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Adjustable-Speed Drives

Author: E.E. Kimberly

Many motor applications require adjustable speed throughout a wide range of speeds. Mechanical speed changers are sometimes satisfactory if the power involved is not very great or if the necessary rate of speed change is low. A speed change of 3 to 1 may be obtained economically by shunt-field control in a d-c motor, if the motor is specially built for adjustable-speed service. When the low speed must be very low compared to the high speed, the speed may best be changed by raising or lowering the voltage applied to the armature while the field strength is held constant. The armature voltage may be varied by simply inserting a control resistor in series with it. This method is used in controlling very small motors, such as sewing-machine and small-fan motors, but the power loss in the control resistor makes the method uneconomical with large motors. Furthermore, the speed regulation is poor and its amount is intolerable in most cases when the load is variable.

Fig. 29-7. Ward-Leonard Motor Control

One method of obtaining adjustable voltage consists of using a generator driven at constant speed but provided with variable separate excitation. The Ward-Leonard system used for this purpose is illustrated in Fig. 29-7. The main generator G and the exciting generator E are driven by a constant-speed motor M (a-c or d-c). The generator-armature leads are connected directly to the motor-armature leads, without an intervening resistor. The field of the main generator is increased, decreased, or reversed by the control rheostat FR. The resulting increase, decrease, or reversal of the generator voltage causes the corresponding desired changes in motor speed and direction of rotation. The cost of this type of speed control is prohibitive for all but special applications, such as large hoists, elevators, and rolling mills.

Alternating-current motors do not serve adjustable-speed applications so well as do direct-current motors. As mentioned on page 220, the speed of a wound-rotor induction motor may be varied by varying the rotor-circuit resistance, but the efficiency is low and the speed regulation is poor. Several motors of the brush-shifting type have been developed, in which a counter voltage is generated by a special winding on the motor itself. This counter voltage is applied to the rotor in such a way as to limit the rotor currents without the power loss that would occur if a rotor resistance were used. The magnitude of the counter voltage, and hence of the speed, is controlled by shifting the brushes around the commutator. The speed regulation of this type of motor is good at all speeds, but the cost is relatively great; and so its use is limited to spinning-frame drive, forced-draft fans in power houses, and similar applications which require accurate speed control.

Last Update: 2010-10-06