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DC Motor Types
Introduction
Basically, four different types of DC motors are used in industrial applications: series wound, shunt wound, compound wound, and permanent magnet. Several factors must be considered when selecting a DC motor for a specific application.
First, decide what the allowable variation in speed and torque can be for a given change in load. Each type of motor has benefits that are advantageous for certain applications. The following review will help you decide which motor may provide better performance in a given application. The DC motor and drive specifications should always be consulted to deter mine the specific speed and torque capabilities of the system. The speed/ torque curves listed below are for illustrative purposes.
Series Wound DC Motors
A series wound DC motor has the armature and field windings connected in a series circuit. FIG. 17 shows a series wound DC motor, with an associated speed/torque curve.
As seen in FIG. 17, this type of motor configuration features very high breakaway torque. Typical applications for this motor would be printing presses, ski lifts, electric locomotives, cranes, and oil drilling.
The starting torque developed can be as high as 500% of the full load rating. The high starting torque is a result of the fact that the field winding is operated below the saturation point.
An increase in load will cause a corresponding increase in both the armature and field winding current, which means that both armature and field winding flux increase together. As you recall, the torque developed in a DC motor is the result of the interaction of armature and field winding fluxes. Torque in a DC motor increases as the square of the current value.
FIG. 17. Series wound DC motor circuit and curve
A series wound DC motor will generate a larger torque increase compared with a shunt wound DC motor for given increase in current.
Conversely, the speed regulation of a series wound DC motor is poorer than that of a shunt wound motor. As stated above, when the load increases, so does the armature and field winding current. When the load is reduced, so is the current, which causes a corresponding decrease in flux density. As a reminder of DC motor basics, when the field flux is reduced once the motor is running, a decrease in "hold-back" electromotive force (EMF) occurs. Therefore, when the load is reduced, speed increases. If the load were completely removed, the speed of the motor would increase to infinity-basically until the motor destroys itself. As a safety precaution, series wound DC motors should always be connected to a load.
Parallel (Shunt) Wound DC Motors
A shunt wound DC motor has the armature and field windings connected in parallel. FIG. 18 shows a shunt wound DC motor, with an associated speed/torque curve.
FIG. 18. Shunt wound DC motor and curve
This type of DC motor is probably the most widely used motor in industrial applications. As indicated in the figure, this type of motor requires two power supplies-one for the armature and one for the field winding.
Typical applications for this motor would be printing presses, ski lifts, plastic extruders, conveyors, and practically any other application where DC motors are used. Because of the need for two power supplies, this type of motor is a prime candidate for a DC drive (converter), which usually includes a low-current field winding exciter (power supply).
With constant armature voltage and field winding excitation, this type of motor offers relatively flat speed/torque characteristics. The starting torque developed can be 250-300% of the full load torque rating for a short period of time. Speed regulation (speed fluctuation due to load) is accept able in many cases between 5-10% of maximum speed, when operated from a DC drive. Regulation of this amount would be typical when operated from a drive controller, open loop (no electronic feedback device connected to the motor shaft). As discussed in Section 5, speed feedback devices such as a tachometer generator can dramatically improve the regulation (down to less than 1%).
Because of the need for two power sources, the shunt wound DC motor offers the use of simplified control for reversing requirements. Direction of any shunt wound motor can be changed by simply reversing the direction of current flow, in either the armature or shunt field winding. The capability of armature or field reversal is standard on many DC drive modules. (In many cases, the reversing of flux and direction is accomplished in the field winding control. The field winding consumes less than one tenth of the current compared with the armature circuit. Smaller components and less stress on circuitry is the result when "field reversal" is used for DC motor control.)
Compound Wound DC Motors
A compound wound DC motor is basically a combination of shunt wound and series wound configurations. This type of motor offers the high starting torque of a series wound motor. In addition, it offers constant speed regulation (speed stability) under a given load. This type of motor is used whenever speed regulation cannot be obtained from either a series or shunt wound motor. FIG. 19 indicates a compound wound DC motor, with an associated speed/torque curve.
The torque and speed characteristics are the result of placing a portion of the field winding circuit in series with the armature circuit. This additional armature winding circuit is not to be confused with the commutating winding or interpoles. The commutation windings also have a few turns, but have the duty of neutralizing armature reaction.
When a load is applied, there is a corresponding increase in current through the series winding, which also increases the field flux. This in turn increases the torque output of the motor.
