Overload Position with VFD Bypass Contactor

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bwat

EE
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When you have a VFD that has bypass contactors, why is it that the overload is in the circuit after the two circuits (VFD and Bypass) are combined? I think I've always seen it this way. But it seems that you could technically only have the OL in the circuit for the Bypassed/DOL starting circuit, and let the VFD handle the overload method for VFD operation.

I have suspicions of why it is where it is, but I don't want to influence any answers.
 
Consider the case where the motor has been running on the VFD and then the bypass is activated. If the overload is not placed after where the VFD and bypass have beeen combined, then the overload would not have experienced the current under VFD operation. Therefore it would not properly take into account the prior heating of the motor when it was under VFD operation.
 
In addition, if there’s nothing to detect a failure of the bypass contractor to engage, the motor could run on the VFD indefinitely without OL protection.
 
bwat said:
?? I don’t follow.
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It’s not likely, since VFDs are capable of providing OL protection, but if the motor parameters were never set, it could happen.

If the coil fails on the bypass contactor, for example, the motor might run continuously on the VFD.
 
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