Control Transformer Winding Resistance Test

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alexm3g

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Greetings,

My retard mind is failing me so any insight would be appreciated.

Recently the electric room in my facility smelled like a burnt transformer. I found a tripped unit located in an MCC and it appeared the smell was coming from within the cabinet. Upon opening the unit, the smell definitely originated from inside and I began troubleshooting. All the fuses were still good, connections tight, and no burnt wires. I pulled the control transformer, did a sniff test and confirmed that this was the culprit. I checked the resistance across the terminals and this is what I found:

Transformer is 0.1KVA
Primary: 480V, 47 Ohms across H1-H4
Secondary: 120V, 6 Ohms across X1-X2

Reading such a short resistance I figured that it was no good. After receiving the new one I measured its resistance and got the same thing :rant: My Ohms law calculations obviously don't match these numbers. So my question is, how can I do a field test/calculation to measure the winding resistance to rule out a bad transformer? Is this possible?

Thanks,
Retard
 
Was an insulation resistance (megger) test performed on the circuit associated with the tripped breaker? Specifically Tx windings to ground?

Could be an internal fault to ground that caused breaker to trip.
 
zog said:
In addition to what Luke said, what did you use to measure winding resistance?
Click to expand...

alexm3g said:
My Ohms law calculations obviously don't match these numbers.
Click to expand...

Remember that when you use a DC powered ohmmeter what you are reading is the DC resistance of the windings. When the transformer is actually in use the current through it will be in two parts:
1. The magnetizing current for the core, which will be almost purely inductive and will depend on the impedance of the primary at 60Hz, not at DC. That will be much higher than 50 ohms. Even in the tens of thousands of ohms.
2. The "transformed" load resistance from the secondary. If you have a turns ratio (voltage ratio) of 4 to one, then a resistance of 120 ohms placed on the 120 volt secondary will look like a load resistance of 120 X 16 to the wires supplying current to the primary.
Power in ~= Power out, so 1202/120 = 120 watts and 4802/120*16 also = 120 watts.
The actual effective impedance will then be the inductance in parallel with the transformed secondary load resistance.
If the secondary is delivering 120 watts, you want the power lost in the windings to be a small fraction of that, so the DC resistance would need to be small compared to 120 ohms. Six ohms is high for that particular power, but I expect that the control transformer is rated for a much lower output current.

Finally, if one or more turns of either winding in the transformer are shorted turn to turn, it will have almost no effect on the DC resistance, but will still burn up the transformer by causing it to draw too much current at 60Hz.

You can get a good idea of the 60Hz impedance of the good and the bad transformers by feeding the primary with say 12 volts AC and measuring the current through the primary. From those two numbers of AC, you can now use Ohm's law. It just will not tell you the phase angle (or power factor) of that impedance.
 
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Or you can use a winding resistance test set. If you try and use a multimeter you can leave it connected all day and will never charge the windings, your readings are meaningless.
 
Thanks for the quick responses. This forum never disappoints.

Zog, Luketrician - I used a Fluke 114 to measure the resistance rather than a megger, which should have been my first choice.

Finally, if one or more turns of either winding in the transformer are shorted turn to turn, it will have almost no effect on the DC resistance, but will still burn up the transformer by causing it to draw too much current at 60Hz.

You can get a good idea of the 60Hz impedance of the good and the bad transformers by feeding the primary with say 12 volts AC and measuring the current through the primary. From those two numbers of AC, you can now use Ohm's law. It just will not tell you the phase angle (or power factor) of that impedance.
Click to expand...


Golddigger- I'll set this up and run the numbers. I just finished apprenticeship school and went over that stuff, however, utilizing the theory and math in the field in new. Appreciate the explanation.
 
alexm3g said:
Thanks for the quick responses. This forum never disappoints.

Zog, Luketrician - I used a Fluke 114 to measure the resistance rather than a megger, which should have been my first choice.



Golddigger- I'll set this up and run the numbers. I just finished apprenticeship school and went over that stuff, however, utilizing the theory and math in the field in new. Appreciate the explanation.
Click to expand...

A winding resistance test set, read this http://www.irispower.com/PDF/WRT-100_english.PDF

Also you should megger the windings and perhaps do a TTR test
 
alexm3g said:
Transformer is 0.1KVA
Primary: 480V, 47 Ohms across H1-H4
Secondary: 120V, 6 Ohms across X1-X2

Reading such a short resistance I figured that it was no good. After receiving the new one I measured its resistance and got the same thing :rant: My Ohms law calculations obviously don't match these numbers. So my question is, how can I do a field test/calculation to measure the winding resistance to rule out a bad transformer? Is this possible?

Thanks
Click to expand...