FIG. 19. Compound wound DC motor and curve
Permanent Magnet DC Motors
A permanent magnet motor is built with a standard armature and brushes, but has permanent magnets in place of the shunt field winding. The speed characteristic is close to that of a shunt wound DC motor. When adding the cost of a DC motor and control system, this type of motor is less expensive to operate, since there is no need for a shunt field winding exciter supply. FIG. 20 indicates a permanent magnet DC motor, with an associated speed/torque curve.
FIG. 20. Permanent magnet DC motor and curve
Along with less expensive operation, this type of motor is simpler to install, with only the two armature connections needed. This motor type is also simpler to reverse-simply reverse the connections to the armature.
The permanent magnet poles are usually constructed of materials such as ceramic or alnico (aluminum, nickel, and cobalt). The ceramic magnets are used for low-horsepower, slow-speed applications because of their low flux level generation. Though this type of motor has good operational characteristics and lower cost, there are several drawbacks to this type of motor compared with the others.
Materials such as ceramic have a high resistance to demagnetization. How ever, permanent magnets do have a tendency to lose some of their magnetic strength over use and time. This reduction in magnetic field strength causes a corresponding reduction in torque output. To counteract this possibility, some higher-cost permanent magnet motors include windings built into the field magnets for the purpose of "re-magnetizing" the mag nets.
In addition to ceramic or alnico magnets, rare earth magnets are also a cost-effective means of generating magnetic field flux. This type of magnetic group includes the "embedded" magnet, which is only one of nine different magnetic materials available.
Though this type of motor has very good starting torque capability, the speed regulation is slightly less than that of a compound wound motor.
The overall torque output makes this motor a prime candidate for low torque applications. Peak torque is limited to about 150%. This limitation is based on the fact that additional "demagnetizing" of the field poles could occur if more torque was developed.
Specialty DC Motors-PM DC Servomotors
Servomotors are considered "specialty" in that they are used in applications that require very fast speed response and accuracy. In many cases, the shaft speed is accelerated from zero to 6000 rpm in hundredths of a second. The same speed profile could be needed in the deceleration mode, as well as an immediate reversal of direction.
These types of motors must be designed to handle the stress of acceleration, plus not fluctuate in speed, once the desired speed is obtained. Special consideration is given to heat dissipation, since these motors must be small, yet generate enough torque to operate the machine. The small size allows this type motor to fit inside small packaging, palletizing, and processing machines. Typically, these motors are long and narrow, in contrast to a standard shunt wound DC motor. The long, narrow design results in low inertia armature assemblies, which can be accelerated quickly. Servo motor design with permanent magnets affords the smallest space possible.
In comparison, shunt field windings must have laminations wide enough to generate the necessary field flux, which adds to the total width of the machine. FIG. 21 indicates the physical appearance of a typical DC servomotor.
As seen in FIG. 21, this type of motor is usually of a totally enclosed design to seal out most moisture, dust, and moderate contaminates. The physical frame of the motor acts as a heat sink to dissipate the heat generated.
FIG. 21. DC servomotor with C face mounting (Baldor Electric Company)
Many servomotors are used expressly for positioning applications. There fore, the motor design allows for a position feedback device such as an encoder or resolver. Mounting of the servomotor can be easily done by means of a "C" face (no flange, but tapped holes to receive mounting bolts) or "D" flange (outside flange with through-holes).
The principle involved in the PM servomotor is exactly the same as the standard PM DC motor. It has an armature, commutator, and the PM field for magnetic interaction. The difference comes in the physical size and shape of the servomotor, as well as the performance and speed characteristics.
Specialty DC Motors--Brushless Servomotors
Another type of DC servomotor uses the high-torque and acceleration characteristics, but without the use of a commutator or brushes. This type, called the brushless DC servomotor, takes input three-phase or single-phase input power and converts it to DC used by the motor windings. The windings create magnetic flux that interacts with the PM field to generate motor speed and torque. FIG. 22 shows the design of the brushless DC servomotor.
As seen in FIG. 22, instead of the permanent magnets being mounted as the field, the magnets are actually part of the rotor. (Note: Since there are no brushes or commutator, the term "rotor" is used instead of armature, indicating an AC-type machine design.) A typical brushless DC servo motor may have multiple poles, such as three N and three S poles. There would also be corresponding windings in the stator to create the magnetic interaction. (Note: Because this is an AC-type machine design, the term "stator" is used instead of "field" or "field windings.")
