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How to test a 60Hz HV transformer

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hsieh

Mon Jul 30 2012, 12:04PM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

I need to test a 60Hz HV transformer.The specifications are: primary:240V secondary:5KV 710mA.I tried to test it with a Multimeter.The primary is 0.4 ohm.The secondary is 200ohm at first but it is changing and finally falls below 100ohm.If I measure it again,it is 200 ohm again and start to fall again.

Is this transformer good?Why the secondary resistance is falling when I measure it?

Thanks

Steve Conner

Mon Jul 30 2012, 12:37PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The secondary has high inductance, maybe the inductance is confusing your meter?

You won't know if it is good until you fire it up!

hsieh

Mon Jul 30 2012, 12:47PM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

Steve Conner wrote ...

The secondary has high inductance, maybe the inductance is confusing your meter?

You won't know if it is good until you fire it up!

Is there any good way to test it?It is installed in a very expensive(20000+USD) HV power supply.I remove all wires connected to it before measure it.

I don't think it is a good idea to test it by drawing arcs. 5KV*0.7A=3.5KW.If I test it with dummy load or resistor,what kind of resistor can dissipate this kind of power at this voltage?

Steve Conner

Mon Jul 30 2012, 01:06PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The last time I had a transformer like this to test (2.2kV, 1A) I connected an electric kettle in series with the primary as a ballast, then switched it on with the secondary open circuit.

It drew high current and made sizzling noises. I also tried drawing arcs off the secondary (should be harmless if the primary is ballasted) and failed completely. This indicated insulation breakdown in the secondary, even though it tested good at low voltage with a multimeter.

Something that costs $20k should have a warranty, a service contract, not a student poking around the guts of it.

Sigurthr

Mon Jul 30 2012, 01:21PM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

What about it are you trying to test? Just that it works? Are you thermally testing it at full load? Are you trying to verify its short circuit current sourcing capacity, open circuit voltage, real life impedance, etc?

Transformers are functionally pretty simple devices, if the insulation is good and pristine, and there is no damage to the winding or core, it generally has no reason to not work. Trying to figure out if the transformer can do what the ratings say it can is a different story though.

kilovolt

Mon Jul 30 2012, 06:17PM

Registered Member #2018 Joined: Tue Mar 10 2009, 09:56AM
Location: Switzerland
Posts: 74

You could test it with very low voltage and take a look, if the turns ratio is as it should. For this purpose you could put in a low voltage (0...10V) on the primary and measure what the secondary puts out. For example at 10V input the output should be about (5000V/240V) *10V = 208V.

Best regards
kilovolt

radiotech

Mon Jul 30 2012, 06:34PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Drawing an arc is a good way to destroy the transformer. Don't do this.

The transformer could be tested for insulation by using a Hi-Pot tester. This is a DC
voltage test. On an older transformer, the DC voltage should not exceed the RMS
rating of the winding.* You might also want to check out, 'index of polarization '
tests. This is to determine moisture in windings. *(opinion ) A 6000 volt Megger
might be used

Other tests that could be done would need data from the manufacturer, or
experience with other units of same model. The no load wattage and power factor
will indicate the possibility of shorted or carbonized turns.

Your experience with the inductance reaction with multimeter is a quick
test to know the transformer isn't shorted. An analog meter, i.e. an AVO,
works best.

Dr. ISOTOP

Mon Jul 30 2012, 08:33PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

You can variac it to avoid the risk of a catastrophic failure. An open-circuit test should be enough to verify that the insulation is still intact.

Unleashed

Mon Jul 30 2012, 09:26PM

Registered Member #5171 Joined: Tue Jun 05 2012, 11:32AM
Location:
Posts: 67

Raise the voltage slowly with a variac and measure.

Dr. Brownout

Tue Jul 31 2012, 12:14AM

Registered Member #2405 Joined: Fri Oct 02 2009, 12:59AM
Location: Wisconsin
Posts: 140

Put 240 volts on the secondary and measure the primary. That will tell you the voltage ratio not necessarily the turns ratio. But either way it should be close enough to see if there is a problem.

240 volts on the secondary with a voltage ratio of ~20:1 you should have 10 or 12 volts on the primary.

You can put in even 120 volts just remember your reading will be the ratio of the turns and that's what your looking for on the primary side.

But watch the current in case the transformer has a shorted turn and is in fact no good.

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