Overload protection on VFD

[MW_EE_2015]

The client would like to install VFDs on cooler motors of 50 HP using the existing equipment. Currently the motors are being powered by NEMA 3 motor starter buckets with thermal overloads and an instantaneous trip breaker (AB Centerline Series).

The plan was to wire the contactor such that it is always commanded on to feed the VFD to meet the thermal protection requirement in the code 430.32(A)(1). I'm thinking you just remove the wire that was controlling the coil and wire it straight to the control transformer so it is always on (not bypassing the thermal overload just giving the coil a 'go' signal all the time.)

I thought this would satisfy the overcurrent protection requirement since it is rated for the same HP as the VFD, and the disconnect requirement per NEC 430.109 as it is still a listed self protected combination controller. I'd like to hear any concerns in this plan to see if it is viable or not.

 

[petersonra]

Generally you cannot use an IT trip CB on a VFD.

The VFD is listed as having built in overload protection for the motor.

Best bet, rip out the starter, overload, and IT only breaker and replace it with a T/M breaker and wire the breaker direct to the VFD.

 

[tom baker]

You gain nothing with your plan, except more equipment to fail. The VFD has internal software overloads that are better than the existing.
The IT breaker may not be allowed for protection of the VFD, TM breakers typically are ok