Oversized VFD for Motor

[tkb]

I have a job that the customer want to use a spare 25 HP VFD to run a 7-1/2 HP fan motor.

I know that according to 430.122 I need to size the feeder to this VFD at 125% of the VFD input current.

Do I size the load side conductors of the VFD at 125% of the motor full-load current as per 430.22 or the same as the input current conductor size?

 

[Jraef]

Load size conductors are OK to be sized per normal. I know, it makes no sense, but there it is ... If it were me I would size the conductors for the possible use of the drive at its full capacity, which is the philosophy behind 430.122 in the first place.

On a side note however, many VFD mfrs will tell you not to use a VFD that is more than 2x the motor rating, so for example your 25HP drive should not be used on anything smaller than 15HP. The problem becomes that the drive's current sensors become out of the range of acceptable error.

 

[tkb]

Load size conductors are OK to be sized per normal. I know, it makes no sense, but there it is ... If it were me I would size the conductors for the possible use of the drive at its full capacity, which is the philosophy behind 430.122 in the first place.

On a side note however, many VFD mfrs will tell you not to use a VFD that is more than 2x the motor rating, so for example your 25HP drive should not be used on anything smaller than 15HP. The problem becomes that the drive's current sensors become out of the range of acceptable error.

I cant find sizing recommendations in the PowerFlex 400 manual.
is there somewhere else that I can find this information?

I would rather sell them the correct size drive.

 

[Jraef]

I cant find sizing recommendations in the PowerFlex 400 manual.
is there somewhere else that I can find this information?

I would rather sell them the correct size drive.

That's one of the few drives that are not Sensorless Vector Control, strictly V/Hz. So it actually would not care, because the only thing the current sensors are doing is monitoring for OL conditions.

 

[kwired]

For whatever it may be worth, I have a client that happened to have a AB 1336 250 hp drive sitting there in a part of the plant they don't use. A part that doesn't suit their needs but still has previous owners equipment in there. That drive I helped install new in maybe about 1994 or 1995 and it did drive a 250 Hp motor at that time.

Maybe about 2000-2003 current owner has an idea to try to vary the speed of a 50 HP motor on one of the machines they have. I said we have a 250 HP drive - lets just use it to see how it goes. Went very well. Even though the drive called for more input then it would ever see while connected to this motor we hooked it to the same 100 amp breaker that the 50 HP was originally supplied from. Motor overload settings only went down to about 50 amps, but that was still enough. Did have some issues with burning out motors, but finally resolved that with proper inverter duty motor and one with an additional blower to cool the motor.

Couple years ago the need came up to vary the speed of a 100 HP motor to see if that would have any benefits to a problem we were having with another machine. Ordered a 50 HP drive to put on the previously mentioned motor and moved the 250 HP drive to the 100 HP motor. So far no problems with it either. Only problems we ever had with the 50 HP machine since putting on the inverter duty motor have not been drive related problems, drive tripping overload has always just been the symptom of other problems.

 

[Cow]

We install oversized drives quite a bit.

I always wire for the 125% input to the drive, with the T leads being sized to the actual motor size.