Capacitor Start Induction Motors

Capacitor Start Induction Motors AU : May-04,08,12,13,16,18, Dec.-06,ll,13

The construction of this type of motor is similar to the resistance split phase type. The difference is that in series with the auxiliary winding the capacitor is connected. The capacitive circuit draws a leading current, this feature used in this type to increase the split phase angle a between the two currents I_m and I_st.

Depending upon whether capacitor remains in the circuit permanently or is disconnected from the circuit using centrifugal switch, these motors are classified as,

1. Capacitor start motors 2. Capacitor start capacitor rim motors

The construction of capacitor start motor is shown in the Fig. 8.7.1 (a).

The current Im lags the voltage by angle m while due to capacitor the current I_st leads the voltage by angle st. Hence there exists a large phase difference between the two currents which is almost 90°, which is an ideal case. The phasor diagram is shown in the Fig. 8.7.1 (b).

The starting torque is proportional to 'a' and hence such motors produce very high starting torque.

When speed approaches to 75 to 80 % of the synchronous speed, the starting winding gets disconnected due to operation of the centrifugal switch. The capacitor remains in the circuit only at start hence it is called capacitor start motors.

In case of capacitor start capacitor run motor, there is no centrifugal switch and capacitor remain parmanently in the circuit. This improves the power factor. The schematic representation of such motor is shown in the Fig. 8.7.2.

The phasor diagram remains same as shown in the Fig. 8.7.2 (b). The performance not only at start but in running condition also depends on the capacitor C hence its value is to be designed so as to compromise between best starting and best running condition. Hence the starting torque available in such type of motor is about 50 to 100 % of full load torque. The torque-slip characteristics is shown in the Fig. 8.7.3.

The direction of rotation, in both the types can be changed by interchanging the connections of main winding or auxiliary winding. The capacitor permanently in the circuit improves the power factor. These motors are more costly than split phase type motors. 

The capacitor value can be selected as per the requirement of starting torque, the starting torque can be as high as 350 to 400 % of full load torque. The torque-speed characteristics is as shown in the Fig. 8.7.3.

1. Applications

These motors have high starting torque and hence are used for hard starting loads. These are used for compressors, conveyors, grinders, fans, blowers, refrigerators, air conditioners etc. These are most commonly used motors. The capacitor start capacitor run motors are used in ceiling fans, blowers and air-circulators. These motors are available upto 6 kW.

Example 8.7.1 The main and auxiliary winding impedances of a 50 Hz, capacitor start single phase induction motor are (3 + j 2.7) ohm and (7 + j 3) ohm respectively. Determine the value of the capacitor to be connected in series with the auxiliary winding to achieve a phase difference of 90° between the currents of the two windings at start. AU : May-04, Marks 8

Solution : Let X_C be the reactance of the capacitor connected in the auxiliary winding.

Z_a = 7 + j 3

Then Z_a = 7 + j 3 - j X_C = 7 - jX

Now Z_m = 3 + j 2.7 = 4.036 ∠42°Ω

Thus I_m lags behind V by 42°.

Since, time phase angle between Im and Ia has to be 90°, 1, must lead V by,

90°-42° = 48°

Example 8.7.2 The equivalent impedances of the main and auxiliary windings in a capacitor motor are (15 + j 22.5) Ω and (50 + j 120) 2 respectively, while the capacitance of the capacitor is 12 uF. Determine the line current at starting on a 230 V, 50 Hz supply. AU : May-13, Marks 8

Solution :

Examples for Practice

Example 8.7.3 The resistance and inductive reactance of each winding of a 50 Hz single phase capacitor induction motor are 80 ohms and 237.58 ohms respectively. Additional resitance "R" and a capacitor "C" are in series with one winding in order to achieve a phase difference of 90 degrees while both windings carry equal current. Calculate the values of R and C.

[Ans.: R = 157.495 Ω, C = 10 µF]

Examples 8.7.4 A 250 watt, 230 V, 50 Hz, single phase capacitor start Induction motor has the following constants for the main and auxiliary windings. Main winding, Zm =(4.5 + j 3.7 ) Ω, awxz'Zza^ winding Za =(9.5 + j 3.5) Ω. Determine the value of the capacitor that will place the main and auxiliary winding currents in quadrature at starting.

[Ans. : 211.40 µF]

Review Question

1. Describe the principle of operation of single phase induction motor using capacitor. Draw the circuit and phasor diagram.