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INDUSTRIAL CONTROL HANDBOOK - 4.1 ELECTRONIC CONTROL OF DIRECT CURRENT MOTORS

High-speed, reliable and inexpensive semiconductor devices have produced a dramatic change in the control of dc motors. In this chapter, we examine some of the basic principles of such electronic controls.

In describing the various methods of control, we shall only study the behavior of power circuits. Consequently, the many ingenious ways of shaping and controlling triggering pulses are not covered here.

 

4.1.1 First quadrant speed control

We begin our study with a variable speed drive for a dc shunt motor. We assume its operation is restricted to quadrant 1.

A gate triggering processor receives external inputs such as actual speed, actual current, actual torque, etc. These inputs are picked off the power circuit by means of suitable transducers. In addition, the processor can be set for any desired motor speed and torque. The actual values are compared with the desired values, and the processor automatically generates gate pulses to bring them as close together as possible. Limit settings are also incorporated so that the motor never operates beyond acceptable values of current, voltage and speed.

Figure 4-1 Armature torque and speed control of a dc motor using a thyristor converter.Figure 4-1 Armature torque and speed control of a dc motor using a thyristor converter.

Three features deserve our attention as regards the start-up period:

  1. no armature resistors are needed; consequently, there are no I2R losses except those in the armature itself;
  2. the power loss in the thyristors is negligible;
  3. consequently, all the active power drawn from the ac source is available to drive the load;
  4. even if an inexperienced operator tried to start the motor too quickly, the current-limit setting would override the manual command. In effect, the armature current can never exceed the allowable preset value.

The converter absorbs a great deal of reactive power when the motor runs at low speed while developing its rated torque. Furthermore, the reactive power diminishes continually as the motor picks up speed. As a result, power factor correction is difficult to apply during the start-up phase.4.1.2 Two-quadrant control-field reversal

We cannot always tolerate a situation where a motor simply coasts to a lower speed. To obtain a quicker response, we have to modify the circuit so that the motor acts temporarily as a generator. By controlling the generator output, we can make the speed fall as fast as we please. We often resort to dynamic braking using a resistor. However, the converter can also be made to operate as an inverter, feeding power back into the 3-phase line. Such regenerative braking is preferred because the kinetic energy is not lost. Furthermore, the generator output can be precisely controlled to obtain the desired rate of change in speed.

To make the converter act as an inverter, the polarity of Ed must be reversed as shown in Fig. 4-2. This means we must also reverse the polarity of E0. Finally, Ed must be adjusted to be slightly less than E0 to obtain the desired braking current Id (Fig. 4-2).

Figure 4-2 Motor control by field reversal.Figure 4-2 Motor control by field reversal.

 

4.1.3 Two-quadrant control-armature reversal

In some industrial drives, the long delay associated with field reversal is unacceptable. In such cases, we reverse the armature instead of the field. This requires a high-speed reversing switch designed to carry the full armature current. The control system is arranged so that switching occurs only when the armature current is zero. Although this reduces contact wear and arcing, the switch still has to be fairly large to carry a current, say, of several thousand amperes.

Figure 4-3 Motor control by armature reversal.Figure 4-3 Motor control by armature reversal.

 

4.1.4 Two-quadrant control -two converters

When speed control has to be even faster, we use two identical converters connected in reverse parallel. Both are connected to the armature, but only one operates at a given time, acting either as a rectifier or inverter (Fig. 4-4). The other converter is on "standby", ready to take over whenever power to the armature has to be reversed. Consequently, there is no need to reverse the armature or field. The time to switch from one converter to the other is typically 10ms. Reliability is considerably improved, and maintenance is reduced. Balanced against these advantages are higher cost and increased complexity of the triggering source.

Figure 4-4 Two-quadrant control using two converters without circulating currents.Figure 4-4 Two-quadrant control using two converters without circulating currents.

