front and rear bearings. Use penetrating oil at first, and then follow up with a heavier machine oil to provide lasting lubrication.

Thomas Edison and his large group of associates, in the latter part of the nineteenth century, conceived of a generation and distribution system that would bring electrical power to individual homes and businesses. The focus at the time was to provide electric lights in these buildings and outside, an improvement over existing gas lights. The concepts of fusing and overcurrent protection, even circuit breakers, had been developed decades earlier, and they were in use in telegraph systems. Therefore, the basic elements were in place. Early electricians fished wires through disused gas pipes, which became the first electrical conduit runs.

The overall system worked, and electrical power was brought into the home and workplace. However, these installations were not safe. Electrical fires and electrocutions became frequent events. Insurance companies sustained great financial loss, and they and other interested organizations developed the first National Electrical Code in 1897. A few years later, it came under the jurisdiction of the National Fire Protection Association (NFPA), which currently releases a revised edition every three years. A large number of committees and individuals work to create each new version, which contains many revisions and a large amount of new material. Electricians have to read and assimilate these new mandates and integrate them into their work on a continuous basis.

The NEC as issued by the NFPA has no legal standing on its own, but is issued so that states, municipalities, and other jurisdictions may enact it into law. Currently, the NEC is the principal electrical authority by statute throughout the United States, Colombia, Costa Rica, Mexico, Panama, Puerto Rico, Venezuela, and other locations. The Canadian Electrical Code (CEC) is substantially similar, and Europe's International Electrotechnical Commission governs electrical design and installation in that region.

The NEC is divided into an Introduction and nine sections. Sections 1 through 4 apply generally. Sections 5 through 7 apply to special occupancies, special equipment, or other special conditions. Section 8 covers communication systems.

Section 4 is titled Equipment for General Use, and here we find Article 430, Motors, Motor Circuits, and Controllers. This article, one of the longest in the Code, is made up of over 50 pages covering the complex subject of motor installation. To diagnose and repair motorized equipment and the systems that support it, it is essential to be aware of Code mandates that are involved because any errors can create a hazardous situation.

Article 430 conforms to the consistent NEC template for articles. What this means is that insofar as the subject matter allows, the structure of each article is the same, with a decimal numbering system for headings and subheadings.

In this spirit, Article 430 begins with Section 430.1, Scope, and then proceeds to list definitions that pertain to the article. Next on the agenda are short discussions of part winding motors (430.4) and Ampacity and Motor-Rating Determination (430.6). The main concept being introduced at this point is that methods for ampacity determination differ for various types of motors. Therefore, if you are planning to wire a motor to the source of power, you need to consult this section in order to determine the ampacity, circuit size, and thence the size of the supply conductors to be used.

Section 430.7, Marking on Motors and Multimotor Equipment, lists the information that manufacturers, in order to be Code compliant, must include on the nameplate. The nameplate is all-important, and the information it displays provides the starting point for designing a new motor installation.

Part II is titled Motor Circuit Conductors, and it covers the information you need to know to size a motor circuit. The basic rule is that the conductors that supply a single motor, as opposed to a group installation, are to have an ampacity of not less than 125 percent of the motor full-load current rating. How is that rating determined? It is determined by consulting Tables 430.247, 430.248, 430.249, and 430.250. These tables are found at the end of the article, and they give full-load currents for various horsepower motors. So the thing to remember is that regardless of what the nameplate may say, to find the full-load current of a motor, take the horsepower off the nameplate and then go to one of the three tables depending on the current type or number of phases. Table 430.247 is for dc motors, Table 430.248 is for single-phase ac motors, Table 430.249 is for two-phase motors, and Table 430.250 is for three-phase motors. (Two-phase systems are rarely used and are mostly obsolete.) Section 430.6(2), Nameplate Values, states that separate motor overload protection is to be based on the motor nameplate current rating.

This is the first mention of separate overload protection, and it is the key concept in wiring motors because it provides the procedure for protecting motors from overcurrent while at the same time allowing for the very high inrush starting current that motors require.

An interesting and valuable table is 430.7(B), Locked-Rotor Indicating Code Letters. This is among the information required to be displayed on the motor nameplate, and it consists of a single letter, A through V. (I is not included because it could be confused with the number 1.) Each letter corresponds to a certain number of kilovolt-amperes per horsepower drawn by a motor with a locked rotor. This becomes clear once a couple of points are clarified.

Kilovolt-amperes are similar to kilowatts. One volt-ampere is equal to 1 watt in a dc circuit, but the former takes into consideration the effect of frequency. At 60 Hz, the effect is not too great, but volt-ampere terminology is more technically correct.

