- When a single phase induction motor fed from a single phase supply, its stator winding produces a flux which is only alternating or pulsating not a synchronously revolving flux, as in case of a two or three phase stator winding fed from two or three phase supply.
- This alternating flux acting on a stationary squirrel cage rotor can not produce rotation which can only revolving flux.
- That's why a single phase induction motor is not self starting.
.png "Why single phase induction motor is not self starting")
According to the double field revolving theory, any alternating quantity can be broken down into two parts. Each component rotates in the opposite direction from each other and has a magnitude that is equal to half the maximum magnitude of the alternating quantity.
For example, suppose after giving single phase power supply to the stator winding there is a flux 𝜙 and as per theory we can divided it into two components.
Each of these components r0rates at synchronous speed and in opposite direction with each others like if -𝜙/2 rotates in forward direction then the 𝜙/2 will rotates in backward direction.
At each moment in time, the sum of these two flux components yields the value of the instantaneous stator flux at that exact time.
During starting time, the forward and backward components of flux are precisely opposite one another. Additionally, the magnitude of both of these flux components is the same. As a result, they cancel one another, and the rotor experiences zero net torque at startup.
Therefore, the single phase induction motor is not self-starting motor.
Starting of a single phase induction motor.
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