# Disconnect ahead of VFD?



## frenchelectrican

Cdn.,

First of all if you really want a isolating or local disconnect switch you should add a interlock microswitch on as well so that way when you hit the local disconnect or isolating switch the microswitch or aux concats will shut the VSD off.

That what I done that in European side.

If you try to disconnect the VSD motour while it still running the voltage spikes may damage the VSD controller I have see it at least couple time when someone try that.

I do not know how strict in your location reguarding of E stop and I know the E stop is NOT a means of disconnecting means. so therefore for true disconnect means just follow as I mention above a isolating switch or local disconnect switch *AVCE* aux concats to trip the E stop at the VSD controller.

I know there are couple other guys in the fourm they will echo simauir comment as my I know two guys for sure they will say something about this.

BBQ are you in the game ?

Merci,
Marc


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## Frank Mc

Hi

Cant say i have had any issues connecting isolators (disconnects) adjacent motors that were controlled by vsd,s....Although i havent used ABB ones to date...

Frank


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## MDShunk

Since I'm mostly an Allen-Bradley guy... I'll respond from an A-B persepective.

Yes, you can absolutely have a disconnect on the load side of a VFD, if you keep certain things in mind. Like the French sparky rightly points out, you MUST have it interlocked with the VFD itself. If you simply disconnect the three line side conductors, you'll toast your VFD. You absolutely need a 4th contact to interlock with the VFD to disable it from trying to run. In Allen-Bradley terms, that would mean installing the "Drive Guard" module in the VFD (standard fare, lately).

I like to use Meneekes IEC-style motor disconnects, which feature a low-current 4th pole as a standard item. Very LOTO friendly also. 
http://www.mennekes.com/product_motordisconnect_features.html
Hubbell makes a suspiciously similar product.


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## BBQ

Ditto what Marc said and here under the NEC a disconnect at the motor is required.


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## cdnelectrician

Thanks for the answers guys, I suppose you could use an IEC type disconnect with an aux contact installed and wired to the stop circuit. I was looking at a brand new tower with a few fans the other day, all danfoss drives and all square D disconnects at the tower which definitely did not have aux contacts.


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## McClary’s Electrical

Marc, you're an AB guy? When's the last time you saw any of these?












We installed 8 of these about 10 years ago, all with remote disconnects and interlocked.


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## Mike in Canada

cdnelectrician said:


> Thanks for the answers guys, I suppose you could use an IEC type disconnect with an aux contact installed and wired to the stop circuit. I was looking at a brand new tower with a few fans the other day, all danfoss drives and all square D disconnects at the tower which definitely did not have aux contacts.


 You can get aux contacts for most full-size disconnects.
It's not that big a deal to put the disconnect ahead of the VFD... as long as it's lockable then it passes CEC even if it's two miles away from the motor.


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## BBQ

cdnelectrician said:


> Thanks for the answers guys, I suppose you could use an IEC type disconnect with an aux contact installed and wired to the stop circuit.


You could, we just order NEMA stuff with AUX contact kits.


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## John Valdes

I would never install any device between the motor and drive except a load reactor. The slightest imbalance of current or voltage will trip a VFD. Violation or not, it's an issue for the control.


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## BBQ

John Valdes said:


> I would never install any device between the motor and drive except a load reactor. The slightest imbalance of current or voltage will trip a VFD. Violation or not, it's an issue for the control.


It is done all the time without trouble, the AUX contacts we use are early break so the VFD is off before the disconnect opens.


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## Big John

As a rule of thumb, I know it's a bad idea, but what actually happens when you open open the load side of a VFD? Does your bus voltage go through the roof? 

-John


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## Wirenuting

I wish they would install the aux switch on those disconnects. We kill a drive every month or two around here on average. The maintenance and AC guys never secure the drive before opening/ closing the disconnects.

You get a spiked wave that travels back to the drive & motor. It can knock a pin hole in the varnish or blow the drive.


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## micromind

BBQ said:


> Ditto what Marc said and here under the NEC a disconnect at the motor is required.


True, a disconnect is usually required between the controller and the motor, but not between a VFD and a motor. 

See 430.102 (B) exception (a) and FPN No.1


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## JRaef

There are technicalities here that are being over-interpreted and under appreciated a bit.

There is technically no problem _*HAVING *_a disconnect device on the load side of the VFD. What _*IS *_a problem is _*opening a device on the load side*_ _*WHILE UNDER LOAD*_. Opening it when the VFD is powered off is not at all a problem. So that means if you have a known procedure of turning the VFD output off BEFORE you open the load side disconnect, that's OK. What the aux. contacts do is to operate slightly ahead of the main contacts and are wired to a _*disable input*_ on the VFD. That way, by the time the main contacts BEGIN to open on the disconnect, the output of the VFD is already turned off.

