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Discussion Starter · #1 ·
I have a customer with drives on three intake and three exhaust fans on a roof. They've asked me to install local disconnects for the motors. I know that disconnects on the output of a drive is not an ideal situation. The current setup is a dedicated emt conduit with THHN to each fan, about 150'.

I'm considering pulling in 600v shielded cable with the 480v to each disconnect using an aux contact to an enable input in the drive. I'm trying to do this as correctly as I can under the circumstances while trying not to smoke the drives in the process. A separate control conduit would be ideal, but the customer said if I have to run another conduit, they'll just scrap the disconnect idea.

What do you guys think?
 

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It is quite common on factory machinery for VFD-controlled motors to have maintenance disconnects fitted between drive and motor. The solution is to use an early-break aux contact to signal the switching ahead of the break, this allows the drive to stop first.
 

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It is quite common on factory machinery for VFD-controlled motors to have maintenance disconnects fitted between drive and motor. The solution is to use an early-break aux contact to signal the switching ahead of the break, this allows the drive to stop first.
Do they make disconnects with that switch built in or do you have to ghetto rig a limit switch in there?
 

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sparky970 said:
Hubbell makes one
Ive used it. It's pretty chincy but it does the job. I don't think it's available over 100 amps either
 

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I have a customer with drives on three intake and three exhaust fans on a roof. They've asked me to install local disconnects for the motors. I know that disconnects on the output of a drive is not an ideal situation. The current setup is a dedicated emt conduit with THHN to each fan, about 150'.

I'm considering pulling in 600v shielded cable with the 480v to each disconnect using an aux contact to an enable input in the drive. I'm trying to do this as correctly as I can under the circumstances while trying not to smoke the drives in the process. A separate control conduit would be ideal, but the customer said if I have to run another conduit, they'll just scrap the disconnect idea.

What do you guys think?
As much as a properly installed disconnect should add very little resistance to the circuit, why are your sources saying not to do it? Also, I believe that it is the law of the land anyway.
 

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RIVETER said:
As much as a properly installed disconnect should add very little resistance to the circuit, why are your sources saying not to do it?
it's not forbidden in practice but it makes it too easy for someone to smoke the drive by throwing the disconnect under load
 

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Discussion Starter · #9 ·
it's not forbidden in practice but it makes it too easy for someone to smoke the drive by throwing the disconnect under load
That's exactly why. And that's what they plan on doing.

As I understand it, they want to leave the fans running, climb up on the roof and then shut the disconnect off with the drive running. Do whatever they are planning to do and then throw the disconnect back on with the drive still running.

I'm just trying to not kill the drive.

I'm hoping to use a Hubbell HBLDS3AC unless someone else has a better choice?

http://www.hubbellcatalog.com/wiring/section-c-datasheet.asp?FAM=Switches&PN=HBLDS3AC

It looks like they make a "break before break" aux contact for it too:

http://www.hubbellcatalog.com/wiring/section-b-datasheet.asp?FAM=Locking_Devices&PN=HBLAC2

So would a regular aux contact work or is a "break before break" aux contact a must?
 

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it's not forbidden in practice but it makes it too easy for someone to smoke the drive by throwing the disconnect under load
Hire people who know what they are doing. If you cannot, then lock the switch on. It is only for maintenance...anyway..
 

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RIVETER said:
Hire people who know what they are doing. If you cannot, then lock the switch on. It is only for maintenance...anyway..
I don't have that problem. I'm only trying to be helpful to someone who might. I like the lock idea though
 

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That's exactly why. And that's what they plan on doing.

As I understand it, they want to leave the fans running, climb up on the roof and then shut the disconnect off with the drive running. Do whatever they are planning to do and then throw the disconnect back on with the drive still running.

I'm just trying to not kill the drive.

I'm hoping to use a Hubbell HBLDS3AC unless someone else has a better choice?

http://www.hubbellcatalog.com/wiring/section-c-datasheet.asp?FAM=Switches&PN=HBLDS3AC

It looks like they make a "break before break" aux contact for it too:

http://www.hubbellcatalog.com/wiring/section-b-datasheet.asp?FAM=Locking_Devices&PN=HBLAC2

So would a regular aux contact work or is a "break before break" aux contact a must?
Sorry, you can't disconnect the motor while the drive is running. The point of the early break contact is to signal the drive to stop (using the Stop input on the drive) before the disconnect opens, so it doesn't destroy itself. When it comes to speed, the electronics in the drive are heaps faster than the mechanical switch contacts, so the drive output will always go off first.
It might be possible to set the drive to restart the motor when the disconnect is closed again, but there may also be safety factors to consider that would indicate a Start button on a console at ground level is a safer option.
The aux contacts for disconnects are sold as add-ons by the switch manufacturers. The main manufacturers all offer EB versions of their aux contacts.
 

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Sorry, you can't disconnect the motor while the drive is running. The point of the early break contact is to signal the drive to stop (using the Stop input on the drive) before the disconnect opens, so it doesn't destroy itself. When it comes to speed, the electronics in the drive are heaps faster than the mechanical switch contacts, so the drive output will always go off first.
It might be possible to set the drive to restart the motor when the disconnect is closed again, but there may also be safety factors to consider that would indicate a Start button on a console at ground level is a safer option.
The aux contacts for disconnects are sold as add-ons by the switch manufacturers. The main manufacturers all offer EB versions of their aux contacts.
That's exactly what the original poster is saying...
 

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Some drives are OK with the switch being OPENED under load, but NO drive is going to withstand being CLOSED while running. You must use that interlock.

Several of the companies selling the "VFD" cable offer a version that has the 3 shielded power conductors and ground, and also has a separately shielded set of 2 conductors specifically embedded in the bundle for this purpose too. That way you do NOT need to run a separate conduit for the signal wires.

This is a picture from Belden, the one I'm referring to is on the far left. Only goes up to #10 power conductors though, you don't say what size HP you have. But if you use a shielded pair for the signal, you shouldn't have to run separate conduit anyway.

 

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Discussion Starter · #17 ·
Some drives are OK with the switch being OPENED under load, but NO drive is going to withstand being CLOSED while running. You must use that interlock.
That's the way I've understood it as well.

Several of the companies selling the "VFD" cable offer a version that has the 3 shielded power conductors and ground, and also has a separately shielded set of 2 conductors specifically embedded in the bundle for this purpose too. That way you do NOT need to run a separate conduit for the signal wires.

This is a picture from Belden, the one I'm referring to is on the far left. Only goes up to #10 power conductors though, you don't say what size HP you have. But if you use a shielded pair for the signal, you shouldn't have to run separate conduit anyway.

Thanks for the pic, I've used the VFD cable before but didn't realize they had a style with a separate shielded pair as well. Good to know.
 
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