N2 Electrical Trade Theory|The Easy Way! 63


Tis, in effect, results in a rotating field similar to that which would be obtained from a two-phase supply. Te torque produced in this manner is enough to start the motor under light starting load conditions. Tese motors are referred to as resistance-start motors.


In order to increase the starting torque, the phase displacement between the current through the running winding and the current through the auxiliary winding must be increased. In the case of capacitor-start motors, the phase displacement between the running winding and auxiliary winding is increased by connecting a capacitor in series with the auxiliary winding.


To increase the running torque of the motor, the auxiliary winding is sometimes leſt connected in the circuit with a continuously rated capacitor while the motor is running.


Two common capacitor-start motors are;


• the capacitor-start, induction run motor, and • the capacitor-start, capacitor-run motor.


4.7.2 Capacitor-start, induction run motor


By connecting a suitable capacitor in series with the starting (auxiliary) winding, the current through the starting winding may be made to lead the voltage. Tus, the time-phase difference in the currents of the two windings at starting may be made nearly 900.


When the rotor reaches a certain speed, the starting winding is disconnected by means of a centrifugal switch, fig. 4.5.


Fig. 4.5 Capacitor-start induction run motor


4.7.3 Capacitor-start, capacitor-run motor


Te capacitor in series with the auxiliary winding may have one fixed value, fig. 4.6 (a), or it may have one value for starting and another value for running. One method of obtaining two values of capacitance is shown in fig. 4.6 (b). As the motor approaches operating speed, the centrifugal switch disconnects one of the capacitors.


Te capacitor-start capacitor-run motor starts and operates in much the same way as a two-phase motor and has the advantage of producing a more constant torque.


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