Control Transformer Winding Resistance Test

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130610-1233 EDT

ATSman:

Your 75 VA transformer has a DC resistance of 603 ohms. A 75 VA transformer 480 V primary has an input current of 0.156 A at full load. This full load current thru 603 ohms dissipates 14.7 W or 39% of the transformer VA rating. A short on the secondary would produces less than 480/603 = 0.8 A input current. Maximum input power with a shorted secondary is less than 382 W, and the transformer would burn out. Maximum power transfer to a load would be less than 240^2/603 = 95 W.

Quite likely the 75 VA transformer was designed to have a high internal impedance to provide a type of current limiting. A transformer designed for efficient power transfer is not designed with this high an internal impedance. This is not a normal control transformer.

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gar said:
130610-1233 EDT

ATSman:

Your 75 VA transformer has a DC resistance of 603 ohms. A 75 VA transformer 480 V primary has an input current of 0.156 A at full load. This full load current thru 603 ohms dissipates 14.7 W or 39% of the transformer VA rating. A short on the secondary would produces less than 480/603 = 0.8 A input current. Maximum input power with a shorted secondary is less than 382 W, and the transformer would burn out. Maximum power transfer to a load would be less than 240^2/603 = 95 W.

Quite likely the 75 VA transformer was designed to have a high internal impedance to provide a type of current limiting. A transformer designed for efficient power transfer is not designed with this high an internal impedance. This is not a normal control transformer.

.
Click to expand...

gar
I have no argument with your analysis. Can you please address the comments I made in my previous post
to answer the OP?
Thanks
 
130610-1343 EDT

ATSman:

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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I believe this is the previous post you referenced.

If you compare relatively comparable transformers between different manufactures there is good similarity between the measurements. So I don't think it is pointless to make comparisons.

I do not know what you mean by the statement that the 75 VA functions properly. And I do not agree that whatever it is is proof that one can not make judgements on a transformer of one manufacturer based on the values of another. This depends upon the accuracy required in the comparison, and the likelihood one is comparing relatively comparable designs.

I have taken some relatively different transformers in size and extrapolated from them to a 100 VA unit and obtained values that would imply that the original post transformer has some shorted turns on the primary. From a resistance perspective the best comparison is with another identical transformer, and that we agree upon, but lacking that comparison at this time we have worked with any reasonable item that is available.

Whether there are shorted turns or not probably can be determined with an input current measurement.

Possibly we will hear from alexm3g with more information.

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gar said:
130610-1343 EDT

I have taken some relatively different transformers in size and extrapolated from them to a 100 VA unit and obtained values that would imply that the original post transformer has some shorted turns on the primary. From a resistance perspective the best comparison is with another identical transformer, and that we agree upon, but lacking that comparison at this time we have worked with any reasonable item that is available.

Whether there are shorted turns or not probably can be determined with an input current measurement.

Possibly we will hear from alexm3g with more information.

.
Click to expand...

gar- I received a replacement transformer and checked the DC resistance of the primary and secondary. It produced the same values as the original (bad) transformer in question. I installed the new transformer and the unit hasn't tripped. I'm at a loss...first year journeyman here.

The insight from this post provided a monumental amount of information. Thanks all for the posts.
 
alexm3g said:
gar- I received a replacement transformer and checked the DC resistance of the primary and secondary. It produced the same values as the original (bad) transformer in question. I installed the new transformer and the unit hasn't tripped. I'm at a loss...first year journeyman here.

The insight from this post provided a monumental amount of information. Thanks all for the posts.
Click to expand...

If you are really serious about finding the answer my thought would be is to obtain a recording device such as a oscilloscope to record the instantaneous current that may tripping the OCPD. There may be some hand held devices available from Fluke that could do this. But unless you get an actual snap shot of the event all that you will be left with is a guess.
I personally feel that this has been beaten to death at this point.
 
jumper said:
At 26 posts this thread this does not even qualify for a good run, need 100+ just to be really noticed.;)
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Jumper, how can we jump start this again? alexm3g received his replacement for which the primary measured the same as the all edged defective one. The replacement worked. It would be fun to kick this around a bit.
 
templdl said:
Jumper, how can we jump start this again? alexm3g received his replacement for which the primary measured the same as the all edged defective one. The replacement worked. It would be fun to kick this around a bit.
Click to expand...
Either shorted turns or a primary to secondary short that he did not check for in his ohmmeter measurements.
One way to tell would be to apply a controlled low AC voltage to the primary. A shorted turn would lead to a much higher no-load primary current in the failed transformer than in the new one.
 
templdl said:
Jumper, how can we jump start this again? alexm3g received his replacement for which the primary measured the same as the all edged defective one. The replacement worked. It would be fun to kick this around a bit.
Click to expand...
I think post #9 summed it up.
 
GoldDigger said:
Either shorted turns or a primary to secondary short that he did not check for in his ohmmeter measurements.
One way to tell would be to apply a controlled low AC voltage to the primary. A shorted turn would lead to a much higher no-load primary current in the failed transformer than in the new one.
Click to expand...

A PRI. to SEC. short would be hard to do since the two are separate in the way a transformer is constructed. But never the less it could be easily be verified if there was.
 
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