The values given in the table are per horsepower, so this means they are consistent for any size motor-large or small. Moreover, the figures are applicable to a motor with a locked rotor, that is, a rotor that is mechanically fixed so that it cannot turn, or encumbered with a very heavy load for the size of the motor so that it will fail to start. Of course, the locked-rotor current will be quite large, especially for a big motor. Going down the code letter alphabet, the locked-rotor current becomes higher. Code Letter A motors have a maximum locked-rotor current of 3.14 kilovolt-amperes per horsepower, while Code Letter V motors have a minimum locked-rotor current of 22.4 kilovolt-amperes per horsepower.

This represents a very large range. A motor with the lower locked-rotor current is a higher impedance motor. Generally, motors should not be run with the rotor locked or they will quickly burn out. However, with sufficiently high impedance and low supply voltage, a motor can operate indefinitely in a locked-rotor state without overheating. Such a motor is called a "torque motor."

Often we will see more than one motor on a single circuit. These are called "group installations." Part II includes information on wiring them. It states that conductors supplying several motors, or a motor and other loads are to have an ampacity not less than the sum of each of the following:

1. 125 percent of the full-load current rating of the highest-rated motor.

2. Sum of the full-load current rating of all the other motors.

3. 100 percent of the noncontinuous non-motor load.

4. 125 percent of the continuous non-motor load.

Part III, Motor and Branch-Circuit Overload Protection, describes the overload protection, which may take the form of a fuse or circuit breaker. Its purpose is to protect the motor from failure to start due to excessive or defective load, single phasing of a three phase motor, or other adverse conditions including problems inside the motor. This protection is in addition to and separate from the branch-circuit short-circuit and ground fault protection at the source of the branch circuit.

To select the overcurrent protection device, we use the motor nameplate full-load current rating, not the value from the tables at the end of the article, which were used to select the branch-circuit short-circuit and ground-fault protection device.

The device is to be selected to trip at no more than the following percentage of the motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater: 125 percent.

Motors with a marked temperature rise of 40°C or less: 125 percent.

All other motors: 115 percent.

Service factor is one of the parameters marked on the motor nameplate. Motors with a larger service factor are more conservatively rated. If you are confronted with an installation where a motor consistently runs hot or trips out, you may want to go to a motor having a higher service factor rather than a motor of higher horsepower. Service factor has to do with the heat tolerance of the internal motor insulation.

It is further provided in Part II that where the sensing element or setting or sizing of the overload device is not sufficient to start the motor or to carry the load, higher size sensing elements or incremental settings or sizings are permitted to be used, provided the trip current of the overload device does not exceed the following percentage of motor nameplate full-load current rating:

Motors with a marked service factor 1.15 or greater: 140 percent.

Motors with a marked temperature rise 40°C or less: 140 percent.

All other motors: 130 percent.

The most important provision having to do with motor overcurrent protection (because it allows motors to actually start) follows.

If not shunted during the starting period of the motor, the overload device is to have sufficient time delay to permit the motor to start and accelerate its load.

An Informational Note states that a class 20 or class 30 overload relay will provide a longer motor acceleration time than a class 10 or class 20 relay, respectively. Use of a higher-class overload relay may preclude the need for selection of a higher trip current.

Table 430.37, Overload Units, specifies the number and location of overload units, such as trip coils or relays.

The numbers and locations vary depending on the current type (ac or dc) and the phase of the motor. What is surprising to many apprentice electricians is that the overload relay may be in just one hot conductor, for example, in a 240-volt single-phase supply. This is because the overload is not intended to perform the disconnect function. However, in a three-phase system there must be an overload in each of the three legs so that the motor will not attempt to run and be damaged if one leg is lost.

Part IV, Motor Branch-Circuit Short-Circuit and Ground-Fault Protection, specifies devices that will protect the entire circuit, including controls, through to the motor. It protects them against short circuits and ground faults at a high level, but does not protect the motor against overloads, which is done at a lower level with time delay built into the overload protective device.

Table 430.52 gives the maximum rating or setting of branch-circuit short-circuit and ground-fault protective devices for seven types of motors, using four types of protective devices. These ratings are given as percentages of full-load current. You will notice that they are quite high. For example, a single-phase motor can be protected at the branch-circuit level at 175 percent of the full-load current using a nontime delay fuse, 250 percent with an inverse-time breaker, 300 percent with a nontime delay fuse, and 800 percent with an instantaneous trip breaker! Additionally, where these values do not correspond to standard sizes, it is permitted to go to the next higher rating.

