Hello,

I've got a question that would pertain to any VFD whose front end is simple rectification and capacitor filtering to create a DC bus. (Specifically, I've got a bunch of old Allen-Bradley 1330 drives, but the question should hold to any drive that uses this type of front end.)

Basically, on the small drives that can be run on single phase inputs, opting to run the drive on a single phase instead of using the normal 3 phase input causes the manufacturer to derate the drive by about 30% to 50%. (e.g. the A-B 1330 is good for 6kW of output if it's being run off 3ph 240 VAC, but only 4kW if it's hooked up to 1ph 240 VAC.) Looking at the drive schematic, it appears that the only things limiting the drive and causing it to be derated are the 1) heaters, 2) copper wiring, and 3) bridge. After the bridge, you're on the capacitor and DC bus, so from that point forward the drive can't tell if you're running it off 1ph or 3ph. So, here's the $24 question: If I 1) change the heaters, 2) upgrade the gauge of the input wiring, and 3) change the bridge to a single, full-wave bridge that can handle the higher amperage, can I safely run the drive with a full-rated motor (6kW in the case of my 1330 drives)?

Thank you very much!
Dave Doerschuk
d.doerschuk@att.net

Hello Dave

Welcome to the forum.
The major problem area in the inverters when you run a three phase inverter off single phase, is the rectifier stage and the Capacitor bank.

If the rectifier has benn correctly selected for the rating of the drive, then you would need to limit the current to that of the rectifier stage. ( this is less than the RMS current ratings of the rectifier due to the thermal effects). Additionally, with a three phase input, the rectified DC voltage (without capacitors) is relatively smooth. There is about 16% ripple voltage at 300 Hz (50Hz system or 360Hz 60Hz system). This result in a relatively low ripple current through the DC Bus capacitors. Int the case of single phase input, you end up with 100% ripple voltage at 100Hz. This results in a very much higher ripple current through the capacitors.

In many drives, the capacitors are being pushed quite hard, and it is not unusual to have an expected life of the capacitors of about 5 years. When you increase the ripple current, you reduce the life expectancy of the capacitors. Additionally, the reduction in the ripple frequency requires an increase in capacitance to achieve the same level of regulation. You will have difficulty achieveing full output voltage due to poor DC bus regulation.

The bottom line? I would expect that you will find problems running at full output with the output voltage sagging considerably, and the life of the capacitors and rectifiers would be severly compromised. If a DC Bus choke is employed, this would also be severly compromised due to the reduction in ripple frequency.

Best regards,

Thank you very much for both your kind welcome to the forum, and your very clearly written answer. I understand now; it is clear that the 3 phase rectification sums the phases and the resulting signal contains far less ripple. I really should have thought of this, thank you for keeping me from destroying the capacitor bank!

Best Regards,

Dave Doerschuk
d.doerschuk@att.net