Motor-Drive Systems: Summary Review and Conclusion

The following information will serve as a final guide summary. This will help the student in preparing for the Final EXAM below. For a thorough review and before the Final EXAM is taken, it would be helpful to review all section summaries and questions.

What is a Drive?

Why Drives are Applied...

There are many reasons to use variable speed drives, but basically they fall into three categories of efficiency gains, process changes and improvements, and system coordination. As an example, efficiency of AC motors can be quite high, which thereby reduces the overall monthly cost of operating the system. Variable speed drives also allow for changes in the process, as well as process improvements. Some processes operate at less than full speed, so optimum product quality can be achieved. System coordination is a major factor in today's industrial environment. AC and DC drive systems are typically applied in a manufacturing process. Computers control the entire process, from in feed rate to output of the machine.

A generic drive system includes the following components: machine, coupler, motor, drive, controller and power source. No matter what type of system is discussed, these main components are involved.

There are various types of variable speed drives available for use in industry. The basic categories are mechanical, hydraulic and electrical/electronic. Electronic drives can be further divided into the following categories of eddy current, rotating DC, DC converters and variable frequency AC.

Review of Basic Principles

All electrical circuits have three main factors: current, voltage and resistance. Current is the actual electrons that cause the work to happen in a circuit. Resistance is the opposition to current flow, which is present in any circuit (almost all electrical circuits have some resistance). Voltage is the electrical "pressure" that tries to overcome resistance.

The two types of voltage (electrical pressure) available are direct current (DC) and alternating current (AC) . DC is typically found in battery circuits, and its output value does not fluctuate, until the battery goes dead.

AC is typically found as a power source for households and industrial operations. The output value fluctuates depending on the time base of transmission (positive to negative to positive output). Frequency is the number of times complete cycles are seen in a power system (e.g. 60 Hz). The two types of AC available are single phase and three phase.

Electromagnetism is the ability to produce a magnetic field through the use of a voltage and a coil. The devices that use this principle are inductors, relays, contactors, and transformers. Inductance is the ability to block AC voltage and allow DC to flow. Capacitance is the ability to block DC and allow AC to flow. Capacitance in an inductive circuit (motors) tends to improve the power factor of the entire system. Power factor is the measure of the efficient use of the current waveform; it’s stated as a ratio between the utility generated voltage and current waveform.

Semiconductors are a mix between conductors and insulators - and need an electrical "push" to drive the device to conduct current. Typical semi conductor devices include diodes (allow current flow in one direction), thyristors (silicon controlled rectifiers [SCRs] that conduct current only when triggered ON), gate turn-off thyristors (GTOs can be latched on or off depending on the polarity of the gate signal), transistors (amount of cur rent flow is determined by the amount of trigger signal), and insulated gate bipolar transistors (IGBTs are specialized transistors that have extremely high speed switch ON and OFF times).

There are three basic types of mechanical loads that are encountered by any AC or DC drive system: constant torque (e.g. conveyors), variable torque (e.g. centrifugal fans/pumps), and constant horsepower (e.g. machine tools). Speed, torque, and horsepower (HP) play a major role in the operation of any application. Speed affects how much HP is required to perform the function; faster acceleration (more speed), requires more HP. Torque is a turning effort, and defines the ability of a system to start and keep moving at a specified rate. Inertia (WK2 or WR2 ) is the measurement of an object's resistance to change in speed. This measurement is needed to determine the acceleration time available from a drive system.

Gears, belts, pulleys (sheaves), chains, and sprockets all work to allow a smooth transmission of mechanical power, and in some cases, change speed and direction. Types of belts and pulleys include flat and V-belts, synchronous belts, and chains and sprockets.

Couplings, gearboxes, and speed reducers offer a positive connection point between the motor and application. Couplings are available in the following designs: flange, sleeve, and flexible (mechanically and elastically). Speed reducers offer an effective means of changing speed delivered to the application, as well as the torque developed by the motor.

Motors

AC and DC motors are the two major types used today in industrial and HVAC applications. These motors provide the speed, torque and HP necessary to operate the application. The motor changes one form of energy (electrical) to rotational or linear motion (mechanical).