On the subject of several motors or loads on one branch circuit, Article 430.53 lays out rules for single-motor taps. NEC tap rules are sometimes difficult for electricians, both on licensing exam and actual design/installation levels, because these rules do not appear in any one Code location. They are spread throughout this heavy volume, which means you have to know where to look. The motor tap rules provide that for motor group installations, the conductors for any tap supplying a single motor are not required to have an individual short-circuit and ground-fault protective device, provided they comply with one of the following:

1. No conductor to the motor is to have an ampacity less than that of the branch circuit conductors.

2. No conductor to the motor is to have an ampacity less than 1/3 that of the branch circuit conductors, the conductors to the motor being not more than 25 ft long and being protected from physical damage by being enclosed in raceway.

3. Conductors from the branch-circuit short-circuit and ground-fault protective device to a listed manual motor controller additionally marked "Suitable for Tap Conductor Protection in Group Installations" or to a branch-circuit protective device are permitted to have an ampacity not less than 1/10 the rating or setting of the branch-circuit and ground-fault protective device. The conductors from the branch-circuit and ground-fault protective device to the controller are to (a) be suitably protected from physical damage and enclosed either by an enclosed controller or by a raceway and be not more than 10 ft long, or (b) have an ampacity not less than the branch-circuit conductors.

You will notice, in interpreting tap rules, that there is a range of permitted reduced sizes available from which to choose. In addition, to compensate there is a range of techniques for required isolation from damage and maximum lengths permitted. For example, with the 1/3 reduction, you can run a 25-ft tap, while with the 1/10 reduction you can go only 10 ft.

All Code tap rules are similar, although they differ in the details.

Part VI, Motor Control Circuits, is especially applicable to the troubleshooting endeavor.

The controller enhances motor function and generally works quite well, but there are times when, rightly or wrongly, it will prevent the main supply current from getting through to the motor. Controllers are easy to diagnose and service because, aside from the main contacts, the amount of current and wire sizes are modest and the controller should be in a good accessible location.

First, we will look at NEC mandates for motor control circuits, and later we will discuss troubleshooting and repair techniques for motor controllers and associated wiring.

Generally, a motor control circuit is tapped from the load side of a motor branch-circuit short-circuit and ground-fault protective device. It functions to control the motor connected to that branch circuit and, of course, the primary motor overcurrent device is much too large to protect the controller, a relatively light load with small conductors, so the conductor must be protected separately. A tapped control circuit is not to be considered a branch circuit, so it may be protected by either a supplementary or a branch-circuit overcurrent device.

Table 430.72(B), Maximum Rating of Overcurrent Protective Device in Amperes, gives these ratings as a function of the control circuit conductor size. The general Code rule is that the minimum conductor size for any circuit is 14 AWG, but this table states that control circuit wires can be 16 AWG or 18 AWG, as long as they are protected by overcurrent devices as shown.

Where a motor control circuit transformer is provided, the transformer is to be protected as follows:

1. Where the transformer supplies a Class 1 power-limited circuit, Class 2 or Class 3 remote-control circuit complying with the requirements of Article 725, protection must be in accordance with that article.

This raises the complex subject of Class 1, Class 2, and Class 3 remote control, signaling, and power-limited circuits. Requirements are found in NEC Article 725, and it is recommended that individuals interested in troubleshooting and repairing commercial electrical equipment study this topic in detail. (It is discussed in my previous McGraw-Hill book, 2011 National Electrical Code Section By Section.) The Relevance of Article 725 Article 725 is frequently misconstrued as dealing with "low-voltage wiring" but this is a misnomer. The subject of Article 725 is remote control, signaling, and power-limited circuits; the voltages (in Class 1 circuits) can run as high as 600 volts and there is not always a power limitation. In some cases, however, there are power and voltage limitations. Accordingly, Article 725 permits a relaxation of some specific Code requirements in some applications, while in other applications more rigid forms of protection are mandated. Motor control circuits may be permitted to conform to any one of these classes, so do not be too surprised if you see unusually small (less than 14 AWG) conductors with splices made outside of enclosures.

1. Compliance with Article 450, Transformers (Including Secondary Ties).

2. Less Than 50 Volt-Amperes-Control circuit transformers rated less than 50 volt amperes and that are not an integral part of the motor controller and located within the motor controller enclosure are permitted to be protected by transformer primary overcurrent devices, impedance-limiting means, or other protective means.

