Disconnect ahead of VFD? - Page 2 - Electrician Talk - Professional Electrical Contractors Forum

BBQ · 07-10-2011, 08:00 AM

Quote:

Originally Posted by 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

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.

Wirenuting · 07-10-2011, 08:08 AM

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.

Jlarson · 07-10-2011, 08:08 AM

Quote:

Originally Posted by BBQ

I disagree with your interpretation

I too disagree

Quote:

Originally Posted by BBQ

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.

Mike in Canada · 07-10-2011, 08:13 AM

Quote:

Originally Posted by Jlarson

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.

BBQ · 07-10-2011, 08:25 AM

Quote:

Originally Posted by Wirenuting

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

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

Replacing drives forever - Priceless.

BBQ · 07-10-2011, 08:28 AM

Quote:

Originally Posted by Jlarson

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"

.

Jlarson · 07-10-2011, 08:28 AM

Quote:

Originally Posted by Mike in Canada

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.

Jlarson · 07-10-2011, 08:31 AM

Quote:

Originally Posted by BBQ

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.

BBQ · 07-10-2011, 08:36 AM

Quote:

Originally Posted by Jlarson

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

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'.

Wirenuting · 07-10-2011, 08:37 AM

Quote:

Originally Posted by BBQ

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

Replacing drives forever - Priceless.

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?

Jlarson · 07-10-2011, 08:56 AM

Quote:

Originally Posted by BBQ

'But it stops it'.

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

BBQ · 07-10-2011, 08:59 AM

Quote:

Originally Posted by Jlarson

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.

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

John Valdes · 07-10-2011, 12:35 PM

Quote:

Originally Posted by 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.

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.

sparkymcwiresalot · 07-10-2011, 07:43 PM

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.

nitro71 · 07-11-2011, 02:27 AM

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.

don_resqcapt19 · 07-11-2011, 07:35 AM

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.

BBQ · 07-11-2011, 08:39 AM

Quote:

Originally Posted by John Valdes

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.

Quote:

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.

John Valdes · 07-11-2011, 11:58 AM

Quote:

Originally Posted by BBQ

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.

JRaef · 07-11-2011, 12:13 PM

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.

BBQ · 07-11-2011, 01:00 PM

Quote:

Originally Posted by John Valdes

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.