FIG. 22. Brushless DC servomotor (design and control)
The rotor of the servomotor is usually laminated iron with magnets inserted and "press-fit" or epoxied into position. Special high-speed bearings support the rotor in position. Instead of a standard conduit box, servomotors usually include a military-style connector. This style features all connections on one plug or receptacle, with a screw-on ring to ensure positive contact. This style of connector is resistant to machine vibration and electrical interference.
The servomotor takes input power and converts it to DC for the main windings in the stator. Depending on the design of the servomotor, the control unit may include transistors that are turned on or off to generate voltage. In the case of a three-phase servomotor, an external servo amplifier is connected to generate the control voltage for the stator windings.
The main disadvantage of this motor is the inability to develop high starting torque. In the case of a single-phase servomotor, half of the main windings are used at any given time. This causes the copper losses to be somewhat high. However, since transistor switching is used for control of the brushless DC servomotor, motor life is mainly limited by the bearings, since there are no commutator segments or brushes to wear out.
AC Motors: General Principles of Operation
Introduction
The squirrel cage induction motor is probably the most widely used motor in industry today. Traditional applications for AC induction motors include fans and pumps. The AC induction motor has been widely accepted in many demanding industrial applications, compared with the DC motor, because less maintenance is required. It is quite common to find AC motors in applications such as compressors, machine tools, conveyors, mixers, crushers, ski lifts, and extruders. With its efficient operation and energy savings characteristics, the AC induction motor will increase in prominence throughout the next several decades.
The basic principles of operation of any manufacturer's motor is essentially the same. Specific designs may differ, such as the air gap between the rotating parts, voltage insulation strength, and resistance to high-voltage spikes. However, the main parts in an induction motor are all the same.
It should be noted here that in the world of AC motors, there are basically two languages: NEMA (National Electrical Manufacturers Association) in North America, and IEC (International Electrotechnical Commission) in most of the rest of the world. Until recently, there was little need to be aware of the differences, both subtle and obvious. However, that is all changing as the motor market becomes more global. This trend gained additional fuel in 1992 when the economies of the European Common Market countries became one.
Later in this section, NEMA versus IEC motor ratings will be explored.
More companies are shipping their electrical products overseas, and vice versa. In the not-so-distant future, it will be difficult to not come in con tact with an IEC-rated motor. Therefore, a review of the comparisons will be useful. In addition, because of industry's wide use of three-phase induction motors, the focus of this section will be on that motor type. However, several other common three-phase motor types will also be explored.
All AC motors can be classified into single-phase and polyphase motors (poly meaning many-phase, or three-phase). Because polyphase motors are the most commonly used in industrial applications, we will take a closer look at the construction of these units. Keep in mind that there are also single-phase AC motors in use for applications such as small appliances, residential fans, furnaces, and many other low-horsepower applications.
For industrial applications, however, mainly three-phase induction motors are used. The main advantage of using three-phase motors is efficiency.
Three-phase motors are much simpler in construction than other types and require less maintenance. A more powerful motor can be built into a smaller frame compared with a single-phase motor. The three-phase motor will operate at a higher efficiency compared with the single-phase motor.
There are several types of polyphase motors: induction, wound rotor, and synchronous. The most common type of motor in this group is the squirrel cage induction motor. This motor type will be the basis for understanding the general AC motor principles.
AC Induction Motor
The main parts in an AC induction motor is the rotor (rotating element) and the stator (stationary element that generates the magnetic flux). The rotor construction looks like a squirrel cage, hence the traditional name: squirrel cage induction motor. FIG. 23 indicates the rotor construction.
FIG. 23. Squirrel cage induction motor rotor
The squirrel cage motor is the simplest to manufacture and the easiest to maintain. The operation of the squirrel cage motor is simple. The three-phase current produces a rotating magnetic field in the stator. This rotating magnetic field causes a magnetic field to be set up in the rotor also. The attraction and repulsion between these two magnetic fields causes the rotor to turn. This concept can be seen in Figures 3-24 and 3-25
FIG. 24. Rotor and stator
The squirrel cage motor is a constant-speed motor with either a normal or high starting torque. These characteristics fulfill the requirements of the majority of industrial applications.
