How to Perform Resistance Test on a Transformer

A basic transformer has two windings that are wound around an electromagnetic core. When AC voltage is applied to one of the windings, it creates a magnetic field, which is amplified by the electromagnetic core. The second winding that is also wrapped on the same core then induces current because of the core’s magnetic field. All this hinge on the fact that the windings are intact and not broken or shorted. A resistance test can help you figure this out. So how to perform a resistance test on a transformer?

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Transformer Windings

Let’s first understand what transformer windings are. Windings are just wires, usually made of copper but any conductor should work, perhaps not the same as copper. So imagine a long copper wire that is wrapped around an electromagnetic core. A copper wire, although an excellent conductor of electricity, will have some resistance because there is no such thing as a pure conductor.

Longer wires have more resistance as compared to a shorter one if all the other variables are consistent. In the same way, thinner wires will have more resistance when compared to thicker ones. You can measure the resistance of wires using ohmmeters.

What are the primary and secondary windings?

As mentioned at the start of the post, a basic transformer has two windings that are wound around an electromagnetic core. When AC voltage is applied to one of the windings, it creates a magnetic field, which is amplified by the electromagnetic core. The second winding that is also wrapped on the same core then induces current because of the core’s magnetic field

The winding where you connect the AC voltage supply is the primary winding, and the side where you would connect a load or where you will receive the output voltage is the secondary winding. But not all transformers have one primary and one secondary

The transformer that we will be testing here is a toroidal transformer with one primary winding with a centre tap and two secondary windings, so it’s a two-output transformer.

Purpose of the Resistance Test on a Transformer

The manufacturers make the transformers to work at certain voltages. For example, the transformer that we’re using here is a 0-115-230V primary to 2 x 7V transformer. Let’s understand this simply

Primary winding can take a voltage of 230 at its full winding and 115V at the centre tap on the primary. There are two secondary windings here, so if the rated voltages are applied at the primary, both secondary windings will give 7V each. This makes it a step down transformer

A step-down transformer gives a reduced voltage on the secondary as compared to the secondary. This is only possible when the secondary winding is less than the primary. As discussed earlier, if the length of one winding is less than the other, the resistance of that winding will also be less.

If the wires are broken, it won’t generate a magnetic field, and if the wires have short-circuited, the magnetic field will be smaller and won’t give the same output.

The two primary purposes of a resistance test on a transformer are

  1. Winding identification – how to identify the start and ends of the primary and secondary windings
  2. Winding continuity – are the windings intact, broken or shorted?

So let’s see the steps to perform the resistance test.

Steps to Perform the Resistance Test on a Transformer

1. Isolate the transformer from the AC supply

You cannot perform a resistance test with the supply connected. All ohmmeters have a built-in power supply, and when you connect it a circuit that has another supply, you may not get the correct measurements or worse, you will damage the meter.

To avoid these problems, it’s the best practice to isolate the transformer. Isolating means disconnecting the AC power supply from the transformer. It’s advisable to remove the transformer from the circuit altogether.

2. Test the ohmmeter

Never assume that your meter works, unless you have been using it immediately before the test that you are about to perform. In any case, I would advise that you test your meter before testing any circuit or component

The best way to test an ohmmeter is to measure the resistance of a known resistance like a resistor. For example, in the video, I’m using a 20 ohm resistor as a test resistor.

3. Identify the test points

Resistance needs to be measured between two test points. The transformer here has 7 terminals, which means that we have 7 test points and we need to decide the combination of test points that you will connect the ohmmeter across.

To make it easier, you should make a test table with some room for results.

4. Perform the resistance test

Connect your ohmmeter to the test points in your table and measure and record the results. Here is the table of our results

Terminal 1Terminal 2Result (Ohms)
VioletBlack1.172 k
VioletYellow1.738 k
VioletBlueOpen circuit
VioletBrownOpen circuit
VioletRedOpen circuit
VioletGreenOpen circuit
BlackYellow566
BlackBlueOpen circuit
BlackBrownOpen circuit
BlackRedOpen circuit
BlackGreenOpen circuit
YellowBlueOpen circuit
YellowBrownOpen circuit
YellowRedOpen circuit
YellowGreenOpen circuit
BlueBrown8.58
BlueRedOpen circuit
BlueGreenOpen circuit
BrownRedOpen circuit
BrownGreenOpen circuit
RedGreen8.63

What do these Results mean?

There are only five measurements that gave some reading.

The resistance between Violet and Yellow is 1.738 kOhms. From the transformer circuit diagram, we know that these points are the ends of the full primary winding. If you compare this measurement to the resistance between Violet and Black, which is 1.172 kOhms, we can see that this is part of the primary winding but not an exact centre tap.

Resistances between Blue and Brown terminals and Red and Green terminals were both close to 8.6 ohms. This shows that both windings will give somewhat similar output and we can see that from the ratings of the transformer.

Open circuits

All open circuit readings mean that there is no continuity between the two test points. For example, there is an open circuit reading between Violet and Green terminals and this makes sense because violet terminal belongs to the primary winding and the Green terminal belongs to one of the secondary windings.

If there were an open circuit reading between Red and Green terminals, that would be a problem because these points are the terminals of one winding.

Conclusion

I hope this post helps you understand the steps to performing a resistance test on a transformer, the purpose behind it and how to interpret the results.

Resistance test is only one type of test that should be done on a transformer; there are a few more. One of the other essential tests is an insulation resistance test, which we will look at in another post.

Please share your experiences with transformer testing in the comments below or if you would like to add to the information here.

Thanks for reading!

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About the Author

Husnen Rupani

Husnen Rupani

I help electrical training organisations increase learner engagement by designing innovative training equipment. I have a saying "Electricity - you cannot see, you cannot hear it, but by the time you feel it, it may be too late." My main aim is to turn this black magic that we call electricity into something that people can understand.

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