Single-phase induction motors are not self-starting because there is no rotating magnetic field produced. It is not practicable to provide initial rotation to the rotor, manually. So in practice, some provision is made to provide a rotating magnetic field at the start, to make the motors self-starting.
Single-phase induction motors are classified according to the methods of starting used for them. They are,- Split phase motor.
- Capacitor start motor.
- Capacitor start capacitor run motor.
- Shaded pole motor.
Split Phase Induction Motor (resistance start motor) :
In this motor, in addition to normal stator winding called Main Winding, the stator carries one more winding called Auxiliary or Starting Winding. The two windings are displaced in space by 90° electrical with each other. Mostly the rotor is of squirrel cage type. The arrangement is shown in the figure.
The time displacement between the two currents is obtained by designing the auxiliary winding as highly resistive in nature. The main winding is reactive and hence carries current, Im which lags supply voltage by large-angle while the auxiliary winding carries a current, Ist which almost phase with supply voltage. Hence there exists a phase difference between the two alternating currents.
The two currents produce the two fluxes which have a phase difference of 'α'. Due to these two fluxes having considerable phase differences between them, a rotating magnetic field is produced and the motor develops a starting torque.
Because of the high resistance character of auxiliary winding, this motor is also known as Resistance Start Motor. The auxiliary winding is not designed for continuous operation to prevent the motor from drawing excessive currents from the line and also to protect the winding from damage due to overheating.
Hence the auxiliary winding is automatically disconnected by the centrifugal switch when the motor speed (75% to 80% of synchronous speed).
Capacitor Start Induction Motor :
In this motor, the main and auxiliary windings are space displaced by 90° electrical. The time displacement between the currents in the main and auxiliary windings is achieved by connecting a capacitor in series with auxiliary winding.
The feature of the capacitive circuit is to draw the leading current. The current drawn by the main winding lags voltage by angle Φm while the current drawn by the capacitive start winding leads voltage by angle Φst. Hence there exists a large phase difference between Im and Ist which is almost 90°. Thus the motor develops a very high starting torque.
When the motor speed reaches near to synchronous motor (75% of its speed). The capacitor and centrifugal switch are disconnected, these processes can be made automatically or manually. The starting torque is required. The value of the capacitor is chosen as per the requirement of the starting torque of the motor.
Advantages :
- High starting torque.
- Better starting power factor.
Speed Reversal :
The direction of rotation can be changed by interchanging the connections of either the main winding or auxiliary winding terminals.
Applications :
These motors are used where high starting torque is required such as in compressors, pumps, air conditioners, conveyors, etc.
Capacitor Start Capacitor Run Induction Motor :
Two Value Capacitor Motor: To increase the starting torque, two capacitors are used in the auxiliary winding. Theoretically, optimum starting and running performance can be achieved by having two capacitors, one for starting and the other for running both the capacitors Cs and CR are in the circuit during starting.
After the motor picks the speed, the centrifugal switch disconnects the starting capacitor, Cs. The auxiliary winding and running capacitor, CR remains in the circuit during running conditions also. The capacitor Cs is larger in value and is of electrolytic type, permitting high starting torque.
The capacitor CR is smaller in value and is of oil type. This motor is more expensive than a capacitor start motor however it provides the best performance. The starting torque is up to 300% of the full load torque.
