Single VFD controlling two motors

[ptonsparky]

If it's bad to disconnect a running motor from a VFD, why isn't it risky to have overload protection between them?

The overload protection should open the Run command to the drive.

 

[petersonra]

If it's bad to disconnect a running motor from a VFD, why isn't it risky to have overload protection between them?

Because the aux contact from the ol is wired into the VFD enable. It results in both motors shutting off if either motor overloads, which is one of many reasons I do not like it.

If you use a mpcb, conceivably you could not wire the aux contact to the drive as the mpcb itself will open

 

[Jraef]

What would the response of a newer VFD be if one of the fuses blew? An orderly shut down with a fault code of some sort?

Not all VFDs have protection for output phase loss, so there is no guarantee that the drive would protect the motor from single phasing. But also, the act of blowing a fuse involves drawing an arc across the fuse element parts until that arc is quenched. Arcing on the output of a VFD runs a risk of damaging the transistors from dV/dt (rapid increase in voltage). While this is not as critical in modern drives as it once was, there's also no guarantee that any particular drive has what it takes to prevent this damage, making it a bad practice. It's kind of like saying it's OK to run your car engine with the oil a quart low for a while; some cars will be OK with that, some will not, so it's a good idea not to do it..

 

[garbo]

If it's bad to disconnect a running motor from a VFD, why isn't it risky to have overload protection between them?

The overload are usually wired in series with the stop circuit so overload trip turns off the electronics. Shutting off a motor disconnect while a VFD is running can damage the drive. Had a contractor use the outdoor 480 volt non fused safety switches to turn four 75 HP cooling tower fans on & off rather then walk down to the drives in third basement level ( over 75 steps climb ) while performing maintenance. First visit he did no damage but next trip he blow up a board on one drive and something else on another drive. The Danfoss drive tech said some times you can disconnect a running motor maybe 8 or even ten times before luck runs out. Should be a NEC article to demand that all disconnects on output side of drives have a Normally Closed switch that opens before the motor leads. Years ago I came across them on a lot of drive safety switches but not one in last ten years or so.

 

[garbo]

Not all VFDs have protection for output phase loss, so there is no guarantee that the drive would protect the motor from single phasing. But also, the act of blowing a fuse involves drawing an arc across the fuse element parts until that arc is quenched. Arcing on the output of a VFD runs a risk of damaging the transistors from dV/dt (rapid increase in voltage). While this is not as critical in modern drives as it once was, there's also no guarantee that any particular drive has what it takes to prevent this damage, making it a bad practice. It's kind of like saying it's OK to run your car engine with the oil a quart low for a while; some cars will be OK with that, some will not, so it's a good idea not to do it..

I always ordered VFD'S with a built in disconnect and made sure they had fuses on input side. Had an input fuse blow while drive was running but never did any damage to the drive. At my hospital had the B phase blow on an Critical AHU return fan. They used the A & C phase to power control transformer to control board. Fan was only running maybe at 60% of full speed. They would not allow me to turn power off for a minute to replace the fuse until around 4 AM the next morning. I went into the parameters and reset the maximum speed to think 60% until I replaced the fuse. The two supply fans provided enough air to not set off any alarms. A three phase 480 volt drive normally produce around 660 volts DC . Of course a VFD with a blown fuse would never be able to run any where near full speed.