Synchronous Motor
A synchronous motor is an AC motor that operates at a constant speed, synchronized with the
frequency of the supply current. Unlike an induction motor, where the rotor speed can lag behind
the rotating magnetic field, the rotor of a synchronous motor rotates in step with the stator's
magnetic field. This means its speed is directly proportional to the supply frequency and is not
dependent on load variations.
Synchronous motors are commonly used in applications where precise and constant speed is
required, such as in clocks, record players, and industrial machines. They also offer higher
efficiency and power factor correction capabilities.
Operating Principle:
Stator Magnetic Field Generation:
When an AC supply is provided to the stator windings, it generates a rotating magnetic
field (RMF). This field rotates at a speed determined by the supply frequency and the
number of poles in the motor, called synchronous speed.
Rotor Magnetic Field:
The rotor is excited by a DC power source, typically through a set of field windings or
permanent magnets, which produces a constant magnetic field around the rotor.
Synchronization Process:
For the motor to operate, the rotor must reach the same rotational speed as the stator's
magnetic field. Initially, if the rotor is at rest, it cannot immediately catch up to the rotating
field. To bring the rotor up to speed, additional starting mechanisms like damper windings
(or a variable frequency drive) may be used.
Magnetic Locking:
Once the rotor reaches the synchronous speed, the magnetic field of the rotor aligns with
the rotating magnetic field of the stator. This process is called magnetic locking or
synchronization, meaning the rotor and stator fields are locked together. But there are
several methods of interlocking processes described later.
Constant Speed Operation:
After synchronization, the rotor continues to rotate in step with the stator's rotating
magnetic field, meaning it operates at a constant speed, known as synchronous speed,
which is independent of load changes.
Figure: Operation of synchronous motor
Ways to interlock:
Three basic approaches can be used to safely start a synchronous motor:
1. Reduce the speed of the stator magnetic field to a low enough value that the rotor can accelerate
and two magnetic fields lock in during one half-cycle of field rotation. This can be achieved by
reducing the frequency of the applied electric power (which used to be difficult but can be done
now).
2. Use an external prime mover to accelerate the synchronous motor up to synchronous speed, go
through the paralleling procedure, and bring the machine on the line as a generator. Next, turning
off the prime mover will make the synchronous machine a motor.
3. Use damper windings or amortisseur windings – Damper windings assist a synchronous motor by
enabling it to start and maintain stable operation. During start-up, they act like the rotor of an
induction motor, generating torque to accelerate the rotor to near-synchronous speed. Once
synchronization is achieved, damper windings continue to stabilize the motor by producing
damping torque, which counters oscillations and helps the rotor remain synchronized with the stator
field during load fluctuations, ensuring smooth and consistent operation.
Q. Why synchronous motor is not self-started?
Synchronous Condenser:
An over-excited synchronous motor running on no-load is called the synchronous condenser. It is also
known as synchronous capacitor or synchronous compensator or synchronous phase modifier. A
synchronous motor can deliver or absorb reactive power by changing the DC excitation of its field winding.
It can be made to draw a leading current from the supply with over-excitation of its field winding and
therefore, it supplies lagging reactive power (or absorbs leading reactive power).