But there are a lot of "gotchas" in that, the main one being that many novices do not understand the concept of "off" on a VFD. For example if you have a Decel function enabled, and you put the disconnect aux. contact in the regular start-stop circuit, opening the aux contact will NOT disable the output of the VFD, because it will be trying to DECELERATE the motor. 

So for this to work properly, what you have to do is *find and understand the input commands on the VFD that will completely turn OFF the output transistors*. The problem with this however, is that there are many different VFD mfrs and many different wys of them describing this functionality, which means there are lots of ways to screw it up. Some call is "E-Stop", some call it "Base Block", some call it "Emergency Power Off", etc. etc. etc. Sometimes it's as simple as using a function that would be there for an external overload or thermistor device (because if something is overloaded, it also must kill output power immediately). *It therefore requires a THOROUGH knowledge of the VFD programming for make sure you are doing it correctly. *Then add into the mix that I have seen several cheap brands of VFD that do NOT provide any such function!

So the SAFE BET is to not install a disconnect (or contactor) on the output side unless you are ABSOLUTELY SURE you know what you are doing in programming the VFD.

How it potentially damages the VFD by opening a load side disconnect or contactor when under load is like this:

When you interrupt the flow of current with an air gap device like a disconnect or contactor, there is an arc formed that lasts until there is enough separation between the contacts to extinguish it. While the arc is still active, there is a very rapid increase in the voltage potential across the gap because of what is essentially a capacitor effect. That rapid change in voltage (called dV/dt for *d*elta (change) in *V*oltage over *d*elta in *t*ime) can cause unpredictable effects in the operation of semiconductor devices, such as the transistors on the output side of the VFD. So if some of those transistors fire at the wrong instant, they can cause a short circuit and become damaged. In addition, the dV/dt can cause tiny little "holes" to be punched through the silicon layers of the transistor material. The effects of that are cumulative. So even if you got lucky and the unpredictable effects are not bad, every time you open a switch on the output you are incrementally damaging the transistors and it's only a matter of time before they fail.


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## 76nemo

JRaef said:


> There are technicalities here that are being over-interpreted and under appreciated a bit.
> 
> There is technically no problem _*HAVING *_a disconnect device on the load side of the VFD. What _*IS *_a problem is _*opening a device on the load side*_ _*WHILE UNDER LOAD*_. Opening it when the VFD is powered off is not at all a problem. So that means if you have a known procedure of turning the VFD output off BEFORE you open the load side disconnect, that's OK. What the aux. contacts do is to operate slightly ahead of the main contacts and are wired to a _*disable input*_ on the VFD. That way, by the time the main contacts BEGIN to open on the disconnect, the output of the VFD is already turned off.
> 
> But there are a lot of "gotchas" in that, the main one being that many novices do not understand the concept of "off" on a VFD. For example if you have a Decel function enabled, and you put the disconnect aux. contact in the regular start-stop circuit, opening the aux contact will NOT disable the output of the VFD, because it will be trying to DECELERATE the motor.
> 
> So for this to work properly, what you have to do is *find and understand the input commands on the VFD that will completely turn OFF the output transistors*. The problem with this however, is that there are many different VFD mfrs and many different wys of them describing this functionality, which means there are lots of ways to screw it up. Some call is "E-Stop", some call it "Base Block", some call it "Emergency Power Off", etc. etc. etc. Sometimes it's as simple as using a function that would be there for an external overload or thermistor device (because if something is overloaded, it also must kill output power immediately). *It therefore requires a THOROUGH knowledge of the VFD programming for make sure you are doing it correctly. *Then add into the mix that I have seen several cheap brands of VFD that do NOT provide any such function!
> 
> So the SAFE BET is to not install a disconnect (or contactor) on the output side unless you are ABSOLUTELY SURE you know what you are doing in programming the VFD.
> 
> How it potentially damages the VFD by opening a load side disconnect or contactor when under load is like this:
> 
> When you interrupt the flow of current with an air gap device like a disconnect or contactor, there is an arc formed that lasts until there is enough separation between the contacts to extinguish it. While the arc is still active, there is a very rapid increase in the voltage potential across the gap because of what is essentially a capacitor effect. That rapid change in voltage (called dV/dt for *d*elta (change) in *V*oltage over *d*elta in *t*ime) can cause unpredictable effects in the operation of semiconductor devices, such as the transistors on the output side of the VFD. So if some of those transistors fire at the wrong instant, they can cause a short circuit and become damaged. In addition, the dV/dt can cause tiny little "holes" to be punched through the silicon layers of the transistor material. The effects of that are cumulative. So even if you got lucky and the unpredictable effects are not bad, every time you open a switch on the output you are incrementally damaging the transistors and it's only a matter of time before they fail.


 
Very well said:thumbsup:


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## John Valdes

JRaef said:


> There is technically no problem _*HAVING *_a disconnect device on the load side of the VFD.