Because one converter is always ready to take over from the other, the respective converter voltages are close to the existing armature voltage, both in value and polarity. Thus, in Fig. 4-5a, converter 1 acts as a rectifier, supplying power to the motor at a voltage slightly higher than the cemf Eo. During this period, gate pulses are withheld from converter 2 so that it is inactive. Nevertheless, the control circuit continues to generate pulses having a delay alpha2 so that Ed2 would be equal to Ed1 if the pulses were allowed to reach the gates (G7 to G12, Fig. 4-4).

 

4.1.5 Two-quadrant control - two converters with circulating current

Some industrial drives require precise speed and torque control right down to zero speed. This means that the converter voltage may at times be close to zero. Unfortunately, the converter current is discontinuous under these circumstances. In other words, the current in each thyristor no longer flows for 120°. Thus, at low speeds, the torque and speed tend to be erratic, and precise control is difficult to achieve.

Figure 4-4 Two-quadrant control using two converters without circulating currents.Figure 4-5a Converter 1 in operation, converter 2 blocked.

To get around this problem, we use two converters that function simultaneously. They are connected back-to-back across the armature (Fig. 22-8). When one functions as a rectifier, the other functions as an inverter, and vice versa. The armature current I is the difference between currents Id1 and Id2 flowing in the two converters. With this arrangement, the currents in both converters flow for 120°, even when I = 0. Obviously, with two converters continuously in operation, there is no delay at all in switching from one to the other. The armature current can be reversed almost instantaneously; consequently, this represents the most sophisticated control system available. It is also the most expensive. The reason is that when converters operate simultaneously, each must be provided with a large series inductor (L1, L2) to limit the ac circulating currents. Furthermore, the converters must be fed from separate sources, such as the isolated secondary windings of a 3-phase transformer. A typical circuit composed of a delta-connected primary and two wye-connected secondaries is shown in Fig. 4-6. Other transformer circuits are sometimes used to optimize performance, to reduce cost, to enhance reliability or to limit short-circuit currents.

Figure 4-5b Converter 2 in operation, converter 1 blocked.Figure 4-5b Converter 2 in operation, converter 1 blocked.

Figure 4-6 Two-quadrant control of a dc motor using two converters with circulating currents.Figure 4-6 Two-quadrant control of a dc motor using two converters with circulating currents.

Figure 4-7 hoist raising a load.Figure 4-7 hoist raising a load.

 

4.1.6 Two-quadrant control with positive torque

So far, we have discussed various ways to obtain torque-speed control when the torque reverses. However, many industrial drives involve torques that always act in one direction, even when the speed reverses. Hoists and elevators fall into this category because gravity always acts downwards whether the load moves up or down. Operation is therefore in quadrants 1 and 2.

Consider a hoist driven by a shunt motor having constant field excitation. The armature is connected to the output of a 3-phase, 6-pulse converter. When the load is being raised, the motor absorbs power from the converter. Consequently, the converter acts as a rectifier (Fig. 4-7). The lifting speed depends directly upon converter voltage Ed. The armature current depends upon the weight of the load.

When the weight is being lowered, the motor reverses, which changes the polarity of E0. However, the descending weight delivers power to the motor, and so it becomes a generator. We can feed the electric power into the ac line by making the converter act as an inverter. The gate pulses are simply delayed by more than 90°, and Ed is adjusted to obtain the desired current flow (Fig. 4-8).

Figure 4-8 hoist lowering a load.Figure 4-8 Hoist lowering a load.

Hoisting and lowering can therefore be done in a stepless manner, and no field or armature reversal is required. However, the empty hook may not descend by itself. The downward motion must then be produced by the motor, which means that either the field or armature has to be reversed.

 

4.1.7 Four-quadrant control

We can readily achieve 4-quadrant control of a dc machine by using a single converter, combined with either field or armature reversal. However, a great deal of switching may be required. Four-quadrant control is possible without field or armature reversal by using two converters operating back-to-back. They may function either alternately or simultaneously, as previously described.

