24V Relay Problem

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tmillard

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Hi...

A few years back we designed a job with about 50 unit ventilators. This past summer they started having issues with control relays in these UV's burning out. This occurred in about 1/2 of them, each at different times. The most recent one to burn out actually caught on fire. The relays have 24V ac control coils and they are fed from 120V/24V control transformers within the unit.

The UV manufacturer is claiming that there is an issue with the 120V power that feeds these units. They believe the voltage is below 120V and therefore the 24V is low, which in turn is causing the relay contacts to chatter. In other words there isn't enough voltage to energize the coil to draw the contacts in. The constant chattering is generating excessive heat unitil the relay either fails or catches on fire.

We had some power monitoring done on site and there have not been any noticable power fluctuations. In fact it was very clean. The monitoring was done on both the 480V distribution panels and 208V distribution panels for about a week. They then moved the metering further downstream and noticiced some harmonic issues.

It appears that they have added a number of computers to the building. I still don't believe this is the cause of the relays burning out.

Does anyone have any thoughts?

Thanks...

Tom
 
tmillard said:
Does anyone have any thoughts?
Click to expand...
Why monitor voltage at the panels? Monitor voltage at the units themselves, at least power. Perhaps monitor control voltage at the relays if they are remotely switched at considerable distance from units. What are the spec's on the relays, specifically minimum pull-in voltage and are contact ratings suitable (i.e. not borderline) for the load? Got a make and P/N?

When you say the relays are burning out, exactly what part of the relay is burning out, the coil, the contacts, the entire contact ass'y?
 
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Tmiller,
'Harmonic currents also increase I2Z heat losses in transformers and wiring. Since transformer impedance is frequency dependent, increasing with harmonic number, the impedance at the 5th harmonic is five times that of the fundamental frequency. So each ampere of 5th harmonic current causes five times as much heating as an ampere of fundamental current."

http://ecmweb.com/mag/electric_harmonic_current_voltage/index.html
If I might suggest, take harmonic measurements on the 24volt line. Additionally take an O scope and see what kind of wave you have.

"Effects of voltage distortion. Besides overheating, the other major effect of current distortion on an electrical system is the creation of voltage distortion. This distortion will have minimal effect on a distribution system, but unlike current distortion, it isn't path dependent. So harmonic voltages generated in one part of a facility will appear on common buses within that facility"
Just my $.02
 
Are the UV's in question 208 volt or 120. I am assuming 208 V. If there are harmonic problems that are affecting the control relays, there would also be problems with the motor contactor's, right? I can't believe a chattering relay would catch fire. I have seen some reeal buzzer's over the years and they lastest quite a while just buzzing along. Something else is causing the failure. The harmonics from computers are usually from the UPS attached to them.

I agree to scope the 24 V controls and see what you have.

Good luck
 
You may want to also check the "transformer VA" required to lock in this particular brand of contactors.If the "seal in" va. is borderline coupled with lower voltage at the step-down transformer,the contactor will not properly magnetically lock-up and will cause high heat generation at the contact surfaces.
 
If the relays are chattering, I'd think it would be obvious in either sound or pitted contacts. Of course, if they're chattering, then whatever they're controlling would chatter, too, and you'd probably be seeing burned-out motors or constant tripping of the overcurrent devices.

I think you get a bum batch of relays.
 
I would be there is one of two things going on - a bad batch of relays, or some kind of design/workmanship flaw in the system. Without a lot more information, it is hard to comment on which it is, but it is rare for that many parts to just fail. I would bet on some design/workmanship flaw somewhere, something like what another poster suggested.
 
Relay Update

Relay Update

Thanks for everyones input. I am on my way up to the site today so hopefully I can answer some of your questions. Here is what I have so far...

- The relays are Potter Brumfield KUHP-11A51-120. I understand that they replaced a few with another manufacturer's equivalent and they still had the same problem.

- The UV's are 120V and draw around 700W each since they are 4 pipe systems (hot water, chilled water)

I'll post more info when I get back.

Tom
 
Several years ago we built some equipment with a situation which may be relevant, not sure it's what you're seeing. We had a 24 v 'power on' relay which was held by auxiliary contacts which the 'power off' button would interrupt. Needed a simple hold function. Some of the machines would hum at the relay itself, on our shop floor. We could 'fix' those by swapping relays. Some customers complained of dropping out or not engaging at all. Everything should have been properly specified.

Empirical testing from several manufacturers showed that the Square D relay held with significantly less (10-20%) current than other brands, despite the same numbers in the literature. And that dropping out (using a variac) was less likely. The rep couldn't come up with any good answers.

What was done, finally, was to retain the Square D relay, and to swap the size 8 transformer for a size 9. Problem solved. We never needed to revisit that one.
 
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