You are a very respected guy around her on stuff like this. At least in my opinion. I for one would enjoy a working relationship with you. I am certain I would learn much from you. Now I will stop the praise (brown nosing) and get to my point.

Tell me why you would allow/encourage a knife switch, breaker or fuses on the VFD output? Are you not concerned about the possibility of high resistance connection (Loose contact between the blade and the pressure slot it goes into)? To be specific the blade and the blade holder. I am not sure if I am using the correct terminology. But the slide in connection in the disconnect. Do you understand my point?

Disconnects and contactors are subject to loose connections in the device. This could impact the control as high current or open phase or phase imbalance. 
Drives as you know are very picky about everything going on with the input and the output. You know the slightest most minor fault will be subject to a drive fault and subsequent trip.
I know you have used fuse clamps before. That is the point I am trying to establish. Just think about why fuse holder clamps are used. We were using them long before drives were available. They make them for a reason.
It is the weak link in the chain. The disconnect. Why increase the possible 3:00 am drive trip?

Lastly. Every drive manufacturer I am familiar with would highly discourage the use of the switch in this location. I am also certain the drive warranty could be compromised if the instructions from the manufacturer are not observed.
We know NEC requirements and electronic control requirements can be confusing. Sometimes good ole common sense rules. I believe that's the case in this debate.


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## 76nemo

John Valdes said:


> You are a very respected guy around her on stuff like this. At least in my opinion. I for one would enjoy a working relationship with you. I am certain I would learn much from you. Now I will stop the praise (brown nosing) and get to my point.
> 
> Tell me why you would allow/encourage a knife switch, breaker or fuses on the VFD output? Are you not concerned about the possibility of high resistance connection (Loose contact between the blade and the pressure slot it goes into)? To be specific the blade and the blade holder. I am not sure if I am using the correct terminology. But the slide in connection in the disconnect. Do you understand my point?
> 
> Disconnects and contactors are subject to loose connections in the device. This could impact the control as high current or open phase or phase imbalance.
> Drives as you know are very picky about everything going on with the input and the output. You know the slightest most minor fault will be subject to a drive fault and subsequent trip.
> I know you have used fuse clamps before. That is the point I am trying to establish. Just think about why fuse holder clamps are used. We were using them long before drives were available. They make them for a reason.
> It is the weak link in the chain. The disconnect. Why increase the possible 3:00 am drive trip?
> 
> Lastly. Every drive manufacturer I am familiar with would highly discourage the use of the switch in this location. I am also certain the drive warranty could be compromised if the instructions from the manufacturer are not observed.
> We know NEC requirements and electronic control requirements can be confusing. Sometimes good ole common sense rules. I believe that's the case in this debate.


 
You missed his point John, almost entirely. It's a matter of design, it might work just fine for some and for many years, but he didn't say he encouraged in the least.


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## MDShunk

JRaef said:


> There are technicalities here that are being over-interpreted and under appreciated a bit.
> 
> There is technically no problem _*HAVING *_a disconnect device on the load side of the VFD. What _*IS *_a problem is _*opening a device on the load side*_ _*WHILE UNDER LOAD*_. Opening it when the VFD is powered off is not at all a problem. So that means if you have a known procedure of turning the VFD output off BEFORE you open the load side disconnect, that's OK. What the aux. contacts do is to operate slightly ahead of the main contacts and are wired to a _*disable input*_ on the VFD. That way, by the time the main contacts BEGIN to open on the disconnect, the output of the VFD is already turned off.
> 
> But there are a lot of "gotchas" in that, the main one being that many novices do not understand the concept of "off" on a VFD. For example if you have a Decel function enabled, and you put the disconnect aux. contact in the regular start-stop circuit, opening the aux contact will NOT disable the output of the VFD, because it will be trying to DECELERATE the motor.
> 
> So for this to work properly, what you have to do is *find and understand the input commands on the VFD that will completely turn OFF the output transistors*. The problem with this however, is that there are many different VFD mfrs and many different wys of them describing this functionality, which means there are lots of ways to screw it up. Some call is "E-Stop", some call it "Base Block", some call it "Emergency Power Off", etc. etc. etc. Sometimes it's as simple as using a function that would be there for an external overload or thermistor device (because if something is overloaded, it also must kill output power immediately). *It therefore requires a THOROUGH knowledge of the VFD programming for make sure you are doing it correctly. *Then add into the mix that I have seen several cheap brands of VFD that do NOT provide any such function!
> 
> So the SAFE BET is to not install a disconnect (or contactor) on the output side unless you are ABSOLUTELY SURE you know what you are doing in programming the VFD.
> 
> How it potentially damages the VFD by opening a load side disconnect or contactor when under load is like this:
> 
> When you interrupt the flow of current with an air gap device like a disconnect or contactor, there is an arc formed that lasts until there is enough separation between the contacts to extinguish it. While the arc is still active, there is a very rapid increase in the voltage potential across the gap because of what is essentially a capacitor effect. That rapid change in voltage (called dV/dt for *d*elta (change) in *V*oltage over *d*elta in *t*ime) can cause unpredictable effects in the operation of semiconductor devices, such as the transistors on the output side of the VFD. So if some of those transistors fire at the wrong instant, they can cause a short circuit and become damaged. In addition, the dV/dt can cause tiny little "holes" to be punched through the silicon layers of the transistor material. The effects of that are cumulative. So even if you got lucky and the unpredictable effects are not bad, every time you open a switch on the output you are incrementally damaging the transistors and it's only a matter of time before they fail.


