VFD conversion question.

[oxford]

I will be getting around to doing a vfd swap on my clausing 5914 very soon and would like to get everything I need rounded up. The lathe has the factory 2 horse motor on it (I know probably not the best insulation on the wires in the motor for vfd). I am going to be using a 230v toshiba drive rated for 5hp and will be powering it with single phase fed from a 30 breaker in the panel.

My question is should I be using a smaller breaker down the line to protect the vfd and motor from over load? Is the sizing still the same when you are powering a 3 phase motor with a drive on single phase? I know at work that the breaker size for 3ph motors is what is stated for FLA on the motor name plate. Thanks.

[Forestgnome]

Your vfd will have built-in overcurrent protection. The 30A breaker will prevent a fire if the vfd shorts out. Shouldn't need anything additional.

[lakeside53]

You size the breakers and/or fuses as per the VFD manf recommendations - They will specify exactly what you need.

[oxford]

You size the breakers and/or fuses as per the VFD manf recommendations - They will specify exactly what you need.

Is it the same when you are putting single phase in to a 3 phase converter. There is nothing in the manual concerning powering up with single phase. The drive will operate on it though, I have already tried it.

[lakeside53]

Probably or less. If it's not listed for single phase operation, you will be derating the drive anyhow, usually 50% but sometimes much more. I have a 7.5hp ABB drive that is only 1.5hp on single phase. You are just protecting for vfd failure though. The VFD takes care of the motor. You do need to program the vfd with the motor FLA though.


It's not so simple as just comparing the 3 phase motor FLA - the vfd is pretty much 95-98% efficient at power conversion, but the input capacitors add a surge load on fuses and breakers.

[oxford]

Thanks lakeside. Yes I know that the drive will be de-rated on single phase. I actually spoke to a toshiba tech on this. They are derating theres at less than 50%. He had told me that one of there 3hp drives I was questioning about would be on the border line of powering the 2hp motor but I would probably be ok with it. I should have no problem with the 5hp drive that I have.