From panel CB, we run circuit to a VFD on 1st floor and extend the wiere to a non-fused disconnect on roof, then to a motor (also located on roof). If the motor is running and we turn off the disconnect, what will happens to VFD? Do we have a low voltage/signal wiring between motor and VFD so when the power is off at motor, the VFD will do something to avoid damage itself?
VFD Connection and Motor
- Thread starter anbm
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Immediately, probably nothing will happen, at least nothing you can see. But opening a circuit while a VFD is running causes high dv/dt to be expressed on the transistors and can incrementally damage them, leading to premature failure.
So yes, a way to avoid that is to use an aux. contact on the disconnect that kills the Run command* to the VFD before the main contacts open. If the VF is off, no damage will occur.
A better way to do this, if the VFD has this feature, is to use that disconnect aux. to feed a VFD input set to preform what is called a "base block" on the VFD output transistors. This way, if you have a decel routine that takes place whenever a Run command is released, the Base Block overrides that continued transistor output.
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