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INDUSTRIAL CONTROL HANDBOOK - 4.2 ELECTRONIC CONTROL OF ALTERNATING CURRENT MOTORS

Whereas dc machines are controlled by varying the voltage and current, ac machines are often controlled by varying the voltage and frequency. Now we may ask, if dc machines do such an outstanding job, why do we also use ac machines? There are several reasons:
  1. AC machines have no commutators; consequently, they require less maintenance;
  2. AC machines cost less (and weigh less) than dc machines;
  3. AC machines are more rugged and work better in hostile environments.

 

4.2.1 Types of ac drives

Although there are many kinds of electronic ac drives, the majority can be grouped under the following broad classes:

  1. Static frequency changers
  2. Variable voltage controllers
  3. Rectifier - inverter systems with natural commutation.
  4. Rectifier - inverter systems with self-commutation

Static frequency changers convert the incoming line frequency directly into the desired load frequency. Cycloconverters fall into this category, and they are used to drive both synchronous and squirrel-cage induction motors.

Variable voltage controllers enable speed and torque control by varying the ac voltage. They are used in squirrel-cage and wound-rotor induction motors. This method of speed control is the least expensive of all, and provides a satisfactory solution for small and medium-power machines used on fans, centrifugal pumps and electric hoists.

Rectifier - inverter systems rectify the incoming line frequency to dc, and the dc is reconverted to ac by an inverter. The inverter may be self-commutated, generating its own frequency, or it may be naturally-commutated by the very motor it drives. Such rectifier-inverter systems with a dc link are used to control squirrel-cage and wound-rotor induction motors and, in some cases, synchronous motors.

 

4.2.2 Squirrel-cage induction motor with cycloconverter

Figure 4-24 shows a 3-phase squirrel-cage induction motor connected to the output of a 3-phase cycloconverter. Consequently, the windings cannot be connected in wye or delta, but must be isolated from each other. Motor speed is varied by applying appropriate gate pulses to the thyristors, to vary the output voltage and frequency. For example, the speed of a 2-pole induction motor can be varied from zero to 1500 r/min, on a 60 Hz line, by varying the output frequency of the cycloconverter from zero to 25 Hz.

Figure 4-24 Squirrel-cage induction motor fed from a 3-phase cycloconverter.

Figure 4-24 Squirrel-cage induction motor fed from a 3-phase cycloconverter.

 Figure 4-25 Typical torque-speed curves of a 2-pole induction motor driven by a cycloconverter. The cycloconverter is connected to a 460 V, 3-phase, 60 Hz line.

Figure 4-25 Typical torque-speed curves of a 2-pole induction motor driven by a cycloconverter. The cycloconverter is connected to a 460 V, 3-phase, 60 Hz line.


Figure 4-26 Operating mode of converter 1 and converter 2 when current lags 30° behind Ea.

Figure 4-26 Operating mode of converter 1 and converter 2 when current lags 30° behind Ea.

 

4.2.3 Squirrel-cage motor and variable voltage controller

We can vary the speed of a 3-phase squirrel-cage induction motor by simply varying the stator voltage. This is particularly useful for a motor driving a blower or centrifugal pump. Suppose the stator is connected to a variable-voltage 3-phase autotransformer (Fig. 4-27).

At rated voltage, the torque-speed characteristic of the motor is given by curve 1 of Fig. 4-28. To simplify the drawing, the curve is shown as two straight lines. If we apply half the rated voltage, we obtain curve 2. Because torque is proportional to the square of the applied voltage, the torques in curve 2 are only 1/4 of the corresponding torques in curve 1. Thus, the breakdown torque drops from 200 % to 50 %. Similarly, the torque at 60 percent speed drops from 175% to 43.75%.

Figure 4-27 Variable-speed blower motor.

Figure 4-27 Variable-speed blower motor.

 Figure 4-28 Torque-speed curve of blower motor at rated voltage (1) and 50% rated voltage (2). Curve (3) is the torque-speed characteristic of the fan.

Figure 4-28 Torque-speed curve of blower motor at rated voltage (1) and 50% rated voltage (2). Curve (3) is the torque-speed characteristic of the fan.

Figure 4-30 shows the resulting current and line-to-neutral voltage for phase A. The thyristors in phases B and C are triggered the same way, except for an additional delay of 120 and 240, respectively.

