Induction motor application guidelines for AC variable frequency drives
Modern variable frequency drives (VFDs) offer an almost dizzying range of capabilities that include output frequencies into the hundreds of hertz. It can be tempting to use a standard AC induction motor with one of these VFDs. But doing so requires a thorough understanding of the intended application and how the VFD will affect the motor. Since the most popular VFDs sold today are pulse-width modulated (PWM) type, the comments and recommendations in this article will assume that is the type used. Also, motor voltage will be 600 volts or less.
Those who have been around for a few decades remember the first motor problems associated with PWM VFDs; that is, winding failures. The lesson learned was that magnet wire insulation needed to be improved to withstand the higher peak voltages that resulted from many PWM VFD applications. This was a somewhat painful time with finger-pointing between the drive and motor manufacturers, if they were different companies.
Topics covered include:
- winding failures
- excessive rotor heating
- bearing failures due to shaft currents
- motor speeds less than or equal to nameplate speed
- motor speeds greater than or equal to nameplate speed
- estimating size equivalent applied to VFDs
The article also touches on other considerations, such as:
- mechanical torsional considerations
- electrical torsional considerations
- bearing size limits
- lubrication limits