VFD Question

[pkbrown0138]

I went out on an ABB drive that operated a 480 volt 3 phase 15hp non potable water pump motor. I discovered while I was there that the A phase fuse was blown on the service disconnect that fed the drives. The VFD ran fine with the pump/ motor under it's normal load. It is possible (I guess it is) for a VFD to operate even if it loses on leg of the three phase power? I saw no fault or error code one the HMI. Thanks for the feed back!

 

[ptonsparky]

Yes, drives don't always care if it is single phase or three coming in. This can be a problem if the VFD is running while other motors have stopped because of the phase loss.

 

[wirenut1980]

I suppose if the load is low enough compared to the drive rating, nothing would be overloaded if it was single phased.

 

[pkbrown0138]

One of the other problems I noted was that the A phase was one of the two phases that fed the step down transformer for the 110 volt control power. The transformer was giving out a voltage of 60 due to the blown fuse. I do not know if the low voltage would affect the drive.

 

[Besoeker]

One of the other problems I noted was that the A phase was one of the two phases that fed the step down transformer for the 110 volt control power. The transformer was giving out a voltage of 60 due to the blown fuse. I do not know if the low voltage would affect the drive.

If the control transformer had one of the two input lines missing how would it produce any output voltage?

 

[Jraef]

If the control transformer had one of the two input lines missing how would it produce any output voltage?

Other motors on-line down stream from that blown fuse will contribute. Think of a rotary phase converter (without the caps to boost the level).

A VFD uses the line power simply as a "raw materials resource" for energy because it just converts the AC to DC; 3 phase or 1 phase is technically irrelevant at the basic level. BUT when you lose a phase, the input CURRENT from the two remaining phases will increase by the sq. root of 3 (x 1.732), so the converter section (diode bridge) devices are usually not sized to handle that added power without over heating. In addition, the ripple on the DC bus will increase because of the missing sequence and the DC bus smoothing capacitors would probably not be capable of taking care of it, which runs a risk of causing problems with the output transistors. For that reason, many 3 phase VFDs have input phase loss protection built-in. Yours apparently doesn't. However, if the LOAD is not pulling full rated current, the line current may still end up low enough to avoid being a problem. That is why you can use a VFD that is double-sized as a phase converter to be able to run 3 phase loads from 1 phase sources. So if it survived without being over sized, you were lucky.

Fuses are often used in VFD systems because they act faster than CBs, but this is a risk you run in to if the VFD lacks phase loss protection. I would add a phase monitor relay down stream of those fuses that will shut down the VFD if you want to avoid this in the future.