INVERTER-FED INDUCTION MOTOR DRIVES:CYCLOCONVERTER DRIVES

CYCLOCONVERTER DRIVES

We conclude this chapter with a discussion of the cycloconverter variable-frequency drive, which has never become very widespread but is sometimes used in very large low-speed induction motor or synchron- ous motor drives. Cycloconverters are only capable of producing ac- ceptable output waveforms at frequencies well below the mains frequency, but this, coupled with the fact that it is feasible to make large induction or synchronous motors with high-pole numbers (e.g. 20) means that a very low-speed direct (gearless) drive becomes practicable. A 20-pole motor, for example, will have a synchronous speed of only 30 rev/min at 5 Hz, making it suitable for mine winders, kilns, crushers, etc.

Most of the variable-frequency sources discussed in this book have been described as inverters because they convert power from d.c. to a.c. The power usually comes from a Wxed-frequency mains supply, which is Wrst rectiWed to give an intermediate stage – the ‘d.c. link’ – which is then chopped up to form a variable-frequency output. In contrast, the cycloconverter is a ‘direct’ converter, i.e. it does not have a d.c. stage (see also Section 2.4.6). Instead, the output voltage is synthesised by switching the load directly across whichever phase of the mains gives the best approxi- mation to the desired load voltage at each instant of time. The principal advantage of the cycloconverter is that naturally commutated devices (thyristors) can be used instead of self-commutating devices, which means that the cost of each device is lower and higher powers can be achieved. In principle cycloconverters can have any combination of input and output phase numbers, but in practice the 3-phase input, 3-phase output version is used for drives, mainly in small numbers at the highest powers (e.g. 1 MW and above).

Textbooks often include bewilderingly complex circuit diagrams of the cycloconverter, which are not much help to the user who is seeking to understand how such systems work. Our understanding can be eased by recognising that the power conversion circuit for each of the three output phases is the same, so we can consider the simpler question of how to obtain a variable frequency, variable voltage supply, suitable for one phase of an induction motor, from a 3-phase supply of Wxed frequency and constant voltage. It will also assist us to bear in mind that cycloconverters are only used to synthesise output frequencies that are low in comparison with the mains frequency. With a 50 Hz supply for example, we can expect to be able to achieve reasonably satisfactory approximations to a sinusoidal output voltage for frequencies from zero (d.c.) up to about 15 Hz; but at higher frequencies the harmonic distortion of the waveform will be so awful that even the normally tolerant induction motor will balk at the prospect.

To understand why we use any particular power electronic circuit conWguration, we Wrst need to address the question of what combination(s) of voltages and currents will be required in the load. Here the load is an induction motor, and we know that under sinusoidal supply conditions the power factor varies with load but never reaches unity. In other words, the stator current is never in phase with the stator voltage. So during the positive half-cycle of the voltage waveform the current will be positive for some of the time, but negative for the remainder; while during the negative voltage half-cycle the current will be negative for some of the time and positive for the rest of the time. This means that the supply to the motor has to be able to handle any combination of both positive and negative voltage and current.

We have already explored how to achieve a variable-voltage d.c. supply, which can handle both positive and negative currents, in Chap- ter 4. We saw that what was needed was two fully controlled 3-phase converters (as shown in Figure 2.11), connected back to back, as shown in Figure 4.8. We also saw in Chapter 4 that by varying the Wring delay angle of the thyristors, the positive-current bridge could produce a range of mean (d.c.) output voltages from a positive maximum, through zero, to a negative maximum; and likewise the negative-current bridge could give a similar range of mean output voltages from negative maximum to positive maximum. Typical ‘d.c.’ output voltages over the range of Wring angles from a ¼ 0o (maximum d.c. voltage) to a ¼ 90o (zero d.c. volt- age) are shown in Figure 2.12.

In Chapter 4, the discussion focused on the mean or d.c. level of the output voltages, because we were concerned with the d.c. motor drive. But here we want to provide a low-frequency (preferably sinusoidal) output voltage for an induction motor, and the means for doing this should now be becoming clear. Once we have a double thyristor con- verter, and assuming for the moment that the load is resistive, we can generate a low-frequency sinusoidal output voltage simply by varying the Wring angle of the positive-current bridge so that its output voltage increases from zero in a sinusoidal manner with respect to time. Then, when we have completed the positive half-cycle and arrived back at zero voltage, we bring the negative bridge into play and use it to generate the negative half-cycle, and so on.

In practice, as we have seen above, the load (induction motor) is not purely resistive, so matters are rather more complicated, because as we saw earlier for some part of the positive half-cycle of the output voltage wave the motor current will be negative. This negative current can only be supplied by the negative-current bridge (see Chapter 4) so as soon as the current reverses the negative bridge will have to be brought into action, initially to provide positive voltage (i.e. in the inverting mode) but later – when the current goes negative – providing negative voltage (i.e. rectifying). As long as the current remains continuous (see Chapter 2) the synthesised output voltage waveform will be typically as shown in Figure 8.9.

We see that the output voltage wave consists of chunks of the incom- ing mains voltage, and that it oVers a reasonable approximation to the fundamental frequency sine wave shown by the dotted line in Figure 8.9. The output voltage waveform is no worse than the voltage waveform from a d.c. link inverter (see Figure 8.2), and as we saw in that context, the current waveform in the motor will be a good deal smoother than the

voltage, because of the Wltering action of the stator leakage inductance. The motor performance will therefore be acceptable, despite the extra losses that arise from the unwanted harmonic components. We should note that because each phase is supplied from a double converter, the motor can regenerate when required (e.g. to restrain an overhauling load, or to return kinetic energy to the supply when the frequency is lowered to reduce speed). This is one of the major advantages of the cycloconverter.

It is not necessary to go into the detail of how the Wring angle scheme is implemented, but it should be clear that by varying the amplitude and frequency of the reference signal to the Wring angle control, we can expect the output voltage to vary in sympathy. We then have the ability to keep the voltage–frequency ratio constant, so that the Xux in the induction motor remains constant and we obtain a constant torque characteristic. It should also be evident from Figure 8.9 that as the output frequency is raised it becomes increasingly diYcult to achieve a reasonable approxi mation to a sine wave, because there are too few ‘samples’ available in each half-cycle of the output. Cycloconverters are therefore seldom op- erated at more than one third of the mains frequency.

With the conWguration described above, each phase of the motor requires its own double-bridge converter, consisting of 12 thyristors, so the complete cycloconverter requires 36 thyristors. To avoid short- circuits between the incoming mains lines, the three motor phase- windings must be isolated from each other (i.e. the motor cannot be connected in the conventional star or delta fashion, but must have both ends of each winding brought out), or each double converter can be supplied from separate transformer secondaries.

In practice there are several power-circuit conWgurations that can be used with star-connected motors, and which diVer in detail from the set- up described above, but all require the same number of thyristors to achieve the waveform shown in Figure 8.9. This waveform is referred to as 6-pulse (see Chapter 2), because the output has six pulses per cycle of the mains. A worse (3-pulse) waveform is obtained with 18 thyristors, while a much better (12-pulse) waveform can be obtained by using 72 thyristors.

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