The following example illustrates 4-quadrant control of an industrial drive.

 

Example 4-1:

An industrial drive has to develop the torque-speed characteristic given in Fig. 4-9. A dc shunt motor is used, powered by two converters operating back-to-back. The converters function alternately (only one at a time). Determine the state of each converter over the 26-second operating period, and indicate the polarity at the terminals of the dc machine. The speed and torque are considered positive when acting clockwise.

Figure 4-9 Torque-speed characteristic of an industrial drive.Figure 4-9 Torque-speed characteristic of an industrial drive.


Figure 4-10 See Example 4-1.Figure 4-10 See Example 4-1.

 

Solution:

The analysis of such a drive is simplified by subdividing the torque-speed curve into the respective 4 quadrants. In doing so, we look for those moments when either the torque or speed pass through zero. These moments always coincide with the transition from one quadrant to another. Referring to Fig. 4-9, the speed or torque passes through zero at 2, 8, 15, 21, and 25s.

We draw vertical lines through these points (Fig. 4-10). We then examine whether the torque and speed are positive or negative during each interval. Knowing the respective signs, we can immediately state in which quadrant the motor is operating. For example, during the interval from 2 s to 8 s, both the torque and speed are positive. Consequently, the machine is operating in quadrant 1. On the other hand, in the interval from 21 s to 25 s, the speed is negative and the torque positive, indicating operation in quadrant 2.

Knowing the quadrant, we know whether the machine functions as a motor or generator. Finally, assuming that a positive (clockwise) speed corresponds to a "positive" armature voltage (Fig. 4-11a), we can deduce the required direction of current flow. This tells us which converter is in operation, and whether it acts as a rectifier or inverter.

Thus, taking the interval from 21 to 25 seconds, it is clear that the machine acts as a generator. Consequently, one of the two converters must function as an inverter. But which one? To answer the question, we first look at the polarity of the armature. Because the speed is negative, the armature polarity is negative, as shown in Fig. 4-11b. Current flows out of the positive terminal because the machine acts as a generator. Only converter 1 can carry this direction of current flow, and so it is the one in operation.

Figure 4-11a Polarities when the speed is positive.Figure 4-11a Polarities when the speed is positive.

Figure 4-11b Interval from 21 s to 25 s.Figure 4-11b Interval from 21 s to 25 s.

A similar line of reasoning enables us to determine the operating mode of each converter for the other intervals. The results are tabulated below; we encourage the reader to verify them.

Converter operation

4.1.8 DC traction

Electric trains and buses have for years been designed to run on direct current, principally because of the special properties of the dc series motor. Many are now being modified to make use of the advantages offered by thyristors. Existing trolley lines still operate on dc and, in most cases, dc series motors are still used. To modify such systems, high-power electronic choppers are installed on board the vehicle. Such choppers can drive motors rated at several hundred horsepower, with outstanding results. To appreciate the improvement that has taken place, let us review some of the features of the older systems.

A train equipped with, say, two dc motors, is started with both motors in series with an external resistor. As the speed picks up, the resistor is shorted out. The motors are then paralleled and connected in series with another resistor. Finally, the last resistor is shorted out, as the train reaches its nominal torque and speed. The switching sequence produces small jolts, which, of course, are repeated during the electric braking process. Although a jolt affects passenger comfort, it also produces slippage on the tracks, with consequent loss of traction. The dc chopper overcomes these problems because it permits smooth and continuous control of torque and speed. We now study some simple chopper circuits used in conjunction with series motors. Figure 4-12 shows the armature and field of a series motor connected to the output of a chopper.

Supply voltage Es is picked off from two overhead trolley wires. The inductor-capacitor combination L1C1 acts as a dc filter, preventing the sharp current pulses Is from reaching the trolley line. The capacitor can readily furnish these high current pulses. The presence of the inductor has a smoothing effect so that current I drawn from the line has a relatively small ripple.

