Synchronous motors operate at a constant synchronous speed determined by the supply frequency. They require an external DC excitation source to start and synchronize the rotor speed with the rotating stator magnetic field. Synchronous motors can develop torque through a wide range of speeds and loads, and are well-suited for applications requiring constant speed operation or power factor correction.

1. Synchronous Motor
 Introduction
 Starting Torque
 Types of Rotors
 Power, Torque developed
 Comparison with Induction motor
 Starting Methods
 Equivalent Circuit
 Torque-speed characteristics
 Changing the Load-Effects
 Phasor diagrams-with Ref. Frame
 Applications

2. Introduction
The synchronous motorrotatesat the synchronous
speed i.e. the speed of theRMF.
Stator is similar in construction to that of an
induction motor, so same principle is appliedto
the synchronous motorrotor.
Field excitation is provided on the rotor byeither
permanent or electromagnets with number of
poles equal to the poles of the RMF caused by
stator

3. Introduction
Faradays Law of Electromagnetic Induction.
The rotor acting as a bar magnet will turn to line up with
the rotating magnet field. The rotor gets locked to the RMF
and rotates unlike induction motor at synchronous speed
under all load condition

4. Construction
Stator:
 Stationary Armature winding
 Stator core use a laminated construction  Special
steel stamping and insulated from each other with
varnish or paper  Reduce the eddy current loss
 Steel material  Reduce hysteresis loss

5. Rotor
: Projected pole type as all the poles are projected out
from the surface of the rotor.
 Rotor have large diameter and small axial length
 The poles have damper winding.
 The damper windings are used to reduce the
‘Hunting’

6. SM types
• Non salient-pole (NSP) synchronous machine
• Salient-pole (SP) synchronous machine
3

7. Rotor with non-salient and salient poles
7

8. Pole numbers and synchronous speed
 The stator and the rotor may have a
higher number (always an even
integer) of poles.
 The winding structure of a 2p pole
motor (p is the number of pole
pairs) is similar to two-pole
winding except that sinusoidally
distributed winding is repeated p
times in 360 mechanical degrees
 For a 2p pole machine, each cycle
of 2π electrical radians of supply
current causes the stator field to
rotate by 2/p mechanical radians.
rev/sec

fs
syn
N
p
electrical  pmechanical
20
a1
a1' a2'
a2
Average speed regulation = 0

9. Synchronous Motor-General
A synchronous motor is electrically identical with an alternator or AC
generator.
A given alternator ( or synchronous machine) can be used as a motor,
when driven electrically.
Some characteristic features of a synchronous motor are as follows:
1.It runs either at synchronous speed or not at all i.e. while running it
maintains a constant speed. The only way to change its speed is to vary
the supply frequency (because NS=120f/P).
2. It is not inherently self-starting. It has to be run up to synchronous
(or near synchronous) speed by some means, before it can be
synchronized to the supply.
3. It is capable of being operated under a wide range of power factors,
both lagging and leading. Hence, it can be used for power correction
purposes, in addition to supplying torque to drive loads.

10. Starting Torque
 Itcannot be started from a standstill byapplying
ac to thestator.
 When AC supply is applied to the stator a high
speed RMF appears around the stator. This RMF
rushes past the rotorpoles so quickly that the rotor
is unable to getstarted.
 It is attracted first in one direction and then in the
other and hence no startingtorque.

11. Improvement of starting torque
 It is started by using a squirrel cage within a rotor
construction and therefore starts as an induction
motor.
 At synchronous speed the squirrel cage has no part to
play.

12. Different Torques of a Synchronous Motor
(a) Starting Torque: It is the torque developed by the motor
when full voltage is applied to its stator (armature)
winding. It is also sometimes called breakaway torque.
(b)Running Torque:As its name indicates, it is the torque
developed by the motor under running conditions.
It is determined by the horse-power and speed of the
driven machine.
The peak horse power determines the maximum torque
that would be required by the driven machine.
The motor must have a breakdown or a maximum
running torque greater than this value in order to avoid
stalling.

13. (c) Pull-in Torque:A synchronous motor is started as induction motor
till it runs 2 to 5% below the synchronous speed.
Afterwards, excitation is switched on and the rotor pulls into step with
the synchronously rotating stator field.
(d) Pull-out Torque: The maximum torque which the motor can
develop without pulling out of step or synchronism is called the pull-
out torque.
Motor develops maximum torque when its rotor is retarded by an angle
of 90o.
Any further increase in load will cause the motor to pull out of step
(or synchronism) and stop.

