SPEED CONTROL OF PERMANENT SPLIT CAPACITOR MOTORS

The speed of a permanently split capacitor motor can be adjusted by connecting it to a variable voltage source such as an auto-transformer. The limitation of this method is that the starting torque developed is very low, especially when the motor is started on low speed. Another limitation is that speed is sensitive to voltage changes on low speed connections. Further speed varies considerably for different loading conditions of the motor.

The motor can be started in three different ways using an auto-transformer as shown in Figure 1. In Figure 1 (a) the voltage across the main winding and the auxiliary winding are varied simultaneously resulting in low starting torque for low speed operation. This shortcoming, however, can be overcome by first starting the motor on high speed and then stepping it down to low speed as shown in Figure 1 (a).
Good starting torque at all speeds can be obtained if the auxiliary winding is connected across the mains supply and voltage adjustment is provided only for the main winding as shown in Figure 1 (b).

In the third method of connections shown in Figure 1 (c), the two windings are connected to a tapped auto transformer so that any voltage change across the main winding is accompanied by an inverse change across the auxiliary winding. This method with a properly designed motor can be applied to an installation whose speed should not be sensitive to variations of normal line voltage.

Figure 1 Voltage control method for speed adjustment of split capacitor motors

In another design of split motor a special winding arrangement is made to eliminate the requirement of an auto transformer. The motor in this case, in addition to the main and the auxiliary windings, is provided with a so called intermediate winding. The intermediate winding so provided is in space phase with the main winding. The main and the intermediate windings occupy the same slots, the later placed directly over the former. They need not have the same number of turns. The wire of the intermediate winding is invariably smaller in size than the main winding. Two different connections for this type of motor design are shown in Figure 2 (a and b).

Figure 2 Speed control of split capacitor motor with intermediate winding

The connection scheme shown in Figure 2 (a) is generally used for 110 volt motors. The main and the intermediate winding are connected in series across the auxiliary winding and the capacitor. The terminal of SPDT (Single Pole Double Throw) switch H and L are wired to the intermediate winding as shown in Figure 2(a).

Connections shown in Figure 2 (b) are used for 230 voltage motors.