fuses between VFD and motor

Hi All,
I've inherited a scenery elevator with a 480v 40HP induction motor which is VFD controlled. In the machinery pit, there is a 3ph, 60A FUSED disconnect between the motor and the drive. I presume it's there to satisfy the requirement for local disconnect for maintenance. I have no problem with that (though I take issue that there's no AUX contact in the disconnect to inhibit the drive when it's off - I'm already going to fix that).

I am wondering about the appropriateness of the fuses in the disconnect though. Are they A) necessary and B) a good idea? To my knowledge, there should be no need for fuses as the drive will protect the motor (assuming the drive is set up correctly, which it is).

Should I replace the disconnect with a non-fused version or slug the fuse holders with a non-fuse element? To my mind, there should be no way to drop a leg on the motor, as that might lead to blowing the drive's output (opening under load).

Thoughts? Thanks!


-rpoet

Fuses are always a good idea for protection. Multi motor VFD applications require fusing to each branch.

If the VFD is "modern" at all, the loss of any motor connection will fault the drive.

If not, the fuses are there to protect the motor from same.

Normally I would not ever think of putting fuses on VFD load wires. However, there may be special codes associated with an elevator, I don't know. More likely someone had a fused disconnect instead of a nonfused one and just used it. I suggest you check with your AHJ just in case there is some special requirement for elevators.

I didn't know Ashland had any buildings high enough for elevators - beautiful town though.

CalG said:

Fuses are always a good idea for protection. Multi motor VFD applications require fusing to each branch.

If the VFD is "modern" at all, the loss of any motor connection will fault the drive.

If not, the fuses are there to protect the motor from same.

Click to expand...

The drive is modern; it's a Altivar 58, installed about 10 years ago. I agree that the drive will sense a fault, but what possible protection could the fuses provide that the drive doesn't already? The drive's going to react faster than any fuse (with the possible exception of a semiconductor fuse).

I worry about the possibility of a fuse opening below its rating due to aging, and fraging the output stage of the drive because it's being disconnected under load.


-rpoet

TConnolly said:

Normally I would not ever think of putting fuses on VFD load wires. However, there may be special codes associated with an elevator, I don't know. More likely someone had a fused disconnect instead of a nonfused one and just used it. I suggest you check with your AHJ just in case there is some special requirement for elevators.

I didn't know Ashland had any buildings high enough for elevators - beautiful town though.

Click to expand...

I will have a chat with the AHJ - we're on good terms with our fire marshal - and see if there are code issues that I don't know about.

The history I've found out so far is that the elevator vendor didn't do the electrical install. IT was handled by the local EC, and I'm sure he had a fused disconnect on hand, so he used it. I just want to know if it was the best choice; if this elevator quits, we will probably loose a couple of shows at least, since we can't get the repertory scenery out of storage in the basement. No one's sure exactly what to call this thing; I guess technically, it's a freight elevator, but it's man-rated with all the appropriate safety equipment installed. But it might be classified as "stage machinery." It's a strange beast to be sure.

Most of the elevator is actually underground, as our newest theater was partially built into a hill. It's a little James Bond Villian-esque in the lower levels.

TConnolly, if you're ever passing through Ashland, drop me a line. I'd love to give a backstage tour at OR Shakespeare Festival if you're interested. I do the electrical design and fab for our Scenery Automation dept.


-rpoet

I just got confirmation from the elevator vendor - they don't want fuses either. Looks like there are some dummy fuse slugs in my future.


-rpoet

I may just do that. I have family in Medford, so I'm up that way somewhat regularly.

TConnolly said:

I may just do that. I have family in Medford, so I'm up that way somewhat regularly.

Click to expand...

Sounds good. First beer is on me.


-rpoet

Generally, when disconnects are located in the drive-to-motor leads, there is a pre-action switch on the handle which goes back to the drive's Stop circuit or Run Enable circuit. This is to shut off the drive output before the main contacts open. The Altivar drive you mention is old enough to probably blow its IGBT's if the motor leads were opened under load. Also, there would be a nasty flash in the disconnect. I'd install the pre-action switch just to be sure. But no fuses!

DickDV said:

Generally, when disconnects are located in the drive-to-motor leads, there is a pre-action switch on the handle which goes back to the drive's Stop circuit or Run Enable circuit. This is to shut off the drive output before the main contacts open. The Altivar drive you mention is old enough to probably blow its IGBT's if the motor leads were opened under load. Also, there would be a nasty flash in the disconnect. I'd install the pre-action switch just to be sure. But no fuses!

Click to expand...

Thanks for the confirmation, DickDV! I'm sure the disconnect wouldn't be operated with the motor energized, as it's below the elevator in the machinery pit - not the place you want to be when the elevator is moving. It would only be used during service and maintenance for lockout tag-out.

I will wire the AUX contact into the e-stop circuit that cuts power to the drive when tripped. It's not the way I'd design a control cabinet (the control PLC cycles input power to the drive through the main contactor whenever someone runs the elevator - dumb, I know. I'm surprised the drive has lasted 10+ years like that), but it'll better than what it is now.

I'll ask my supplier if there's an early-action option on that AUX contact kit. Thanks for the recommendation. The fuses are on the way out too. Now to convince the bean counters.


-rpoet

DickDV said:

The Altivar drive you mention is old enough to probably blow its IGBT's if the motor leads were opened under load. Also, there would be a nasty flash in the disconnect.

Click to expand...

Why would they blow? How are older IGBT's different than newer type? Newer types won't blow?

Nothing will "BLOW" on an inadvertent/ unplanned disconnect.

But a rapid reconnect of a running motor into an unfaulted VFD can burn tiny holes inside semiconductors that can add up over time.

If the VFD has fly start provision, starting into a spinning motor won't be any problem either.

VFD's have so may parameters these days .......mind boggling!

There is a long history of IGBT failure when the motor leads are opened under load. I suspect that the voltage rating of the IGBT's is exceeded when the contacts first open and the flash occurs.

What has changed on newer AC drives is the flyback protection. Modern IGBT protection is faster and less likely to fail under flash conditions itself. Since the protective devices are in parallel across the IGBT's, it makes no difference if the IGBT or the protection shorts. Either way, the drive is toast unless it is large enough to be repaired.

Some manufacturers still advise against opening the motor leads in their instruction manuals but most are now tolerating the practice.

If you manage to open a contactor on a running motor and the drive is set up in V/hz or Scalar mode (not Sensorless or Flux Vector), the drive may continue to run on as if the motor was still connected. But reclosing will result in a fault every time as the drive cannot deal with the starting inrush current going into the stationary motor. Flying Start doesn't help because the drive is already running and up to speed.

And, yes, I certainly agree that I wouldn't want to be in an elevator pit with the elevator running! That just may be an application where a preaction switch on the disconnect is not warranted.

Many old elevators did not have variable speed drives (not invented yet). It could be that the fused disconnect was there long before the VFD was installed, so it was easier and cheaper to leave it in place when the elevator was upgraded.

Now you all have got me wondering about something: In the case of a single VFD upstream of multiple motors, what problems would there be with opening the contactor or disconnect for just one motor while the drive is running?

Example: Four fan cooling tower, one fan to be shut down for maintenance, other three to continue running. Or, one fan motor trips the overloads, the other three to continue running.

What happens if the motor is placed back online without stopping the drive?