Exactly, which is why I advocate NOT interlocking with the stop terminal, and using something like A-B's Drive Guard module, which is especially designed for this purpose. It's a quarter-size accessory board you install in the drive.


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## BBQ

John Valdes said:


> Tell me why you would allow/encourage a knife switch, breaker or fuses on the VFD output?


Well for me it is because it is clearly code required in all most all cases where the VFD is located remotely from the motor.

In all most all applications the NEC requires a disconnecting means at the motor and if the VFD is back at some MCC s disconnecting means must be installed. 



> Are you not concerned about the possibility of high resistance connection (Loose contact between the blade and the pressure slot it goes into)? To be specific the blade and the blade holder. I am not sure if I am using the correct terminology. But the slide in connection in the disconnect. Do you understand my point?



No more so than I am about a disconnect switch located in any circuit.



> Disconnects and contactors are subject to loose connections in the device. This could impact the control as high current or open phase or phase imbalance.


Of course it could, it could cause problems on the line side of a VFD as well. A chunk of Skylab might fall on it as well. 





> Drives as you know are very picky about everything going on with the input and the output. You know the slightest most minor fault will be subject to a drive fault and subsequent trip.


Isn't that what those trips are for, to protect the drive when things fail?





> Lastly. Every drive manufacturer I am familiar with would highly discourage the use of the switch in this location. I am also certain the drive warranty could be compromised if the instructions from the manufacturer are not observed.



I imagine as long as you install the AUX interlock your warranty will remain.




> We know NEC requirements and electronic control requirements can be confusing. Sometimes good ole common sense rules. I believe that's the case in this debate.


There is nothing 'commonsense' about not providing a disconnecting means at the motor in order to protect the drive or maybe eliminate a false trip.

Protect humans first, drives second.


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## BBQ

micromind said:


> True, a disconnect is usually required between the controller and the motor, but not between a VFD and a motor.
> 
> See 430.102 (B) exception (a) and FPN No.1


I disagree with your interpretation, just because there is a VFD involved does not automatically allow the use of that exception. You would have to demonstrate that adding the disconnect is MORE hazardous than not providing the disconnect.

Considering the addition of an AUX contact to kill the VFD eliminates the hazard associated with havering a disconnect on the load side of a VFD I say that exception is almost worthless a far as VFDs are concerned.


In some locations exception (b) might be able to be applied.


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## Wirenuting

I don't like using the exception for VFD's. To many of my co-workers walk in and just turn things back on. They don't use LOTO and don't seem to know any better. 
But I can't get management to add the aux switches. They claim a cost factor. But yet we replace drives non-stop.


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## Jlarson

BBQ said:


> I disagree with your interpretation


I too disagree




BBQ said:


> Considering the addition of an AUX contact to kill the VFD...


I like to put in aux contacts and reinforce the fact that the disconnect is not a normal freaking stop switch. Trying to get that idea through some people's heads is not easy though.


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## Mike in Canada

Jlarson said:


> I like to put in aux contacts and reinforce the fact that the disconnect is not a normal freaking stop switch. Trying to get that idea through some people's heads is not easy though.


 If this is an at-all dangerous area then there will be an e-stop within a reasonable distance, so put a lock on the disconnect, and put the keys for all of the disconnects in the MCC so they have to have it shut down to open the MCC to get the keys to close the disconnects.
It won't work in all cases, but it might be workable with adaptation.


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## BBQ

Wirenuting said:


> But I can't get management to add the aux switches. They claim a cost factor. But yet we replace drives non-stop.


:laughing:

Install auxiliary switch - One time cost of maybe $500-$1000


Replacing drives forever - Priceless. :laughing:


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## BBQ

Jlarson said:


> I like to put in aux contacts and reinforce the fact that the disconnect is not a normal freaking stop switch.



That is the sign you need to put on the disconnect.


"This is not a freaking stop switch"​







.


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## Jlarson

Mike in Canada said:


> If this is an at-all dangerous area then there will be an e-stop within a reasonable distance, so put a lock on the disconnect, and put the keys for all of the disconnects in the MCC so they have to have it shut down to open the MCC to get the keys to close the disconnects.
> It won't work in all cases, but it might be workable with adaptation.