To reduce the motor voltage, we delay the firing angle 9 still more. For example, to obtain 50 percent rated voltage, all the pulses are delayed by about 100°. The resulting voltage and current waveshapes for phase A are given very approximately in Fig. 4-31. In actual fact, both the voltage and current are distorted, and the current lags considerably behind the voltage. The distortion increases the losses in the motor compared to the autotransformer method. Furthermore, the power factor is considerably lower, because of the large phase angle lag 9. Nevertheless, to a first approximation, the torque-speed characteristics shown in Fig. 4-28 still apply.

Figure 4-29 Variable-voltage speed control of a squirrel-cage induction motor.

Figure 4-29 Variable-voltage speed control of a squirrel-cage induction motor.

Figure 4-29 Waveshapes at rated voltage.

Figure 4-29 Waveshapes at rated voltage.

Owing to the considerable I2/R losses and low power factor, this type of electronic speed control is only feasible for motors rated below 20 hp. Small hoists are well suited to this type of control, because they operate intermittently. Consequently, they can cool off during the idle and light-load periods.

Figure 4-32a Current-fed frequency converter.

Figure 4-32a Current-fed frequency converter.


Figure 4-32b Motor voltage and current.

Figure 4-32b Motor voltage and current.

 

Figure 4-32c Asynchronous generator voltage and current.

Figure 4-32c Asynchronous generator voltage and current.

 

4.2.4 Voltage-fed self-commutated frequency converter

In some industrial applications, such as in textile mills, the speeds of several motors have to move up and down together. These motors must be connected to a common bus in order to function at the same voltage. The current-fed frequency converter is not feasible in this case because it tends to supply a constant current to the total ac load, irrespective of the mechanical loading of individual machines. Under these circumstances, we use a voltage-fed frequency converter. It consists of a rectifier and a self-commutated inverter connected by a dc link (Fig. 4-33a).

Figure 4-32c Asynchronous generator voltage and current.  Figure4-33a Voltage-fed frequency converter.

Figure4-33a Voltage-fed frequency converter.

 

Figure 4-33b Motor line-to-line voltages.

Figure 4-33b Motor line-to-line voltages.

A 3-phase bridge rectifier produces a dc voltage E1. An LC filter ensures a "stiff" dc voltage at the input to the inverter. The inverter successively switches this voltage across the lines of the 3-phase load. The switching produces positive and negative voltage pulses of 120° duration (Fig. 4-33b).

The amplitude of the inverter output voltage E is varied in proportion to the frequency so as to maintain a constant flux in the motor (or motors). Because the peak ac voltage is equal to the dc voltage E2, it follows that rectifier voltage E1 must be varied as the frequency varies. The speed of the motor can be controlled from zero to maximum while developing full torque.

Regenerative braking is possible but, owing to the special arrangement of the auxiliary components in converter 2, the link current Id reverses when the motor acts as a generator. Voltage E2 does not change polarity as it does in a current-fed inverter. Because converter 1 cannot accept reverse current flow, a third converter (not shown) has to be installed in reverse parallel with converter 1 to permit regenerative braking. The third converter functions as an inverter and, while it operates, converter 1 is blocked. As a result, voltage-fed drives tend to be more expensive than current-fed drives, thus making them less attractive.

Figure 4-34 a. Three mechanical switches could produce the same

Figure 4-34 a. Three mechanical switches could produce the same


Figure 4-34 b and c:

Figure 4-34 b and c:

b: Table showing the switching sequence of the switches

c: Voltages produced across the motor terminals

The switching action of converter 2 can be represented by the three mechanical switches shown in Fig. 4-34a. The opening and closing sequence is given in the table (Fig. 4-34b) together with the resulting ac line voltages. An X indicates that the switch contact is closed. This again illustrates that the thyristors and other electronic devices in converter 2 merely act as high-speed switches. The switching action is called 6-step because the switching sequence repeats after every 6th step. This is evident from the table.

