Capacitor starts induction motor

Capacitor starts induction motor: Capacitors are used to improve the starting and running performance of the single phase inductions motors.

The capacitor start induction motor is also a split phase motor. The capacitor of suitable value is connected in series with the auxiliary coil through a switch such that Ia the current in the auxiliary coil leads the current Im in the main coil by 90 electrical degrees in time phase so that the starting torque is maximum for certain values of Ia and I_m. This becomes a balanced 2- phase motor if the magnitude of I_a and I_m are equal and are displaced in time phase by 90° electrical degrees. Since the two windings are displaced in space by 90 electrical degrees as shown in Fig. 9.6 maximum torque is developed at start. However, the auxiliary winding and capacitor are disconnected after the motor has picked up t5 per cent of the synchronous speed. The motor will start without any humming noise. However, after the auxiliary winding is disconnected, there will be some humming noise.

Since the auxiliary winding and capacitor are to be used intermittently, these can be designed for minimum cost. However, it is found that the best compromise among the factors of starting torque, starting current and costs results with a phase angle somewhat less than 90° between I_m and I_a. A typical torque-speed characteristic is shown in Fig. 5.6 (c) high starting torque being an outstanding feature.

Here the auxiliary winding and capacitor could be so designed that the motor works as a perfect 2-phase motor at anyone desired load. With this the backward rotating magnetic field would be completely eliminated. The double stator frequency torque pulsations would also be eliminated, thereby the motor starts and runs as a noise free motor. With this there is improve- ment in p.f. and efficiency of the motor. However, the starting torque must be sacrificed as the capacitance is necessarily a compromise between the best starting and running characteristics. The torque-speed characteristic of the motor is shown in Fig. 9.7 (b).

(c) Capacitor start capacitor rum motor. If two capacitors are used with the auxiliary winding as shown in Fig. 5.8 (a), one for starting and other during the start and run, theoretically optimum starting and running performance can both be achieved.