Identifying Motor Stress Points 

Motors controlled by VFDs installed at long distances often fail due to high voltage induced insulation breakdown. VFDs use insulated gate bipolar transistors (IGBTs). These devices offer many advantages and improve the control of AC motors, but there are some trade-offs.  Some IGBT characteristics, particularly their fast-switching capability, combined with long lead lengths between the drive and motor, can shorten motor life.

IGBTs let drives turn voltage on and off at a very high frequency: 4,000 to 16,000 times a second. This means the voltage rise time is short and usually less than a few microseconds. These short rise times combined with long lead lengths between the drive and motor can produce voltage reflections, also called reflected waves, that have high peak voltages. If the voltages are large enough, they will produce stresses in the motor insulation.

Shorter voltage rise time and longer motor lead length produce higher amplitudes of the reflected wave. The voltage can reach two times or more than the DC bus voltage. Therefore, if a drive has a 650 Vdc bus (normal for 480V input), voltages at the motor may reach 1,300 or higher, enough to cause some motors to fail.

Using Output Load Reactors to Close the Gap

Most VFD manufacturers recommend a maximum distance between drive and motor. Most distances are between 100 to 300 ft.  Installing output reactors between the drive output and the motor reduces the rate of voltage rise (increases the voltage rise time). This limits the reflected wave amplitude and extends the allowable distance for motor cables.

The general “rule of thumb” is that an output reactor should be used if the motor wiring extends over 100 ft., but this value can vary depending on the motor. For any motor, if the distance is between 300 to 500 ft., a load reactor should be used.