Converter output impedance: overlap
So far we have tacitly assumed that the output voltage from the converter was independent of the current drawn by the motor, and depended only on the delay angle α. In other words we have treated the converter as an ideal voltage source.

In practice the a.c. supply has a finite impedance, and we must therefore expect a volt-drop which depends on the current being drawn by the motor. Perhaps surprisingly, the supply impedance (which is mainly due to inductive leakage reactances in transformers) manifests itself at the output stage of the converter as a supply resistance, so the supply volt-drop (or regulation) is directly proportional to the motor armature current.

It is not appropriate to go into more detail here, but we should note that the effect of the inductive reactance of the supply is to delay the transfer (or commutation) of the current between thyristors; a phenomenon known as overlap. The consequence of overlap is that instead of the output voltage making an abrupt jump at the start of each pulse, there is a short period when two thyristors are conducting simultaneously. During this interval the output voltage is the mean of the voltages of the incoming and outgoing voltages, as shown typically in Figure 4.5.

Figure 4.5 Distortion of converter output voltage waveform caused by rectifier overlap.

It is important for users to be aware that overlap is to be expected, as otherwise they may be alarmed the first time they connect an oscilloscope to the motor terminals. When the drive is connected to a 'stiff' (i.e. low impedance) industrial supply the overlap will only last for perhaps a few microseconds, so the 'notch' shown in Figure 4.5 would be barely visible on an oscilloscope.

Books always exaggerate the width of the overlap for the sake of clarity, as in Figure 4.5: with a 50 or 60 Hz supply, if the overlap lasts for more than say 1 ms, the implication is that the supply system impedance is too high for the size of converter in question, or conversely, the converter is too big for the supply.

Returning to the practical consequences of supply impedance, we simply have to allow for the presence of an extra 'source resistance' in series with the output voltage of the converter. This source resistance is in series with the motor armature resistance, and hence the motor torque"speed curves for each value of α have a somewhat steeper droop than they would if the supply impedance was zero.

Four-quadrant operation and inversion
So far we have looked at the converter as a rectifier, supplying power from the a.c. mains to a d.c. machine running in the positive direction and acting as a motor. As explained in Chapter 3, this is known as one-quadrant operation, by reference to quadrant 1 of the complete torque"speed plane shown in Figure 3.16.

But suppose we want to run the machine as a motor in the opposite direction, with negative speed and torque, i.e. in quadrant 3; how do we do it? And what about operating the machine as a generator, so that power is returned to the a.c. supply, the converter then 'inverting' power rather than rectifying, and the system operating in quadrant 2 or quadrant 4. We need to do this if we want to achieve regenerative braking. Is it possible, and if so how?

The good news is that as we saw in Chapter 3 the d.c. machine is inherently a bidirectional energy converter. If we apply a positive voltage V greater than E, a current flows into the armature and the machine runs as a motor. If we reduce V so that it is less than E, the current, torque and power automatically reverse direction, and the machine acts as a generator, converting mechanical energy (its own kinetic energy in the case of regenerative braking) into electrical energy. And if we want to motor or generate with the reverse direction of rotation, all we have to do is to reverse the polarity of the armature supply. The d.c. machine is inherently a four-quadrant device, but needs a supply which can provide positive or negative voltage, and simultaneously handle either positive or negative current.

This is where we meet a snag: a single thyristor converter can only handle current in one direction, because the thyristors are unidirectional devices. This does not mean that the converter is incapable of returning power to the supply however. The d.c. current can only be positive, but (provided it is a fully controlled converter) the d.c. output voltage can be either positive or negative (see Chapter 2). The power flow can therefore be positive (rectification) or negative (inversion).

For normal motoring where the output voltage is positive (and assuming a fully controlled converter), the delay angle (α) will be up to 90°. (It is common practice for the firing angle corresponding to rated d.c. voltage to be around 20° when the incoming a.c. voltage is normal: if the a.c. voltage falls for any reason, the firing angle can then be further reduced to compensate and allow full d.c. voltage to be maintained.)

Figure 4.6 Average d.c. output voltage from a fully-controlled thyristor converter as a function of the firing angle delay α.

When α is greater than 90°, however, the output voltage is negative, as indicated by equation (2.5), and is shown in Figure 4.6. A single fully controlled converter therefore has the potential for two-quadrant operation, though it has to be admitted that this capability is not easily exploited unless we are prepared to employ reversing switches in the armature or field circuits. This is discussed next.

Single-converter reversing drives
Single-converter reversing drives
We will consider a fully controlled converter supplying a permanent-magnet motor, and see how the motor can be regeneratively braked from full speed in one direction, and then accelerated up to full speed in reverse. We looked at this procedure in principle at the end of Chapter 3, but here we explore the practicalities of achieving it with a converter-fed drive. We should be clear from the outset that in practice, all the user has to do is to change the speed reference signal from full forward to full reverse: the control system in the drive converter takes care of matters from then on. What it does, and how, is discussed below.

When the motor is running at full speed forward, the converter delay angle will be small, and the converter output voltage V and current I will both be positive. This condition is shown in Figure 4.7(a), and corresponds to operation in quadrant 1.

Figure 4.7 Stages in motor reversal using a single-converter drive and mechanical reversing switch.

