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Author Topic: testing transformers  (Read 5184 times)
N0SOY
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« on: October 04, 2012, 05:22:08 PM »

I have a piece of equipment I suspect that the main power transformer is bad.  I blew a fuse as soon as I started applying voltage with the variac. How do I test the transformer.  It has been years since I worked on anything that may have had that issue.

thanks
N0soy
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W8JI
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« Reply #1 on: October 04, 2012, 05:52:50 PM »

Connect a low wattage 120V lamp in series as a fuse. Use a 25 watt lamp for small transformers, and a 60 watt lamp for large ones. Pull the rectifier tube and disconnect any HV CT wire or disconnect the HV wires from a solid state rectifier system. Pull the filament winding or any high current winding off on one side, and power up the primary through the lamp.

If the lamp glows fairly bright you have a bad xfm. If it does not glow, you MIGHT have a good one.

Connect the HV secondary leads ***one at a time*** to the chassis with the chassis ***grounded*** to the power line safety ground. If grounding any single HV lead makes the light light up, you have a bad transfomer.

There are a dozen more tests to make, but this will usually find most bad ones.
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KA4POL
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« Reply #2 on: October 04, 2012, 10:08:08 PM »

I have a piece of equipment I suspect that the main power transformer is bad.
There are up- and down-transformers, i.e. HV or LV. Can't you let us know what the 'piece of equipment'' is?
Quote
I blew a fuse as soon as I started applying voltage with the variac.
Does the fuse sit on the primary or on the secondary side? This has a major influence on testing.
Quote
How do I test the transformer.  It has been years since I worked on anything that may have had that issue.
Things didn't change that much so you should be able to locate the problem now.
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N0SOY
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« Reply #3 on: October 05, 2012, 04:55:14 PM »

the fuse is on the primary side.  the device is a OM-3 Oscilloscope.  the circuit on the primary is simple in that on one branch there is the fuse then the transformer and on the other branch there is a power on/off switch and then the transformer.  Since it is a tube type device I would think that the Primary is 110 and the secondary are a mix of voltages. 

I tried the scope today with a new fuse and it started the heaters on the tube and after about 30 second to a minute the fuse blew with a bright flash.  I am suspecting there is a short in the primary side of  the transformer that may be shorting as it starts to warm. 

Thanks for any help. 

David
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W8JI
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« Reply #4 on: October 05, 2012, 07:07:37 PM »

the fuse is on the primary side.  the device is a OM-3 Oscilloscope.  the circuit on the primary is simple in that on one branch there is the fuse then the transformer and on the other branch there is a power on/off switch and then the transformer.  Since it is a tube type device I would think that the Primary is 110 and the secondary are a mix of voltages. 

I tried the scope today with a new fuse and it started the heaters on the tube and after about 30 second to a minute the fuse blew with a bright flash.  I am suspecting there is a short in the primary side of  the transformer that may be shorting as it starts to warm. 

Thanks for any help. 

David

Nothing in what you said makes me think it is the transfomer, nor does it rule the transformer out.

BUT, if you keep running it and blowing fuses it might wind up needing a transformer!!

Remove the 6X4 and the 1V2 tubes, and see if it still blows the fuse. It it still blows the fuse with those tubes removed, it is more likely a transformer. If it does not blow the fuse with those tubes removed, it is more likely something like a bad capacitor in the high voltage lines.
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KA4POL
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« Reply #5 on: October 05, 2012, 11:44:31 PM »

Thanks for the information. This makes it much easier to help. In case you need parts, there is a Heathkit Group in Yahoo. They can at least point you in the right direction.
Let us know of your findings.
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KB3HG
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« Reply #6 on: October 06, 2012, 03:31:32 AM »

If it does not blow fuses with the tubes out. It could be possibly a shorted tube, or a potentially bad filter cap. As Goofy as it seams I'd take a few minutes with a meter, Ohm it out. (not under power) There should be some resistance there even at dc from the meter. A shorted xformer will be a short at dc as well as 60 hz. Leaky caps are in my mind. It took time to blow. Was the scope idle for a long time without being powered up?

Tom Kb3hg
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W8JI
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« Reply #7 on: October 06, 2012, 06:30:09 AM »

A shorted xformer will be a short at dc as well as 60 hz.

Very rarely does a shorted transformer show on a meter.

For example, a single turn shorted on a 100 turn winding would be immeasurable at DC with a meter, but would draw grossly excessive primary current and eventually melt the transformer down.

