- 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. The synchronous speed of a motor depends on the supply frequency and the number of poles in the motor.
Synchronous speed is given by
Where, f = supply frequency and p = number of poles.
We can change the synchronous speed of the motor by changing the supply frequency and the number of poles. But the motor would always run with this speed for a given supply frequency and the number of poles.
Synchronous motors offer many benefits but their inability to start on their own, unlike 3 phase induction motors, is a significant drawback. In these motors, the stator is powered by a 3-phase supply and has 3-phase windings that create a rotating flux, while the rotor, excited by DC supply, generates a constant flux.
The torque produced on the rotor is a pulsating one and not uni-directional. Considering the frequency to be 50 Hz, from the above relation we can see that the 3 phase rotating flux rotates about 3000 revolutions in 1 min or 50 revolutions in 1 sec. At a particular instant rotor and stator poles might be of the same polarity (N-N or S-S) causing a repulsive force on the rotor and the very next second it will be N-S causing attractive force. But due to the inertia of the rotor, it is unable to rotate in any direction due to attractive or repulsive force and remain in standstill condition. Due to this, the motor cannot start on its own. The rotor of the synchronous motor has to be brought to synchronous speed by using external means.
Below are the techniques used for starting a synchronous motor:
Starting a Synchronous Motor Using an Induction Motor
Before starting the synchronous motor, its rotor must reach synchronous speed. To achieve this, we couple it with a smaller induction motor, known as a pony motor. The induction motor must have fewer poles than the synchronous motor to reach and match its synchronous speed, as induction motors typically operate at speeds below synchronous speed. After the rotor of the synchronous motor is brought to the synchronous speed, we switch on the DC supply to the rotor. After that, we simply de-couple the induction motor from the synchronous motor shaft.
Starting a Synchronous Motor Using a DC Machine
It is similar to above method with a slight difference between the two. A DC machine is coupled to the synchronous motor. The DC machine works like a DC motor initially and brings the synchronous motor to synchronous speed. Once it achieves the synchronous speed, the DC machine works like a DC generator and supplies DC to the rotor of the synchronous motor. This method offers easy starting and better efficiency than the earlier method.
Starting a Synchronous Motor Using Damper Windings
In this popular method, damper windings help start the motor as an induction motor. These windings, made of copper bars in the pole faces, act like an induction motor’s rotor. Initially, when 3-phase power is applied, the motor operates below synchronous speed. Once near this speed, DC is applied, pulling the motor into synchronism and it begins running as a synchronous motor. At synchronous speed, damper windings no longer induce emf, ceasing to affect the working of the motor.
Starting a Synchronous Motor Using Slip Ring Induction Motor
Here we connect one external rheostat in series with the rotor. The motor is first started as a slip ring induction motor. The resistance is gradually cut-off as the motor gains speed. When it achieves near synchronous speed, DC excitation is given to the rotor, and it is pulled into synchronism. Then it starts rotating as a synchronous motor.