3. Primary Less Than 2 Amperes-Where the control circuit transformer rated primary current is less than 2 amperes, an overcurrent device rated or set at not more than 500 percent of the rated primary current is permitted in the primary circuit.

4. Other Means-Protection is permitted to be provided by other approved means.

"Approved" is Code terminology. It means allowed by the local electrical inspector or other authority having jurisdiction.

Section 430.74, Electrical Arrangement of Control Circuits, brings up a significant safety issue. It states that if one conductor of the motor control circuit is grounded, the motor control circuit is to be arranged so that a ground fault in the control circuit remote from the motor controller will not start the motor nor will it bypass manual or automatic safety shutdown devices.

Part VII, Motor Controllers, lays out requirements for controllers, as opposed to motor control circuits, covered previously. The overriding principle, stated at the outset, is that all motors that are powered by a premises electrical system must have controllers. However, for a stationary motor of 1/8 horsepower or less that is normally left running and is constructed so that it cannot be damaged by overload or failure to start, such as clock motors and the like, the branch-circuit disconnecting means, usually a circuit breaker, will serve as the controller.

Similarly, for a portable motor rated at 1/3 horsepower or less, the attachment plug and receptacle or cord connector will serve as the controller.

Larger motors require more advanced controllers, and the principle is that any motor controller must be capable of starting and stopping the motor it controls, and interrupting the locked-rotor current of that motor.

A branch-circuit inverse time circuit breaker rated in amperes is permitted to serve as a controller for any motor. In addition, a molded-case switch rated in amperes may serve as controller for any motor. A molded-case switch looks like a circuit breaker and it fits in a circuit breaker enclosure, receiving its power from the busbar and having screw terminals for outputs. It does not necessarily include overcurrent protection. It may serve as a motor disconnecting means, and must be sized at 115 percent of the motor full-load current.

Other devices are also permitted as motor controllers, notably air-break ("ordinary") switches, inverse-time circuit breakers, manually or power operated, and oil switches.

It is further provided that every motor must have an individual disconnecting means, although exceptions allow a single disconnecting means to serve a group installation under certain conditions, such as when a group of motors is in a single room within sight of the disconnecting means.

Part X, Adjustable-Speed Drive Systems, uses this generic term to cover the VFDs that we discussed earlier in Section 2.

The point is made right away that the installation requirements of the previous nine parts of Article 430 are applicable to Part X except where modified or supplemented therein.

Section 430.122, Conductors-Minimum Size and Ampacity, states that the electrical supply conductors for the power conversion equipment that is part of the adjustable drive system are to have an ampacity not less than 125 percent of the rated input current to the power conversion equipment. This means that you go by the adjustable drive system nameplate, not the motor nameplate, and add an additional 25 percent for good measure.

Some adjustable drive systems incorporate a bypass device, where power from the electrical supply can be shunted directly to the motor. The supply conductors for these systems are sized based on 125 percent of the rated input current to the adjustable drive system or 125 percent of the motor full-load current rating, whichever is higher.

Section 430.124, Overload Protection, states that overload protection for the motor is required. Some adjustable-speed drive systems include the overload protection and, if it is marked to so indicate, additional overload for the motor is not required. However, if a bypass device is installed to allow the motor to operate at full load speed, then overload protection must be included in the bypass circuit.

Section 430.126, Motor Over-Temperature Protection, makes note of the fact that the relationship between motor current and motor temperature changes when a motor is operated by an adjustable-speed drive. There can be a problem when an adjustable-speed drive causes a motor to run slower than its rated speed, especially when there is a fan attached to the motor shaft. There will be less cooling, and the motor can overheat.

Protection against motor heating is to be provided by one of the following:

1. Motor thermal protector

2. Adjustable-speed drive system with load- and speed-sensitive overload protection and thermal-memory retention upon shutdown or power loss

3. Over-temperature protection relay utilizing thermal sensors embedded in the motor

4. Thermal sensor embedded in the motor whose communications are received and acted upon by an adjustable-speed drive system Additional adjustable-speed drive provisions apply to equipment operating at over 600 volts, protection of live parts, and grounding.

The 2011 NEC section that covers adjustable-speed drives is brief because the Code does not have jurisdiction over the internal workings of factory-made electrical equipment. It is up to the authority having jurisdiction to approve or not approve a given piece of electrical equipment. Since the local inspector lacks the test equipment and knowledge to evaluate each factory-made unit in an installation, he or she will rely on listings by a testing organization such as Underwriters Laboratories, which has extensive testing facilities and resources to evaluate products.