The only way to test a transformer with any reliability is with AC, and with a current limited full voltage supply. Also with a high pot tester for winding-to-winding or winding-to-core faults.
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G3RZP
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« Reply #8 on: October 06, 2012, 08:11:02 AM »

A variac and an ammeter in the primary circuit with a voltmeter across the primary. No secondary load: measure the current. Wind up the variac slowly  and if there's a shorted turn you'll see lots of current. Figure how many watts the transformer is going to be providing - you can get an idea from the fuse rating which could be as a first guess, twice the full current drain. Then the no load full voltage primary current should be, depending on transformer size, of the order of  10%. Run the transformer like that and see if it gets hot. It shouldn't - it might get warm.  If it draws the same current as the fuse is rated for at any voltage, then you have a bad transformer.

Measure the resistance between windings and windings to frame. Should be very very high. Now test those resistances  at high voltage with an ionisation tester (preferred -if you can find one) or a hi - pot. Should be very high still. Typical voltages would be 500 to 1000 for primary, filament windings and low voltage secondaries, 2 to 3 000 for something like a 350 - 0 - 350 type winding. The advantage of the ionisation tester over the hi pot is that you can hear the ionisation starting before there's any measurable current flow, so it's much better for non-destructive testing.
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W8JI
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« Reply #9 on: October 06, 2012, 09:32:59 AM »

My 15 kV high pot is current limited at 30 uA, and has a 50 uA meter.

I wouldn't have any other kind.

I can switch a .01 uF cap across the output if I want to blow whiskers off a cat.
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K8AXW
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« Reply #10 on: October 06, 2012, 09:45:58 AM »

This question is a great subject that might help a lot of hams.   Please note, I said HAMS. 

It would be great if answers or suggestions could be confined to methods and means as simple as possible, depending on the question and problem.  Most hams don't have access to sophisticated test gear.

So could we keep it simple?  I certainly would appreciate it as well as SOY.  And, SOY and others reading this, whenever you ask a question, provide information on what your are working with, what you have done, if anything, and the results.
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N8CMQ
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« Reply #11 on: October 06, 2012, 09:08:26 PM »

If you have an audio oscillator and a scope, you can see what the inductance of the windings are.
You can also sweep the frequency from 40 to 100 Hz and see where the resonant frequency is as well.
Meggers can be made so you don't spend tons of money on test gear, and can be used for caps too.
Be sure to use the DC mode on transformers so you don't over-voltage the windings!
With the transformer disconnected, are the output AC voltages near normal till the fuse opens?
I would suspect a winding to case short. You can measure from each lead to the case and see if any are shorted.
But it could still be a winding to winding short...
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KA4POL
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« Reply #12 on: October 06, 2012, 09:43:35 PM »

If you have an audio oscillator and a scope, ...
He got a scope, the OM-3 which we are just trying to fix  Grin
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G3RZP
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« Reply #13 on: October 07, 2012, 04:38:53 AM »

You need a certain amount of test gear for any fault finding. An analogy is that you wouldn't try taking the engine out of a car without a set of wrenches of various types.

So assume that you have a cheap DVM. Un-hook the transformer, put an old fashioned incandescent 60 watt light bulb in series with the primary and apply power. If the bulb glows brightly, the transformer has had it.  Now measure all the voltages - primary, and all secondaries, and check the ratios. If these are about right and the bulb is not glowing, or only very dimly, short out the bulb and measure the voltages again, and they should be about right - possibly a little high because it's unloaded.

Use the DVM on resistance to determine if there's a short between any of the windings or windings and frame. This isn't a very definitive test, because the volts are low. An analogue meter may show some volts when connected between windings or windings to frame with power on the primary, but if is a fairly insensitive one (say 1 to 10 kohm/volt), capacitive effects will be negligible. In that case, if voltages show, you have a leakage, and the insulation has probably failed somewhere.

And that is as far as you can go without more test gear.
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K8AXW
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« Reply #14 on: October 07, 2012, 07:06:22 AM »

Thank you Peter.  Great answer!  A ham should have a VOM and a dummy antenna at the very least. 

Your descriptive procedure for testing the transformer is great Peter.

Your analogy about the car and removing the engine is a bit over the top but a more accurate analogy would be to check the air in the tires, you need a tire pressure gauge.   Cheesy

A great deal of trouble shooting can be done with a simple VOM (as opposed to a DVM) although the DVM can be bought for as little as $5.00.  Either is a "must have" for a ham shack.
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