Why stored AC drives can be dangerous

As a company in the variable speed drive service business, a client recently asked us to come to its facility to enter a program in a replacement drive. A 40-hp, 600-volt PWM (pulse-width-modulated) inverter had failed. The company had installed a spare unit from its stockroom. The installation was checked and it seemed safe to turn the power on.

The switch was turned on, and perhaps 30 seconds later, the room lit up and there was a noise like a shotgun blast! The capacitor bank had short-circuited. As a result, the unit was not repairable. A new unit was ordered and installed the next day.

The original replacement drive looked new but it turned out that it had been in the stockroom for 10 years. AC drives that have not been powered up for a period of years need to have the capacitor banks reformed. This procedure involves charging the capacitor bank slowly.

There are two rules of thumb that I have followed over the years:

1. Place resistance in series with the capacitor bank so that when full voltage is applied, the RC time constant limits the charging time to five minutes for the first year the unit has not been powered up, plus one minute for each additional month.

2. Apply line voltage to the inverter in stages. In the case of a 600-volt inverter, apply 220 volts for 15 minutes, 440 volts for 15 minutes and finally apply 600 volts. Note that there is a danger of burning out the precharge resistor if the capacitor bank is shorted.

The best approach is to rotate the spares with the production units during scheduled maintenance shutdowns. This will help ensure the integrity of the spares inventory.

The damage caused to a drive when the capacitor bank short circuits is often so extensive that the drive is not repairable. There is also a danger to personnel when a capacitor shorts. Often there are pieces of capacitor, bus bar, connectors, etc., projected through the air. Be sure that all covers are in place before powering up an AC drive.

J. Daryl Keys, C.E.T., is president of Industrial Drive Service Inc., Hensall, Ont. He can be reached at 519-262-2837 or 866-917-9929. For more information, visit www.idscontrols.com.