VFD Over Current Protection

[SteveWag]

I have a C-H MCC with a section containing a 50 Amp circuit breaker and a C-H VFD dseigned to control a 25HP motor, all new, not yet powered. Turns out the device being powered has a 10HP motor. Any problems? I am thinking of NEC motor protection.

Thanks;
Steve

 

[ozmosis]

Steve
Is there a PTC sensor (thermistor) fitted to the VFD or is it relying on electronic thermal characteristics?
The issue is not just over current protection but motor thermal protection.
I would check with C-H to determine if this meets class 20 protection for the motor if using purely the electronics of the drive to protect.

 

[SteveWag]

The motor is not fitted with temperature monitoring. I am OK with software monitoring of motor running current. I am concerned about NEC 430.52 requirements.
Thanks;
Steve

 

[jraef]

The VFD is going to provide all of the protection for the motor, Overload and Short Circuit, it's part of the UL listing requirements and because the VFD is in an MCC, you know that it is UL listed. So when you program the motor nameplate data, the VFD will automatically adjust its protection parameters to take care of everything DOWN STREAM of it regarding that specific motor, it doesn't matter if the motor is smaller than the VFD regardless of the size of the CB ahead of the drive (which if you think about it, is always sized for the VFD max amp rating, not the motor rating).

That said, some VFD mfrs have a restriction as to HOW MUCH smaller the motor can be, because of the resolution of the type of current sensing devices they have on the output. Some use cheaper methods than others. I have seen some that limit the motor to no less than 1/2 the rating of the drive, so in this case, 10HP is below that threshold. But it varies from mfr to mfr, A-B PowerFlex drives for instance have no such restriction, you can program a 400HP drive to protect a 1HP motor if you want to. So only C-H can answer that for you, or it will say so in the manual if you look up the programming of the motor FLA.

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