The document provides information about synchronous motors, including their construction and working principles. It discusses that synchronous motors: 1. Work at a constant synchronous speed determined by the supply frequency, and cannot operate at other speeds. 2. Consist of a stator wound with three-phase coils that generate a rotating magnetic field, and a rotor containing field windings. 3. Do not self-start because the rotor must be magnetically locked to the rotating stator field to achieve synchronous speed; external methods are needed at startup.

1. Department of Electrical Engineering
SHRINATHJI INSTITUTE OF TECHNOLOGY & ENGINEERING
UPALI ODEN, NATHDWARA, DISTT. RAJSAMAND (RAJ.)
Rajasthan Technical University, KOTA
APRIL 2020

2. Contents
Introduction
Construction of Synchronous Machine
Working Principle of Synchronous Motor
Why Synchronous motor is not self starting
Method of starting synchronous motor

3. Introduction
 Such a device which converts an electrical energy into a mechanical energy
running at synchronous speed is called synchronous motor.
 Synchronous motor works only at synchronous speed and can not work at a
speed other than the synchronous speed.
 Its speed is constant irrespective of load.
 Types of Synchronous motor:
1. Three phase synchronous motors.
2. Single phase synchronous motors
Single phase synchronous motor are further classified as reluctance motor and
hysteresis motor.
The three phase synchronous motor works on the concept of rotating magnetic
field. Its speed is always synchronous and given by,
pfNs /120

5. Structure of Machine

6. Construction of Three Phase
Synchronous Motor
 Similar to d.c. Machine where there is no construction difference
between a generator and motor.

7. Construction of Three Phase
Synchronous Motor
 It consists of two parts
I. Stator:
 Consisting of a three phase star or delta connected winding .This is
excited by three phase supply.
 The stator core is uses a laminated construction .It is build up of
special steel stamping insulated from each other with varnish and
paper.
 The laminated construction is basically to keep down eddy current
losses.
 Generally choice of material is steel to keep down hysteresis losses.
 The entire core is fabricated in a frame made of steel plates .
 The core has slots on its periphery for housing the armature
conductors.
 Frames does not carry any flux and serve as support the core.
 Ventilation is maintained with the help of holes cast in the frames.

8. Construction of Three Phase
Synchronous Motor
It consists of two parts
II.Rotor:
There is two types of rotor used in alternators
1.Salient pole type
2.Smooth cylindrical type

9. Construction of Three Phase
Synchronous Motor
It consists of two parts
II.Rotor:
1. Salient Pole type

10. Construction of Three Phase
Synchronous Motor
 This is called projected pole type as all poles are
projected out from the surface of the rotor.
 The poles are built up of thick steel laminations.
 The field winding is provided on the pole shoe.
 The rotors have large diameters and small axial
lengths.
 As mechanical strength of salient pole type is less.
 This is preferred for low speed alternators ranging
from 125 r.p.m. To 500 r.p.m.
 The prime movers used to drive such rotors are
generally water turbines and I.C. Engines.

11. Construction of Three Phase
Synchronous Motor
2. Smooth Cylindrical Type

12. Construction of Three Phase
Synchronous Motor
 This is also called non-salient type or non projected type rotor.
 Cylindrical rotors are made from solid forgings of high-grade
nickel chrome molybdenum steel.
 It has a comparatively small diameter and long axial length.
 They are useful in high-speed machines.
 Such a construction provides a greater mechanical strength and
permits more accurate dynamic balancing.
 The smooth rotor of the machine makes less windage losses and
the operation is less noisy because of the uniform air gap.
 hey are driven by steam or gas turbines.
 Cylindrical synchronous rotor synchronous generators are called
turbo alternators and turbo generators.
 The unslotted portions of the cylindrical itself act as the poles.

13. Difference Between Rotor

14. Difference Between Rotor

15. Principle of Working
 Synchronous motor works on the principles of the magnetic locking.
 When two unlike poles brought near each other ,if magnets are strong,
there exists a tremendous force of attraction between two poles.
 In such conditions magnets are said to be magnetically locked.
 Consider a three phase synchronous motor whose wound for 2 poles.
 The two magnetic fields are produced in the synchronous motors by
exciting both the winding stator and rotor with three phase a.c. Supply and
d.c, supply respectively.
 When three phase winding is excited by a three phase a.c. Supply then flux
produced by three phase winding is always of rotating type and their speed
is synchronous speed and its called rotating magnetic field.
 The stator poles are N1 and S1 which are rotating at a speed of Ns.The
direction of rotation of rotating magnetic field is say clockwise.
 When the field winding on vrotor is excited by a d.c. Supply it produced
two ploes .Let the poles be N2 and S2.

16. Principle of Working
Now one magnet is rotating at Ns having poles
N1 and S1 while at start rotor is stationary second
magnet is stationary having poles N2 and S2 .
If some how the unlike poles N1 and S2 or S1 and
N2 are brought near each other, the magnetic
locking may get established between stator and
rotor poles.
As stator poles are rotating ,due to magnetic
locking rotor will rotate in the same direction as
that of stator poles as in the direction of rotating
magnetic field.,with the same speed.

17. Why Synchronous motor is not Self
Starting

18. THANK YOU
FOR
LISTENING