- Synchronous Motor Definition: A synchronous motor is defined as a machine whose rotor speed is synchronized with the frequency of the current supply; it requires external methods for starting.
- Self-Starting Challenge: Due to alternating magnetic forces that fail to move the rotor from a standstill, synchronous motors are not self-starting.
- Starting Methods of Synchronous Motor: Various techniques, such as using pony motors, DC machines, or damper windings, are employed to bring the rotor to synchronous speed.
- Damper Windings Function: Damper windings allow the motor to start as an induction motor and transition to synchronous operation upon reaching the right speed.
- Efficiency and Application: Different starting methods offer varying efficiencies and are chosen based on the specific requirements of the motor application.
Synchronous motors run at synchronous speed. That speed depends on supply frequency and the number of poles in the motor. They cannot produce a steady starting torque as synchronous machines, so they need a start method.
Synchronous speed is given by
where f = supply frequency and p = number of poles.
For a given frequency and pole count the shaft speed in synchronous operation stays at that value. Changing frequency (a drive) or pole number changes the set speed.
Synchronous motors have useful running properties. Starting still needs dampers, a pony motor, a DC set or a drive. Plants compare that with self-starting 3 phase induction motors. The stator takes a 3-phase supply and its 3-phase winding produces a rotating flux. The rotor, excited by DC, produces a steady field.
Torque on the rotor then pulses rather than staying in one direction. At 50 Hz a 2-pole field rotates at 3000 r/min, or 50 r/s. A 4-pole field at the same frequency is 1500 r/min. At one instant stator and rotor poles may be N-N or S-S (repel); a fraction of a cycle later they are N-S (attract). The rotor inertia cannot follow that rapid reversal, so the shaft stays still. The rotor must be brought near synchronous speed by some other means, then the DC field is applied so the machine can lock in.
Below are the techniques used for starting a synchronous motor:
Starting a Synchronous Motor Using an Induction Motor
Before the synchronous motor can lock in, its rotor must be near synchronous speed. One way is to couple a smaller induction motor, a pony motor. That induction motor is given fewer poles than the synchronous motor so its own synchronous speed is higher, and with slip it can still bring the main rotor up to the required speed. DC is then applied to the main rotor field. The pony motor is uncoupled from the shaft.
Starting a Synchronous Motor Using a DC Machine
The same idea can use a coupled DC machine. The DC machine first runs as a DC motor and brings the synchronous machine up to speed. Once at speed, the DC machine can run as a generator and feed the rotor of the synchronous motor. Plants sometimes prefer this to a pony motor because the same DC set can also supply field current.
Starting a Synchronous Motor Using Damper Windings
Damper windings are the usual built-in start method. Copper bars in the pole faces act like an induction motor cage. With 3-phase power on the stator the machine accelerates below synchronous speed. Near that speed, DC is applied to the field and the rotor is pulled into step as a synchronous motor. At synchronous speed the dampers see no steady induced emf, so they produce no steady torque in the normal working of the motor. They still damp hunting if the load angle swings.
Starting a Synchronous Motor Using Slip Ring Induction Motor
Here an external rheostat is connected in series with the rotor. The motor is first started as a slip-ring induction motor. The resistance is cut out as the motor gains speed. Near synchronous speed, DC excitation is applied to the rotor and the machine is pulled into step. It then runs as a synchronous motor.
