OCPD's for Variable Frequency Drives

[mshields]

I'm in the practice of sizing my upstream breaker feeding a drive operated motor at 175% and sizing the conductors at 125%. I have a situation where there is a desire to use an existing breaker which will give me 150% of FLA. I know that drives have very little inrush and so I "assume" this will work fine.

What I'd appreciate feedback on is A) Do you agree that 150% should work fine and B - what is the worst case inrush to a VFD. e.g. could I set the OCPD at 125%

Does it depend on how quickly it starts up? If you start it up rapidly can you see inrush magnitudes approaching or equaling an across the line. I wouldn't think so since it's toying the frequency to get there. But I don't know.


thanks,

Mike

 

[iwire]

Mike the NEC requires the supply to a VFD to be based on the size of the drive not the driven motor. 430.122.(A)

The feeder or branch circuit must be at least 125% of the drives rating.

Any inrush will be directly related to how aggressively you accelerate the motors. I have worked on 500 HP motors that started with virtually no inrush. I would hit start, the current would jump to 40 or 50 amps, then gradually rise to a max running load of around 500 amps.

 

[don_resqcapt19]

Mike,

I'm in the practice of sizing my upstream breaker feeding a drive operated motor at 175% and sizing the conductors at 125%.

There is no provision that permits the conductors to have a rating less than that of the OCPD for this application.
Don

 

[ron]

Mike,
If the drive has a bypass, it would essentially be an across the line start and you need to size like a regular starter.

 

[mshields]

thanks

thanks

Yes - it sounds like the bypass question is the 64,000 dollar.

Mike