LEESON Micro Series Compact Inverters - Manual (Page 19)

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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