The two major components of a DC motor are the armature and field winding. The armature is the rotating part that is physically connected to the shaft and develops magnetic flux around its windings. The field winding is the part of the stationary frame and provides the flux necessary to interact with the armature flux to produce rotation. The commutator acts as an electrical switch and always ensures that a repelling force is taking place in the armature flux circuit. This repelling force against the field winding flux causes rotation of the armature. Brushes are the devices that physically connect the voltage supply to the Armature circuit. Brushes are constructed of carbon material and require routine maintenance or replacement in order to reduce arching at the commutator segments.

Two separate voltage supplies are connected to the DC motor, one for the armature (variable DC voltage armature supply) and one for the field winding (fixed voltage field exciter). Speed of the DC motor is directly controlled by the magnitude of the armature supply voltage. Speed is also inversely proportional to the magnitude of the field flux. Torque is a direct result of the interaction of armature and field winding flux.

Various types of enclosures safeguard the DC motor against harm. For example, drip-proof motors provide a degree of protection against vertical falling materials, and also allows for the ventilation of cool outside air.

Totally enclosed motor frames provide a higher degree of protection, but are not practical for large frame motors due to the inability to remove heat.

Motors are listed with many types of ratings that indicate the torque generating ability, altitude, heat capability, vibration, and electrical specifications. DC motors are constructed with several different types of field winding circuit: series wound, shunt (parallel wound), and compound wound.

AC motors are listed with one of two ratings: NEMA or IEC. All motors can be classified into single phase or polyphase categories. The main components of the AC motor are the rotor and stator. The rotor is the rotating part and the stator is the stationary part connected to the frame. Only one power source is required to set the rotor into motion. The stator windings create magnetic flux that causes a magnetic field (flux) to be induced in the rotor. The attracting forces of the rotor and stator flux produce torque and rotation of the rotor.

Speed of an AC induction motor is related to the frequency applied and the number of pole pairs. The number of pole pairs causes an inverse relationship in speed, but the frequency applied has a direct relationship to speed. The AC motor will always operate a speed less than synchronous.

This is due to the requirement of magnetic flux in the rotor to be attracted to the rotating magnetic flux in the stator.

AC motors typically draw 600% inrush current upon start-up. Once the speed has increased to near synchronous, the current draw drops closely in line with the torque being produced. All AC motors are designed with a specific torque producing characteristic in mind (V/Hz). If the Volts per Hz relationship is kept constant, the motor will develop the rated torque it was designed to produce.

A common rating scale for AC Induction motors is that of a NEMA design classification: A, B, C, D, and E. Each classification indicates a different motor torque producing category. AC Induction motor nameplates have similar designs to DC motors, only referring to AC input power. A major indication of motor durability is the temperature class of the Stator windings. IEC ratings differ with NEMA in most categories.

AC motor types range from the standard induction motor, to wound rotor, synchronous, and multiple pole motors. Specialty motors include stepper, AC vector, servomotors, linear stepper, and linear motors.

Drives

In DC Drive systems, an armature and field exciter are used to control the two separate elements in a shunt wound DC motor. SCRs are used to vary the DC voltage output, which has a direct bearing on the speed of the motor. The DC Drive is the simplest of drive systems. The disadvantage of using SCRs for the drive power structure is the inherent nature of line notching. This notching is caused by the "phasing on and off" of the (6) SCRs located in the drive input section.

Digital DC drives are the latest major development in DC Drive technology. With digital technology, precise control of speed and torque can be realized. Speed and current controller circuits use feedback to make small changes in armature supply, and field exciter operation. Measuring and scaling circuits sample the actual speed and current output. Summing circuits take the error signal and translate those signals into corrective actions. Higher speed accuracy (< 1% regulation) is obtained by the use of a tachometer generator or tach. When operated in the speed regulation mode, the drive closely monitors the speed feedback signal. Armature voltage (or electromagnetic force or EMF) control would give a 1 to 2% regulation characteristic. Induction regulator (IR) compensation will improve the drooping speed due to load. When operated in the current regulation mode, the drive closely monitors the value of the current measuring circuit. The drive ignores the speed controls and calculates the cur rent (torque) required by the load. The drive will automatically operate at the speed required to allow the motor to develop the desired torque.

Field exciters are constructed of SCRs or the newer IGBT power semiconductor technology. Some supplies are powered by single phase or 2-phase power and have the ability to operate in "field control" mode. This mode is the weakening of the shunt field strength, to allow above base speed operation. Form factor is the term used to compare the purity of the DC output from the armature or field exciters.