FIG. 25. End view of two rotor segments (magnetic interaction with
stator)
The concept of the rotating magnetic field is shown in FIG. 26. This figure shows the relationship of the three-phases versus pole magnetic fields. Each magnetic pole pair is wound in such a way that allows the stator magnetic field to "rotate." The stator of the motor consists of groups of coils wound on a core, which are enclosed by a frame. The simple two pole stator shown in FIG. 26 has three coils in each pole group. (A two-pole motor would have two poles × three phases = six physical poles.) Each coil in a pole group is connected to one phase of the three-phase power source. One characteristic of three-phase power is that the phase current reaches maximum value at different time intervals. FIG. 26 also indicates the relationship between maximum and minimum values.
FIG. 26. Basic two-pole stator
For the sake of example, our focus will be an instant in time when the cur rent is almost at maximum in the "A" coils. (Use the upper left corner of FIG. 25.) The magnetic fields of these coils will also be at almost maxi mum value. At this same instant, the currents of phase "B" are at zero and phase "C" currents are slightly more than "A." At a later instant in time, the current in the "B" coils is close to maximum with consequent maximizing of the magnetic field of the "B" coils. At this same instant, the field of the "C" phase is slightly less than maximum. The "A" coil fields are at zero value.
This same process repeats as the magnetic field of each of the phases reaches maximum, all at different times (different degrees of magnetic field rotation). The maximum field thus sequentially repeats at "A," "C," and "B" continuously around the stator and essentially defines a rotating magnetic field.
The coils of the stator are wound such that they are diametrically opposite coils. This means that they carry the same phase current but are connected so their magnetic fields are of opposite polarity. Again, the motor shown in FIG. 26 would be a configuration of a two-pole winding.
Magnetic Field (Rotor)
The rotor is the rotating part of the motor. The rotor consists of copper or aluminum bars, connected together at the ends with end rings. Refer to FIG. 27.
FIG. 27. Induction motor rotor construction
The inside of the rotor is filled with many individual disks of steel, called laminations. The revolving field set up by the stator currents, cut the squirrel cage conducting aluminum bars of the rotor. This causes voltage in these bars, called induced voltage. This voltage causes current to flow in the aluminum bars. The current sets up a magnetic field around the bars with corresponding north and south poles in the rotor. Torque is produced from the attraction and repulsion between these poles and the poles of the revolving stator field.
FIG. 28 shows the assembly of the parts into a complete induction motor unit.
FIG. 28. Induction motor construction
Eddy Current Generation
The rotating stator magnetic field and induced voltage in the rotor bars also causes voltage in the stator and rotor cores. The voltage in these cores cause small circulating currents to flow. These currents, called eddy currents, serve no useful purpose and result only in wasted power. To reduce these currents, the stator and rotor cores are constructed with laminations. (discussed in the previous section). These laminations are coated with insulating varnish and then welded together to form the core. This type of core substantially reduces eddy current losses, but it does not eliminate them entirely.
Induction Motor Design
Engineers can design motors for almost any application by changing the design of the squirrel cage rotor and stator coils. Characteristics such as speed, torque, and voltage are just a few of the features controlled by the designer.
To standardize certain motor features, the National Electrical Manufacturers Association (NEMA) has established standards for a number of motor features. The following section contains many of the features that will be helpful in selecting the right motor for a specific application.
Control of Speed, Torque, and Horsepower
Control of Speed
The speed of a squirrel cage motor depends on the frequency and the number of poles for which the motor is wound. The higher the frequency, the faster the motor operates. The more poles the motor has, the slower it operates. The smallest number of poles ever used in a squirrel cage motor is two. A two-pole 60-Hz motor will run at approximately 3600 rpm. As soon will be seen, the motor will always operate at a speed less than 3600 rpm.
To find the approximate speed of any squirrel cage motor, the formula for synchronous speed can be used, which is actually the speed of the rotating magnetic field:
N = F × 120 / P
N = synchronous speed (rpm)
F = frequency of the power supply (Hertz)
P = number of stator poles
Squirrel cage induction motors are wound for the synchronous speeds found in Table 1.
Table 1. Motor synchronous speeds vs. number of poles
Most standard induction motors (NEMA 143T through 445T frame sizes) are wound with a maximum of eight poles.
The actual speed of the motor shaft is somewhat less than synchronous speed. This difference between the synchronous and actual speeds is defined as slip. If the squirrel cage rotor rotated as fast as the stator field, the rotor bars would be standing still with respect to the rotating magnetic field. No voltage would be induced in the rotor bars, and no magnetic flux would be cut by the rotor bars. The result would be no current set up to produce torque. Since no torque is produced, the rotor will slow down until sufficient current is induced to develop torque. When torque is developed, the rotor will accelerate to a constant speed. FIG. 29 is a graphical representation of slip.