As far as the motor is concerned, the total inductance of the armature and series field is large enough to store and release the energy needed during the chopper cycle. Consequently, no external inductor is required. When the motor starts up, a low chopper frequency is used, typically 50 Hz. The corresponding "on" time Ta is typically 500s. In many systems, Ta is kept constant while the switching frequency varies. The top frequency (about 2000 Hz) is limited by the switching and turn-off time of the thyristors.

Other choppers function at constant frequency, but with a variable "on" time Ta. In still more sophisticated controls, both the frequency and Ta are varied. In such cases, Ta may range from 20s to 800•. Nevertheless, the basic chopper operation remains the same, Direct-current series motor driven by a chopper. The chopper is not a switch as shown, but a force-commutated SCR, no matter how the on-off switching times are varied.

Figure 4-12 Direct-current series motor driven by a chopper. The chopper is not a switch as shown, but a force-commutated SCR.Figure 4-12 Direct-current series motor driven by a chopper. The chopper is not a switch as shown, but a force-commutated SCR.

Figure 4-13aFigure 4-13a

 

Figure 4-13b Current pulses Is drawn by the chopper from the 700V source when the motor is stalled.Figure 4-13b Current pulses Is drawn by the chopper from the 700V source when the motor is stalled.

 

4.1.9 Current-fed dc motor

Some electronic drives involve direct current motors that do not look at all like dc machines. The reason is that the usual rotating commutator is replaced by a stationary electronic converter. We now discuss the theory behind these so-called "commutatorless" dc machines.

Consider a 2-pole dc motor having 3 independent armature coils, A, B, and C spaced at 120° to each other (Fig. 4-15). The two ends of each coil are connected to diametrically opposite segments of a 6-segment commutator. Two narrow brushes are connected to a constant-current source that successively feeds current into the coils as the armature rotates. A permanent magnet N, S creates the magnetic field.

With the armature in the position shown, current flows in coil A and the resulting torque causes the armature to turn counterclockwise. As soon as contact is broken with this coil, it is immediately established in the next coil. Consequently, conductors facing the N pole always carry currents that flow into the page, while those facing the S pole carry currents that flow out of the page (towards the reader). The motor torque is therefore continuous and may be expressed by:

T = kIB (Equation 4-1)

where

T = motor torque (N-m)

I = current in the conductors (A)

B = average flux density surrounding the current-carrying conductors (T)

k = a constant, dependent upon the number of turns per coil, and the size of the armature

Figure 4-14a Conditions when the motor is running at rated torque and speedFigure 4-14a Conditions when the motor is running at rated torque and speed

 Figure 4-14b Corresponding current pulses drawn by the chopper from the 700 V source.Figure 4-14b Corresponding current pulses drawn by the chopper from the 700 V source.

Figure 4-15 Special current-fed dc motor.Figure 4-15 Special current-fed dc motor.

If the current and flux density are fixed, the resulting torque is also fixed, independent of motor speed.

The commutator segments are 60° wide; consequently, the current in each coil flows in 60° pulses. Furthermore, the current in the coil reverses every time the coil makes half a turn (Fig. 4-16). The alternating nature of the current is of crucial importance. If the current did not alternate, the torque developed by each coil would act first in one, then the opposite direction, as the armature rotates. The net torque would be zero, and so the motor would not develop any power.

Figure 4-16 shows that the ac currents in the 3 coils are out of phase by 120. Consequently, the armature behaves as if it were excited by a 3-phase source. The only difference is that the current waveshapes are rectangular instead of sinusoidal. Basically, the commutator acts as a mechanical converter, changing the dc current from the dc source into ac current in the coils. The frequency is given by:

f = pn/120 (Equation 4-2)

where p is the number of poles and n the speed (r/min). The frequency in the coils is automatically related to the speed because the faster the machine rotates, the faster the commutator switches from one coil to the next. In effect, the commutator generates a frequency which at all times is appropriate to the instantaneous speed.