 

4.2.5 Chopper speed control of a wound-rotor induction motor

We have already seen that the speed of a wound-rotor induction motor can be controlled by placing three variable resistors in the rotor circuit. Another way to control the speed is to connect a 3-phase bridge rectifier across the rotor terminals and feed the rectified power into a single variable resistor. The resulting torque-speed characteristic is identical to that obtained with a 3-phase rheostat. Unfortunately, the single rheostat still has to be varied mechanically in order to change the speed.

We can make an all-electronic control (Fig. 4-35) by adding a chopper and a fixed resistor R0 to the secondary circuit. In this circuit, capacitor C supplies the high current pulses drawn by the chopper.

Figure 4-35 Speed control of a wound-rotor induction motor using a load resistor and chopper.

Figure 4-35 Speed control of a wound-rotor induction motor using a load resistor and chopper.

Figure 4-36 Speed control using a variable-voltage battery.

Figure 4-36 Speed control using a variable-voltage battery.

 

Figure 4-37 Speed control using a rectifier and naturally-commutated inverter.

Figure 4-37 Speed control using a rectifier and naturally-commutated inverter.

Figure 4-38a Torque-speed characteristics of a wound-rotor motor for two settings of voltage ET.

Figure 4-38a Torque-speed characteristics of a wound-rotor motor for two settings of voltage ET.

Figure 4-38b Rotor voltage and current in Fig. 4-35.

Figure 4-38b Rotor voltage and current in Fig. 4-35.

Figure 4-39 Speed control by pulse width modulation.

Figure 4-39 Speed control by pulse width modulation.

 

4.2.6 Pulse width modulation

The frequency converters discussed so far create substantial harmonic voltages and currents. When these harmonics flow in the windings, they produce torque pulsations that are superimposed on the main driving torque. The pulsations are damped out at moderate and at high speeds owing to mechanical inertia. However, at low speeds, they may produce considerable vibration. Such torque fluctuations are unacceptable in some industrial applications, where fine speed control down to zero speed is required. Under these circumstances, the motor can be driven by pulse width modulation techniques.

To understand the technique, consider the voltage-fed frequency converter system shown in Fig. 4-39. A 3-phase bridge rectifier 1 produces a fixed voltage E1 which appears essentially undiminished as E3 at the input to the self-commutated inverter 2. The inverter is triggered in a special way so that the output voltage is composed of a series of short positive pulses of constant amplitude, followed by an equal number of short negative pulses (Fig. 4-40a). The pulse widths and pulse spacings are arranged so that their weighted average approaches a sine wave. The pulses as shown all have the same width, but in practice, the ones near the middle of the sine wave are made broader than those near the edges. By increasing the number of pulses per half cycle, we can make the output frequency as low as we please. Thus, to reduce the output frequency of Fig. 4-40a by a factor of 10, we increase the pulses per half-cycle from 5 to 50.

The pulse widths and pulse spacings are specially designed so as to eliminate the low-frequency voltage harmonics, such as the 3rd, 5th, and 7th harmonics. The higher harmonics, such as the 17th, 19th, etc., are unimportant because they are damped out, both mechanically and electrically. Such pulse width modulation produces output currents having very low harmonic distorsion. Consequently, torque vibrations at low speeds are greatly reduced.

Figure 4-40a Voltage waveform across one phase.

Figure 4-40a Voltage waveform across one phase.


Figure 4-40b Waveform yielding the same frequency but half the voltage.

Figure 4-40b Waveform yielding the same frequency but half the voltage.

In some cases, the output voltage has to be reduced while maintaining the same output frequency. This is done by reducing all the pulse widths in proportion to the desired reduction in output voltage. Thus, in Fig. 4-40b, the pulses are half as wide as in Fig. 4-40a, yielding an output voltage half as great, but having the same frequency. We can therefore vary both the output frequency and output voltage using a fixed dc input voltage. As a result, a simple diode bridge rectifier can be used to supply the fixed dc link voltage. The power factor of the 3-phase supply line is therefore high.

Regenerative braking can be achieved, but during such power reversal, current Id reverses while the polarity of E2 remains the same. Consequently, an extra inverter 3 has to be placed in reverse parallel with rectifier 1 in order to feed Power back to the line (Fig. 4-39). Rectifier 1 is automatically blocked while inverter 3 is in operation, and vice versa. Pulse-width modulation is effected by computer control of the gate triggering. It can be used to control induction motors up to several hundred horsepower.

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