The operation of the DC drive can be compared to a relationship of "quad rants." A four quadrant drive would allow forward and reverse operation as well as regenerative capability (regenerating voltage back to AC line power.). This type of armature supply includes (12) SCRs in a bridge con figuration.

Braking methods include coast and ramp to stop, which are typically the longest method of bringing a motor to a stop. Dynamic braking causes the back-fed voltage to be dissipated in heat, through a high wattage power resistor. The fastest "electronic" method of stopping a motor is through regenerative braking. This allows full voltage to be fed directly back to the AC line. Mechanical braking also allows fast braking of the motor armature, through a brake pad assembly similar to that of an automobile.

A concern of DC drive use is the generation of harmonics, line distortion, and power factor. A line reactor is required ahead of the drive unit to reduce the distortion back to the utility system. In order to avoid the generation of RFI, shielded control cable, and power cable are used. In addition, a shielded transformer is used ahead of the drive.

There are three basic designs of AC variable-frequency drives (VFDs): cur rent source inverters, variable voltage inverters (input inverters), and pulse width modulation (PWM). All AC drives operate under the same characteristic. They change a fixed incoming voltage and frequency to a variable voltage and frequency output. With PWM drives, the incoming voltage and frequency is rectified to a fixed DC voltage. That voltage is "inverted" back to AC, with the output, 0-460V and 0-60 Hz (or 0-230V). Braking methods for AC motors are similar to that of DC. In addition, the AC drive has the capability to "injecting" DC voltage into the stator windings. In doing so, the drive sets up a definite N and S polarity in the motor, causing high reverse torque, and bringing the rotor to a fast stop.

Torque control AC drives basically fall into two categories: flux vector (feedback required) and sensorless flux vector (no feedback required). Vector control drives have the capability of full torque at zero speed. The drive forces the motor to develop the torque required, to effectively handle the load.

Since the introduction of IGBTs into the power technology ranks, the size of AC drives has been reduced to less than half of its counterpart 10 years ago. With IGBT technology comes a challenge in AC motors-high voltage spikes caused by "voltage reflection." PWM drives produce an inherent oscillation between the drive output and motor input. Precautions to be taken against this motor damage possibility include: use of inverter duty motors, output reactors, or DV/DT output filters.

Harmonics are generated back to the AC line, due to the technology of pulling voltages in bursts. The rectifier that accomplishes this is termed a

"switch mode power supply." All electronic devices with this type of supply cause harmonics back to the utility system. Line reactors, harmonic trap filters, and higher pulse drives are a few of the corrections to the harmonics issue. Improper shielding and grounding of AC drives can cause bearing current damage after prolonged use, and can cause immediate radio frequency interference (RFI) and electromagnetic interference (EMI). Proper installation methods drastically reduce in conducted and radiated noise.

Package designs, Digital I/O, IGBT technology and multi-function keypads make AC drives easy to set-up. Programming panels are removable and are able to store all drive values in flash memory.

Drive Control and Feedback Devices

The two types of drive control methods are open-loop and closed- loop. Open loop is operating a motor, directly from the drive unit. Closed loop would include some type of feedback device that is connected directly to the motor shaft. The speed regulation of the system can be improved by using a feedback device.

Typical drive feedback devices include tachometers (tachs), encoders (or pulse tachs), and resolvers. Tachometers are available in AC or DC versions. AC tachs are used in a variety of industrial applications, since they are able to indicate direction and are generally more accurate than AC Tachs.

Encoders use a system of light beams, directed toward a light sensor. The light beams are created by shining a light source through a rotating, slotted disc. The disc is connected to the encoder shaft, which is connected directly to the motor shaft, through a coupling.

Resolvers are position type devices. These devices are typically used on servo applications where precise feedback of the rotor position is required.

Resolvers are similar to an AC induction motor, as well as acting like a revolving transformer.

DC drive control methods include closed loop control using a tach feed back device. It would also include armature voltage feedback or EMF control. With EMF control, a sampling of the output voltage is used as feedback to correct for output speed and current (torque).

AC drive control methods also include tach or encoder feedback, as well as operating the VFD by open loop methods. A variety of transducers can be used as feedback devices. Pressure, flow, level, temperature, and humidity are just a few of the controlling elements sensed by transducers.

Dynamic and static speed regulation are part of the set-up process of the performance drive application. The accuracy of the speed or torque is dependent on the feedback device, as well as the closed loop control circuit within the drive. The amount of "dampening" will determine if the system will be stable, or produce oscillations.

