AC Drives are fed by AC mains and produce an AC power output for the motor. Why then is “DC Bus Over Voltage” one of the most common faults on a drive system?
It turns out that most of the Variable Frequency Drives (VFD) used in web handling convert AC mains to DC using a rectifier. Then the DC voltage is chopped into a Pulse Width Modulated (PWM) AC signal using an inverter.
Common VSD’s use a simple bridge rectifier to create the DC bus voltage. Rectifying 3 phase AC voltage increases the dc voltage by a fixed ratio of 35%. For example, with 460 VAC mains, the dc bus voltage will be 621 volts. This voltage is not regulated and assumes that power is being delivered to a motor.
It turns out that the rectifier is not the only source of power for the DC bus inside the drive system. If the motor acts like a generator, the inverter back feeds power to the DC bus. Capacitors and resistors in the DC circuit can absorb a bit (<10%) of the generated power. If the generated power is too much, the DC volts have to rise.
In all cases, you will find the drive was generating when the DC Over Voltage fault occurred. The situations vary but may include:
- tension is over-hauling the roller
- unwind is generating all the time
Sometimes the situation is only a very brief event. For instance, when an accumulator completely empties, the web snaps and may cause the motor of an adjacent roller to generate for only a fraction of a second. This may be enough to create the fault.
Solutions are dynamic braking resistors, common dc bus for multiple drives, and VFD’s specifically engineered for regenerative applications.