FIG. 29. Slip in an induction motor
To summarize: There must be a difference between the rotating magnetic stator field and the actual rotor bars' position. This allows the rotor bars to cut through the stator magnetic fields and create a magnetic field in the rotor. The interaction of the stator and rotor magnetic fields produce the attraction needed to develop torque.
When the load on the motor increases, the rotor speed decreases. Then the rotating field cuts the rotor bars at a faster rate than before. This has the effect of increasing the current in the rotor bars and increasing the magnetic pole strength of the rotor. Basically, as the load increases, so does the torque output.
Slip is usually expressed as a percentage and can easily be calculated using the following formula:
Percent slip = Synchronous speed - Actual - speed / speed Synchronous speed x 100
Squirrel cage motors are built with the slip ranging from about 3-20%. Motors with a slip of 5% or higher are used for hard-to-start applications.
A motor with a slip of 5% or less is called a normal slip motor. A normal slip motor is often referred to as a constant speed motor because the speed changes very little with variations in load.
In specifying the speed of the motor on the nameplate, most motor manufacturers use the actual speed of the motor at rated load. The term used is base speed. Base speed is a speed somewhat lower than the synchronous speed. It is defined as the actual rotor speed at rated voltage, rated hertz, and rated load.
Direction of Rotation
The direction of rotation of a squirrel cage induction motor depends on the motor connection to the power lines. Rotation can easily be reversed by interchanging any two input leads.
Control of Torque and Horsepower
As discussed earlier, horsepower takes into account the speed at which the shaft rotates. It takes more horsepower to rotate the shaft fast, compared with rotating it slowly. Note: Horsepower is a rate of doing work.
By definition, 1 HP equals 33,000 ft-lb per minute. In other words, lifting a 33,000-pound weight 1 foot, in 1 minute would take 1 HP.
By using the familiar formula below, we can determine the horsepower developed by an AC induction motor.
HP = TxN/5252
T = torque in lb-ft
N = speed in rpm
For example, a motor shaft turns at 5 rpm and develops 3 lb-ft of torque.
By inserting the known information into the formula, we calculate that the motor develops approximately 0.003 HP (3 × 5 ÷ 5252 = .0028). As the formula shows, horsepower is directly related to the speed of motor shaft.
If the shaft turns twice as fast (10 rpm), the motor will develop almost .006 HP, twice as much.
We can see the general rules of thumb for torque developed versus speed by reviewing Table 2.
Torque developed will vary slightly on lower HP and rpm motors or non standard motors.
As seen in Table 2, at higher synchronous speeds, the induction motor develops less torque compared with lower speeds. We can also see that the higher the number of poles, the larger the amount of torque developed.
Table 2. AC induction motor-speed vs. torque developed
Basically, more poles mean stronger magnetic fields that will be produced.
With more magnetic flux interacting with rotor flux, a stronger twisting motion will result, thereby developing more torque.
Regarding the issue of motor torque, there are several areas on the standard speed/torque curve that should be reviewed. An induction motor is built to supply this extra torque needed to start the load. The speed torque curve for a typical induction motor is seen in FIG. 30.
FIG. 30. Speed/torque curve for a NEMA design B motor
FIG. 30 shows the starting torque to be about 250% of the rated-load torque.
Peak (Breakdown) Torque
Occasionally a sudden overload will be placed on a motor. To keep the motor from stalling every time an overload occurs, motors have what is called a breakdown torque. The breakdown torque point is much higher than the rated load torque point. For this reason, it takes quite an overload to stall the motor. The speed/torque curve shown in FIG. 30 indicates the breakdown torque for a typical induction motor to be about 270% of the rated load torque.
Operating a motor overloaded for an extended period of time will cause an excessive heat buildup in the motor and may eventually burn up the motor windings.
The NEMA definitions and ratings for an induction motor's characteristic torque is given later in this section.
Locked Rotor Torque (Starting or Breakaway Torque)
The locked rotor torque of a motor is the minimum torque, which it will develop at rest for all angular positions of the rotor. This capability is true with rated voltage and frequency applied.
Pull-Up Torque
The pull-up torque of a motor is the minimum torque developed by the motor when accelerating from rest to the breakdown torque point. For motors that do not have a definite breakdown torque, the pull-up torque is the minimum torque developed up to rated speed.