In order to brake the motor, the torque has to be reversed. The only way this can be done is by reversing the direction of armature current. The converter can only supply positive current, so to reverse the motor torque we have to reverse the armature connections, using a mechanical switch or contactor, as shown in Figure 4.7(b). (Before operating the contactor, the armature current would be reduced to zero by lowering the converter voltage, so that the contactor is not required to interrupt current.)

Note that because the motor is still rotating in the positive direction, the back e.m.f. remains in its original sense; but now the motional e.m.f. is seen to be assisting the current and so to keep the current within bounds the converter must produce a negative voltage V which is just a little less than E. This is achieved by setting the delay angle at the appropriate point between 90° and 180°. (The dotted line in Figure 4.6 indicates that the maximum acceptable negative voltage will generally be somewhat less than the maximum positive voltage: this restriction arises because of the need to preserve a margin for commutation of current between thyristors.)

Note that the converter current is still positive (i.e. upwards in Figure 4.7(b)), but the converter voltage is negative, and power is thus flowing back to the mains. In this condition the system is operating in quadrant 2, and the motor is decelerating because of the negative torque. As the speed falls, E reduces, and so V must be reduced progressively to keep the current at full value. This is achieved automatically by the action of the current-control loop, which is discussed later.

The current (i.e. torque) needs to be kept negative in order to run up to speed in the reverse direction, but after the back e.m.f. changes sign (as the motor reverses), the converter voltage again becomes positive and greater than E, as shown in Figure 4.7(c). The converter is then rectifying, with power being fed into the motor, and the system is operating in quadrant 3.

Schemes using reversing contactors are not suitable where the reversing time is critical, because of the delay caused by the mechanical reversing switch, which may easily amount to 200"400 msec. Field reversal schemes operate in a similar way, but reverse the field current instead of the armature current. They are even slower, because of the relatively long time-constant of the field winding.

Double-converter reversing drives
Where full four-quadrant operation and rapid reversal is called for, two converters connected in anti-parallel are used, as shown in Figure 4.8. One converter supplies positive current to the motor, while the other supplies negative current.

Figure 4.8 Double-converter reversing drive.

The bridges are operated so that their d.c. voltages are almost equal thereby ensuring that any d.c. circulating current is small, and a reactor is placed between the bridges to limit the flow of ripple currents which result from the unequal ripple voltages of the two converters. Alternatively, the reactor can be dispensed with by only operating one converter at a time.

The changeover from one converter to the other can only take place after the firing pulses have been removed from one converter, and the armature current has decayed to zero. Appropriate zero-current detection circuitry is provided as an integral part of the drive, so that as far as the user is concerned, the two converters behave as if they were a single ideal bidirectional d.c. source.

Prospective users need to be aware of the fact that a basic single converter can only provide for operation in one quadrant. If regenerative braking is required, either field or armature reversing contactors will be needed; and if rapid reversal is essential, a double converter has to be used. All these extras naturally push up the purchase price.

Power factor and supply effects
One of the drawbacks of a converter-fed d.c. drive is that the supply power factor is very low when the motor is operating at high torque (i.e. high current) and low speed (i.e. low armature voltage), and is less than unity even at base speed and full load. This is because the supply current waveform lags the supply voltage waveform by the delay angle a,as shown (for a 3-phase converter) in Figure 4.9, and also the supply current is approximately rectangular (rather than sinusoidal).

Figure 4.9 Supply voltage and current waveforms for single-phase converter-fed d.c. motor drive.

It is important to emphasise that the supply power factor is always lagging, even when the converter is inverting. There is no way of avoiding the low power factor, so users of large drives need to be prepared to augment their existing power factor correcting equipment if necessary.

The harmonics in the mains current waveform can give rise to a variety of interference problems, and supply authorities generally impose statutory limits. For large drives (say hundreds of kilowatts), filters may have to be provided to prevent these limits from being exceeded.

Since the supply impedance is never zero, there is also inevitably some distortion of the mains voltage waveform, as shown in Figure 4.10 which indicates the effect of a 6-pulse converter on the supply line-to-line voltage waveform. The spikes and notches arise because the mains is momentarily short-circuited each time the current commutates from one thyristor to the next, i.e. during the overlap period discussed earlier.

Figure 4.10 Distortion of line voltage waveform caused by overlap in three-phase fully-controlled converter.(The width of the notches has been exaggerated for the sake of clarity.)

For the majority of small and medium drives, connected to stiff industrial supplies, these notches are too small to be noticed (they are greatly exaggerated for the sake of clarity in Figure 4.10); but they can pose a serious interference problem for other consumers when a large drive is connected to a weak supply.

Coming up in Part 3: Control arrangements for DC drives.

Printed with permission from Newnes, a division of Elsevier. Copyright 2005. "Electric Motors and Drives" by Austin Hughes. For more information about this title and other similar books, please visit www.elsevierdirect.com.

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Basics of the Electric Servomotor and Drive - Part 1: Basic Magnetics and Motor Control Overview | Part 2: Permanent-Magnet Brush Motors | Part 3: Brushless PM Motors
Tutorial: Motion-control architectures adapt to technology changes
Implementing Embedded Speed Control for Brushless DC Motors: Part 1 | Part 2 | Part 3
Open-Source Robotics and Process Control Circuit Examples - Part 3: Speed-controlled DC motor