Motor fed from MCC changing to VFD

[Designer69]

I have some existing 1HP and 5HP motors that are fed from starters in individual MCC buckets.

Now they are replacing the motors and the new ones will have VFDs.


Is standard convention to replace the starter buckets with just circuit breakers? (Put a brand new breaker bucket in or rip out the starter components and just mount a new breaker in?)

The VFDs will also have local disconnects.

Replace the existing feeder cables?

Thank You

 

[petersonra]

I have some existing 1HP and 5HP motors that are fed from starters in individual MCC buckets.

Now they are replacing the motors and the new ones will have VFDs.


Is standard convention to replace the starter buckets with just circuit breakers? (Put a brand new breaker bucket in or rip out the starter components and just mount a new breaker in?)

The VFDs will also have local disconnects.

Replace the existing feeder cables?

Thank You

It is hard to tell you the best choice for your situation. If your existing buckets have fuses or MCCBs chances you can remove (or just not use) the contactor and feed the new VFD from the fuses or MCCB. If it has an MCP you will probably need to remove the MCP and replace it with a MCCB.

As for the "feeder" cables, presumably you mean the motor branch circuit conductors. I might or might not replace them with VFD cables. It is a somewhat complicated decision that mixes in technical factors and cost factors. Keep in mind that despite what the VFD manufacturers and VFD cable manufacturers tell you, my guess is there are ten times as many VFDs wired up with normal wire as with VFD cables and they generally work just fine.

it may be more cost effective to just remove the buckets and replace them with one or more buckets that just have MCCBs in them to supply the new VFDs. that might well result in you gaining a few spare spaces.

 

[ptonsparky]

Another thread touches on multiple VFD outputs in the same conduit. In short don’t do it. I would guess they shouldn’t be bundled in a wireway or gutter either.

For us it is less expensive to order a MCC column the way we want it as if it is a new install vs order just the buckets.

 

[Jraef]

But definitely don’t leave the starters there and feed the VFDs from the starters. It’s OK to cycle power to VFDs with a contactor or breaker occasionally, but doing it all the time will eventually damage the VFD.

As to replacing the branch conductors out to the motor, there is another thread on this that I suggest you read. But I never advocate ripping out existing conductors if they are fine, you already own those so you may as well get some remaining value. But if they do fail, replace them with RHH/RHW or XHHW conductors.

If however these existing X-Line starter outputs are in cable tray or PVC conduit, you WILL need to use shielded VFD cables for the output from the VFD to the motor. That’s absolute. You need shielding on those cables, either steel conduit (properly grounded) or shielded cable, shields grounded at BOTH ends (unlike signal shields).