For a transformer, or any other wound component else designed to operate on AC (motors, relays etc), a DC resistance test is of limited value. It will tell you is the winding has continuity but it wouldn't, for example, tell you if there was a coupled shorted turns.
You can't simply apply Ohm's law to the secondary voltage and its resistance - your 120V and 6 ohms. If there there was no load on the secondary there would be no secondary current.
The 120V is an induced voltage from the primary, not an applied voltage to the secondary.
It's a 120V source.
 
zog said:
... Also you should megger the windings and perhaps do a TTR test
Click to expand...
zog -
It's a 100va control xfm and it smells bad.

Are you suggesting a troubleshooting technique or a science fair project for learning?

ice
 
alexm3g said:
Greetings,

My retard mind is failing me so any insight would be appreciated.

Recently the electric room in my facility smelled like a burnt transformer. I found a tripped unit located in an MCC and it appeared the smell was coming from within the cabinet. Upon opening the unit, the smell definitely originated from inside and I began troubleshooting. All the fuses were still good, connections tight, and no burnt wires. I pulled the control transformer, did a sniff test and confirmed that this was the culprit. I checked the resistance across the terminals and this is what I found:

Transformer is 0.1KVA
Primary: 480V, 47 Ohms across H1-H4
Secondary: 120V, 6 Ohms across X1-X2

Reading such a short resistance I figured that it was no good. After receiving the new one I measured its resistance and got the same thing :rant: My Ohms law calculations obviously don't match these numbers. So my question is, how can I do a field test/calculation to measure the winding resistance to rule out a bad transformer? Is this possible?

Thanks,
Retard
Click to expand...

I just measured some good xfmrs with a Fluke in my shop and found the following:
50 VA primary:125 Ohms sec: 8 Ohms
75 VA primary: 603 Ohms, sec: 3 Ohms.
100 VA primary 70 Ohms sec 4 Ohms
As you can see, there is no pattern to the readings on different VA units. But readings on identical xfmrs
should be very close to each other.
What I would do is measure another good identical xfmr in another bucket and compare the readings.
Like others mentioned, you can't use Ohms law with DC resistance values.

I suspect the primary on yours is fried.
 
130608-1406 EDT

DC resistance of transformer windings can be of use, but possibly not for the purpose of checking the original post transformer problem. We will see if the following is useful in this case.

Here are a couple small transformer I have with nominal 117 to 120 V primaries at room temperature.


A Stancor P-8688 56 VA filament transformer rated at 28 V and 2 A, 117 V in.
The 117 V primary DC resistance reads 6.4 ohms, and
the 28 V secondary reads 0.69 ohms. This measured resistance ratio is 6.4/0.69 = 9.28 .

The approximate turns ratio would imply a resistance ratio of (117/28)^2 = 17.4 .
The actual turns ratio is closer to 121/32 = 3.78, and 3.78 squared is 14.3 . This 14.3 would be the predicted resistance ratio.

Actual resistance ratio to predicted is 9.28/14.3 = 0.65 . This transformer has the primary inside of the secondary.



A Signal Transformer A41-175-16 is rated 175 VA, 120 V input, and 16 V output.
The 120 V primary DC resistance reads 1.71 ohms, and
the 28 V secondary reads 0.049 ohms. The measured resistance ratio is 1.71/0.49 = 34.9 .

The approximate turns ratio would imply a resistance ratio of (120/16)^2 = 56.3 .
The actual turns ratio is closer to 121/18 = 6.72, and 6.72 squared is 45.2 . This 45.2 would be the predicted resistance ratio.

Actual resistance ratio to predicted is 34.9/45.2 = 0.77 . This transformer has the primary and the secondary in separate side by side bobbin areas.


If we apply the 0.65 ratio to the original post transformer, then this implies 16 * 0.65 * 6 = 62.4 ohms for an estimate of the 480 V primary resistance using DC secondary resistance as the reference. If the 0.77 ratio is used, then estimated primary resistance is about 74 ohms.

Both of these values are much higher than the original post measured primary DC resistance of 47 ohms. But if the 120 V secondary resistance is not really 6 ohms but somewhat smaller, then the predicted primary resistance would be lower.


Now suppose that no turns shorted, but the transformer just smells, then you want to replace it anyway. If it smells burned, then it is damaged. An insulation test will probably fail.


You could find out if the transformer still works by applying a low input voltage and measuring input current. Unloaded the primary current is low compared to full rated load. Even if it worked you do not want to use it.


Photo P6 thru P8 at my website http://beta-a2.com/EE-photos.html show the Signal transformer no load current. Power input at no load is quite small, about 4 W.


How is DC resistance of a transformer coil useful. It can be used to measure internal average coil temperature rise. The coils are measured at room temperature after the transformer has had time to obtain a uniform internal temperature equal to room temperature. Then the transformer is loaded and temperature is allowed to stabilize again. Power is removed and the coil resistances are measured. Knowing the temperature coefficient of resistance the average temperature per coil can be calculated. Maximum hot spot temperature requires thermocouples to be inserted into the expect hot spot locations in the device under test.


.
 
130609-0748 EDT

Some more information.