This part of the Code, therefore, applies primarily to the branch circuit wiring that supplies power to the adjustable-speed drive, and to the conductors that run from the output of the drive to the motor.

The National Electrical Code says a run of conduit has to be put in as a complete system, terminated at both ends, before installing conductors. You can put in a pull rope as you hook up the conduit lengths, but not conductors. Sometimes the easiest way is to use a shop vac to pull a piston (so-called "mouse"). This is a foam rubber cylinder sized to fit the pipe, with string attached. If a piston is not available, make your own by carving it out of foam or similar material.

The foregoing has been a brief survey of Article 430, Motors, Motor Circuits, and Controllers. There are many provisions that have not been mentioned here. If your troubleshooting and repair involve any alteration of the original installation, it is suggested that you carefully review Article 430 and, indeed, the entire Code to ensure that all elements of your work are in compliance.

Concerning troubleshooting a motor controller and repairing it, these devices are not overly complex for many ordinary applications. You should be able to look at the controller and understand how it works without need of a manual or schematic. However, for a more complex controller, full documentation is required. The controller may be designed to start, stop, reverse rotation, change speed or torque, or any number of other motor parameters. A timer may be included to control the duration of sequential actions, and some elaborate controllers sense motor overload and temperature.

Types of Motor Controllers

Controllers may be human operated from one or more remote stations, or they may automatically receive instructions relating to position and the state of other sensors that are part of the driven equipment. As a generic example, we can conceive of a motor controller that has two electrical circuits-a control circuit powered by a low-voltage source such as a 24-volt transformer and a power circuit connected to the motor. The control signal will activate a magnet that will close three sets of contacts to power a three-phase motor.

Schematics often do not show the power circuit in the interest of simplicity.

The troubleshooting procedure, after looking over the controller for visible signs of damage, is to check input and output voltages of the power circuit with the contacts open and closed. If the power circuit appears functional but the contacts will not pull in, the magnet could be at fault or it is not being energized. See if there is voltage at the transformer secondary. (The transformer is probably located some distance away from the controller, between the load center and the actuator.) If there is no voltage at the secondary, check the primary. If you find the transformer is dead, which is often the case, the problem is solved. If the transformer is good, just out of curiosity compare the measured secondary voltage to the marked secondary voltage on the transformer. A few volts difference is not critical, but if there is a large discrepancy, there are shorted windings, and this can cause the controller to become erratic. The most common transformer fault, however, is zero volts at the secondary-an open primary or secondary. Of course, the whole thing could be a tripped breaker supplying the primary, perhaps indicating a shorted power supply line. In addition, there may be a low-ampere plug fuse associated with the transformer. Ohm readings, with the transformer taken out of circuit, are useful as well.

If the transformer is good, now that you know the secondary voltage put your meter on the controller input with the actuator properly set. If you do not read voltage, unhook one wire and try an ohm reading. It is possible that the controller input is shorted, pulling the voltage down to zero, the transformer and line having sufficient impedance so that the current level would not become too high.

In an industrial setting, with vibration and corrosion possible factors, control line faults are common. If you disconnect the line at both ends and temporarily tie the wires together at one end, you can do an ohmmeter check.

If there is voltage at the control input but the power contacts do not close, there can be an internal wiring fault or defective coil.

Returning to the power circuit, a frequent fault is bad relay contacts. A prelude to this is an audible 60-Hz hum. You can sometimes renew the contacts by pressing them gently on a thin file and rubbing the file back and forth. Afterward, a piece of paper has just the right abrasive property to burnish the contacts. Avoid using sandpaper because it can leave particles embedded in the metal, which will later make hot spots.

If wear is pronounced, the contacts can be replaced. Contact kits are readily available.

We have been talking about magnetic mechanical relays. Many of them are still in service.

The overloads are often "heaters," which come in various ampere ratings and are easily replaced.

Today many controllers are solid-state and they are quite reliable, although faults may occur.

If the motor is cutting out periodically, check the heater value or solid-state overload setting against the motor full-load current (with permitted Code adjustment) to find out if the motor is protected at the correct level. Remember that a motor will draw more current and run a little hotter as it ages, and some of this is acceptable.

In industrial facilities, motors are a major part of the environment, and maintenance electricians spend a good part of their time working on them. You need to become a motor expert. Examine, visually and with test equipment, good running motors and keep dated records of the results. Read the documentation and, if warranted, make sure that good replacement motors are stocked. Where used motors are available for replacement purposes, clean and test run them, and label them. Check the bearings and if there is significant play, replace them.