cdnelectrician

Awhile ago I was working with a company that was doing a lot of cooling tower retrofit work, a cooling tower company would replace the motors and fans and anything else mechanical. We would install drives on all the motors, all the drives I was installing were ABB ACH drives. Most if not all cooling towers have a disconnect for the motor right at the fan or belt access area or somewhere near it. I was dealing with an ABB rep at the time and he told me that there can not be a disconnect between the motor and drive, so what we ended up doing was installing lockable stop switches wired into the drives E stop control circuit so the conductors from the motor to the drive were continuous. Personally, I do not think this is a safe way to do it since a failure in the controls portion of the drive COULD cause the motor to start even with the switch locked in the off position. This was all passed by the AHJ and an engineer involved with the project.

I still see lots of towers using drives that have disconnects between the motor and drive and I still think this is the safest way to keep things safe when someone is working on the motor or fan. I asked the ABB tech rep why you could not have a disconnect between the motor and drive and he said that turning the disconnect off without stopping the drive first could damage the drive.

Is this true? I find it hard to believe since I see it done all the time, can this really damage a drive? Or is this just something specific to ABB drives.

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_moto..._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.

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.

mcclary's electrical

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



29.jpg


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

Mike in Canada

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

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

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.

BBQ

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.

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

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

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 delta (change) in Voltage over delta in time) 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.

76nemo

Very well said

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John Valdes

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.

76nemo

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

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

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.


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

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.



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




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


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.