Peak (Breakdown) Torque
The breakdown torque of a motor is the maximum torque that it will develop. This capability is true with rated voltage and frequency applied, without an abrupt drop in speed.
Rated Load Torque
The rated load torque of a motor is the torque necessary to produce the motor's rated horsepower at rated-load speed. (Note: Rated load speed is normally considered base speed. Base speed means actual rotor speed when rated voltage, frequency, and load are applied to the motor.) The above torque designations are all very important to the motor designer. Essentially, motors can be designed with emphasis on one or more of the above torque characteristics to produce motors for various applications. An improvement in one of these torque characteristics may adversely affect some other motor characteristic.
Enclosure Types and Cooling
Motors are often exposed to damaging atmospheres such as excessive moisture, steam, salt air, abrasive or conducting dust, lint, chemical fumes, and combustible or explosive dust or gases. To protect motors, a certain enclosure or encapsulated windings and special bearing protection may be required.
Motors exposed to the following conditions may require special mountings or protection: damaging mechanical or electrical loading such as unbalanced voltage conditions, abnormal shock or vibration, torsional impact loads, excessive thrust, or overhung loads.
Many types of enclosures are available. A few of the most common types are listed here, many of which are the same designations as for DC motors.
It is strongly recommended that personnel actively involved with applying induction motors be familiar with, and adhere to, the contents of NEMA MG2 ("Safety Standard for Construction and Guide for Selection, Installation and Use of Electric motors and Generators").
Open Motor
The open motor enclosure type has ventilation openings that permit pas sage of external cooling air over and around the windings.
Open Drip-proof Motor
The open drip-proof (ODP) enclosure type is constructed so that drops of liquid or solids falling on the machine from a vertical direction cannot enter the machine. (This vertical direction is not at an angle greater than 15º.) Guarded Motor (This could be abbreviated DPFG-drip-proof fully guarded.) The guarded enclosure type has all ventilation openings limited to specified sizes and shapes. This enclosure prevents insertion of fingers or rods and limits accidental contact with rotating or electrical parts.
Splash-proof Motor
The splash-proof enclosure type is so constructed that drops of liquid or solid particles falling on the motor cannot enter. (These liquid or solid particles can be in a straight line or at any angle not greater than 100º from the vertical.)
Totally Enclosed Motor
The totally enclosed enclosure type prevents the free exchange of air between the inside and outside of the case, but is not airtight.
Totally Enclosed Non-Ventilated (TENV) Motor
The totally enclosed non-ventilated enclosure type is not equipped for cooling by external devices.
Totally Enclosed Fan-Cooled (TEFC) Motor
The totally enclosed fan-cooled enclosure type has a shaft-mounted fan to blow cooling air across the external frame. It is a popular motor for use in dusty, dirty, and corrosive atmospheres.
Totally Enclosed Blower-Cooled (TEBC) Motor
The totally enclosed blower-cooled enclosure type is totally enclosed and is equipped with an independently powered fan to blow cooling air across the external frame. A TEBC motor is commonly used in constant torque, variable-speed applications.
Encapsulated Motor
The encapsulated enclosure has windings that are covered with a heavy coating of material to protect them from moisture, dirt, abrasion, etc.
Some encapsulated motors have only the coil noses coated. In motors with pressure-embedded windings, the encapsulation material impregnates the windings, even in the coil slots. With this complete protection, the motors can often be used in applications that demand totally enclosed motors.
Explosion-Proof (TEXP) Motor
The explosion-proof enclosure is totally enclosed and built to withstand an explosion of gas or vapor within it. It also prevents ignition of gas or vapor surrounding the machine by sparks, flashes, or explosions that may occur within the machine casing.
Protection and Ratings Motor Protection
The typical method of starting a three-phase induction motor is by connecting the motor directly across the power line. Line starting a motor is done with a three-phase contactor. To adequately protect the motor from prolonged overload conditions, motor overloads are installed, typically in the same enclosure as the three-phase contactor. These overloads (OLs) operate as heater elements-heating to the point of opening the circuit, and mechanically disconnecting the circuit (FIG. 31).
FIG. 31. Line starting a motor
Overloads can be purchased with a specific time designed into the element. Classes 10, 20, and 30 are the usual ratings for industrial use. A class 10 overload indicates that the overload will allow 600% inrush current for 10 s before opening the circuit. Class 20 overloads would allow 600% inrush current for 20 s, and a class 30 would allow 30 s of operation. The current draw from a typical induction motor, as well as the torque produced can be seen in FIG. 32.
