AC Motors
AC MOTOR THEORY
Torque
The torque of an AC induction motor is dependent upon the strength of the interacting rotor and
stator fields and the phase relationship between them. Torque can be calculated by using
Equation (12-3).
T = K F IR cos qR
(12-3)
where
T
= torque (lb-ft)
K
= constant
F
= stator magnetic flux
IR
= rotor current (A)
cos qR
= power factor of rotor
During normal operation, K, F, and cos qR
Figure 4 Torque vs Slip
are, for all intents and purposes, constant,
so that torque is directly proportional to
the rotor current. Rotor current increases
in almost direct proportion to slip. The
change in torque with respect to slip
(Figure 4) shows that, as slip increases
from zero to ~10%, the torque increases
linearly.
As the load and slip are
increased beyond full-load torque, the
torque will reach a maximum value at
about 25% slip. The maximum value of
torque is called the breakdown torque of
the motor. If load is increased beyond
this point, the motor will stall and come
to a rapid stop. The typical induction
motor breakdown torque varies from 200
to 300% of full load torque. Starting
torque is the value of torque at 100% slip
and is normally 150 to 200% of full-load torque. As the rotor accelerates, torque will increase
to breakdown torque and then decrease to the value required to carry the load on the motor at
a constant speed, usually between 0-10%.
Rev. 0
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