Justify not self starting; two starting methods

A synchronous motor with excited field has zero average starting torque, so it is not self-starting.

Justification

• On switching on, the stator field rotates at $N_s = 120f/P$ immediately, while the excited rotor poles are stationary and the rotor has high inertia.
• At one instant a stator N pole faces a rotor S pole and the rotor is pulled, say, clockwise. Half a cycle later (0.01 s at 50 Hz) a stator S pole faces the same rotor pole and pushes it anticlockwise.
• The torque reverses every half cycle; the rotor cannot follow, so the average torque is zero and it only vibrates.

 t = 0   : N over S -> attract -> CW
 t = T/2 : S over S -> repel   -> CCW
 average torque = 0

Method 1: Damper winding (induction motor starting)

• Bars in pole-face slots, shorted by end rings, form a squirrel cage.
• Field winding is shorted through a discharge resistor; reduced voltage is applied to the stator.
• The motor starts as an induction motor and reaches about 95-98 % of $N_s$.
• DC excitation is then switched on; rotor poles lock with stator poles and the motor runs at $N_s$. The damper then carries no current, except during hunting, which it damps.

Method 2: Pony (auxiliary) motor

 [Pony motor] ==shaft== [Sync motor] ---> load
 (small IM or DC motor)     |
                    synchroscope/lamps

• A small induction motor (with fewer poles) or DC motor coupled to the shaft runs the unloaded synchronous motor up to about synchronous speed.
• The DC field is excited, and the stator is connected to the supply when the voltage, frequency and phase sequence match (checked with a synchroscope or lamps), as in synchronising an alternator.
• The pony motor is then disconnected (or left running idle on the shaft).
• Suitable only for starting without load.

Other methods: slip-ring induction motor starting, variable frequency (inverter) starting, and using the DC exciter as a motor.