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6.0 THEORY
6.1 DESCRIPTION OF AC MOTOR OPERATION
Three phase AC motors are comprised of two major components, the stator and
the rotor. The stator is a set of three electrical windings held stationary in the
motor housing. The rotor is a metal cylinder, fixed to the motor drive shaft, which
rotates within the stator. The arrangement of the stator coils and the presence of
three phase AC voltage give rise to a rotating magnetic field which drives the rotor.
The speed at which the magnetic field rotates is known as the synchronous speed
of the motor. Synchronous speed is a function of the frequency at which the
voltage is alternating and the number of poles in the stator windings.
The following equation gives the relation between synchronous speed, frequency,
and the number of poles:
Ss = 120 f/p
Where: Ss = Synchronous speed (rpm ), f = frequency (Hz),
p = number of poles
In three phase induction motors the actual shaft speed differs from the synchronous
speed as load is applied. This difference is known as “slip”. Slip is commonly
expressed as a percentage of synchronous speed. A typical value is three percent
at full load.
The strength of the magnetic field in the gap between the rotor and stator is
proportional to the amplitude of the voltage at a given frequency. The output
torque capability of the motor is, therefore, a function of the applied voltage
amplitude at a given frequency. When operated below base (rated) speed, AC
motors run in the range of “constant torque”. Constant torque output is obtained
by maintaining a constant ratio between voltage amplitude (Volts) and frequency
(Hertz). For 60 Hz motors rated at 230, 460, and 575 Vac, common values for this
V/Hz ratio are 3.83, 7.66, and 9.58 respectively. Operating with these V/Hz ratios
generally yields optimum torque capability. Operating at lower ratio values results
in lower torque and power capability. Operating at higher ratio values will cause
the motor to overheat. Most standard motors are capable of providing full torque
output from 3 to 60 Hz. However, at lower speeds, where motor cooling fans
become less effective, supplemental cooling may be needed to operate at full
torque output continuously.
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