480/208 Transformer Hum/Buzz

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geofhowe

New User
Location
San Diego, CA
Bad buzzing from the main facility transformer!!!
Long story short...
I am troubleshooting this. My maintenance supervisor brings a shop vac over, vacuums the transformer off and around the inside of the enclosure (power on).

Buzzing gone. Extra heat gone. Clean that old transformer!

I work in a 30+ year old facility. Our main transformer located inside the wall starting BUZZing loudly this morning and seemed to be producing extra heat. I know the difference between healthy hum (low bassy, solid sound) and a problem sounding buzz (sharp, mechanical). What was it?
I took a piece a plastic hose, held it to my ear and poked around. Definitely the middle coil area more at the top.
Hoping it is not plate delamination - this is annoying but not a show stopper. Must check load balance and then leakage, right?
Loads are pretty close. I started planning a no-load test for the weekend (a very big deal). With no output, I will check for balanced leakage current on each primary. Should be within 5-10%. What about the heat?

Decades of engineering experience...learned something NEW!
 
One thing that can cause noise, excess input current and overheating without obvious effect on the output voltage would be a shorted turn within one of the windings.
This can sometimes be recognized by a sufficiently precise DC resistance measurement but is very clear when you use an AC impedance measuring device such as a reactance bridge.
 
One thing that can cause noise, excess input current and overheating without obvious effect on the output voltage would be a shorted turn within one of the windings.
This can sometimes be recognized by a sufficiently precise DC resistance measurement but is very clear when you use an AC impedance measuring device such as a reactance bridge.

The instrument we normally use in this case is a TTR turns ratio tester:

https://www.google.com/search?q=ttr...ome..69i57.17654j1j8&sourceid=chrome&ie=UTF-8

We have also used a DLRO digital low resistance ohmmeter by Megger(Biddle):

https://www.google.com/search?rlz=1.....gws-wiz.....6..0i71j35i39j0i131.OhKBlCJ-tqA
 
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