IGBT-based ac drives tend to contribute to the partial discharge problem. The transistors switch on and off at such a rapid pace that they can produce spikes that travel to the motor. As the signals travel along the cable, they can increase in magnitude until they reach a level that is past a motor's rated corona inception voltage (CIV) level. For most 400-V motors, the CIV rating is 1,600 V. These scope traces are from a 10-hp, 460-V drive that connects to a motor through a 500-ft cable. The top wave shows the frequency at the drive output terminals. The bottom wave represents the frequency at the motor terminals.

Frequent nuisance tripping usually indicates a problem in a drive or motor. For the drive, a cause may be improper settings for the sensitive electronics. For an ac motor, it's often a sign that the motor insulation system is under attack and degrading. Eventually, the motor will exhibit stronger clues, such as a drop in torque output, that will indicate growing insulation failure.

Solutions to this problem continue to evolve, but so far, the one solution has not been found.

Partial discharge, or corona, is one cause of insulation degradation and is peculiar to motors controlled by digital drives, especially those using IGBT switching transistors. Partial discharge occurs when there's an excessive voltage gradient (dV/dt) in the presence of air. According to NEMA standard MG1, Part 31, this is when peak voltage reaches a magnitude greater than 1,600 V in less than 0.1 μsec.

The arcing stabs holes in the insulation, eroding it and exposing the coil wire within. Current can then flow phaseto- phase, turn-to-turn, or coil-to-coil.

One of the keys to preventing partial discharge is to eliminate air within the motor coils. This means filling empty space with resin or other material. But there is no single solution that works. Effectively eliminating air requires a true systems approach, from the design of the motor to the method of manufacture. Therefore, most manufacturers use combinations of special winding techniques, various types of insulations, and multiple resin applications or vacuum impregnation.

Question & Answers

Q. How can I reduce or prevent partial discharge from a motor I've already installed in my plant?
A. There are several steps you can take. First be sure to follow the manufacturer's guidelines on the length of cable between the drive and motor. Length is important because the longer a cable is, the more opportunity a voltage signal has to increase in magnitude beyond the CIV level. If you can't follow the length limits, consider placing reactors or filters at the motor. Some filters actively shut out high voltages.

Q. Are multiple resin applications always better for a motor?
A. It really depends on the overall design and manufacture of the motor. Resin dipping, flowing, or vacuum pressure impregnation - when applied properly - can reduce the air within the wound stator. One application can be sufficient if the process ensures thorough penetration of the resin to all areas of the stator.

Q. My problem is partial discharge on the motor conductors. Are there any solutions?
A. Sure. Several cable manufacturers offer power cables that handle not only the PWM signals but the high voltages that can occur when standing waves develop on the conductors. These cables have additional electrical insulation on the conductors to decrease the chances of discharge.