Synchronous Motor : Concepts -- 1 ( sai saikumar jn)

Synchronous Motor : Concepts

Introduction of Synchronous Motors:

A three-phase ac voltage is applied to the stator winding and a DC source supplies the rotor field. The rotor is assumed stationary. The stator field rotates at synchronous speed.

In order to develop a continuous torque, the two fields must be stationary with respect to each other. As we now appreciate, this occur only when the rotor is also turning at synchronous speed. It is only then when stator and rotor fields "lock in", hence the name synchronous motor. Figure shows this condition ; the south pole of the rotor will lock in with the stator north pole, and vice versa. There may be momentary fluctuations in speed, but on the average the speed is constant. If the average speed of the rotor is different from synchronous value, even by a small amount, the poles lose their "grip" and the machine will come to a standstill. The bond between stator and rotor poles is then lost, which is the essential criterion for the development of torque.

DIFFERENCE BETWEEN INDUCTION MOTOR AND SYNCHRONOUS MOTOR:

	Induction Motors	Synchronous Motor
1.	These motors have wound rotor with slip rings or a squirrel cage rotor.	These motors have dc poles on rotor energized by excitation system.
2.	Rotor current is ac and is induced by magnetic induction.	The field current (excitation current) can be changed to vary the power factors.
3.	These motors run at less than synchronous speed. Full load slip is about 4%.	These motors always run at synchronous speed without slip.
4.	These motors take lagging power factor current.	These motors take different p.f. currents depending upon the excitation level.
5.	These motors have inherent starting torque.	These motors do not have any, inherent starting torque.
6.	These motors start unaided.	These motors have to be started by suitable means and brought to synchronous speed and then synchronized.
7.	These motors are used for variable speed and variable load drives.	These motors are used for constant speed and constant load drives.
8.	For the same size, these motors are cheaper.	These motors are costly due to additional cost of excitation system.

 

METHODS OF STARTING Synchronous Motor

(1) Using a starting motor:

This motor is directly coupled to the motor. It may be an induction motor which can run on a synchronous speed closer to the synchronous speed of the main motor.

(2) Starting as an induction motor:

This is the most usual method in which the motor is provided with a special damper winding on rotor poles. The stator is switched on to supply either directly or by star delta/reduced voltage starting. When the rotor reaches more than 95% of the synchronous speed, the dc circuit breaker for field excitation is switched on and the field current is gradually increased. The rotor pulls into synchronism. Change of direction of rotation. For this a running motor may be stopped. By interchanging any two phase sequences of stator terminals, the motor will run in reverse direction.

Synchronous Motor: TORQUE DEFINITIONS

(A) Synchronous torque:

It is the steady state torque required to drive the motor and the load at the synchronous speed.

(B) Pull-in torque:

It is the maximum constant load torque under which the motor will pull into synchronism at the rated rotor supply voltage and rated frequency, when the rated field current is applied.

(C) Nominal pull in torque:

It is the value of pull in torque at 95% of the synchronous speed with the rated voltage and frequency applied to the stator when the motor is running with the winding current.

(D) Pull out torque:

It is the maximum sustained torque which the motor will develop at synchronous speed for 1 minute with rated frequency and with rated field current.

(E) Pull up torque:

It is the minimum torque developed between standstill and pull in point. This torque must exceed the load torque by sufficient margin to ensure satisfactory acceleration of the load during starting.

(F) Reluctance torque:

It is fraction of the total torque with the motor operating synchronously. It results from saliency of the poles. It is approximately 30% of the pull-out torque.

(G) Locked rotor torque:

It is the maximum torque which a synchronous motor will develop at rest for any angular positions of the rotor at the rated voltage and frequency.

Losses. Various losses occurring in the motor are :
1. Armature Copper Loss I_a²R_a

2. Iron and friction losses.

Torque. In a synchronous motor, the power per phase

P_m =( E_b.V)/ Z_s cos (θ - α) – E_b² / Z_s cos (θ)

from ur's -- Bellapuri saikumar
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