Lenze MC1000 Series - Manual (Page 18)

14 13435742_EDBM101_v24
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:
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.
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.