ECN Forum Forums General Topics of the Trade General Discussion Area Control transformer - burn up from excess current?

Last week I was involved in troubleshooting a motor failure that was running a supply air fan. Initially it was thought that the motor failed but it turned out to be the controls transformer - burnt up bad. The transformer was 600V:120V @ 60VA. The starter equipment is possibly 1980's vintage. Besides the transformer burning-up there was a blown fuse on the controls side that was rated for 3A/250V. We didn't have a 60VA trans. so we used a 100VA. That's when it occurred to me - why was the 60VA trans. secondary fused @ 3A? I went back the next day and measured the current on the conductors at the load side of the fuse. There are two conductors soldered on the load side of the fuse - one going to the controller for the supply air fan motor and another going to another controller (? haven't traced it out yet) that is interlocked with the first. Current to the controller on the first = 0.4A. Current on the second conductor that goes to another controller - 1.8A. Am I possibly misreading something here or was the 60VA transformer being overworked which might explain why it burnt up? I've been told that it's been like that for years so it was simply an old transformer that finally quit. Something doesn't seem right with that explanation.

It could have been overloaded all the years until it finally died. Also, the secondary controllers might have gone bad and draw more amps than they used to in 1980 - do you have datasheets for them?
your new 100va trans is still overloaded!
did you measure current with a true RMS tool?

you can also measure the line side current of the transformer (true rms!) and see if your secondary measurements are correct (remember transformer losses).

My guess is that the original installation that drew 0.4 Amps at 120V (48VA) is what the 60 VA CPT was sized for.
Someone one day decided to add a power feed to another controller and grossly overloaded the 60 VA CPT and it just finally got fed up with the mistreatment and gave up the ghost.

I refer to this syndrome as "The Bob Vila Approach" since someone who knows very little about electricity just looks for the nearest, cheapest, simplest place to get 120V from and assumes that the available power at that point is infinite.

I actually had an owner tell me that I should just tap the 30A receptacle circuit to get the 100A feed that I needed for a new panelboard...priceless.

Used my Fluke clamp-on DMM to measure current.

How long seems reasonable that a transformer like that could be overloaded before finally burning up - 1 year, 10 years, more?

Yes, it appears load was added without careful consideration. The calculations are simple and the numbers don't lie.

I am wondering if the original fuse was replaced when they added the extra load.

Maybe but I took a look at an identical starter minus the interlock and it has the same size fuse. Here is an image of that identical starter (Cutler-Hammer 9586H6045G Model 6-1-3 contactor):



For comparison I metered the current on the secondary of this starter - 0.4 amps.

Just my two cents...

Back when there were mfg facilities here and I had tools, there were some plant maintenance guys that would 'fix' things, instead of calling the plant EC. Fuses were replaced with 'that's all we had', forget the ratings. "Why use the time delay ones" "Isn't 15 better then 2", etc. (No reflections meant to you Potseal)

The second tap for another control circuit? You didn't mention if it looked like it was done by 'qualified person'.

As to the load killing the control transformer, IMHO the 'old' ones sure could take a lot of excess for a long time. In a printing plant that I worked in for 20+ years, some of there original DC equipment was over 50 years old & still functioning. Some replacement contacts were 'plant made' in the machine shop, as Cuttler Hammer laughed when I tried to order replacements.

Bottom line IMHO, it was 'time' for the old transformer to rest in peace.

IBM used to get pretty serious about fuses. They were sized fairly precisely to the actual load plus a little in the hopes that when they blew, someone would figure out what changed.
They usually labelled the holder with the right fuse size and type. There are lots of fuse types with different trip curves. I really trust fuses more than breakers if it is critical.

Funny that you mention that... I've got two related stories involving manufacturers of electrical equipment. A couple years back we were having trouble with control circuit fuses blowing in Danfoss VLTs. Contacted tech support and they recommended using a higher rated fuse - from a 5 amp to a 15 amp! More recently we were testing new compressor motors for medical air supply. Once all three compressor motors were running simultaneously the fuses on the low voltage side of the electronic power supplies blew. The two fuse holders were marked "0.5" and "1" amps. We found a 1 amp fuse in the fuse holder marked "1" and a 4 amp fuse in the holder marked "0.5". Contacted the manufacturer and after some back and forth discussions they sent us replacement fuses that they recommended - 10 amps for both! Makes a person wonder if quality engineering is becoming a lost art.

Concerning these Cutler-Hammer motor starters for supply/exhaust air fans - I looked at three starters including the one that we replaced the transformer in. I checked the VA, current on the secondary of the transformer, fuse size on the secondary and checked operating temp with a FLIR IR scanner.

