Since a single phase induction motors do not have self-starting torque it needs special method of starting. These motors are classified according to their starting methods & they are usually referred to by name descriptive to their starting methods such as followings.
- Split Phase induction motor
- Capacitor start induction motor
- Capacitor start and capacitor run motor
- Permanent split capacitor motor
- Shaded pole motors
All the mentioned starting methods modify the single phase induction motor. So that it become two-phase motor at the time of starting. The stator is provided with two windings such as main winding and auxiliary windings.
The auxiliary winding is excited by a current which is out of phase with the current in main windings. If the phase difference is 90 ⁰ the maximum torque will be produced. This auxiliary winding is also disconnected from the circuit by a centrifugal switch after motor achieves about 75 % speed of its full speed. The various starting methods differ in the arrangement at the phase difference between the current in main and auxiliary windings.
Split phase single phase induction motors
Ia – Current in the auxiliary winding
I’m – Current in the main winding
The main winding of this split phase induction motor occupies ⅔ of slots and auxiliary winding is design with thin wire to obtain a higher resistances and low resistances than main winding. Does the time displacement is obtained between Im and Ia as above making the motor equivalent to the two-phase induction motor and it adopts a revolving flux. By interchanging the auxiliary winding connections the direction of motor rotation can be reversed.
Capacitor Start Induction Motor
When a proper capacitor is chosen Ia can be made to lead by Im by 90 ⁰ angles at standstill.
Motor torque ∝ Sin (∝ + B)
Thus the motor develops a torque which is much larger than the split-phase motor.
Capacitor Start and Capacitor Run Motor
This has two capacitors C1 and C2 both are available while the motor starts and one will be taken out of the circuit. After the motor achieved the speed of 75% of full speed.
The auxiliary winding remains in the circuit together with during running conditions. This arrangement offers several advantages such as.
- C2 Improves power factor
- Got higher efficiency compared with other starting methods
- High Starting torque is possible.
Permanent Split Capacitor Motor
This single phase induction motors has a huge run type capacitor which is permanently connected in series with the auxiliary winding. It is applicable for with intermittent cycle use such as adjusting mechanism, gate operators. Etc.