Single phase motor not self-starting; two starting methods

Why a single-phase induction motor is not self-starting

A single-phase supply to the stator winding produces only a pulsating field (equivalent to two equal rotating fields of $\phi_m/2$ turning in opposite directions). At standstill, the torques due to the forward and backward fields are equal and opposite, so the net starting torque is zero. A rotating field is needed at start, and it is obtained by using an auxiliary winding with a phase shift.

Starting methods

1. Split-phase (resistance start) motor

• The stator has a main winding (low resistance, high reactance) and a starting winding (thin wire, high resistance, low reactance), placed $90^\circ$ apart in space.
• The currents $I_m$ and $I_a$ differ in phase by about $30^\circ$ to $40^\circ$, giving an elliptical rotating field and a moderate starting torque (1.5 to 2 times full load).
• A centrifugal switch disconnects the starting winding at about 75 % of synchronous speed.
• Use: fans, blowers, washing machines, grinders.

 Main (R low, X high) ----+
                          +-- supply
 Aux (R high, X low)--[S]-+
 Im lags V by large angle; Ia lags by small angle

2. Capacitor-start motor

• A capacitor is connected in series with the starting winding, so that $I_a$ leads $I_m$ by nearly $90^\circ$.
• A nearly circular rotating field is produced, and the starting torque is high (3 to 4.5 times full load) with a low starting current.
• The centrifugal switch cuts out the capacitor and the auxiliary winding at about 75 % speed.
• Use: compressors, refrigerators, pumps, air conditioners.

 Main -------------------+
                         +-- supply
 Aux --[Capacitor]--[S]--+

Shaded-pole and permanent-split capacitor motors are other options for small loads.