AC Motor Drives -- Review / QUIZ

Review

Drives are at the heart of an automation system. They are the controlling element for DC and AC motors, which develop the torque to turn the wheels of industry.

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 six SCRs located in the drive input section.

Digital DC drives are the latest entry into 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 EMF) control would give a 1-2% regulation characteristic. 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 current (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 two 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 with a relationship of quadrants. 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 configuration.

Braking methods include coast-to-stop 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-watt age 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 auto mobile.

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. To avoid the generation of RFI, shielded control and power cables are used. In addition, a shielded transformer is used ahead of the drive.

DC drives have been the object of technology improvements in recent years. An all-in-one package design allows users access to all the required circuitry within one location. Digital I/O and multi-function keypads make drive setup and adjustment easy, along with programming macros and setup routines called wizards by one manufacturer. The self-tuning of the armature circuit and the interface with PLCs make this new breed of DC drive much easier to commission compared with older analog designs.

There are three basic designs of AC VFDs: current source inverters, variable voltage inverters (input inverters), and pulse width modulation. All AC drives operate under the same characteristic. They change a fixed incoming voltage and frequency to a variable voltage and frequency output. Most of the AC VFDs built today are of the PWM variety. With PWM drives, the incoming voltage and frequency is rectified to a fixed DC volt age. That voltage is "inverted" back to AC, with the output, 0- 460 V and 0-60 Hz (or 0-230 V).

Braking methods for AC motors are similar to that of DC. In addition, the AC drive has the capability to "inject" 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 generating full torque at zero speed. The drive forces the motor to develop the torque required to effectively handle the load. Special circuits (DSPs and ASICs) perform calculations every 25 µs. They allow the flux reference controller to respond to very small changes in torque requirements at the shaft.

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 this technology improvement 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. The oscillating voltage actually amplifies itself into a value that is beyond the volt age insulation strength of the stator windings. The windings either suffer a short circuit between windings or an open phase. 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 because of 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 causes harmonics back to the utility system. The local utility is very concerned about TDD (demand distortion) that affects other users on the power 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 RFI and EMI. By using proper installation methods, drastic reduction in conducted and radiated noise can result.

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. Macros and software pro grams like PID allow the user to perform more functions within the drive unit, rather than require a separate PLC program. Self-tuning, communications, and bypass capability make the VFDs of today a cost-effective choice in variable speed.

QUIZ:

1. What are the two main circuits in a DC drive unit?

2. What are the main power components used in each circuit, and what are the characteristics of each?

3. How is line notching corrected in a DC drive system?

4. Describe what the following circuits are used for in a DC drive: Summing circuit, Current controller, Current measuring/scaling

5. Explain the difference between single-, two-, and four-quadrant systems.

6. What is dynamic braking and how is it accomplished?

7. What is RFI and how is it controlled in a DC drive system?

8. What is a macro? Describe its use.

9. What is the difference between a VVI, CSI, and PWM AC drive?

10. What is the carrier frequency or switch frequency?

11. What is motor cogging? What causes it?

12. What is injection braking and how is it accomplished?

13. What is the difference between scalar and vector drives?

14. How do sensorless flux vector drives differ from standard flux vector drives?

15. What is voltage reflection and how is it corrected?

16. What are harmonics and what are the corrective actions to reduce them?

17. What are the effective shielding and grounding methods used with AC drives?

18. What is PID and how is it used?

-- Answers --

1. Armature supply and field exciter unit.

2. SCRs can be turned on with a small milliamp pulse, but must be forced off several amps. IGBTs operate on the same principle, with milliamps required to gate them on and off.

3. Line notching is corrected through use of a line reactor, connected ahead of the drive.

4. The summing circuit takes the speed feedback signal and matches it against the speed reference to obtain an error signal used for drive speed control. The current controller controls the firing angle of the SCRs and signals the firing unit how long and when to gate the SCRs on. Current measuring/scaling takes a sample of the current feedback signal and sends it to the summing junction error processing and correction of current output.

5. Single-quadrant systems allow for motoring in the forward direction only.

Two-quadrant systems allow forward and reverse direction, with reverse torque available for braking; four-quadrant systems allow for forward and reverse direction operation, plus the capability of forward and reverse torque available for braking in either direction.

