Steady-state operation of motor:
Torque-speed curve
The maximum pullout torque occurs when δ = 900:
Normal full-load torques are much less than that (usually, about 3 times smaller).
When the torque on the shaft of a synchronous motor exceeds the pullout torque,
the rotor can no longer remain locked to the stator and net magnetic fields. It starts
to slip behind them. As the motor slows down, the stator magnetic field “laps” it
repeatedly, and the direction of the induced torque in the rotor reverses with each
pass. As a result, huge torque surges of alternating direction cause the motor
vibrate severely. The loss of synchronization after the pullout torque is exceeded is
known as slipping poles.
max
3 A
R net
m S
V E
kB B
X
τ φ
ω
= = (7.73.1)
ELEN 3441 Fundamentals of Power Engineering Spring 2008
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