Dynamic Braking Configuration

The run-away best-read blog I have ever posted was DC Bus Over Voltage. The sheer number of comments indicate dc bus overvoltage for AC drives is a severe problem in many industries. Recently I learned more about successfully using a Dynamic Braking (DB) resistor with a drive. The education came courtesy of a knowledgeable service rep for the drive exhibiting problems at the time.

The drives had been in successful service for about 8 years. One was a turret drive and the other was a roll off-loading lifting table. Both drives had adequate DB resistors which were enabled with default parameters.

Following a line upgrade, the cycle time for the turret and lifting table was increased. Higher speeds and accel/decel rates were required. The drives were tuned for better response. Faster deceleration rates and speeds were attempted. We found limits in the speeds and ramp rates. Consequences were the turret overshot its level position and the lifting table also went past its stop position intermittently. DC Volts increased, but not to alarm levels.

It turned out, that the DB resistor, paid for, wired, and enabled, was not being fully utilized. Correctly entering the resistor ohms and watts did not improve performance. It was necessary to enter the DB pulse watts using two parameters. We monitored the DB resistor mounted outside the drive cabinet to ensure it glowed red and crackled during decelerations and stops.

All in all, 7 parameters were needed to derive benefits from the DB resistor. These parameters were not all grouped in the same section. A knowledgeable service rep and a white paper from the manufacturer were required to get the DB right. We now have the drive performance advertised in the brochures.