Theory; Description Of Ac Motor Operation - Lenze AC Tech M3103S Installation & Operation Manual

6

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:
Where: Ss =
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 .
If the frequency applied to the motor is increased while the voltage remains constant,
torque capability will decrease as speed increases . This will cause the horsepower
capability of the motor to remain approximately constant . Motors run in this mode when
operated above base speed, where drive output voltage is limited by the input line
voltage . This operating range is known as the "constant horsepower" range . The typical
maximum range for constant horsepower is about 2 .3 to 1 (60 to 140 Hz) . The diagram
below depicts the characteristics of a typical AC induction motor with a 60 Hz base speed .
WARNING!
Consult motor manufacturer before operating motor and/or driven
equipment above base speed .
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Ss = 120 f/p
Synchronous speed (rpm)
f = frequency (Hz)
p = number of poles
M301L