Reason: 1-phase IM not self-starting

A single-phase induction motor is not self-starting because a single-phase winding carrying alternating current produces a pulsating field, not a rotating field.

• The flux φ = φ_m cos ωt stays on a fixed axis and only alternates in magnitude and direction. A stationary rotor in an alternating field has an emf induced in it by transformer action only. The resulting currents and forces are equal on both sides of the axis, so no net torque appears.
• By the double revolving field theory, the pulsating field can be considered as two fields of equal magnitude (φ_m/2) revolving at synchronous speed in opposite directions.
• At standstill, the slip is 1 with respect to each field, so the rotor currents are equal and the forward torque T_f equals the backward torque T_b.
• Net starting torque = T_f − T_b = 0.

So, the rotor stays at rest. However, if the rotor is turned by hand in either direction, the field rotating in the same direction produces a bigger torque and the motor keeps running in that direction. Therefore a starting device (such as an auxiliary winding with capacitor or resistance, or shading ring) is used to produce a phase difference between two fluxes, which creates a rotating field and a starting torque.