Synchronous motor starting methods and damper winding

A synchronous motor has no net starting torque when its field is excited and the stator is switched on, so it must be brought near synchronous speed by some other means.

Why it is not self-starting

• The stator 3-phase supply produces a field rotating at $N_s = 120f/P$ (1500 rpm for 4-pole, 50 Hz) instantly.
• The rotor poles are fixed by DC excitation, and the heavy rotor is at rest.
• Suppose a stator N pole is just ahead of a rotor S pole: attraction pulls the rotor one way. Half a cycle (0.01 s) later a stator S pole sits there and pushes the rotor the opposite way.
• Due to rotor inertia it cannot follow these rapid reversals, so the average torque is zero; the rotor only vibrates.

 t = 0      : stator N over rotor S -> torque CW
 t = T/2    : stator S over rotor S -> torque CCW
 average over one cycle             -> 0

Starting methods

1. Damper (amortisseur) winding / induction motor starting - most common.
2. Pony (auxiliary) motor - a small induction motor or DC machine on the same shaft brings the rotor near $N_s$, then the field is excited and the machine is synchronised.
3. Slip-ring induction motor method - rotor has a 3-phase winding connected through slip rings and resistances; starts as a slip-ring induction motor, then DC is fed to the rotor.
4. Variable frequency supply (inverter) - frequency is raised slowly from zero so the rotor follows the field from rest.
5. DC exciter on shaft used as a DC motor to run the machine up.

Damper winding starting method

• Copper or brass bars are placed in slots in the pole faces and shorted at both ends by end rings, forming a partial squirrel cage.
• Step 1: Field winding is not connected to DC; it is shorted through a discharge resistance (to avoid very high induced voltage in it). Reduced voltage (autotransformer or star-delta) is applied to the stator.
• Step 2: The rotating field induces currents in the damper bars, producing induction-motor torque. The rotor accelerates to about 95-97 % of $N_s$.
• Step 3: DC excitation is switched on. The rotor poles lock with the stator poles (synchronising or "pull-in" torque) and the rotor runs at $N_s$.
• Step 4: At synchronous speed there is no relative motion, so no current flows in the damper bars; they carry current only when speed changes, which also damps hunting.

 3-ph AC --> [Autotransformer] --> Stator
                                     |
 Rotor: pole faces with damper bars  |
 Field --[discharge R]-- (start)     |
 Field --[DC exciter ]-- (near Ns)   |

The motor starts with low torque because the cage is small; hence this method suits pumps, fans and compressors started at light load.