VFD as a phase convertor

[FORK]

I have a 15hp 3 phase motor that needs a vfd. There is only single phase available but have been informed that you can use a vfd as a phase convertor as long as you derate and size the vfd larger than your motor load. Wondering if anyone has had success with this?

 

[Rick Christopherson]

Yes. Because the VFD takes the incoming AC power and converts it to DC power, the limitation of using one as a phase converter is based on derating the amperage through the input rectifier. The derating would be 1.7 times (squareroot-3) the motor size, but you can probably squeeze a little more out of this depending on the actual motor nameplate FLA, percent load, and advise from the VFD manufacturer.

 

[Jraef]

Actually there are two issues:

1. The current drawn from the 1 phase source is 1.732 times the 3 phase motor current, and the diodes in the front end of the VFD have to be large enough to handle it.
2. The 1 phase input ends up causing a higher ripple on the DC bus and you need more capacitance inside the VFD to smooth it out. Sizing the VFD only for 1.732X the motor current doesn't cover this.

For that last reason, most VFD manufacturers will tell you to DOUBLE the HP size of the motor. So for a 15HP motor, you will need a 30HP VFD to be safe.

Another "gotcha" is that not all larger VFDs can be used this way. Some have Phase Loss Protection built-in to the VFD controls in order to protect their diodes from just this sort of thing, and some of those some don't allow you to disable that Phase Loss Trip, which renders them incapable of being used as phase converters. Just be thorough in checking out what you are buying. If your supplier doesn't know, ask for the manual and check out the trip functions and see if there is a defeat option for any Phase Loss Trips.

 

[a.bisnath]

yes,I have done this

yes,I have done this

I have done this ,an engineer did the design,we installed and commissioned,we used 220volt single phase to feed the vfd off a double pole 50 amp breaker to the vfd rated at 48.0 amps to 2 ,3phase compressor motors each with a FLA of 10 amps via 20 amp 3 phase sub circuit breakers and DOL starters worked well ,has never given any trouble