Appendix C How to Select the Right AC Motor Drive_VFD-B Series
Revision 10/2005, BE13, SW V4.08 C-5
C.3 How to Choose a Suitable Motor
Standard motor
When using the AC Motor Drive to operate a standard 3-phase induction motor, take the following
precautions:
1. The energy loss is greater than for an inverter duty motor.
2. Avoid running 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 external
forced motor cooling.
3. When the standard motor operates at low speed for long time, the output load must be decreased.
4. The load tolerance of a standard motor is as follows:
3 6 20 60
100
82
70
60
50
0
60%
40%
25%
torque(%)
continuous
Frequency (Hz)
Load duty-cycle
5. If 100% continuous torque is required at low speed, it may be necessary to use a special inverter duty
motor.
6. Motor dynamic balance and rotor endurance should be considered once the operating speed exceeds
the rated speed (60Hz) of a standard motor.
7. Motor torque characteristics vary when an AC Motor Drive instead of commercial power supply drives
the motor. Check the load torque characteristics of the machine to be connected.
8. Because of the high carrier frequency PWM control of the VFD series, pay attention to the following
motor vibration problems:
 Resonant mechanical vibration: anti-vibration (damping) rubbers should be used to mount
equipment that runs at varying speed.
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