Transformer Identification

[charlatandude]

I have a low voltage transformer with no identification on it. My guess was that it is a 120V input 12V output, but when I tested it showed 14.5V output. I'm trying to figure out the watts using W=(E^2)/R. It has the following wires black/white/green on one side and two yellow on the other side. Should I be able to figure out the resistance using the two yellow wires?

 

[augie47]

The 14.5 volt is probably "normal" for a 12v transformer with no load.
The smaller transformers tend to have a no load voltage that runs a bit high.
I know of no way that you can determine the wattage by measuring winding resiistance.

 

[charlatandude]

The 14.5 volt is probably "normal" for a 12v transformer with no load.
The smaller transformers tend to have a no load voltage that runs a bit high.
I know of no way that you can determine the wattage by measuring winding resiistance.

What I read is that if you measure the transformers windings with a DMM you can determine the ohms. If you know the voltage output and the ohms, would't you be able to use the formula W=(E^2)/R, where E=volts and R=ohms? Thanks for your help!

 

[LarryFine]

Charlie, welcome to the forum!

I agree with the dog. It sounds like a 12v (nominal, as they say) secondary. You can get an idea of its ampacity from its size and weight.

You could compare its unloaded temperature with that at various load levels, adding, say, one amp of load at a time, an hour or so apart.

 

[augie47]

From a mathematical standpoint your question makes sense, but, from a reality standpoint it does not work that way. Be patient, someone here will be.... this Forum has a lot of "smarts".
I am not smart enough to tell you why..perhaps the fact that the transformer load is a winding and not purely a resistive load.
Same is true with motors.

 

[jim dungar]

...would't you be able to use the formula W=(E^2)/R, where E=volts and R=ohms? Thanks for your help!

Put simplistically,
That data provides you with the watts consumed by the transformer during a short circuit, not the watts that can be generated to supply a load.

 

[mityeltu]

You can tell alot about a transformer by looking at it. You don't really mention much other than the number and color of the wires.

It's a safe bet that this is a single phase xfmr from your decription. The green wire is probably connected to the frame and is the gound wire. You might also notice that one of the coil wingings terminates to the same lug as the green wire, or it might be soldered to the frame elsewhere.

The yellow wires are probably the secondary. Wire guage can tell you what the wires can handle from an ampacity standpoint. From there you can start running some tests: short circuit, open circuit. If you're not familiar with these, the internet covers them pretty well, or you can ask here.

Next would be size.... The physical size can tell you alot about what kind of kva you're looking at... is this xfmr the size of a coffee cup or a filing cabinet? The bigger it is, the higher, typically, the kva rating.

Without any markings, it's hard to say, but I'd start with secondary wire size and work my way from there.

 

[mityeltu]

What I read is that if you measure the transformers windings with a DMM you can determine the ohms. If you know the voltage output and the ohms, would't you be able to use the formula W=(E^2)/R, where E=volts and R=ohms? Thanks for your help!

You can measure the dc resistance of the coil, but that is small compared to the reactance. Each coil will have reactance (X) and resistance (R). The reactance is going to be the predominant factor in a transformer. It's the reactance that keeps the primary from carrying current when the secondary is open-circuited. If it were only resistance, the current would flow like it were a resistor.

 

[augie47]

Often times on the smaller transformers the voltage/wattage are stamped into the case or the insulator board (if there is one). The data is not apparent withoiut a real close inspection.