Figure 4-15 Special current-fed dc motor.  Figure 4-16 The dc current changes to ac current in the coils.Figure 4-16 The dc current changes to ac current in the coils.

As the coils rotate, they cut across the magnetic field created by the N, S poles. An ac voltage is therefore induced in each coil, and its frequency is also given by Eq. 4-2. Furthermore, the voltages are mutually displaced at 120 owing to the way the coils are mounted on the armature. The induced ac voltages appear as a dc voltage between the brushes. The reason is that the brushes are always in contact with coils that are moving in the same direction through the magnetic field; consequently, the polarity is always the same.

If the brushes were connected to a dc voltage source E, the armature would accelerate until the induced voltage E0 is about equal to E. What determines the speed when the armature is fed from a current source, as it is in our case? The speed will increase until the load torque is equal to the torque developed by the motor. Thus, while the speed of a voltage-fed armature depends upon equilibrium between induced voltage and applied voltage, the speed of a current-fed armature depends upon equilibrium between motor torque and load torque. The torque of a mechanical load always rises with increasing speed. Consequently, for a given motor torque, a state of torque equilibrium is always reached, provided the speed is high enough. Care must be taken so that current-fed motors do not run away when the load torque is removed.

 

4.1.10 Commutator replaced by reversing switches

Recognizing that each coil in Fig. 4-15 carries an alternating current, we can eliminate the commutator by connecting each coil to a pair of slip rings and bringing the leads out to a set of mechanical reversing switches (Fig. 4-17). Each switch has 4 normally open contacts. Considering coil A, for example, switch contacts 7 and 8 are closed during the 60° interval when coil side 1 faces the N pole (Fig. 4-18). The contacts are then open for 120° until coil side 4 faces the N pole, whereupon contacts 9 and 10 close for 60°. Consequently, by synchronizing the switch with the position of coil A, we obtain the same result as if we used a commutator.

Figure 4-17 The commutator can be replaced by an array of mechanical switches and a set of slip rings.Figure 4-17 The commutator can be replaced by an array of mechanical switches and a set of slip rings.

Figure 4-18 Circuit showing how current is controlled in coil A.Figure 4-18 Circuit showing how current is controlled in coil A.

Figure 4-19 The armature is now the stator, and the switches have been replaced by thyristors.Figure 4-19 The armature is now the stator, and the switches have been replaced by thyristors.

Coils B and C operate the same way, but they are energized at different times. Figure 4-17 shows how the array of 12 contacts and 6 slip rings are connected to the current source. The reversing switches really act as a 3-phase mechanical inverter, changing dc power into ac power. The slip rings merely provide electrical contact between the revolving armature and the stationary switches and power supply.

Clearly, the switching arrangement of Fig. 4-17 is more complex than the original commutator. However, we can simplify matters by making the armature stationary and letting the permanent magnets rotate. By thus literally turning the machine inside out, we can eliminate 6 slip rings. Then, as a final step, we can replace each contact by a thyristor (Fig. 4-19). The 12 thyristors are triggered by gate signals that depend upon the instantaneous position of the revolving rotor.

The dc motor in Fig. 4-19 looks so different from the one in Fig. 4-15 that we would never suspect they have the same properties. And yet they do.

For example:

If we increase the dc current I or the field strength of poles N, S, the torque increases, and consequently, the speed will increase.

If we shift the brushes against the direction of rotation in Fig. 4-15, current will start flowing in each coil a little earlier than before. Consequently, the ac current in each coil will lead the ac voltage induced across its terminals. We can produce exactly the same effect by firing the thyristors a little earlier in Fig. 4-19. Under these circumstances, the machine furnishes reactive power to the three thyristor bridges, at the same time as it absorbs active power from them.

If we shift the brushes by 180°, the current in each coil flows in the opposite direction to that shown in Fig. 4-15. However, the induced voltage in each coil remains unchanged because it depends only on the speed and direction of rotation. Consequently, the machine becomes a generator, feeding dc power back into the current source.

