Inverter Duty Failures

It has been documented that some electric motors fail in inverter applications. This has often been attributed to inverter voltage “spikes.” While this is relatively correct, it misses some important aspects to the mode of failure.
The number of pulses that a PWM drive fires in order to control the current waveform to the drive is known as the carrier frequency. The carrier frequency tends to run from 2 to 18 kHz in most modern PWM drive. In addition, each voltage pulse is not a square waveform. They have a tendency to overshoot on startup, causing a “ringing” effect at the peak voltage of the pulse. Insulation systems are designed, not only for temperature, but also for “rise time,” how fast the voltage increases over time.

Initially, it was thought that inverter duty failures occurred only on the first few turns of the electric motor winding. It was later found that this was not correct for all cases. Instead, it was discovered, a phenomenon normally seen in electric motors rated at 6,000 VAC, and above, known as Partial Discharge, was now occurring in motors rated as low as 460VAC. This phenomenon is similar to a lightning storm within the windings themselves. Within voids in the winding insulation, charges build up, then discharge (much like a capacitor). The end result is ozone, which begins to break down the insulation on the wires, eventually causing a current path, or short.
The mode of failure for motors in this environment is as follows:
· The motor and drive are placed a distance apart and the carrier frequency is set high (ie: above 8kHz) in order to keep the motor quiet. The lower the carrier frequency the louder the motor noise. No filtering is put in place.
· The pulses from the drive travel out to the motor. Based upon the impedance of the cable and motor, a reflection of the pulse travels back to the drive. This cycles through the “free-wheeling” diodes of the inverter and travel back out with the normal pulses. This adds on to the peak voltage, causing a greater peak (as much as 2 to 4 times, usually 2) with an extremely fast rise time. (ie: less than .1 u-sec per 500 V versus the 1 u-sec per 500 V recommended by NEMA).
· In some cases, the voltage spikes will cause the weakest part of the winding insulation to fail and the motor shorts.
· In other cases, small voids in the insulation begin to have partial discharge problems, the ozone eats away at the insulation, until, finally, the insulation becomes weak enough for the spikes to break through.

It should be pointed out that this tends to be a rare problem. Following are measures to avoid the chance of this problem occurring to you:
· Check with the motor manufacturer to ensure that the motor can operate in an inverter environment.
· Use filters in the inverter system (ie: from line reactors to spike arrestors, designed for inverter use).
· Read the VFD operators manual. It will often state the minimum distances and frequency settings.
· Use proper wire sizes.

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