Short Cirucit in Power Transformer

[nyrock]

Short Circuit in Power Transformer

I am currently repairing an amp which I suspect has a blown power tranformer. I did a conductivity test on the primaries and it indicated a short circuit. I am aware that the same test will yield a positive result on the secondaries -- but is it a safe assumption that if there is conductivity on the primaries that a short does in fact exist?

 

[awright]

It is really difficult to determine whether a winding is shorted using an ordinary multimeter or DVM because the DC resistance of most transformer windings is below the range that most instruments can accurately measure. A shorted turn that can lead to destructive heating within the winding may reduce the resistance of the winding by less than the resolution of the ohmmeter. And if you did not have a precise measurement of the winding resistance before the suspected failure, what are you going to compare your present measured value to?

Depending upon the rating of your transformer, you may be measuring mostly the resistance of your clip leads. I just measured the resistance of the 115 volt primary of a 150 watt transformer to be 2.955 ohms. This was using a 4-wire "Kelvin Connection" that eliminates the influence of clip lead resistance. The same measurement using a standard 2-lead connection with the same instrument indicated 6 or 7 ohms with the reading fluctuating too much over time to get a reasonable reading.

A possibly better way of determining whether a winding is shorted might be to resonate a winding of the suspect transformer with a shunt capacitor and observe the ringing waveform on an oscilloscope after pulsing the parallel LC circuit with a DC source. Very rapid decay with only one or two cycles visible on the oscilloscope would indicate that a shorted turn is absorbing energy from the LC circuit. Ringing sustained for a dozen or more cycles would indicate no shorted turn. These values are pure guesses and should be confirmed with a known good transformer of similar power rating to the suspect transformer if you try the technique. Actually the capacitor can be paralleled with any winding and any winding can be pulsed because the effect of the shorted turn is transformed to all windings.

How did you perform the measurement of the winding resistance and what was your observation?

awright

 

[Nigel Goodwin]

A big transformer will have a very low primary resistance anyway, the way to test for shorted turns is to 'ring' the transformer - you can build items to do this, or you can get creative with a scope.

 

[nyrock]

Thank you for the info. I took the measurement because the amp (a Fender Deluxe reissue) is blowing out rectifiers and fuses.

 

[Nigel Goodwin]

Thank you for the info. I took the measurement because the amp (a Fender Deluxe reissue) is blowing out rectifiers and fuses.

A faulty transformer wouldn't blow the rectifier - but a faulty rectifer could (and would) blow the transformer unless there's a suitable fuse between transformer and rectifier.

Simply disconnect the secondaries, and plug it in - if the primary fuse blows, then the transformer is duff (as there's no loads to blow the fuse).

 

[nyrock]

Nigel, thank you for the info. Actually, I'm off the power transformer trail. I tested the voltage on the secondaries with my variac at 25%. Everything appears okay (I got about 175VAC). I tested the tubes and a power tube (6V6) is blown. I've read that this can cause the B+ to behave as if there is a short, blowing rectifiers, fuses, etc. I have a solid state rectifier on order and a spare 6V6 in stock. I plan to carefully measure voltages once the rectifier comes in. Does this sound reasonable to you?

 

[Grossel]

How big (power rating) is your transformer?

If there is a short between two adjacent windongs inside a transformer, it will consume more power (active effect) and get hot, litterly. The winding(s) between the points that accidentally touches each other will take up a lot of current. And if that's on primary side (110/230V) the thread use to be quite thin, so it will probably burn out and eventually become a dangerous fire source.

And yes, it's very difficult to prove a winding fault in a transformer ONLY using a regular test instrument. The fault winding might only measure a houndred of an ohm, and you can't tell that apart if the primary windings measures t.ex 3 ohms.

