VFD mitigate harmonics?

T

Tank11

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CO
An industrial facility has (2) 100hp motors and their breakers are overheating and failing. A specialist was brought in for load monitoring and is stating the system is seeing harmonic disruptions that are the cause of the problem. The local utility acknowledges the harmonics but states they are within their allowed parameters. The specialist is stating adding a VFD to the motors will mitigate the harmonics. Ownership does not want to spend the money unless it will solve the problem. The current starting method is across the line.

Will a VFD mitigate harmonics? Does a specific type of VFD need to be specified?
 
Tank11 said:
An industrial facility has (2) 100hp motors and their breakers are overheating and failing. A specialist was brought in for load monitoring and is stating the system is seeing harmonic disruptions that are the cause of the problem. The local utility acknowledges the harmonics but states they are within their allowed parameters. The specialist is stating adding a VFD to the motors will mitigate the harmonics. Ownership does not want to spend the money unless it will solve the problem. The current starting method is across the line.

Will a VFD mitigate harmonics? Does a specific type of VFD need to be specified?
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Not an answer.

I would want to see FOP results for each breaker and starter along with full details of the feeder/branch circuit involved.

How would a VFD help with harmonics that the utility has?
 
Are the harmonics are coming from the utility (so they're measurable with the motor not running) or inside the plant? (That's not clear from the description. either way, it's hard for me to imaging how they would cause motor breakers to fail.)

It's unlikely that the motors themselves are creating a harmonics problem. Are the breakers correctly sized? Are their bus connections tight? How long has this been happening? Were there change around the plant soon before the failures started happening?
 
Complete bunk!!!

A VFD will make harmonics WORSE!
If these 100HP motors are running across-the-line, they are NOT contributing significant harmonics to the system** and they are barking up the wrong tree. This “specialist” is likely a scam artist.

If they ALREADY have older VFDs on these motors and have no harmonic mitigation associated with them, then maybe an argument can be made for IMPROVING that. But still, causing breakers to “fail” (whatever that means)? Unlikely. Nuisance trip, maybe. So the definition of failing is important here.

I like the idea of having someone perform an FOP test on the breakers, it might be as simple as their needing to be replaced. But also, a big culprit I have seen out there is that someone used highly flexible wire, like DLO, when building a drive cabinet and they didn’t realize that the factory lugs on breakers are NOT rated for that high stand count wire, so the lug connections are over heating and that heat travels up into the breaker itself. I’ve even seen it melt the terminals!

** If a motor has a serious internal defect, such as a broken rotor bar or separated rotor end ring, that can cause some harmonic distortion. But something like that significant enough to show up as an overheating breaker would be immediately obvious in the motor performance and sound.
 
IMHO the harmonics thing is a red herring.

It is true that harmonics coming from the utility can increase motor current and motor heating, and that this increased current could cause increased breaker heating.

It is true that if the the utility is supplying harmonic rich power then adding a VFD could reduce the harmonics seen by the motor. This is because the VFD rectifies the incoming AC to DC and then regenerates AC to run the motor. The synthesized AC is rich in very high frequency harmonics, but pretty clean of low frequency harmonics.

But the input rectifier itself will increase harmonic current drawn on the utility side of things, and this would presumably increase any problems at the supply breaker.

The supply breaker should be able to tolerate the harmonic current if the motor can tolerate the harmonics. At worst the motor overload protection should trip. The breakers should not be failing with any reasonable level of harmonics.

I suspect that with all of the changes to the feeders and circuit necessary to switch over to VFDs, it is very likely that the real problem will get fixed or at least discovered during the process. But that is an expensive way to avoid a step by step diagnostic approach.
 
I do not believe that the problem is harmonics. Might as well blame it on bad grounds.

You can put inductors in line before the breakers to reduce the harmonics and see what happens but a VFD is not going to change the incoming harmonics any.

What might happen if you install a VFD is that the line current may go down by a substantial amount resulting in less heating at the CB terminals. Thus the VFD solves the problem.

