Speed Control of Induction Motor

To operate efficiently an induction motor must run with a small slip. It follows that any efficient method of speed control must be based on varying the synchronous speed of the field, rather than the slip. The two factors that determine the speed of the field, are the supply frequency and the pole number.

The pole number has to be an even integer, so where continuously adjustable speed control over a wide range is called for, the best approach is to provide a variable-frequency supply.

In this article we are concerned with constant frequency mains operation, so we have a choice between pole-changing, which can provide discrete speeds only, or slip control which can provide continuous speed control, but is inherently inefficient.

Pole-Changing Motors

For some applications continuous speed control may be an unnecessary luxury, and it may be sufficient to be able to run at two discrete speeds. Among many instances where this can be acceptable and economic are pumps, lifts and hoists, fans and some machine tool drives.

We know that the pole number of the field was determined by the layout and interconnection of the stator coils, and that once the winding has been designed, and the frequency specified, the synchronous speed of the field is fixed. If we wanted to make a motor, that could run at either of two different speeds, we could construct it with two separate stator windings (say 4-pole and 6-pole), and energise the appropriate one.

There is no need to change the cage rotor since the pattern of induced currents can readily adapt to suit the stator pole number. Early 2-speed motors did have 2 distinct stator windings, but were bulky and inefficient.

It was soon realised that if half of the phase belts within each phase-winding could be reversed in polarity, the effiective pole number could be halved. For example, a 4-pole MMF pattern (N-S-N-S) would become (N-N-S-S), i.e. effectively a 2-pole pattern with one large N pole and one large S pole.

By bringing out six leads instead of three, and providing switching contactors to effect the reversal, two discrete speeds in the ratio 2:1 are therefore possible from a single winding. The performance at the high (e.g. 2-pole) speed is relatively poor, which is not surprising in view of the fact that the winding was originally optimised for 4-pole operation.

It was not until the advent of the more sophisticated pole amplitude modulation (PAM) method in the 1960s that 2-speed single-winding high-performance motors with more or less any ratio of speeds became available from manufacturers. This subtle technique allows close ratios such as 4/6, 6/8, 8/10 or wide ratios such as 2/24 to be achieved.

Close ratios are used in pumps and fans, while wide ratios are used for example in washing machines where a fast spin is called for. The beauty of the PAM method is that it is not expensive. The stator winding has more leads brought out, and the coils are connected to form non-uniform phase belts, but otherwise construction is the same as for a single-speed motor.

Typically six leads will be needed, three of which are supplied for one speed, and three for the other, the switching being done by contactors. The method of connection (star or delta) and the number of parallel paths within the winding are arranged so that the air-gap flux at each speed matches the load requirement.

For example, if constant torque is needed at both speeds, the flux needs to be made the same, whereas if reduced torque is acceptable at the higher speed the flux can obviously be lower.

Voltage Control of High-Resistance Cage Motors

Where efficiency is not of paramount importance, the torque (and hence the running speed) of a cage motor can be controlled simply by altering the supply voltage. The torque at any slip is approximately proportional to the square of the voltage, so we can reduce the speed of the load by reducing the voltage. The method is not suitable for standard low-resistance cage motors, because their stable operating speed range is very restricted, as shown in Figure 1(a) .

But if special high-rotor resistance motors are used, the slope of the torque–speed curve in the stable region is much less, and a rather wider range of steady-state operating speeds is available, as shown in Figure 1(b).

The most unattractive feature of this method is the low efficiency, which is inherent in any form of slip control. We recall that the rotor efficiency at slip s is (1 – s), so if we run at say 70% of synchronous speed (i.e. s = 0.3), 30% of the power crossing the air-gap is wasted as heat in the rotor conductors.

The approach is therefore only practicable where the load torque is low at low speeds, so that at high slips the heat in the rotor is tolerable. A fan-type characteristic is suitable, as shown in Figure 1(b), and many ventilating systems therefore use voltage control.