No, they'd unlock them all eventually. Mental re training is the only way.


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## Jlarson

BBQ said:


> That is the sign you need to put on the disconnect.
> 
> 
> "This is not a freaking stop switch" ​
> 
> 
> 
> 
> 
> 
> 
> 
> .


I wrote that on a few before. I removed the "rea" in freaking and substituted in a "uc" though.:laughing:


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## BBQ

Jlarson said:


> I wrote that on a few before. I removed the "rea" in freaking and substituted in a "uc" though.:laughing:


The real problem is that we tell them it is not a stop switch and they look at us like we are crazy and say 'But it stops it'. :laughing:


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## Wirenuting

BBQ said:


> :laughing:
> 
> Install auxiliary switch - One time cost of maybe $500-$1000
> 
> Replacing drives forever - Priceless. :laughing:


I'm glad that I'm "Dumb like rock" , and can't see the bigger picture. 

You would think that a few $$ = less then priceless?


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## Jlarson

BBQ said:


> 'But it stops it'. :laughing:


Following up with something like "well, jamming a monkey wrench in there would stop it too but you don't do that..." might work.


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## BBQ

Jlarson said:


> Following up with something like "well, jamming a monkey wrench in there would stop it too but you don't do that..." might work.


Yeah. :laughing:

Driving my truck into a light pole would stop it but that does not mean light poles are brakes. :laughing:


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## John Valdes

BBQ said:


> Well for me it is because it is clearly code required in all most all cases where the VFD is located remotely from the motor.
> In all most all applications the NEC requires a disconnecting means at the motor and if the VFD is back at some MCC s disconnecting means must be installed.
> 
> *I understand about the safety issue and NEC requirement. *
> 
> No more so than I am about a disconnect switch located in any circuit.
> 
> *Then you nothing about the output of a VFD. Have you ever witnessed a drive output on a scope? You don't have to live with the drive and all of it's possible and perceived problems. Your customer does. *
> 
> Of course it could, it could cause problems on the line side of a VFD as well. A chunk of Skylab might fall on it as well.
> 
> *I guess we could make jokes about a real issue concerning drives and other electronic equipment. Of course we need a switch in front of the control. How else would we be able to kill the power and lock it out?*
> 
> 
> Isn't that what those trips are for, to protect the drive when things fail?
> 
> *Sure. But I for one, would rather sleep all night than have to go into the plant at 3:00 am to reset a fault.* *Your job is to install and leave. My job was to baby sit this equipment when you are long gone.*
> 
> 
> I imagine as long as you install the AUX interlock your warranty will remain.
> 
> *Warranties are subject to the person doing the warranty inspection, and customer explanation. I would not withhold warranty benefits solely for the use of the disco or any other safety device.
> Manufactures want to work with their customers. But if the instructions discourage the use of any switching device on the load side of the drive, then it should be discussed prior to the installation as to the need for the switch.
> *
> 
> There is nothing 'commonsense' about not providing a disconnecting means at the motor in order to protect the drive or maybe eliminate a false trip.
> 
> Protect humans first, drives second.


*Agree. Maybe it makes more sense to install the control where it is visible. I understand drives are installed in remote locations and classified locations would be a good example.

Clearly this is a debate between disciplines. Electronic people tend to look at things differently than a construction electrician would. Someone that relies on a license to operate must follow the rules (NEC).
But in the real world this is not always the case with plant personnel. I am not saying they are right, I am saying they are doing what they were taught.
I am caught in the middle. I have a duty to uphold the NEC and the forethought to install as to minimize downtime.
*


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## sparkymcwiresalot

It's pretty standard on the systems my company installs to always have a local disconnect at all motors. There is always an aux contact in the disconnect that is an input back. I didn't realize that it was such a big deal to disconnect a vfd under load. Granted I rarely deal with any motor over 3hw.


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## nitro71

Is there anything code about using the breaker as the disconnect if it's qualified persons servicing the gear? 

Seems to me that have VFD's fail from opening a disconnect at the motor is a operational, training, procedural problem. If I was in charge of a place I'd have the disconnects labeled with a warning about securing the VFD first. Smoke test a VFD and you'd be on the short list for the next round of lay offs.


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## don_resqcapt19

This is an issue when we try to use electronic equipment in the real world....the electronic equipment needs to adapt to the real world...not the other way around. I see no way that the motor disconnect exception would apply just because you installed a drive and not a starter.


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## BBQ

John Valdes said:


> Then you nothing about the output of a VFD. Have you ever witnessed a drive output on a scope? You don't have to live with the drive and all of it's possible and perceived problems. Your customer does.


John you are the only person I have ever seen be concerned that an untouched knife switch will trip a VFD.

I don't believe it will regardless of the fact I have never put a scope on the output of a VFD.




> I guess we could make jokes about a real issue concerning drives and other electronic equipment.


