# Motor Disconnect before VFD or between VFD and Motor?



## dronai (Apr 11, 2011)

Derman0524 said:


> 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?
> 
> ...



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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## Derman0524 (Feb 19, 2020)

dronai said:


> 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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## tates1882 (Sep 3, 2010)

AB has VFD rated discos that have aux contacts to send a stop signal to VFD.


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## tates1882 (Sep 3, 2010)

Derman0524 said:


> 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?


You could but repeated cycle of line power to a VFD isn't recommended.


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## dronai (Apr 11, 2011)

Derman0524 said:


> 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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## Derman0524 (Feb 19, 2020)

dronai said:


> 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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## bill39 (Sep 4, 2009)

​


tates1882 said:


> 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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## Derman0524 (Feb 19, 2020)

tates1882 said:


> You could but repeated cycle of line power to a VFD isn't recommended.


That definitely answers my questions. Thanks!


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## oliquir (Jan 13, 2011)

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


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## Derman0524 (Feb 19, 2020)

oliquir said:


> 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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## just the cowboy (Sep 4, 2013)

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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## gpop (May 14, 2018)

I agree with cowboy you probably have a brake or a gearbox back stop.


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## CMP (Oct 30, 2019)

A sample of a Schnider aux contact
https://www.se.com/us/en/product/MDSAN20/md-motor-disconnect-switch,-auxiliary-contact-module,-2-no/


Instruction Sheet
https://download.schneider-electric...ile_Name=IS+536776_01.pdf&p_Doc_Ref=IS+536776











Disconnect Type
*MD Enclosed Disconnect Switch*



HD Safety Switch Type


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## JRaef (Mar 23, 2009)

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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## Wirenuting (Sep 12, 2010)

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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## glen1971 (Oct 10, 2012)

oliquir said:


> 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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## gpop (May 14, 2018)

glen1971 said:


> 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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## glen1971 (Oct 10, 2012)

gpop said:


> Weird all 200+ of them i have worked on have a disconnect and some even have the aux contacts.


Maybe it's a spec thing? Just another piece of the electrical world that is different from place to place.


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## Wirenuting (Sep 12, 2010)

I add the interlocks all the time to disconnects that are down stream of a drive.


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## paulengr (Oct 8, 2017)

If you don’t have a disconnect for the VFD how do you work on it?

The only issue I commonly see with VFD front ends is that the input section is almost always supposed to be protected by fast fuses or a fast breaker. There are two situations where this matters. On some drives if you have a large enough transformer feeding it and you get a fault on the load side, the drive front end is destroyed. On others the concern is “minimizing” damage with a fault on the front end.

Second as to power in the caps it’s DC. The danger here is on the DC bus for electricians. Once you get above about 100 HP or so there is enough energy to theoretically burn flesh and break bones. DC does not induce fibrillation, And the light is not a reliable indicator of no charge. Usually this is not a problem. The low voltage signals are physically located away from power and the DC bus on most VFDs is fairly well protected to where you have to make an effort to touch it. It takes seconds to verify with a multimeter and if anyone is reading so deep into codes and specs to put an output disconnect in they probably should be aware that it is a major regulatory violation not to test anything electrical before touching it (test before touch). Plus it’s just plain common sense, whether it has an idiot light or not.

Something I often do if the drive isn’t programmed to shut off on loss of line power is to purposely start a drive when I need to work on it. This puts the remaining energy into the motor instead of waiting for the VFD to “die”. As many times as i have done this though I have never seen the motor actually move. There just isn’t enough power there to do anything mechanical. What this sounds like is urban legend or some idiot engineer.

In a three phase VFD one phase is almost always “on” so the DC link is quite small. It only has to be a little larger than the inductance of the motor (kvars). If I start it from dead stop with no line power I have just enough energy to flux up the motor before the DC link energy is depleted. So not even enough power to run for one cycle. This sounds like pure urban legend. 

There is nothing preventing the DC link from being oversized other than pure economics but I’ve only done it once. We tried to purposely generate leading power factor but in the end the line reactor on this drive (12 MW) was undersized and we had to sacrifice the leading power factor to compensate at peak output (modulation index issue).


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