Mechanical Systems 371
would be no relative motion between the rotor and stator, and, therefore,
no induced rotor current and torque would develop. The rotor speed (op-
erating speed) of an induction motor is always somewhat less than the ro-
tating stator field developed by the applied AC voltage.
The speed of the rotating stator field may be expressed as:f × 120
S = ———
n
where:
S = the speed of the rotating stator field in rpm,
f = the frequency of the applied AC voltage in hertz,
n = the number of poles in the stator windings, and
120 = a conversion constant.A two-pole motor operating from a 60-Hz source would have a sta-
tor speed of 3600 revolutions per minute. The stator speed is also referred
to as the synchronous speed of a motor. The difference between the re-
volving stator speed of an induction motor and the rotor speed is called
slip. The rotor speed must lag behind the revolving stator speed in order
to develop torque. The more the rotor speed lags behind, the more torque
is developed. Slip is expressed mathematically as:Ss – Sr × 100
% slip = ——————
Ss
where:
Ss = the synchronous (stator) speed in rpm, and
Sr = the rotor speed in rpm.As the rotor speed becomes closer to the stator speed, the percentage
of slip becomes smaller.
Another factor, referred to as rotor frequency, affects the operational
characteristics of an induction motor under load. As the load on the shaft
of the motor increases, the rotor speed tends to decrease. The stator speed,
however, is unaffected. When a two-pole induction motor connected to a
60-Hz source operates at 10 percent slip, the slip will equal 360 rpm (3600
rpm × 10%). Functionally, this means that a revolving stator field sweeps
across a rotor conductor 360 times per minute. Current is induced into
a rotor conductor each time the stator field revolves past the conductor.
As slip is increased, more current is induced into the rotor, causing more