Page 146 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The rotor, which is as yet unexcited is speeded up to synchronous / near synchronous speed by some arrangement
and then excited by the D.C. source. This produces alternate N and S poles around the circumference of the
rotor. If the poles at this moment happens to be facing poles of opposite polarity on the stator, a strong magnetic
attraction is set up between them. The mutual attraction locks the rotor and stator poles together and the rotor
is literally yanked into step with the revolving field. The torque developed at this moment is called pull in torque.

The pull in torque of a synchronous motor is powerful, but the DC current must be applied at the right moment. If it
should happen that the emerging N, S poles of the rotor are opposite to the N, S poles of the stator, the resulting
magnetic repulsion produces a violent mechanical shock. The motor will immediately slow down and the circuit
breakers will trip. In practice, starters for synchronous motors are designed to detect the precise moment when
excitation should be applied. The motor then pulls automatically and smoothly into step with the revolving field.
Once the motor turns at synchronous speed, no voltage is induced in squirrel cage windings and so it carries no
current. Consequently, the behaviour of a synchronous motor is entirely different from that of an induction motor.
Basically a synchronous motor rotates because of the magnetic attraction between the poles of rotor and the
opposite poles of the stator.
However, it is important to understand that the arrangement between the stator and rotor poles is not an absolutely
rigid one. As the load on the motor is increased, the rotor progressively tends to fall back in phase (but not in
speed as in D.C. motors) by some angle (Fig 4) but it still continues to an absolutely rigid one. As the load on
the motor is increased, the rotor progressively tends to fall back in phase (but not in speed as in D.C. motors) by
some angle (Fig 4) but it still continues to run synchronously. The value of this load angle or coupling angle (as
it is called) depends on the amount of load to be met by the motor. In other words, the torque developed by the
motor depends on this angle, say between N & S.
S
The working of a synchronous motor is, in many ways, similar to the transmission of mechanical power by a
shaft. In Fig 5 are shown two pulleys P and Q transmitting power from the driver to the load. The two pulleys are
assumed to be keyed together (just as stator and rotor poles are interlocked) hence they run at exactly the same
(average) speed. When Q is loaded, it slightly falls behind owing to the twist in the shaft (twist angle corresponds
to in motor), the angle of twist, in fact, being a measure of the torque transmitted. It is clear that unless Q is so
heavily loaded as to break the coupling, both pulleys must run at exactly the same (average) speed.

Various torques associated with a synchronous motor
1 Starting torque
2 Running torque
3 Pull-in torque and
4 Pull-out torque

1 Starting torque
It is the torque (or turning effort) developed by the motor when full voltage is applied to its stator (armature) winding.
It is also sometimes called breakaway torque. Its value may be as low as 10% as in the case of centrifugal pumps
and as high as 200 to 250% of full-load torque as in the case of loaded reciprocating two-cylinder compressors.

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CITS : Power - Electrician & Wireman - Lesson 76-85 CITS : Power - Electrician & Wireman - Lesson 76-85