Drive Systems

Drive systems operate in a coordinated fashion-with control between the controller (PLC) and the drive unit. A variety of sensors, switches, and transducers are a part of the overall scheme of automation.

Proportional-integral-derivative (PID) control is used when automatic control of some quantity is required. Temperature, pressure, humidity, and level are just a few of the variables that can be conveniently controlled by PID. Tension control is a major part of any coordinated system that processes web material. Dancer control is similar to tension control, in that a separate regulator signal is fed back to the drive in order for correction to take place. Proportional gain and integration time play a part in the tuning of a web system.

A variety of remote operator devices are available for interfacing signals to the drive unit. Remote operator stations are the simplest form of remote control. Standard inputs and outputs (I/O) would include start /stop, speed reference, digital inputs, analog outputs, and relay outputs. Both sinking and sourcing control are used in industry today.

Serial communications is the simplest form of communication link to a drive. Typically, multiple drives are controlled by one system controller, which could be a computer that is set up to talk to the protocol that is installed in the drive. Fiber optic communications has the highest immunity to noise, compared to other forms of drive communications. Optical fibers are connected in a ring structure, and can be connected with plastic or glass fiber. Building automation systems or Ethernet systems are able to talk to many drives on the market today.

Maintenance / Troubleshooting of Drive Systems

To avoid having to troubleshoot a drive, there are several steps in maintaining a "healthy" drive. Keep it clean, dry, and keep the connections tight.

The heatsink should be routinely sprayed with compressed air to reduce any buildup of dust and particles on the chassis. Avoid inducing com pressed air into the drive electronics, unless it’s ionized air or compresses air, specially packaged to reduce electrostatic discharge (ESD) contamination.

Ensure that the drive is not located in an area where moisture could be of concern. Less than 95% humidity, no condensation allowed is the standard rating for all drives. Moisture is a major enemy of electronics, as is excessive heat.

Check tightness of connections as part of a routine periodic maintenance procedure. Don't over-tighten power connections. Mechanical overstress of the bolt or nut could actually reduce the clamping power of the device.

Routine inspection of the drives electronic and mechanical parts is helpful.

Power up spare VFDs once every 6 months or so. The DC bus capacitor's ability diminishes if it’s not electrically stressed periodically. In general troubleshooting, start at the motor and work backwards. It must be deter mined first, if the motor should be turning. Once that is determined, then move to the drive and verify the status.

Finally…

The use of electronic motor speed controls has grown tremendously since the early 1960s. The increased use can be attributed to technology improvements not seen in years prior to the 1060s. Motor speed control is a multi-billion dollar industry worldwide. The use of AC and DC drives, brushless DC drives, permanent magnet AC drives, and stepper drives, all play a part in today' s industrial and HVAC marketplaces.

The use of electronic motor speed controls is expected to grow in non-traditional applications. Applications like automotive subsystems, household appliances, electric vehicles, people movers and marine propulsion units can all benefit from the technology drives offer.

As of this printing, AC drives (VFDs) comprise over 50% of the control method in use on standard induction AC motors. That trend will only increase in the years to come, due to increased focus on energy saving devices. The vast consumption of energy by fixed speed motors is an invitation for the installation of VFDs as part of an energy management pro gram.

The trend in AC drives appears to include several factors that all users can benefit from. First of all, drives will continue to have increased intelligence. Drives of today and tomorrow are more than just a "motor turner." They are a mini-PLC and personal computer, rolled into one unit. The ability to customize drive functions is becoming easier, due to windows based development tools. Custom application software and custom programming tools will continue to be a trend in AC drive development.

Another trend in AC drives, is the use of communication options. If communications is used, the drive can be less reliant on its own intelligence.

External control devices provide the drive with the information, I/O and feedback that it needs to control process variables in a closed loop system.

The use of micro-drives will also increase in the future. These types of drives will present increased functionality and tout the ability to be flexible during process changes. Industry demands the ability to make changes quickly and efficiently. This is no exception when micro-drives are in use.

Since standard consumer products will continue to demand energy savings and flexibility during use, user-oriented programming tools will be the norm, rather than the exception.

As more consumers purchase vector and torque controlled drives, the retail cost will continue to decrease. The manufacturer will realize cost effective techniques and the consumer will reap the benefits of technology improvement at a reduced price. Who knows where electronic adjustable speed drives will be in the future. The future is only limited by the imagination of results-oriented consumers.