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Discussion Starter · #1 ·
Hi everyone,

Just stumbled upon this forum today and it looks amazing. I have a question for a project that I'm on. There's currently a system with a VFD and a Motor but the client wants to isolate the Motor from the VFD for when maintenance personnel want to work on the motor and safely lock out. I read on another thread here that if putting a disconnect after the VFD, you have to wire an auxiliary contact to the disable function of the VFD. I understand that part, it makes sense to me but doesn it matter which type of auxiliary contact? or any contact will do?

Also, why couldn't you just put the disconnect before the VFD? Wouldn't that achieve the same thing without having to wire the additional auxiliary contact?

Any info would be great.

but the original issue is that there is a transfer car that is run by a motor and apparently the maintenance guys went to go work on the transfer car and they just pulled the fuses before the VFD but the transfer car jolted every so slightly during their maintenance and they got freaked out so I'm trying to implement a safe method of locking out with a disconnect. Any info would be awesome.

Thanks!
 

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Hi everyone,

Just stumbled upon this forum today and it looks amazing. I have a question for a project that I'm on. There's currently a system with a VFD and a Motor but the client wants to isolate the Motor from the VFD for when maintenance personnel want to work on the motor and safely lock out. I read on another thread here that if putting a disconnect after the VFD, you have to wire an auxiliary contact to the disable function of the VFD. I understand that part, it makes sense to me but doesn it matter which type of auxiliary contact? or any contact will do?

Also, why couldn't you just put the disconnect before the VFD? Wouldn't that achieve the same thing without having to wire the additional auxiliary contact?

Any info would be great.

but the original issue is that there is a transfer car that is run by a motor and apparently the maintenance guys went to go work on the transfer car and they just pulled the fuses before the VFD but the transfer car jolted every so slightly during their maintenance and they got freaked out so I'm trying to implement a safe method of locking out with a disconnect. Any info would be awesome.

Thanks!

Resolution:
A disconnect can be used between the drive and the motor. This is general practice to assure that power does not reach the motor during maintenance and other non-operating times. Note that the disconnect should not be opened or closed while the VFD is running (outputting voltage from T1,T2,T3). This is sometimes referred to as "slamming the motor." This can damage the output IGBT components of the drive and such damage is not covered under warranty. We recommend adding a fast-acting auxiliary contact on the disconnect that will trigger a freewheel stop on the drive before the main 3 phase contacts open. Refer to your drive's Programming Manual for instructions on programming a logic/digital input as freewheel stop.
 

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Discussion Starter · #3 ·
Resolution:
A disconnect can be used between the drive and the motor. This is general practice to assure that power does not reach the motor during maintenance and other non-operating times. Note that the disconnect should not be opened or closed while the VFD is running (outputting voltage from T1,T2,T3). This is sometimes referred to as "slamming the motor." This can damage the output IGBT components of the drive and such damage is not covered under warranty. We recommend adding a fast-acting auxiliary contact on the disconnect that will trigger a freewheel stop on the drive before the main 3 phase contacts open. Refer to your drive's Programming Manual for instructions on programming a logic/digital input as freewheel stop.
Ya i saw that from the Schneider website but what is a fast acting auxiliary contact? and why can't I put the disconnect before the VFD?
 

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Ya i saw that from the Schneider website but what is a fast acting auxiliary contact? and why can't I put the disconnect before the VFD?
JReaf is the drive expert around here, amongst others to ask. Usually the disconnect is near the motor, and the drive is in a control cabinet somewhere.

"what is a fast acting auxiliary contact"- I think it's an aux contact in the disconnect that will actuate first, before that main handle will disconnect the motor load.
 

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Discussion Starter · #7 ·
JReaf is the drive expert around here, amongst others to ask. Usually the disconnect is near the motor, and the drive is in a control cabinet somewhere.

"what is a fast acting auxiliary contact"- I think it's an aux contact in the disconnect that will actuate first, before that main handle will disconnect the motor load.
that helps, thanks! This is a project in Chile so I have to find out what the electrical codes are when it comes to motor disconnect placements.
 

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You could but repeated cycle of line power to a VFD isn't recommended.
Aww, shucks. Just put a sign on the disconnect warning of this and I’m sure all of the operators and maintenance people will pay attention to it.

Addendum #1: Put it in several languages to make the sign look more official.


more
 

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Discussion Starter · #11 ·
with a disconnect before vfd, there will be voltage for 2-5 min after drive shutoff (time for discharging caps) so disconnnect must be at motor for isolation
that explains the original sudden movement the maintenance guys found then when they went to go work on it. They took out the fuses before the VDF, locked out, went to go work on the motor and the motor moved on them suddenly. so that residual voltage at he caps explains it all.

You guys are the best!
 

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Two things
First the Aux contacts are I think called "make before brake" or " brake before brake" contacts.

Second Check if there is a mechanical brake in the circuit that may of caused the motion, pulling the fuses is no different than opening a disconnect, except for drive power for safety reasons not motion reasons.

Read this https://www.mennekes.com/pdf/white-papers/MENNEKES_WP_MotorDisconnects.pdf

Cowboy
 

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On knife switch type disconnects, ALL aux contacts are going to change state before the main contacts, its a requirement of the NEMA KS-1 and UL98 standards for that design. On the IEC style rotary disconnects that are listed under UL508, that is not always true so you must check.


What the others have said is all true; if you put it ahead of the VFD, you run the risk of causing long term problems by cycling power to the VFD too often. In addition, the motor disconnect usually has to be within sight of the machine / motor.
 

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I pretwist and then use wire nuts. Solder pots rule.
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Like @JRaef has told us many times before, when you connect the aux switch that's inside the disconnect to the drive, Insure you have landed it on the proper terminal. Also Insure the drive is programmed to act upon the open contact properly.
He has mentioned to us before that not all drives react the same nor do they all call the stop command the same.
 

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with a disconnect before vfd, there will be voltage for 2-5 min after drive shutoff (time for discharging caps) so disconnnect must be at motor for isolation
None of the motors we've got on drives have a disconnect at the motor. Off the top of my head, I don't know that I've seen one on other locations.For isolations, we open the breaker feeding the drive and lock it out for the duration of the work. They (mostly PF700's) get shut off and isoalted at least twice a year to replace the auto greasers on the fans mainly with no issues.
 

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None of the motors we've got on drives have a disconnect at the motor. Off the top of my head, I don't know that I've seen one on other locations.For isolations, we open the breaker feeding the drive and lock it out for the duration of the work. They (mostly PF700's) get shut off and isoalted at least twice a year to replace the auto greasers on the fans mainly with no issues.
Weird all 200+ of them i have worked on have a disconnect and some even have the aux contacts.
 
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