Line starting an induction motor, as shown in FIG. 32, would allow the motor to develop rated torque, as soon as the motor starter button is pressed. This is because across the line, the motor has the benefit of full voltage, current, and frequency (Hz). As long as the input power is of rated value, the motor would develop the torque as seen in FIG. 32, from zero to base speed.
FIG. 32. Motor torque and inrush current (line starting)
If the ratio of voltage to hertz is maintained, then the motor will develop the rated torque that it was designed to produce. This relationship can be seen in FIG. 33 and is designated the volts per hertz ratio (V/Hz).
FIG. 33. Volts per Hz ration (V/Hz)
As seen in FIG. 33, the V/Hz ratio is calculated by simply dividing the input voltage by the hertz. This characteristic is an important ingredient of AC drive design, which will be covered in the next section.
There may be applications where full torque is not desirable when the motor is started: a conveyor application in a bottling line, for example. If the feed conveyor has uncapped full bottles on the conveyor, full torque when the conveyor is started would be a not-so-good situation. (The bottles would spill all of their contents.) In cases like that, a reduced torque type of start would be required. There are also cases where full voltage and hertz, which causes 600% inrush current, would cause a serious power dip on the utility system. High-horsepower motors connected to compressors would be an example. In these cases, a reduced voltage start would be required. If the voltage is less than rated value, the motor would not develop rated torque (according to the V/Hz ratio listed in FIG. 32). Reducing the V/Hz ratio also reduces the starting current, which means there is less of a power dip.
Reducing the starting current may be accomplished in any one of the following ways.
Primary Resistor or Reactance
The primary resistor or reactance method uses series reactance or resistance to reduce the current during the first seconds. After a preset time interval, the motor is connected directly across the line. This method can be used with any standard induction motor.
Auto Transformer
The auto transformer method uses an auto transformer to directly reduce voltage and the current for the first few seconds. After a preset time interval, the motor is connected directly across the line. This method can also be used with any standard induction motor.
Wye-Delta
The wye-delta method applies the voltage across the Y connection to reduce the current during the first few seconds. After a preset time interval, the motor is connected in delta mode permitting full current. This type of induction motor must be constructed with wye-delta winding connections.
Part-Winding
The part-winding method uses a motor design that has two separate winding circuits. Upon starting, only one winding circuit is engaged and current is reduced. After a preset time interval, the full winding of the motor is connected directly across the line. This type of motor must have two separate winding circuits. To avoid winding overheating and damage, the time between first and second winding connections is limited to 4 seconds maximum.
Motor Ratings
When reviewing ratings, it is also necessary to review several design features of the induction motor. Induction motor design classifications, characteristics, and ratings will now be reviewed in detail.
Because of the variety of torque requirements, NEMA has established different "designs" to cover almost every application. These designs take into consideration starting current and slip, as well as torque. These motor design classes should not be confused with the various classes of wire insulation, which are also designated by letter.
Table 3. NEMA torque designs for polyphase motors
FIG. 34 indicates the relative differences in torque, given a specific motor NEMA design class. The motors indicated are all line started.
As seen in FIG. 34, the major differences are in the starting torque and peak or breakdown torque capabilities.
FIG. 34. Comparison of NEMA Designs (speed/torque characteristics)
Efficiency
The efficiency of a motor is simply the ratio of the power "out" to the power "in," expressed in percentage.
FIG. 35 illustrates the general relationship between current, slip, efficiency, and power factor.
Generally, motor efficiency is relatively flat from rated load to 50% of rated load. Some motors exhibit peak efficiency near 75% of rated load.
Power Factor
Power Factor (P.F.) is the ratio of real power to apparent power, or kW/ kVA. Kilowatts (kW) are measured with a wattmeter, and kilovolt amperes (kVA) are measured with a voltmeter and ammeter. A power factor of one (1.0) or unity is ideal. Power factor is highest near rated load, as seen in FIG. 35. Power factor at 50% load is considerably less and continues to dramatically decrease until zero speed.
FIG. 35. Speed, slip, efficiency and power factor relationships
Current Draw
Current draw in amperes is proportional to the actual load on the motor in the area of rated load. At other loads, current draw tends to be more non linear (FIG. 35).
