Syncronous Motor

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Team members:

NIRMAL MEHTA

SAMBHAV JAIN

ASHWIN .V.M

RAGHAVENDRA.M.JADHAV


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Synchronous Motor-Introduction

Synchronous Motor-principleChanging the Load

Starting Torque

Method of starting

Synchronous Motor Construction

Main features

Equivalent circuit and phasor diagram

V curvesTorque versus Speed

Advantages and disadvantages

Application

References 2


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Like most rotating machine a synchronous machinecan also operate as both a generator and a motor.

A synchronous motor is a machine that converts acelectrical power to mechanical power at a constant

speed called synchronous speed. A synchronous motor is DOUBLE EXCITED

MACHINE. Its rotor poles are excited by DC and itsstator windings are connected to the ac supply.

The air gap flux is, therefore, the resultant of fluxesdue to both rotor current and stator current, Animportant feature of this motor is that it can draw eitherlagging or leading reactive current from the ac supplysystem.


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The synchronous motor rotates at the synchronous

speed i.e. the speed of the RMF

Stator is similar in construction to that of an

induction motor, so same principle is applied to thesynchronous motor rotor.

Field excitation is provided on the rotor by either

permanent or electromagnets with number of

poles equal to the poles of the RMF caused bystator

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When the stator windings of a 3-phase synchronous motor

are supplied with rated 3-phase supply, a rotating field of

constant magnitude travelling at synchronous speed, given

by the expression 120f/P is developed.

As the rotor is excited from dc supply, so the poles of the

rotor retain the same polarity throughout but the polarity

of the stator poles changes as it is connected to an ac

supply. As a result the torque acting on the rotor is not

unidirectional but pulsating one (the direction ofinstantaneous torque on the rotor reverses every half cycle)

and due to inertia of rotor, it does not move in any direction.

So synchronous motor has got no self starting torque. But if

once the rotor starts rotating at synchronous speed, which is

possible by external means, the rotor poles get lockedmagnetically with stator poles and the rotor also rotates at

synchronous speed. Thus electrical power is converted into

mechanical power.


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The rotor acting as a bar magnet will turn to line

up with the rotating magnet field. The rotor gets locked tothe RMF and rotates unlike induction motor at

synchronous speed under all load condition


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An increase in the load will cause the rotor to lag the stator field but still maintain

synchronous speed. Increase in load has increased the torque component, but the

field strength has decreased due to the increase in length of the air gap between the

rotor and the stator.

If the synchronous motor is overloaded it pulls out of synchronism and comes torest. The minimum amount of torque which causes this is called the pull out

torque.

Lightly

loaded

motor

Heavily

loaded

motor


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It cannot be started from a standstill by applying ac

to the stator. When ac is applied to the stator a high

speed RMF appears around the stator. This RMF

rushes past the rotor poles so quickly that the rotoris unable to get started. It is attracted first in one

direction and then in the other and hence no

starting torque.

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Using a small dc motor.

Ac motor.

Self starting: damper winding or squirrel cage

AC supply given to the stator produces a rotatingmagnetic field which causes the rotor to rotate.

Therefore, in the beginning synchronous motor

provided with damper winding starts as a squirrel

cage induction motor.

Rotor gets magnetically locked by the rotating

field of the stator at about 95% of synchronous

speed and motor runs as synchronous motor.

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The three phase armature winding is on the

stator and is wound for the same number of poles

as the rotor.

The rotor of a synchronous motor can be of

salient pole or cylindrical pole type of

construction.(generally salient pole-high speed

m.c).

Additional winding,called the damper winding is

mounted on the rotor.

Damper windings provide a means of starting the

synchronous motor and stability during loadtransient.

Armature winding is energized from a.c source

and its field winding from d.c source


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It runs either at synchronous speed or not at

all.That is,while running it maintains a

constant speed.The speed is independent of

load.

It is not inherently self-starting.It has to be

run upto synchronous speed by some means

before it can be synchronized to the supply.

It can be operated under wide range of

power factors both lagging and leading.

It will stall if,while running,the countertorque is increased beyond the maximum

torque that the machine can develop.


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Advantages.1) These motors can be made to operate at leading power

factor and thereby improve the power factor of an

industrial plant from one that is normally lagging to

one that is close to unity.

2) This motor operates at a constant speed, irrespective ofload, from no-load to full load.

3) Electromagnetic power varies linearly with the applied

voltage.

4) These motors can be constructed with wider air gapesthan induction motors making them mechanically

better.

5) Efficiency of operation is usually high, especially in

the low speed and unity power factor ranges.


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Disadvantages

1) These motors cannot be used for variable speed jobs asthere is no possibility of speed adjustment.

2) It requires external source for supplying dc excitation

3) It cannot be started under load, the starting torque being

zero

4) It may fall out of synchronism and stop when over-loaded.

5) Collector rings and brushes are required.


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The self-commutated synchronous motor or BLDC

motor has applications from several micro-watt to

several dozens of kW,and more rarely

beyond,generally for variable speeds:

For low power:applications watch and clock

drives,peripheral devices for computers,medical

and automotive domains.

For medium-sized power:automotive,machinetools,robotics,production installations,etc.


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For high power:some electric

vehicles(cars,trolleybuses),large production facilities,etc.

power limitation is mainly linked to the cost and

implementation of magnets.


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www.google.com

www.wikipedia.com

www.whatis.com

Electric Machines(second edition)-Ashfaq Husain

Electric Drives-Marcel Jufer
http://www.google.com/http://www.wikipedia.com/http://www.whatis.com/http://www.whatis.com/http://www.wikipedia.com/http://www.google.com/


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Syncronous Motor

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