Similar to a three-phase induction motor, a single-phase induction motor has two parts; stator and rotor. Generally, the stator winding distributed winding.
Single-phase supply is fed to the stator winding. And the flux produced is alternating, not synchronously rotating like a three-phase induction motor.
If the stator is stationary, this type of alternating field cannot produce a torque. Therefore, the single-phase induction motor is not self-starting.
But if some initial torque is given in either direction, immediately a torque is produced in the motor. Then the motor accelerates to its final speed. The running speed of the motor is less than synchronous speed.
The working of a single-phase induction motor explained by the double revolving theory.
Double Field Revolving Theory
When the stator supplied from the single-phase supply, a sinusoidal distributed MMF is produced in the air gap.
This variable flux is the sum of two rotating fluxes. The magnitude of both fluxes is equal and half of the alternating flux. But the direction of both fluxes is opposite to each other. This figure shows the alternating or pulsating flux varying with time or angle.
The flux is rotating at a speed of synchronous speed in the same direction of the rotor taken as positive induces emf in the rotor conductors. In this condition, the direction of flux is rotating in an anticlockwise direction.
Generally, the rotor used in a single-phase induction motor is a squirrel cage rotor. And the rotor bars short-circuited via end rings.
The current flows in the rotor conductor and the torque produced in the same direction as given. And this direction termed as positive. The other part of flux at the same speed and opposite direction is termed as negative.
Two torques the same magnitude and opposite direction. Hence, the resultant flux will be zero.
But anyhow if the rotor rotated, the one torque is more than the other. Let say, the rotor rotates in anticlockwise direction or forward direction, the forward torque is greater than the backward torque.
Due to resultant torque, the motor accelerates in the anticlockwise direction, and the speed of the motor decides by the load supplied to the motor.
The equation of distributed MMF is,
θ=angle measured from the winding axis
Now, peak MMF is,
The resultant MMF expressed as,
The above equation shows that a pulsating field can be considered as the sum of two synchronously rotating fields.
The forward rotating field Ff is,
The backward rotating field Fb is,
Each field has the same magnitude and that is half of the maximum flux produced in the air gap. The torque produced due to this field is zero.
Now, due to external means, the rotor rotated in a forward direction. Hence, in this condition, the forward torque is greater than the backward torque. And it will produce the resultant torque. That will start the motor rotating.
As we know, the single-phase induction motor is not self-starting. The single-phase induction motor has no starting torque. But if the rotor rotates except synchronous speed.
We need two alternative fluxes at some phase difference angle to produce resultant flux. The resultant flux is rotating flux in either direction.
This can be done by additional flux. The additional flux can be adding by inserting a capacitor or resistor in the circuit or it can be done by changing the construction of the motor.
Depending upon the methods to make single-phase induction motor self-starting, there are many types of motor available. And that listed below.
- Split phase induction motor
- Shaded pole induction motor
- Capacitor start inductor motor
- Capacitor start capacitor run induction motor
- Permanent split capacitor motor or single value capacitor motor
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