I don't see the 'real issue' there are thousands of drives installed with disconnects on the output side and as long as they are equipped with aux switches I do not believe they are causing problems to the drives.


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## John Valdes

BBQ said:


> John you are the only person I have ever seen be concerned that an untouched knife switch will trip a VFD.
> I don't believe it will regardless of the fact I have never put a scope on the output of a VFD.
> I don't see the 'real issue' there are thousands of drives installed with disconnects on the output side and as long as they are equipped with aux switches I do not believe they are causing problems to the drives.


When special VFD cable is used, does it make any sense to interrupt the run?
A disconnect adds 9 added points of connection to fail or cause performance issues.

3 - input conductor terminals
3 - output conductor terminals
3 - Mechanical switching parts (blades).

We could argue this all day long. The NEC says you need one. So you must use one.


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## JRaef

There are situations in MANY companies where the issue of an air-gap disconnecting means is a *non-negotiable requirement*; period. Ford Motor Co. comes to mind, they require disconnects within sight of all motors. In addition, the machine Safety level certifications that are already in force everywhere but North America require not one, but TWO contactors in series on ANY motor that needs the highest level of Safety category implementation, regardless of where the power comes from, including VFDs. these rules are coming to a theater (of operation) near you in the not so distant future. So there is a need for these practices to be understood and implemented, WHERE NECESSARY, in the proper way.


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## BBQ

John Valdes said:


> When special VFD cable is used, does it make any sense to interrupt the run?
> A disconnect adds 9 added points of connection to fail or cause performance issues.
> 
> 3 - input conductor terminals
> 3 - output conductor terminals
> 3 - Mechanical switching parts (blades).


The only thing special about the cable is it is shielded, I don't see any more of an issue with terminations on the load side of a VFD compared to a starter.

The only issue seems to be in your head.


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## lukejenson

Have you guys seen *ABB OTDC Series*. That's their latest disconnects & DC rated for PV systems.


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## jude

spoke to an inspector recently about this issue as we were planning on installing VFD's and had been informed that a disconnect opening when drive running caused damage he said there was no need of a disconnect beside the motor as long as there was a notice at the motor clearly stating where the disconnect was located and again a notice on the disconnect clearly identifying the location of the motor


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## BBQ

That maybe true in Canada but not allowed under the NEC.


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## marcusandreus

Sir, good day! If i have a three water cooled chillers which each are having power capacity is 251kw, FLA is 421.14Amp and starting current is 1086 Amp. What should be the size of circuit breaker that I'm going to use each machine?

Thank you in advance and More Power.


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## JRaef

marcusandreus said:


> Sir, good day! If i have a three water cooled chillers which each are having power capacity is 251kw, FLA is 421.14Amp and starting current is 1086 Amp. What should be the size of circuit breaker that I'm going to use each machine?
> 
> Thank you in advance and More Power.


You should start another new thread, this is not germaine to the original thread that you posted in. That said, this is a forum for North American electrical engineering issue for the most part. What you are describing is something that would relate to whatever the electrical codes and standards are in your country. What we would tell you that applies to North America may not be correct in your country. You should hire a qualified electrical engineer.


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## socalelect

JRaef said:


> You should start another new thread, this is not germaine to the original thread that you posted in.
> 
> 
> The goddamn germans got nothing to do with it, don't you hear good?


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## marcusandreus

Im so sorry coz im new here. abyway,thanks for your professionalism.


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## JRaef

socalelect said:


> JRaef said:
> 
> 
> 
> You should start another new thread, this is not germaine to the original thread that you posted in.
> 
> 
> The goddamn germans got nothing to do with it, don't you hear good?
> 
> 
> 
> Not German, GERMAINE!
> 
> 
> As in Germaine Jackson, Michael's brother.
Click to expand...


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## socalelect

U totally missed it dude  apparently u don't know who Buford t justice is. :laughing:


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## 1up3down

Alot of the places I work at have a kaleidoscope of drives from Cutler hammer to AB. We have disconnects at each motor or (some of you are going to get excited) receptacles. The operators constantly turn those disconnects on and off and sometimes the drive trips but they just reset it. You can have almost all the faults talk to your plc so the operator can see it on the HMI and reset it. *we log that information and review it* thats for 5 hp or less. Over that hp, either run a feed all the way to the motor disconnect first and then back to the drive. OR we install a custom box where the operator has to key off the drive before its possable for him to physically get at the disconnect. Of course there are lots of variables so its hard to say this will work or will not we almost exclusively use coast to stop.


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## sseivard

430.102 B 2 Says that the controller disconnecting means Shall be permitted to serve as the disconnecting means for the motor and exception 2 makes the allowance for a locking means when located out of sight of the motor, also FPN 1 seems to make a pretty obvious clarification for vfd's as being subject to additional hazards to property. It seems to me that a lockable disconnect anywhere up stream of the drive including the circuit breaker would satisfiy the NEC. What am I missing?