The same result occurs if we fire the thyristors 180° later in Fig. 4-19. The thyristors then behave as inverters feeding power back to the dc current source.

It is now clear that the machines in Figs. 4-15 and 4-19 behave the same way. The only difference between them is that one is equipped with a rotating mechanical commutator, while the other has a stationary electronic commutator composed of 12 thyristors. By firing the thyristors earlier or later, we produce the same effect as shifting the brushes.

 

4.1.11 Synchronous motor as a commutatorless dc machine

The revolving-field motor in Fig. 4-19 is built like a 3-phase synchronous motor. However, because of the way it receives its ac power, it behaves like a "commutatorless" dc machine. This has a profound effect upon its performance.

First, the "synchronous motor" can never pull out of step because the stator frequency is not fixed, but changes automatically with speed. The reason is that the gates of the SCRs are triggered by a signal that depends upon the instantaneous position of the rotor. For the same reason, the machine has no tendency to oscillate or hunt under sudden load changes.

Second, the phase angle between the ac current in a winding and the ac voltage across it can be modified by altering the timing of the gate pulses. This enables the synchronous motor to operate at leading, lagging, or unity power factor.

Third, because the phase angle between the respective voltages and currents can be fully controlled, the machine can even function as a generator, feeding power back to the dc current source. The thyristor bridges then operate as rectifiers.

Currents i1, i2, i3 in Fig. 4-19 flow only during 60 degree intervals, as they did in the original dc machine. In practice, the conduction period can be doubled to 120°, by connecting the coils in wye and exciting them by a 3-phase, 6-pulse converter (Fig. 4-20). This reduces the number of thyristors by half. Furthermore, it improves the current-carrying capacity of the windings because the duration of current flow is doubled.

Figure 4-20 Commutatorless dc motor being driven by a converter.

Figure 4-20 Commutatorless dc motor being driven by a converter.

Figure 4-21 This elementary dc motor is equivalent to the entire circuit of Fig.4-20.Figure 4-21 This elementary dc motor is equivalent to the entire circuit of Fig.4-20.

 

4.1.12 Standard synchronous motor and commutatorless dc machine

The machine shown in Fig. 4-20 can be made to function as a conventional synchronous motor by applying a fixed frequency to the SCR gates. Under these conditions, the input to the gate triggering processor no longer depends on rotor position or rotor speed.

 

4.1.13 Synchr0onous motor drive using current-fed dc link

Figure 4-22 shows a typical commutatorless dc motor circuit. It consists of two converters* connected between a 3-phase source and the "synchronous" motor. Converter 1 acts as a controlled rectifier, feeding dc power to converter 2. The latter behaves as a naturally-commutated inverter whose ac voltage and frequency are established by the motor.

*Readers familiar with feedback theory will recognize that the basic distinction between the two machines is that one functions on open loop while the other operates on closed loop.

A smoothing inductor L maintains a ripple-free current in the so-called dc link between the two converters. Current I is controlled by converter 1, which acts as a current source. A smaller bridge rectifier (converter 3) supplies the field excitation for the rotor.

Converter 2 is naturally-commutated by voltage Es induced across the terminals of the motor. This voltage is created by the revolving magnetic flux in the air gap. The flux depends upon the stator currents and the exciting current If. The flux is usually kept fixed; consequently, the induced voltage Es is proportional to the motor speed.

Figure 4-21 This elementary dc motor is equivalent to the entire circuit of Fig.4-20.  Figure 4-22 Commutatorless dc motor driven by a converter with a dc link. The output frequency can be considerably greater than 60 Hz, thus permitting high speeds.Figure 4-22 Commutatorless dc motor driven by a converter with a dc link. The output frequency can be considerably greater than 60 Hz, thus permitting high speeds.

Figure 4-23 Typical voltage and current waveshapes in Fig.4-22.Figure 4-23 Typical voltage and current waveshapes in Fig.4-22.

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