Testing transformer

[camerart]

Hi,

I need a power supply for a battery charger, and have dug out a transformer of unknown quantity. I can easily check it's output voltage, but how do I check what current it will output?

Camerart.

 

[Nigel Goodwin]

You monitor the temperature rise of the transformer under different loads.

But the physical size of the transformer gives you some indication of it's power rating.

 

[jpanhalt]

This may help you quantify that relationship: https://www1.eere.energy.gov/buildi...former_draft_tsd_appendix5b_version080105.pdf

Try to find another transformer of known ratings that you can compare.

John

 

[camerart]

Hi,

Thanks John, but that looks far beyond my reading skills, so I'll use Nigel's method of monitoring it's temperature, it's much more my style.

Thanks.

C.

 

[Diver300]

Primary and secondary resistance will give you a good idea.

If you can measure the power that it takes with no load, the resistive power will probably be of the same magnitude as the power with no load. Unfortunately, the power that a transformer takes with no load can change a lot for a small change in supply voltage.

 

[camerart]

Ok Diver, I'll try that too.

C.

 

[jpanhalt]

Hi,

Thanks John, but that looks far beyond my reading skills, so I'll use Nigel's method of monitoring it's temperature, it's much more my style.

Thanks.

C.

I understand.

John

 

[BobW]

Quick and simple test:
Measure the primary resistance when the transformer is at room temperature. Then apply a load to it and allow the transformer temperature to stablize (about 45 minutes). Now disconnect power and measure the primary resistance again. As long as the hot resistance is less than 1.27 times the cold resistance, you should be within its design limits. Obviously you should start with a modest load on the secondary, and then you can slowly increase it until you reach the 1.27x hot temperature resistance limit.

The theory behind this is that the resistivity of the copper winding changes according to a well defined coefficient, and assuming the room temperature resistance reading is done at 20°C, then 1.27 times that resistance reading corresponds to 90°C which should be a safe operating temperature for virtually any modern transformer. This method also has the advantage that you're measuring the average internal temperature which will be higher than the surface temperature.

 

[Diver300]

Quick and simple test:
Measure the primary resistance when the transformer is at room temperature. Then apply a load to it and allow the transformer temperature to stablize (about 45 minutes). Now disconnect power and measure the primary resistance again. As long as the hot resistance is less than 1.27 times the cold resistance, you should be within its design limits. Obviously you should start with a modest load on the secondary, and then you can slowly increase it until you reach the 1.27x hot temperature resistance limit.

The theory behind this is that the resistivity of the copper winding changes according to a well defined coefficient, and assuming the room temperature resistance reading is done at 20°C, then 1.27 times that resistance reading corresponds to 90°C which should be a safe operating temperature for virtually any modern transformer. This method also has the advantage that you're measuring the average internal temperature which will be higher than the surface temperature.

I think that it would be a good idea to start the test with no load. There can be quite a bit of heating due to losses in the iron core.

 

[Colin]

A simple relationship is 500gm for 15VA 1kg for 30VA.

 

[chemelec]

Hi,

I need a power supply for a battery charger, and have dug out a transformer of unknown quantity. I can easily check it's output voltage, but how do I check what current it will output?

Camerart.

What are its Physical Dimensions?
And If Possible, Post a Picture of it.

 

[The Electrician]

Hi,

I need a power supply for a battery charger, and have dug out a transformer of unknown quantity. I can easily check it's output voltage, but how do I check what current it will output?

Camerart.

This has been dealt with in detail here: https://forum.allaboutcircuits.com/threads/assessing-an-unknown-transformer.38273/

 

[dr pepper]

I design transformers with a current density of 4 amps per square mil, measure the secondary diameter and calculate its square mil and go from there, if you pack it with more than 6a per square mil it'll get hot.