COMMON FAILURE MODES
These are the most common failure modes of VSDs (most common listed first):
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.