The current on the secondary was 0.4 amps for the two comparison starters and 1 amp on the starter with the new transformer (I measured 1.8 amps prior but that was on the load side of the fuse - need to double check that).

The fuse size varied from 2A-3A @ 250V.

The transformer size varied from 100VA to 125VA.

The operating temp for the new 100VA transformer that I put in a week ago was 50 degrees Celcius. in one of the comparison starters there was an identical 100VA transformer, age unknown, that measured 82 degrees Celcius (plus I noticed that some of the various conductors insulation was cracked to the point you could see the copper showing through). The third control transformer was the 125VA and it had temp of 75 degrees Celcius.

After a little research I now understand that these starters are mid 70's vintage. I am suggesting that we start going through all of these starters and do some upgrading hopefully before another one fails.

I tend to agree with Greg on fuses providing better protection for certain loads.

And, yes by all means someone is supposed to check to determine why the fuse blew, or the breaker tripped. This is dependent on way too many variables to discuss now. Back in the days of Edison base plug fuses, you would learn really quick to check the loads before you screwed in a new fuse. The same for <30 amp 250 volt cartridge fuses with the line side still 'hot'. Today we have 'finger safe' fuseholders, and plug fuses are mostly artifacts.

I once showed an apprentice class that I was doing a slide of the various fuse amperage, and fuse types. There were some heads spinning. Not just the standard 'sizes' within the NEC, but the 'stock' sizes from the mfg.

When a contactor is first energized, before it pulls in, it draws many times it's running current. When it is fully pulled in and the magnetic parts are in contact, the completed magnetic circuit limits current to the coil.

If movement of the contactor is restricted or the voltage drops below the level that will pull the contactor in, the coil draws too much power and the fuse will blow, or the coil will burn out or, maybe, both.

Perhaps someone increased the size of the fuse because the contactor wasn't pulling in fast enough and the fuse was blowing. Another good reason to increase the size of a fuse is because the electrician ran out of smaller fuses and forgot to go back to correct the temporary fix.

http://www.cooperindustries.com/con...ets-b/Bus_Ele_DS_1019_FRN-R_1_10-60A.pdf

I also agree with Greg and John about getting better protection with proper fusing selection. My pet peeve on many time - current curves, is that they run out of time. I like to see curves where the fuse will actually blow near its rating. For instance, I have an FRN-R 60A fuse. When will it blow??? I pull out the Bussmann curves and page 4 tells me that it should blow in a second at over 750 amps. Hey look, it should blow in 10 minutes at around 90 amps. So why don't they tell me when it should blow at anything less than 150% of its rating?

Looking at different families of curves, you see how you can easily get into more trouble with the same amperage rating of a different type fuse, than with a close rating of the same type of fuse.
Joe

A few real good points....

Joe said..."proper type fuse"

Twh said..."the electrician ran out of smaller fuses, and forgot to go back to correct the temporary fix"

That brings back memories of the 1/2" copper pipe 'fuses' somebody installed!!

.......

That starter is 1960's vintage, sometimes in the early 1970's C-H replaced that style with the now obsolete Citation series starters & contactors.

Update

After doing a fair bit of leg work I inquired with my boss about the possibility of upgrading these old C/H starters with new combination starters. They gave the green light and right now I am looking at a quote for two Eaton combination starters with electronic overloads. I do not have experience with electronic overloads so I've been researching the subject. It appears to me that there are several advantages over bimetallic overload relays. Beyond these advantages I started looking into the additional communication module that was not a part of the original quote. I like this option but I wonder how far to take this upgrade. The existing starters that I would like to replace are all for 600V building supply/exhaust fan motors. Obviously these are critical to the function of the building and fire safety. So if you have the option to exit the dark ages of monitoring and control and can now enter the modern era why would you not proceed that direction? Basically what I am asking is can anyone give some of their own thoughts on the pitfalls of going high tech for this application?

Options can be a trap.

First you need to figure out if you have any need to continuously monitor whatever the starter is doing.
Then you need to know if you have the capabilities to monitor it.
Then you need to see if you have a way to connect everything up.

If the salesman talks you into buying the communications node he won't tell you that you now need to buy a SCADA system...even if there's only 1 thing to monitor. Then after you have the new starters and the new SCADA system you need to run wiring (or fiber) to connect things together.

My first question is "since we have never monitored this before why do we need to now"?
If there's no good reason to monitor it in real time the communications option is just a waste of money.

Potseal

I agree with Ghost307 on the 'upgrades sales pitch', or in your situation, your quest to jump into 'present time'.

Real time monitoring from scratch is a lot of work, and costly. Unless the investment can be justified by 'future' upgrades, or modernization, it does not sound like a path to take.