Specifications
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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.