What I believe is a 1000 VA control transformer has a 480 V primary resistance of 5.2 ohms, and a 240 V secondary of 1.1 ohms. Actual weight 19#. The Signal 175 VA is 5#. Multiply 4 by 1.1 = 4.4 ohms and that is in the range of 5.2 . If my VA guess on the 1000 VA is correct, then 10 times 5.2 is 52 ohms for a 100 VA.

Direct scaling will not work over a large range, but it does provide some ballpark figures.

Really need an identical transformer to the original one on which to make a primary resistance measurement.

Note: Because of the law of squares and cubes as transformers or machines get larger the efficiency improves and you can expect the relative primary resistance to increase more slowly than the kVA rating. Thus, all of my meassurements and comments seem to imply that the 100 VA transformer 480 primary should be moderately above 47 ohms.

.
 
ATSman said:
I just measured some good xfmrs with a Fluke in my shop and found the following:
50 VA primary:125 Ohms sec: 8 Ohms
75 VA primary: 603 Ohms, sec: 3 Ohms.
100 VA primary 70 Ohms sec 4 Ohms
As you can see, there is no pattern to the readings on different VA units. But readings on identical xfmrs
should be very close to each other.
What I would do is measure another good identical xfmr in another bucket and compare the readings.
Like others mentioned, you can't use Ohms law with DC resistance values.

I suspect the primary on yours is fried.
Click to expand...

Some added info to my previous post:
Manufacturer/ Model Voltage
50 VA primary:125 Ohms sec: 8 Ohms Micron Impervitran 480:120V
75 VA primary: 603 Ohms, sec: 3 Ohms. Allen-Bradley 1497-75A 480:120V
100 VA primary 70 Ohms sec 4 Ohms Micron Impervitran 480:120V
 
130609-1438 EDT

ATSman:

There is something strange about the 75 VA transformer. Secondary is too low, and primary is way too high.

I have now found a 500 VA 480 V primary. This measures 10 ohms for the primary, and 5 times 10 = 50 ohms for a 100 VA without consideration of the law of squares and cubes.

My smaller transformers that bracket each side of 100 VA are probably better estimators of the 100 VA transformer.

However, there are always design considerations that juggle core and copper losses, and this could have a substantial effect on trying to extrapolate from some other transformer to a particular transformer.

.
 
With distribution transformers turns ratio tests are done and not winding resistant tests because the winding ratio between the PRI. and sec. are known.
With a CPT the ratios are not 4:1, 2:1, etc because the ratios are compensated for no load to load voltage drops. As an illustration ayou would expect that 1ph 480-120 CPT would have 120v on the sec. when 480v is applied to to PRI. Not so as the sec. voltage will be higher. It you attempt to apply 120v on the sec. You will be surprised to find the you will end up with noticeably less than 480v on the primary. As such with a CPT that has been wound with compensated windings it may be necessary to consult with the manufacture with regard to what the actual turns ratio is.
It is of my opinion that trying to determine what the winding resistance should be is like measuring the contact resistance of a circuit breaker. If you had 2 identical transformer you may expect both to have similar winding resistances. I may have missed something but what did the transformer manufacturer have to say regarding winding resistance? What you would expect would be that the PRI. c onsists of a given length and size of wire which would result in a given DC resistance.
 
130609-1723 EDT

templdl:

You are correct that turns ratio of a power transformer is not the same as calculated from the nameplate voltages. However, as a first order approximation you can assume the voltage ratio is the turns ratio. There are also other factors that influence the relationship of primary to secondary resistance values.

Nevertheless I used the question to investigate some relationships. From what I found it is reasonable to judge that the smelly transformer primary resistance is a little low implying shorted turns. I looked at a number of transformer data sheets and none provided DC winding resistance.

If you apply a test voltage to an unloaded transformer primary and the primary current is larger than expected, then in all probability there is turn shorting somewhere. Obviously you only excite the primary and have the transformer frame insulated from any other conductive path.

When you check some general characteristics of transformers of different ratings and manufacturers it is surprising how similar they are.

.
 
gar said:
130609-1438 EDT

ATSman:

There is something strange about the 75 VA transformer. Secondary is too low, and primary is way too high.

I have now found a 500 VA 480 V primary. This measures 10 ohms for the primary, and 5 times 10 = 50 ohms for a 100 VA without consideration of the law of squares and cubes.

My smaller transformers that bracket each side of 100 VA are probably better estimators of the 100 VA transformer.

However, there are always design considerations that juggle core and copper losses, and this could have a substantial effect on trying to extrapolate from some other transformer to a particular transformer.

.
Click to expand...

gar,
I think we are in agreement that their are so many design variables between manufacturers that it is pointless to make comparisons. The 75VA unit is proof of this since it does function properly. That is why I would like to emphasize that it only has merit when you compare the resistances of identical units (same manufacturer, VA, part#, etc.), which I have done and the readings come very close between them.
 
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