Appendix C How to Select the Right AC Motor Drive
VFD-EL-W
C-5
C.3 How to Choose a Suitable Motor
Standard Motors
When using the AC motor drive to operate a standard three-phase induction motor, follow these
precautions.
The energy loss is greater than that for an inverter-duty motor.
Avoid running the motor at low speed for a long time. Under this condition, the motor temperature may rise
above the motor rating due to limited airflow produced by the motor’s fan. Consider adding external forced
motor cooling.
When the standard motor operates at low speed for a long time, the output load must be decreased.
The load tolerance of a standard motor is according to the following diagram.
3 6 20 60
100
82
70
60
50
0
60%
40%
25%
to
rq
ue(%)
continuous
Frequency (Hz)
Load duty-cycle
If 100% of continuous torque is required at low speed, it may be necessary to use a special inverter-duty
motor.
Motor dynamic balance and rotor endurance should be considered once the operating speed exceeds the
rated speed (60Hz) for a standard motor.
Motor torque characteristics vary when driving the motor with an AC motor drive instead of a commercial
power supply. Check the load torque characteristics of the machine connected to the motor.
Because of the high carrier frequency PWM control of the VFD series, pay attention to the following motor
vibration problems:
Resonant mechanical vibration: use anti-vibration dampers to mount equipment that runs at varying speed.
Motor imbalance: special care is required for operation at 60 Hz and higher frequencies.
The motor fan is very noisy when the motor speed exceeds 60 Hz or above.
Special Motors
Pole-changing (Dahlander) motor:
The rated current differs from that of a standard motor. Check before operation and carefully choose the
capacity of the AC motor drive. When changing the number of poles, stop the motor first. If over-current
occurs during operation or the regenerative voltage is too high, let the motor free run to stop (coast).
Submersible motor:
The rated current is higher than that of a standard motor. Check before operation and carefully choose the
capacity of the AC motor drive. A long motor cable between the AC motor drive and the motor reduces the
available motor torque.
Explosion-proof (Ex) motor:
Must be installed in a safe place and the wiring should comply with the (Ex) requirements. Delta AC Motor
Drives are not suitable for (Ex) areas that require special precautions.
Gear reduction motor:
The lubricating method of the reduction gearbox and the speed range for continuous operation are different
and depend on the motor brand. Carefully consider the lubricating method when operating for a long time
at low speed and for high-speed operation.
Synchronous motor:
The rated current and the starting current are higher than those of standard motors. Check before
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