But I like the idea of looking at the wiring first as suggested by Jraef. Use of fine stranded wire with terminals not designed for it often results in overheating of the terminals. It just takes a few years or decades to show up.
 
petersonra said:
I do not believe that the problem is harmonics. Might as well blame it on bad grounds.

You can put inductors in line before the breakers to reduce the harmonics and see what happens but a VFD is not going to change the incoming harmonics any.
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Worked for me for quite a few years.
 
winnie said:
Agreed if the goal is to reduce harmonics caused by a VFD. But there is no VFD or rectifier here.

OP has across the line motors and supply breakers that are failing.
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My apologies, kind sir/
 
said before, bunk - harmonics can cause a LOT of problems but not on simple thermal/mechanical breakers.
of course, OP left out what type breakers are involved or if they have zero sequence trip characteristic, etc..
 
ptonsparky said:
You would think an industrial facility would have someone on hand that could trouble shoot breakers that are tripping without calling in a specialist.
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Troubleshooting is a difficult skill to learn and even more difficult to teach. Very few electricians are any good at it beyond the very basic stuff. That's why you get nonsense about better grounds, harmonics, etc. They just don't know but are unwilling to say so. You don't need to go beyond the basic stuff very often. Most times if a breaker trips it is an overload or a short circuit. Once in a blue moon it is a failed breaker. Anything else is pretty darn rare and a typical electrician could easily never run across anything else in his or her entire career.
 
petersonra said:
Troubleshooting is a difficult skill to learn and even more difficult to teach. Very few electricians are any good at it beyond the very basic stuff. That's why you get nonsense about better grounds, harmonics, etc. They just don't know but are unwilling to say so. You don't need to go beyond the basic stuff very often. Most times if a breaker trips it is an overload or a short circuit. Once in a blue moon it is a failed breaker. Anything else is pretty darn rare and a typical electrician could easily never run across anything else in his or her entire career.
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That doesn't say much for the 'specialist' that was brought in.

I was going to add FOP was a standard procedure at our small shop, but it wasn't. They would have had to taken an 87 with them and I only had two. Otherwise they knew how and why.
 
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ptonsparky said:
That doesn't say much for the 'specialist' that was brought in.

I was going to add FOP was a standard procedure at our small shop, but it wasn't. They would have had to taken an 87 with them and I only had two. Otherwise they knew how and why.
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Can somebody please translate that?
 
My crew knew how to take a fall of potential test on circuit breakers. They would need to make sure they had an adequate meter in their possession. In our case, we had two Fluke 87s for this use. They also had handheld thermal scanners that could pinpoint hot spots. Followed up with Fall of Potential testing if they felt it was needed. Basically I'm saying it was normal trouble shooting for us.
 
ptonsparky said:
My crew knew how to take a fall of potential test on circuit breakers. They would need to make sure they had an adequate meter in their possession. In our case, we had two Fluke 87s for this use. They also had handheld thermal scanners that could pinpoint hot spots. Followed up with Fall of Potential testing if they felt it was needed. Basically I'm saying it was normal trouble shooting for us.
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The problem with IR cameras is they give mostly meaningless results unless they are way crazy high.

My limited sample size of electricians suggests to me electricians that routinely work with MV are better troubleshooters. I don't know why. Maybe more methodical.

I think what you are calling FOP is really just measuring the voltage drop across the contacts. I don't know that is especially meaning unless it is way out of whack.
 
I think the point is to see if something is out of whack. I also don't think it's likely to turn up much but the test is quick and easy and you can check the voltages at the same time.

Troubleshooting.... a lot of people doing electrical work are electrical mechanics; they're great at the physical work but not so much at applying whatever theory they might know.
 
dkidd said:
Fall of potential testing is traditionally used for grounding

Fall-of-Potential Measurement | Fluke

The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site.
www.fluke.com www.fluke.com
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Same term used for two totally different things. When talking about fall of potential testing on a breaker you are measuring the voltage drop under load from the line side to the load side of the breaker. The result is typically millivolts. The biggest indicator of a problem is where the readings of voltage drop are substantially different between phases of the same device.
 
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