VFDs...breaking the load side.

[iwire]

Hey can anyone tell me why it is bad to open the load side of a VFD while operating?

I recall it has been posted here before but I can not find it.

Thanks, Bob

 

[peteo]

"While operating" is the issue, if I understand the question. Contacts are not made for the pulsed discharges out of IGBT modules. If the sparking doesn't get the contacts, it may get the transistors, etc. Even at 60 Hz, contacts versus zero-crossing solid state devices are night versus day, in terms of motor reliability.

That said, there's nothing wrong with running VFD output through relays or such. Y/D switching to 5-30 HP is a good way to get more bang out of a spindle motor on a machine tool. Just need to provide a dead spot (bb, or Base Block for Yaskawa; FRS, or Free Run Stop, Mitsubishi, etc) while you're doing it. Have built many thousands of such units.

 

[benaround]

Bob,

It lets the factory sealed smoke out!

 

[steve66]

I'm not sure what the exact process is. But I think opening the output can damage the output transistors on the VFD. It might be that opening the circuit causes an inductive kick from the motor as it tries to dump its magnetic field. This could create a reverse polarity voltage across the transistors, and maybe that's what fries the transistors.

Or maybe its the inductance and capacitance in the branch circuit wiring itself. The VFD output is already high in high frequency harmonics, and already has very high voltage spikes (or so I've heard). So somehow breaking the circuit causes a spike that can damage the VFD or even the motor insulation.