6. Inertia built up in the motor is transferred back to the drive by means of regenerative voltage. The voltage fed back to the drive is sent to a power resistor for dissipation as heat. A DB contactor is used to connect the motor voltage to the resistor. At the same time, the output contactor of the drive is opened so no voltage can be fed back into the drive output section.

7. RFI is radio frequency interference. This interference is caused by the control circuits, contactors, and oscillators used to control the drive. The best corrective action is to use shielded control cable, as well as shielded input and output power cable in extreme cases. The use of a shielded transformer will also reduce the conducted noise that could be transmitted back to the power line.

8. A macro is a predetermined list of parameter values. These values are designed to allow the user to match the drive parameters to the application. Macros such as three-wire, hand-auto, and PID make drive set-up quicker, since the default values closely approximate the needed programming.

9. VVI and CSI drives use a variable voltage DC bus circuit. The bus voltage is accomplished through an SCR bridge rectifier in the converter section of the drive. Displacement PF of the units drops as the speed drops. PWM drives use a fixed diode bridge rectifier in the converter section. This causes the DC bus to be a fixed voltage. The DC bus feeds the inverter section, where the variable output voltage and frequency is generated. PWM drives operate at a high displacement PF and could be considered power factor correction devices.

10. Carrier frequency is the speed at which the output IGBTs switch on and off. The higher the carrier frequency, the smoother the output waveform will be, and the closer it approximates that of sine wave power.

11. Cogging is the pulsations of the motor shaft at very low speeds. It’s caused by a VVI or CSI drive output that sends out pulses in steps. The motor translates these steps into specific magnetic poles. The rotor flux searches for the next available stator pole, which causes the shaft to jump whenever it finds the next position.

12. Injection braking is the process of inducing a DC voltage into the AC stator winding. The amount of voltage will determine the amount of pole flux that will be set-up in the stator. The rotor is attracted to the definite polarity and stopping torque is the result.

13. Scalar is the term given to standard operation of a PWM voltage con trolled drive. With this drive mode, the motor must have several percent slip to rotate. The drive simply supplies the volts and hertz output, and the motor responds with rotation, per the designed slip characteristic.

Vector is the term given to a specialized drive that causes full motor torque at zero speed. Flux vector is another term used to describe the control of flux, which is also the control of torque.

14. Standard flux vector drives require feedback from the motor shaft to sup ply information to the controlling elements in the drive. The drive must know the position of the rotor at all times. The drive generates its control changes, based on the feedback from the shaft. Sensorless flux vector drives receive their name from the fact that no feedback device is used.

All the motor data (flux constants, hysteresis curves, temperature coefficients) is stored in a motor model in the drive. The drive responds to small amounts of information fed back by the DC bus, output current, and the actual IGBT switch positions.

15. Voltage reflection is the phenomenon of drive output voltages combining with voltage bouncing back from the motor. These combined peak volt ages can cause damage to the stator windings if they are not rated to handle the voltage stress. Precautions would include using an inverter duty motor, adding drive output reactors and/or dv/dt filters. Inductors will reduce the high-voltage spikes that can occur, and keep the values within the range of the motor lacquer insulation.

16. Harmonics are described as distortion; they provide no usable work, yet are fed back to the AC line. This distortion is superimposed on the fundamental waveform of 60 Hz. For a standard six-pulse drive, the 5th, 7th, 11th, and 13th harmonic will be generated and will be the most destructive in value. Line reactors and isolation transformers help in the mitigation of voltage harmonics. Trap filters and 12-pulse drive units will reduce the harmonic content to a significant level.

17. Control wiring needs to be shielded, with the shield cut back and taped at the signal source end. In addition, control wiring must be kept away from power wiring or at a minimum of 12" away. Input and output power wiring also need to be separated. A continuous ground wire throughout the entire system is a requirement and will reduce the possibility of conducted noise.

18. PID stands for "proportional integral derivative." PID is the ability to automatically control temperature, pressure, level, humidity, or any other medium that can be supplied as an electrical feedback signal. The drive has the ability to make corrections in speed, due to the error given by the summing junction. This is considered a closed loop system. It operates at a very high performance rate, with small feedback errors translating to thousands of an inch rotation of the shaft.