14. Method of Starting
There are several methods to start the synchronous motor such as:
(a) Auxiliary drive (induction motor or dc motor),
(b) Induction start (using damper winding), etc
General Starting Procedure: The rotor (which is as yet
unexcited) is speeded up to synchronous or near synchronous speed by
some arrangement and then excited by the DC source.
The moment this (near) synchronously rotating rotor is excited, it is
magnetically locked into position with the stator i.e. the rotor poles are
engaged with the stator poles and both run synchronously in the same
direction.
It is because of this inter-locking of stator and rotor poles that
the motor has either to run synchronously or not at all.
However, it is important to understand that the arrangement
between the stator and rotor poles is not an absolutely rigid one.

15. Procedure for Starting a Synchronous Motor
Using the Damper Winding
While starting a modern synchronous motor provided with
damper windings, following procedure is adopted.
1. First, main field winding is short circuited.
2. Reduced voltage with the help of auto-transformers is
applied across stator terminals. The motor starts up.
3. When it reaches a steady state speed (as judge by its sound), a
weak DC excitation is applied by removing the short-
circuit on the main field winding. If excitation is sufficient,
then the machine will be pulled into synchronism.
4. Full supply voltage is applied across stator terminals by
cutting out the auto-transformers.
5. The motor may be operated at any desired power factor by
changing the DC excitation.

16. Advantages of SM over IM
1. SM can be used for power factor correction
2. SMs are more efficient (at unity power factor)
3. The field pole rotor of SM can permit the use of wider
airgap than the squirrel cage IM.
4. SM can operate lagging, leading and unity power factor
5. SM may less costly for the same horsepower, speed
and voltage ratings.
6. They give constant speed from no-load to full- load.

17. Disadvantages of SM over IM
1. They require dc excitation which must be
supplied from external source.
2. They have tendency to hunt.
3. They cannot be used for variable speed jobs
as speed adjustment cannot be done.
4. They cannot be started under load. Their
staring torque is zero.
5. They may fall out of synchronism and stop
when overloaded.

18. Starting Synchronous Motors
Three basic approaches can be used to safely start a
synchronous motor:
 Motor Starting by Reducing Electrical Frequency
 Motor Starting with an External Prime Mover
 Motor Starting by Using Amortisseur Windings

19. The per phase power development in a synchronous machine is
as under:

21. Equivalent Circuit of a Synchronous Motor
Fig. 36.9(a) shows the equivalent
circuit model for one armature phase
of a cylindrical rotor synchronous
motor.
It is seen from Fig. 36.9(b) that the
phase applied voltage V is the vector sum
of reversed back emf i.e. –Eb and the
impedance drop IaZs.
In other words, V=(-Eb+IaZs).
The angle  between the phasor for V and Eb is called the load angle or power angle
of the synchronous motor.

22. Power Developed by a Synchronous Motor
The armature resistance of a synchronous motor is negligible as compared to its
synchronous reactance.
Hence the equivalent circuit for the motor becomes as shown in Fig. 36.10 (a).
From the phasor diagram of Fig. 36.10(b), it
is seen that AB  Eb sin  Ia XS cos
EbV sin =VIa XS cos
Now, VIacos = motor power input/phase
 Pin 
EbV
sin  per phase
XS XS
 Pin
3EbV
sin  forthree phase
Since Stator Cu losses have been neglected, Pin also
represents the gross mechanical power (Pm)
developed by the motor.
XS
 Pm Pin
3EbV
sin
The gross torque developed by the motor is Tg=9.55Pm/NS N-m -NS inrpm.
Tg  9.55
Pm 
9.55 3EbV
sin  28.65
EbV sin N m
NS NS XS NS XS

23. The Effect of Load Changes on
a Synchronous Motor
X s
V : fixed (from electrical course)
EA : fixed (when I F fixed or using permanent magnets)
A
V E
P  3V I cos   3  A
sin 
   (leading  lagging)
A A A
2
P  (load increases)  sin   I  more heat (3I R )

jXsIA P

24. The Effect of Field Current Changes on a
Synchronous Motor
jX sI A
V  E A  RAI A 
 E A  jX sIA


25. Phasor diagram and the stator reference frame
Xq Iq V sin, Iq V sin/ Xq
E f  X d Id V cos,
P V cos Iq  V sin  I
d
d d q
V 2 Xd
VEf  Xq 
P  sin  
X 2 X X

sin2
I cos  Iq cos   Id sin



q-axis
d-axis
Id
V
jIdXd
Ef
I
jI X
q q
Iq
43

26. 51
Summary
Non-SP SM Reluctance
SM
SP PM SP SM (Wound
Rotor)
inductance Lq = Ld = Ls Lq < Ld Lq > Ld Lq < Ld
excitation Ef Ef =0 Ef Ef
-δ|Tmax (deg) 90 45 >90 <90

27. Synchronous Motor Applications
Synchronous motors find extensive application for
the following classes of service:
1. Power factor correction;
2. Constant speed, constant load drives
3. Voltage regulation.