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.