Voltage control became feasible only when relatively cheap thyristor a.c. voltage regulators arrived on the scene during the 1970s. Previously the cost of autotransformers or induction regulators to obtain the variable voltage supply was simply too high. The thyristor hardware required is essentially the same as for soft starting, and a single piece of kit can therefore serve for both starting and speed control.

Where accurate speed control is needed, a tachogenerator must be fitted to the motor to provide a speed feedback signal, and this naturally increases the cost significantly. Applications are numerous, mainly in the range 0.5–10 kW, with most motor manufacturers offering high-resistance motors specifically for use with thyristor regulators.

Speed Control of Wound-Rotor Motors

The fact that the rotor resistance can be varied easily allows us to control the slip from the rotor Side, with the stator supply voltage and frequency constant. Although the method is inherently inefficient it is still used in many medium and large drives such as hoists, conveyors and crushers because of its simplicity and comparatively low cost.

A set of torque–speed characteristics is shown in Figure 2, from which it should be clear that by appropriate selection of the rotor circuit resistance, any torque up to typically 1.5 times full-load torque can be achieved at any speed.

Voltage Control

In addition to their use for soft-start and speed control, thyristor voltage regulators are sometimes marketed as power factor controllers and/or energy optimisers for cage motors. Much of the hype surrounding their introduction has evaporated, but users should remain sceptical of some of the more extravagant claims, which can still be found.

The fact is that there are comparatively few situations where considerations of power factor and/or energy economy alone are sufficient to justify the expense of a voltage controller. Only when the motor operates for very long periods running light or at low load can sufficient savings be made to cover the outlay. There is certainly no point in providing energy economy when the motor spends most of its time working at or near full load.

Both power factor control and energy optimisation rely on the fact that the air-gap flux is proportional to the supply voltage, so that by varying the voltage, the flux can be set at the best level to cope with the prevailing load. We can see straightaway that nothing can be achieved at full load, since the motor needs full flux (and hence full voltage) to operate as intended. Some modest savings in losses can be achieved at reduced load, as we will see.

If we imagine the motor to be running with a low-load torque and full voltage, the flux will be at its full value, and the magnetising component of the stator current will be larger than the work component, so the input power factor will be very low, as shown in Figure 3(a).

Now suppose that the voltage is reduced to say half (by phasing back the thyristors), thereby halving the air-gap flux and reducing the magnetising current by at least a factor of two. With only half the flux, the rotor current must double to produce the same torque, so the work current reflected in the stator will also double. The input power factor will therefore improve considerably (see Figure 3(b)). Of course the slip with ‘half-flux’ operation will be higher (by a factor of four), but with a low-resistance cage it will still be small, and the drop in speed will therefore be slight.

The success (or otherwise) of the energy economy obtained depends on the balance between the iron losses and the copper losses in the motor. Reducing the voltage reduces the flux, and hence reduces the eddy current and hysteresis losses in the iron core. But as we have seen above, the rotor current has to increase to produce the same torque, so the rotor copper loss rises. The stator copper loss will reduce if (as in Figure 3) the magnitude of the stator current falls.

In practice, with average general-purpose motors, a net saving in losses only occurs for light loads, say at or below 25% of full load, though the power factor will always increase.

Slip Energy Recovery (Wound Rotor Motors)

Instead of wasting rotor circuit power in an external resistance, it can be converted and returned to the mains supply. Frequency conversion is necessary because the rotor circuit operates at slip frequency, so it cannot be connected directly to the mains.

In a slip energy recovery system, the slip frequency a.c. from the rotor is first rectified in a 3-phase diode bridge and smoothed before being returned to the mains supply via a 3-phase thyristor bridge converter operating in the inverting mode. A transformer is usually required to match the output from the controlled bridge to the mains voltage.

Since the cost of both converters depends on the slip power they have to handle, this system (which is known as the static Kramer drive) is most often used where only a modest range of speeds (say from 80% of synchronous and above) is required, such as in large pump and compressor drives.

Speed control is obtained by varying the firing angle of the controlled converter, the torque–speed curves for each firing angle being fairly steep (i.e. approximating to constant speed), thereby making closed-loop speed control relatively simple.