Page 225 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

VARIABLE-SPEED DRIVES

10.24 CHAPTER TEN

1. Motor bearings
2. Motor insulation
3. Drive electrolytic capacitors
4. AC line transients that damage the IGBTs and diodes

MOTOR APPLICATION GUIDELINES

Never use a “standard” induction motor with a VSD. All motors used with VSDs must be
inverter-rated. They must be designed and manufactured for use with a VSD. These are the
modifications from the standard induction motor design:
1. The quality and voltage capability of the winding insulation must be higher than that of
a standard motor.
2. Thinner and higher-quality steel laminations are needed to reduce eddy current losses.
3. Improved motor cooling generates more heat due to nonsinusoidal PWM waveform.
4. Better-quality bearings are needed with shaft grounding brush or an electrostatic shield
to reduce bearing current and increase bearing life. Consideration should be given to the
motor cooling at low speed. Since the fan is mounted on the shaft, the motor cooling
will be reduced at low speed. If the VSD is used with a pump or a compressor in which
the load drops significantly with speed, overheating will not be a problem. However, if
the VSD is used with a constant-torque load, a separate blower is required for cooling
the motor. This will ensure that motor cooling is provided at low speed.

REFERENCE

1. A.C. Stevenson, Power Converter Application Handbook, Institute of Electrical and Electronics
Engineers, New York, 1999.

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