Unit V : Synchronous Motors Course Name : BEE

1. Synchronous Motors
By
Dr. Ajay Kumar M
Associate Professor,
Dept. of EEE.

2. Definition: The motor which runs at synchronous speed is known as
the synchronous motor.
Synchronous motor runs at synchronous speed (Ns = 120f/P).
The only way to change its speed is to alter the supply frequency
(Ns = 120 f/P).
Function: The synchronous motor is used for converting the electrical
energy into mechanical energy.
Therefore, a synchronous motor can be made to carry the
mechanical load at constant speed and at the same time improve the
power factor of the system.
The synchronous motor is not self-starting. The prime mover is
used for rotating the motor at their synchronous speed.

3. Construction of Synchronous Motor
• The stator and the rotor are the two main parts of the synchronous
motor.
• The stator becomes stationary, and it carries the armature winding
of the motor.
• The armature winding is the main winding because of which the
EMF induces in the motor.
• The rotator carry the field windings.
• The main field flux induces in the rotor.
• The rotor is designed in two ways, i.e., the salient pole rotor and the
non-salient pole rotor.

5. • The synchronous motor uses the salient pole rotor.
• The rotor of the synchronous motor is made with the laminations of
the steel.
• For obtaining the high-speed, cylindrical rotor is used in the motor.

6. Working & operation of Synchronous Motor
• The stator and rotor both
are excited by AC and DC
respectively.
• When the three phase
supply is given to the stator,
the rotating magnetic field
developed between the
stator and rotor gap.

7. • Because of the rotating magnetic field, the north and south poles
develop on the stator.
• Now the rotor is excited by the DC supply.
• The DC supply induces the North and South poles on the rotor. As
the DC supply remains constant, the flux induces on the rotor
remains same. Thus, the flux has fixed polarity.
• The north pole develops on one end of the rotor, and the south
pole develops on another end.
• When the rotor and stator both have the same pole on the same
side, they repel each other.
• If they have opposite poles, they attract each other.

8. • The rotor attracts towards the pole of the stator for the first
half cycle of the supply and repulse for the second half cycle.
• Thus the rotor becomes pulsated only at one place.
• This is the reason because of which the synchronous motor
is not self-starting.
• The prime mover is used for rotating the motor.
• The prime mover rotates the rotor at their synchronous
speed.
• The synchronous speed is the constant speed of the
machine whose value depends on the frequency and the
numbers of the pole of the machine.

9. • When the rotor starts rotating at their synchronous speed, the prime
mover is disconnected from the motor.
• The DC supply is provided to the rotor because of which the north
and south pole develops at their ends.
• The north and south poles of the rotor and the stator interlock each
other. Thus, the rotor starts rotating at the speed of the rotating
magnetic field and the motor runs at the synchronous speed.
• Also, In order to make the motor self-starting, a squirrel cage
winding (also called damper winding) is provided on the rotor.
Q: Why the SM is not self starting? (or) How do yo make the SM self
starting? (or) Explain the SM operation and Working principle clearly.

10. Power flow in synchronous Motor
• Let Ra = Armature resistance/phase
Xs = Synchronous reactance/phase
then Zs = Ra + jXs = Synchronous Impedance/phase
Motor Input per phase = VIa cosɸ
Total input for a star connected 3-phase synchronous motor is given by

11. • The Mechanical power developed by the motor is given by
Pm

12. Different power stages in Synchronous motor

13. Equivalent circuit of Synchronous motor
The terminal voltage
V = -E+Ia Zs
where E= Back emf
Zs = Ra +j Xs
Ia = armature current
Stator Rotor

14. Applicationsof Synchronous motor
• Over excited Synchronous motors are used as synchronous
condensors in generating stations and in substations connected to
improve the power factor.
• Synchronous motors are used to regulate the voltage at the end of
transmission lines.
• In textile and paper industries synchronous motors are employed to
attain wide range of speeds with variable frequency drive system.

15. Hunting in Synchronous Motor
The word hunting is used because after the sudden application of load,
the rotor has to search or ‘hunt’ for its new equilibrium position. That
phenomenon is referred to as hunting in a synchronous motor.

16. Causes of Hunting in Synchronous Motor
1.Sudden change in load.
2.Sudden change in field current.
3.A load containing harmonic torque.
4.Fault in supply system.
Effects of Hunting in Synchronous Motor
1.It may lead to loss of synchronism.
2.Produces mechanical stresses in the rotor shaft.
3.Increases machine losses and cause temperature rise.
4.Cause greater surges in current and power flow.
5.It increases possibility of resonance.
Reduction of Hunting in Synchronous Motor
• Two techniques should be used to reduce hunting. These are –

17. • Use of Damper Winding: It consists of low electrical resistance copper /
aluminum brush embedded in slots of pole faces in salient pole
machine. Damper winding damps out hunting by producing torque
opposite to slip of rotor. The magnitude of damping torque is
proportional to the slip speed.
• Use of Flywheels: The prime mover is provided with a large and heavy
flywheel. This increases the inertia of prime mover and helps in
maintaining the rotor speed constant.
• Designing synchronous machine with suitable synchronizing power
coefficients.