AC MOTOR THEORY
AC Motors
where
NS = synchronous speed (rpm)
NR = rotor speed (rpm)
The speed of the rotating magnetic field or synchronous speed of a motor can be found by using
Equation (12-2).
(12-2)
NS
120 f
P
where
Ns =
speed of rotating field (rpm)
f
=
frequency of rotor current (Hz)
P
=
total number of poles
Example:
A two pole, 60 Hz AC induction motor has a full load speed of 3554 rpm. What
is the percent slip at full load?
Solution:
Synchronous speed:
NS
120 f
P
NS
120 (60 Hz)
2
NS
3600 rpm
Slip:
SLIP
NS
NR
NS
x 100%
SLIP
3600
3554 rpm
3600 rpm
x 100%
1.3%
ES-12
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