240 V Motor on 208 V supply with VFD

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Calrissian45

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A client ordered a 240 V motor with a VFD and they want to connect it to a 208 V panelboard. Will this not matter since the motor is ran by the VFD? How would I calculate the FLA since the nameplate data is based on 240 V?
 

GoldDigger

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A client ordered a 240 V motor with a VFD and they want to connect it to a 208 V panelboard. Will this not matter since the motor is ran by the VFD? How would I calculate the FLA since the nameplate data is based on 240 V?

The VFD approximates its desired output by summing up a large number of on-off pulses of DC directed to the proper windings as the motor turns.
The DC is obtained by rectifying the input AC. That means that the DC available will not be more than the peak of the input AC waveform. For 208V AC that would be 294V DC.
When you reverse the process, there is no way that the VFD can take that 294V DC and use it to replicate a 240VAC waveform which peaks at 340V DC.

If the motor is not fully loaded, you may be able to get away with the VFD feeding it only 208V or less when it is running. Or maybe not.

You may have to install a boost transformer set on the input of the VFD.
FLA(208) (fully loaded, which you may not have or the motor may not tolerate) will be 240/208 times the 240V FLA.

Bottom line: If the motor can run directly off 208V, then you can run it off the VFD. If not, you probably cannot, unless the motor will be operating at reduced speed and voltage only.
 

Jraef

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There is a good chance the motor will be fine at 208V. Most of them have 240/208V ratings or say "Usable at 208V".
10HP and below, that's just about standard now. The motor nameplate often has a different FLC for running at 208V.

You have to pay attention to the little details. If the VFD is DESIGNED for a 230V motor of the HP you are using, then if the FLC at 208V is higher, the VFD may end up too small and forced into limiting the current at full load, which means limiting the speed you can run.

if you provide details on the motor, load and VFD, we may be able to help more.
 
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