Transformer Problem

[iwire]

Hi all, a friend of mine went on a call for a loud transformer.

It is a K rated 480 delta 208Y/120 transformer, I do not know the size, probably 45 to 75 KVA.

Here are the true RMS readings he got.

Primary

A-B 496 volts
A-C 497 volts
B-C 497 volts

Secondary

A-B 214.8 volts
A-C 214.7 volts
B-C 213.9 volts


Primary

A 15.6 amps
B 18 amps
C 21 amps

Secondary

A 2.4 amps
B 21 amps
C 5.4 amps
N 13.7 amps

I have confidence in my friends ability to use his high quality meters.

He would like to know if these readings suggest any particular problem, something more than 'it's junk'.

[ December 26, 2004, 11:02 AM: Message edited by: iwire ]

 

[bwyllie]

Re: Transformer Problem

I am not a trransformer expert but I believe I read somewhere that transformer "hum" can be cause by a small load on the transformer and the hum may go away when the load is increased.

 

[jim dungar]

Re: Transformer Problem

Your voltage ratio of 497->215 is okay based on the transformer 480->208 (about 2.3:1), but I would like to know what the secondary line to neutral and line to ground voltages are.

However, your input and output currents seem questionable. The primary currents are not very close to balanced for a delta connection. And, the primary to secondary current in no way reflects the turns ratio of the transformer. Is it possible to measure the currents at different amounts of loading, especially at no load?

And of course, I need to ask if mechanical problems have been eliminated as a source of magnification of normal transformer noise.

 

[captal]

Re: Transformer Problem

Jim, thanks for answering iwire. I'm the guy that went on the call. The transformer in question is a GE K-rated 225 KVA. The secondary line to neutral voltage you are asking for is 118v, line to ground was the same. There were no mechanical problems that I could see.

Al

 

[templdl]

Re: Transformer Problem

"loud transformer" Noise as is heat is relative.
You have to have a benchmark when referring to either.
When one expect something to be quiet or cool and it emits a humming sound or is warm it is often referred to as noisy or hot.
What is the db level just as one would provide a temperature?
NEMA ST 1-4 (ANSI-C69.) section 2.7 covers Audible Sound Level Tests.
NEMA maximum allowable averages of the readings are 10-50kva, 45db, and 51-150kva, 50db.
When doing a sound level test the ambient sound must also be considered. Tests should be done at rated voltage under no load condition. My referrence suggests that the sound level in the room be 5db but preferably 10db below the ambient level plus the transformer level.
Some things that should be considered:
a)Pads made of cork or rubber reduce the transmission of noise to the floor.
b)Locating the transformer near multiple reflective surfaces such as in a corner or in a hallway often amplifies noise.
c)Locating a transformer in a very quiet are often times makes it appear that the transformer is noisy because there is no competition with other ambient noise.
d)Loose parts inside the enclosure can vibrate and cause noise, the transformer should be connected with flex. conduit.
e) Also check to assure that the mounting pads between the core/coil assembly and where is is attached to the enclosure are intact and that any shipping supports are removed as they may defeat the pupose of the sound dampening pads.
f)Loose sheet metal
g)Stressed enclosure due to mounting method, i.e. floor not level, etc.
h)Check for possible harmonic problems.
i)High primary voltage.
Incorrectly set taps.
If you can varify a noise problem with documentation you will have the amo to discuss the potential problem with the manufacturer.

 

[charlie tuna]

Re: Transformer Problem

did the original installer back the shipping bolts back to allow the transformer to float on the factory sound isolation pads??????????

 

[rattus]

Re: Transformer Problem

Reminds me of a neighbor who had a window AC that sounded like a diesel locomotive. Backed off the bolts, and the sound level dropped dramatically.

 

[iwire]

Re: Transformer Problem

Hi guys, hey this is not Al's first transformer and as far as sound being relative this transformer is located in a room with a couple more identical transformers, they all went in at the same time and all sounded about the same.

This particular one has been getting nosier over the past month.

I thought the amp readings looked way off and was thinking we had a problem.

 

[charlie tuna]

Re: Transformer Problem

iwire,
i know what your talking about--- and i believe some transformers are just noisy! check that the shipping bolts are backed off sufficently -- that something is transfering the noise out of the transformer---and i have even had them drastically change their noise pattern when the support rods are adjusted one way or the other. load doesn't seam to enter into it as much as you would think?

 

[dereckbc]

Re: Transformer Problem

Bob, something is wrong with current ratios. As Jim suggested is it possible to look at the primary currents with no seconday load?

Does the Xfmr have a shield and is it connected?

 

[captal]

Re: Transformer Problem

dereckbc, I agree the current ratios are wrong. Yes a shutdown of the secondary loads can be arranged. Yes it is shielded and connected. There is a second transformer in this room connected the same way operating normally.

I spoke with the original installer today. This transformer would cause the primary over current breaker to trip each time he tried to energize it. The secondary over current device was in the open position when this was happening. The installer then switched the primary breaker with the other transformer in the room and had no problem energizing the noisy transformer. The primary breaker that was tripping worked fine on the second transformer. The current ratio is correct on the second transformer. My question is do I have a bad core in the first transformer?

 

[dereckbc]

Re: Transformer Problem

Originally posted by captal:
My question is do I have a bad core in the first transformer?

Sure sounds like it, assuming all the wiring is correct.

 

[charlie tuna]

Re: Transformer Problem

the tripping is common on new transformer installations and caused by large inrush currents required to set up the initial magnetic field in the transformer. its an instantainious thing!

 

[rattus]

Re: Transformer Problem

Tuna, I don't see the inrush current thing in a transformer. With an unloaded transformer you would see the primary inductance and the equivalent core loss resistance and some capacitance. None of these would lead to a current surge as we see in incandescent lamps and motors.

If the xfrmr is loaded with motors and incandescent lamps, there would be a surge in load current which might trip breakers.

 

[charlie tuna]

Re: Transformer Problem

rattus sir,
many times --- maybe 20 per cent??? --- when you energize a new transformer --- with no connected load --- the primary breaker will trip! this is due to the energy required to alien the magnitism in the transformer's core. i have had the breaker trip two or three times before holding and then operate normally. last one was on a 225 amp feeder breaker on a step up installation...

 

[al hildenbrand]

Re: Transformer Problem

this is due to the energy required to alien the magnitism

It's an Abduction thing. . .

 

[al hildenbrand]

Re: Transformer Problem

Charlie, apologies. . .I simply couldn't resist.

Rattus, try to show that inrush-on-initial-energization in an equivalent circuit.

 

[Ed MacLaren]

Re: Transformer Problem

this is due to the energy required to alien the magnitism in the transformer's core.

I agree with Charlie T on this.

The amount of inrush current depends on the exact time in the AC cycle that electrical connection to the source is made. If the transformer core magnetic "domains" happen to remain aligned in the opposite polarity, from the previous energization, at the moment of connection to the source, the inrush would be even higher.

More here

Ed

 

[rattus]

Re: Transformer Problem

Touche' Al,

You forced me to look up inrush current, and it makes sense. Also, there are methods of predicting this current, but they hurt my head. True, the steady state xfrmr model would not predict this current.

Also, I would think that in this day and age, the switch closure could be so timed as to minimize this current.

 

[templdl]

Re: Transformer Problem

rattus,
The only product that I can think of that has a fairly consistant as well as fast closing to is a vavuum contactor which does not make sense for this application.
Dave