 

[MrAl]

Hello,

Another way to test the primary (and secondary at the same time really) is to apply a much smaller than usual AC voltage to the primary and read the secondary voltage. If you know the turns ratio, you can see if you are getting close to the correct voltage. At the same time, measure the primary input current. A shorted turn will cause a much higher than usual AC input current as the voltage is increased which indicates something is wrong. You may have to estimate the excitation current too but with a shorted turn the input current should go up quite a bit even with small voltages like 12vac (when normally you would apply 120vac). You can current limit the small AC voltage supply by using a small value series resistor like 10 ohms, then perhaps bring it down to 1 ohm if everything starts to look ok.
Probably the best way to do this is with a variac so you can increase the input voltage slowly and check the input current as the voltage is increased, but a constant shorted turn will show up with any input voltage.

 

[nyrock]

Thank for the info. Actually, if you look at my second post (2:08 PM, two up from yours), I already did this and determined that the transformer is okay. Again, I appreciate the advice, regardless.

 

[tvtech]

It is really difficult to determine whether a winding is shorted using an ordinary multimeter or DVM because the DC resistance of most transformer windings is below the range that most instruments can accurately measure. A shorted turn that can lead to destructive heating within the winding may reduce the resistance of the winding by less than the resolution of the ohmmeter. And if you did not have a precise measurement of the winding resistance before the suspected failure, what are you going to compare your present measured value to?

Depending upon the rating of your transformer, you may be measuring mostly the resistance of your clip leads. I just measured the resistance of the 115 volt primary of a 150 watt transformer to be 2.955 ohms. This was using a 4-wire "Kelvin Connection" that eliminates the influence of clip lead resistance. The same measurement using a standard 2-lead connection with the same instrument indicated 6 or 7 ohms with the reading fluctuating too much over time to get a reasonable reading.

A possibly better way of determining whether a winding is shorted might be to resonate a winding of the suspect transformer with a shunt capacitor and observe the ringing waveform on an oscilloscope after pulsing the parallel LC circuit with a DC source. Very rapid decay with only one or two cycles visible on the oscilloscope would indicate that a shorted turn is absorbing energy from the LC circuit. Ringing sustained for a dozen or more cycles would indicate no shorted turn. These values are pure guesses and should be confirmed with a known good transformer of similar power rating to the suspect transformer if you try the technique. Actually the capacitor can be paralleled with any winding and any winding can be pulsed because the effect of the shorted turn is transformed to all windings.

How did you perform the measurement of the winding resistance and what was your observation?

awright

Really easy. Unplug all Secondary supplies. Only two leads from your Transformer going directly to Mains and Neutral. Switch on.

If it goes bang or a fuse blows, the transformer has a short on the Primary.

Cheers

Edit. Nigel beat me to it.

 

[Nigel Goodwin]

Edit. Nigel beat me to it.

Only by 7 hours 18 minutes

But who's counting

 

[Grossel]

If it goes bang or a fuse blows, the transformer has a short on the Primary.

It doesn't always act like a dead short. It may just draw more current than it would if there was no load. Fuse don't blow, but heat builds up from inside.

 

[Nigel Goodwin]

It doesn't always act like a dead short. It may just draw more current than it would if there was no load. Fuse don't blow, but heat builds up from inside.

A shorted turn WILL blow the fuse, it increases current consumption MASSIVELY -unless of course the fuse has been replaced by a nail

 

[Grossel]

Not always!

A shorted turn WILL blow the fuse, it increases current consumption MASSIVELY -unless of course the fuse has been replaced by a nail

Point is: The dangerous situation is when there is a fault in windings and the fuse DONT blow. When it blow, the owner of the device probably KNOW that there is a defect or cannot use the device due to the broken fuse.

Nigel, I don't think you understood what I meant. I draw the fail situation:

 

[Nigel Goodwin]

Point is: The dangerous situation is when there is a fault in windings and the fuse DONT blow. When it blow, the owner of the device probably KNOW that there is a defect or cannot use the device due to the broken fuse.

Nigel, I don't think you understood what I meant. I draw the fail situation:

I think you're missing the point, a transformer can only fail in a very limited number of ways:

1) O/C - pretty obvious, as it doesn't work.

2) Shorted turns - massive over current, and it WILL blow any correctly rated fuse.

Even a single shorted turn will almost always blow the fuse - and even if it doesn't, the transformer rapidly cooks, more turns go shorted, and the fuse blows.

The chances of a 'hidden' failing in a mains transformer is VERY, VERY slight.