Locked Rotor (kVA/HP)
Another rating specified on motor nameplates is locked rotor kVA per horse power. (Some manufacturers use the designation locked rotor amps.) A letter appears on the nameplate corresponding to various kVA/HP ratings. Refer to Table 4 for letter designations.
The nameplate codes are a good indicator of the starting current in amperes. A lower code letter indicates a low starting current and a high code letter indicates a high starting current for a specific motor horse power rating. Calculating the starting current can be accomplished using the following formula:
Example: What is the approximate starting current of a 10-HP, 208-V motor with a nameplate code letter of "K" ?
Solution: From Table 4, the kVA/HP for a code letter of "K" is 8.0 to 8.9.
Taking a number approximately halfway in-between and substituting in the formula, we get:
Therefore, the starting current is approximately 236 amperes. The starting current is important because the purchaser of the motor must know what kind of protection (overload) to provide. The installation must also include power lines of sufficient size to carry the required currents and properly sized fuses.
Insulation Systems
An insulation system is a group of insulating materials in association with conductors and the supporting structure of a motor. Insulation systems are divided into classes according to the thermal rating of the system. Four classes of insulation systems are used in motors: class A, B, F, and H. Do not confuse these insulation classes with motor designs previously discussed. Those design classes are also designated by letter.
Another confusion factor is the voltage insulation system classes of the stator windings. Those classes are also designated by class B, F, and H, for example. NEMA, standard MG1, part 31 indicates the voltage insulation classes, relative to use on AC drives. More review of motor voltage insulation characteristics will be done in Section 4.
At this point, we will review the temperature insulation classes, common in standard industrial induction motors operated across the line.
Class A. Class A insulation is one in which tests have shown suitable thermal endurance exists when operated at a temperature of 105ºC. Typical materials used include cotton, paper, cellulous acetate films, enamel coated wire, and similar organic materials impregnated with suitable sub stances.
Class B. Class B insulation is one in which tests have shown suitable thermal endurance exists when operated at a temperature of 130ºC. Typical materials include mica, glass fiber, asbestos, and other materials, not necessarily inorganic, with compatible bonding substances having suitable thermal stability.
Class F. Class F insulation is one in which tests have shown suitable thermal endurance exists when operated at a temperature of 155ºC. Typical materials include mica, glass fiber, asbestos, and other materials, not necessarily inorganic, with compatible bonding substances having suitable thermal stability.
Class H. Class H insulation is one in which tests have shown suitable thermal endurance exists when operated at a temperature of 180ºC. Typical materials used include mica, glass fiber, asbestos, silicone elastomer, and other materials, not necessarily inorganic, with compatible bonding sub stances, such as silicone resins, having suitable thermal stability.
Usual Service Conditions
When operated within the limits of the NEMA-specified "usual service conditions," standard motors will perform in accordance with their ratings.
For service conditions other than usual, the precautions listed below must be considered.
Ambient or room temperature not over 40ºC. If the ambient temperature is over 40ºC (104ºF), the motor service factor must be reduced or a higher horsepower motor used. The larger motor will be loaded below full capacity so the temperature rise will be less and overheating reduced.
(Note: Service factor refers to rated motor power and indicates permissible power loading that may carried by the motor. For example, a 1.15 service factor would allow 15% overload power to be drawn by the motor.) Altitude does not exceed 3300 feet (1000 meters). Motors having class A or B insulation systems and temperature rises according to NEMA can operate satisfactorily at altitudes above 3300 feet. However, in locations above 3300 feet, a decrease in ambient temperature must compensate for the increase in temperature rise, as seen in Table 5.
Table 5. Temperature vs. altitude
Motors having a service factor of 1.15 or higher will operate satisfactorily at unity service factor and an ambient temperature of 40ºC at altitudes above 3300 feet up to 9000 feet.
Voltage Variations. A voltage variation of not more than ±10% of name plate voltage:
Operation outside these limits or unbalanced voltage conditions can result in overheating or loss of torque and may require using a larger-horse power motor.
Frequency Variations. A frequency variation of not more than ±5% of nameplate frequency: Operation outside of these limits results in substantial speed variation and causes overheating and reduced torque.
A combination of 10% variation in voltage and frequency provided the frequency variation does not exceed 5%.
Mounting Surface and Location. The mounting surface must be rigid and in accordance with NEMA specifications. Location of supplementary enclosures must not seriously interfere with the ventilation of the motor.