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## JRaef

sseivard said:


> 430.102 B 2 Says that the controller disconnecting means Shall be permitted to serve as the disconnecting means for the motor and exception 2 makes the allowance for a locking means when located out of sight of the motor, also FPN 1 seems to make a pretty obvious clarification for vfd's as being subject to additional hazards to property. It seems to me that a lockable disconnect anywhere up stream of the drive including the circuit breaker would satisfiy the NEC. What am I missing?


IIRC, it has to be a facility with an established LOTO safety procedure that is reviewed and monitored, something to that effect. So it doesn't universally apply to every installation.


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## sseivard

Thats Exception 2 b Which I took to be totally seperate.


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## Peewee0413

MDShunk said:


> Since I'm mostly an Allen-Bradley guy... I'll respond from an A-B persepective.
> 
> Yes, you can absolutely have a disconnect on the load side of a VFD, if you keep certain things in mind. Like the French sparky rightly points out, you MUST have it interlocked with the VFD itself. If you simply disconnect the three line side conductors, you'll toast your VFD. You absolutely need a 4th contact to interlock with the VFD to disable it from trying to run. In Allen-Bradley terms, that would mean installing the "Drive Guard" module in the VFD (standard fare, lately).
> 
> I like to use Meneekes IEC-style motor disconnects, which feature a low-current 4th pole as a standard item. Very LOTO friendly also.
> http://www.mennekes.com/product_motordisconnect_features.html
> Hubbell makes a suspiciously similar product.


how about a disconnect with an aux. one that interrupts the run cmd.


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## Wirenuting

Peewee0413 said:


> how about a disconnect with an aux. one that interrupts the run cmd.


That's what we are talking also about. I know Square D has an add on kit for their newer disconnects. I'm sure the others do also.


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## Peewee0413

Hubble makes one that I have found to be reliable on an 7 hp motor.


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## electric mike

To add to all this, I wired a roof top exhaust fan that is controlled by a vfd back at the range hood. There was a factory installed disconnect on the fan and it had a sticker on it warning that the vfd must be shut off before the disconnect is shut off or "damage to drive may occur and warranty can be voided". 

It seemed more to cover the manufacturers azz but at least they did post a warning.


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## denny3992

socalelect said:


> U totally missed it dude  apparently u don't know who Buford t justice is. :laughing:


Jr, when i get home, im gonna punch ur momma right in the mouth!


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## JRaef

Peewee0413 said:


> how about a disconnect with an aux. one that interrupts the run cmd.


If you look at my long previous post, I addressed this. If you just interrupt the Run command, the VFD output may still be On if the drive is programmed to Decel. You need to use an input on the drive that KILLS the output completely. They all have it, you just need to find it or program one of the inputs to do that.


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## JRaef

denny3992 said:


> Jr, when i get home, im gonna punch ur momma right in the mouth!


Just heard an interview with Tyler Perry because he's in a new movie. They played a line of his from another movie where a father and son are talking.

"Daddy, why you being so mean?"
"Son, I wish I'd been meaner with your Mamma, then I wouldn't have to put up with you."

Looks like he stole that line concept from Smokey and the Bandit.

I just saw SatB on TV again last week, I admit now to being a little slow on the uptake of the references to Buford T. Justice. Great character. 

I ran into a cop like him outside of Huntsville AL once a long time ago. Unlit cigar stub in his mouth and all. Pulled me over for doing 50mph, I said the last sign I saw said it was a 50mph limit. He said "Son, speed limit on this here stretch is 35, because I say so and I'm the law 'round here. But being that I see y'all are from California, I'm gonna take pity on ya 'n let ya off with a warnin' this time. You people are in too big a hurry to die as it is, I don't reckon I'll see you in this town again anyway."


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## Vintage Sounds

JRaef said:


> If you look at my long previous post, I addressed this. If you just interrupt the Run command, the VFD output may still be On if the drive is programmed to Decel. You need to use an input on the drive that KILLS the output completely. They all have it, you just need to find it or program one of the inputs to do that.


Just wondering, how does someone become such a complete VFD jedi? Your posts on the subject, especially the longer ones, blow me away. In trade school when I started asking about VFDs and PLCs the instructors were happy to answer but most of my classmates looked at me like "why isn't this guy happy just chaining up fluorescent lights?". I think somewhere along the line I would chain myself up by the neck if that's all I did throughout my career.


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## Peewee0413

If the motor is disconnected how will it fry the drive if no current is passing, its just sitting idle. PF's have current limits, as well as other VFD's. If anything you will get a fault..but programming should make it safe. Only problem I see is starting the motor again while the drive is running. That's why interrupting the run cmd is smart.


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## JRaef

Peewee0413 said:


> If the motor is disconnected how will it fry the drive if no current is passing, its just sitting idle.


