Starting methods of synchronous motor

A synchronous motor has no starting torque because the stator field rotates at $N_s = \frac{120f}{P}$ while the rotor is stationary. The rotor poles are attracted and then repelled in each half cycle, so the average torque is zero. The motor must be brought near synchronous speed before DC field excitation is applied. The common methods are given below.

1. Using a pony (auxiliary) motor

A small induction or DC motor, mechanically coupled to the shaft, runs the rotor close to $N_s$ without load. The rotor is then excited and synchronised with the supply in the same manner as an alternator (lamp or synchroscope method), the pony motor is disconnected, and the load is applied. It needs an extra motor and is used for large machines.

2. Using damper (amortisseur) winding

Short-circuited copper bars are placed in slots in the pole faces and joined by end rings, similar to a squirrel-cage rotor.

• Reduced voltage is applied to the stator. The field winding is closed through a resistance (about 10 times the field resistance) to avoid dangerous high voltage in the many-turn field.
• The motor starts as an induction motor and rises to about 95 % of $N_s$.
• DC field is applied, and the rotor locks into synchronism.
• Once running at $N_s$ there is no relative motion, so the damper winding carries no current. It also reduces hunting.

  stator ---(autotransformer / star-delta)--- supply
  rotor: pole-face damper bars + end rings
  field winding --[discharge resistor]--

3. Using variable frequency supply

An inverter supplies the stator at very low frequency at first, so the rotating field is slow and the rotor locks to it. The frequency is gradually raised to the rated value. This is used in modern drives.

4. Using the DC motor method

The exciter or a DC machine on the same shaft runs the machine as a DC motor to bring it to speed.

Of these, the damper-winding method is the most common.