It's not the fact that the circuit is _already_ open. It is the ACT of opening the circuit under load. As I had said previously, the arc that happens when you open a set of contacts can damage the silicon layers of the transistors in the drives, as well as cause inadvertent firing of the transistors, which can cause brief but damaging short circuit currents. So if you have a disconnect down stream, sooner of later, IN SPITE OF whatever warning labels you put on it, someone is going to ignore it or not read it and is going to shut that running motor down by opening that disconnect. So if you kill the VFD output first, no problem.


> PF's have current limits, as well as other VFD's. If anything you will get a fault..but programming should make it safe.


Yes, PowerFlex drives (and most others) have current limit, but that's not the issue. The SC currents that can flow if the transistors misfire are WAY too fast for the Current Limit function to react to. That's why they can damage the transistors. 


> Only problem I see is starting the motor again while the drive is running. That's why interrupting the run cmd is smart.


_Once again_, killing the Run command does NOT guarantee that the VFD output power is OFF! If you have the VFD programmed to decelerate the motor or brake the motor to a stop, killing the Run command INITIATES that process and the VFD transistors are still firing. You have to make sure the function you choose KILLS THE OUTPUT of the VFD. All drives have that ability, but they all get you there in different ways (see my responses on the ATV71 thread). You just have to find the way that's appropriate for your drive.


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## Wirenuting

JRaef said:


> It's not the fact that the circuit is already open. It is the ACT of opening the circuit under load. As I had said previously, the arc that happens when you open a set of contacts can damage the silicon layers of the transistors in the drives, as well as cause inadvertent firing of the transistors, which can cause brief but damaging short circuit currents. So if you have a disconnect down stream, sooner of later, IN SPITE OF whatever warning labels you put on it, someone is going to ignore it or not read it and is going to shut that running motor down by opening that disconnect. So if you kill the VFD output first, no problem.
> 
> Yes, PowerFlex drives (and most others) have current limit, but that's not the issue. The SC currents that can flow if the transistors misfire are WAY too fast for the Current Limit function to react to. That's why they can damage the transistors.
> Once again, killing the Run command does NOT guarantee that the VFD output power is OFF! If you have the VFD programmed to decelerate the motor or brake the motor to a stop, killing the Run command INITIATES that process and the VFD transistors are still firing. You have to make sure the function you choose KILLS THE OUTPUT of the VFD. All drives have that ability, but they all get you there in different ways (see my responses on the ATV71 thread). You just have to find the way that's appropriate for your drive.


It happens here all the time. 
More drives are replaced because of people opening the down stream disconnect. 
It is only recently that the aux contacts are being installed. Been asking for years. 
But having them landed properly is the next issue. I had even printed out what you have stated and yet the best I can get is a break in the run command. 
At least it's a step in the right direction. 

Can this also cause a wave form to rebound and punch a hole thru the insulation of a conductor?


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## JRaef

Vintage Sounds said:


> Just wondering, how does someone become such a complete VFD jedi? ....


20+ years of doing it, one F' up at a time but learning from my mistakes... 

I did my first VFD installation in 1985 when I stuffed a bunch of Saftronics VFDs and soft starters into Klockner Moeller MCCs for a saw mill. I had no idea what I was doing, but neither did anyone else in those days, so nobody knew any different. I made it all work, but I had to eat my learning curve. After that I liked VFDs and kept at it, it was job security for the exact reason you mentioned; most guys ran away from them.


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## JRaef

Wirenuting said:


> ...Can this also cause a wave form to rebound and punch a hole thru the insulation of a conductor?


Maybe. I've seen it discussed in papers, because there is already a feeling that the higher voltage of the PWM DC pulses is technically exceeding the voltage rating of the insulation, and especially when you get standing (reflected) waves in the circuit. But I've never seen it proven with examples, just theoretical stuff usually promoted by someone with an agenda, like wire mfrs trying to sell us on higher voltage rated cable insulation or filter mfrs trying to prevent standing wave generation.

Doesn't mean it's not true, I just take that stuff with a grain of salt when I see where it comes from.


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## Peewee0413

76nemo said:


> Very well said:thumbsup:


I agree


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## Jabberwoky

I have rebuilt/repaired/reverse engineered VFDs for a few years. What JRaef says about the control settings for a VFD is very true. I want to point out that I was taught to never use a contactor on the output side of VFDs. Although many brands will still fire the outputs with no connection on the output leads it will cause serious damage to your VFD to open them under load. I had a client at one point send me the same drive 3 times in one month with blown output power transistors due to this issue. If you want to get into a field that is technically challenging and pays fairly well, drive technicians are in demand in almost any industrial area in North America. I could go into the pros and cons of the field but that would be best for another thread.


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## ace24wright

Thanks for this thread, you guys gave me more food to digest! Always hungry!


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