How should I test a unknown power transformer ?

Thread Starter

Sparkyrobbin

Joined Feb 26, 2018
4
Hi everyone,
I have got a old transformer for building a variable dc power supply, the transformer is in good condition and has 5 different taps for 5 different voltage outputs.
But the problem is that the transformer doesn't have a tag over it for the rated current, thus before using it in a circuit, First I would like to test the maximum current output of the transformer, I tried measusing it with multimeter and some resistance but it didn't worked for me as the multimeter is showing nothing.I have no idea how to measure the maximum current output of a unknown transformer, pls help me !
 

MrChips

Joined Oct 2, 2009
21,687
We can help you if you provide us with a number of vital pieces of information.

Where are you located?
What is the AC mains voltage in your country?
What does the transformer look like? Post a photo.
How heavy is the transformer?
What are the dimensions?

@MaxHeadRoom will soon come along and give you some directions.

We presume that you have already identified the primary winding and secondaries.
Also you have already connected AC mains to the primaries. What was the voltage on the secondaries?
The reason you got no current readings on your meter is likely because the fuse in the meter is blown. Do you know how to use the ammeter to measure current without blowing the fuse?

You will need some high wattage power resistors or a power rheostat in order to present a sizable load on the output. Do you have these?
 

dendad

Joined Feb 20, 2016
3,597
here is part of a forum discussion...


http://amfone.net/Amforum/index.php?topic=16537.0;wap2
Tom WA3KLR:
Just some quick off-the-top-of-the-head answers for now:
I've always used a rule of thumb based on some transformers here, lower than 1 KW, but 15 - 30 Watts per pound (range due to duty cycle and how conservative/reliability) for 60 Hz. iron-laminated transformers.
An electrical emperical test would be to have high value power resistors on hand and see what load across the HV secondary it takes to produce a 10 - 15% drop from no load versus loaded.
I will take a look at what transformer design notes I have, but as I recall we need the actual core cross-sectional area. Is this what your H & W dimensions are?

Another BIG clue is to measure the primary inductance, no loads. This is a fundamental design parameter.
From the Radiotron Designer's Handbook 4th Edition page 235:
core sized needed (for 60 Hz.)
cross-sectional area in inches = (V*A)1/2 / 5.58
So, solving for Volt*Amps, knowing core cross-sectional area:
(5.58 x core area)2 = Volt*Amps capacity
If your numbers are correct for the core cross-section, then your core area is 7.266 square inches yielding 1644 Volt*Amps.



Also, this link has lots of info...
https://en.wikibooks.org/wiki/Electronics/Transformer_Design
 

eetech00

Joined Jun 8, 2013
1,967
Hi everyone,
I have got a old transformer for building a variable dc power supply, the transformer is in good condition and has 5 different taps for 5 different voltage outputs.
But the problem is that the transformer doesn't have a tag over it for the rated current, thus before using it in a circuit, First I would like to test the maximum current output of the transformer, I tried measusing it with multimeter and some resistance but it didn't worked for me as the multimeter is showing nothing.I have no idea how to measure the maximum current output of a unknown transformer, pls help me !
Do you have a sine wave signal generator?
You should be able to find the turns ratio and figure out the secondary voltages. Set the generators output to 1v RMS, 60hz and connect to primary. Then CAREFULLY (by that I mean treat it with respect!) measure the output voltage of each secondary with an RMS voltmeter.

et
 

Janis59

Joined Aug 21, 2017
1,205
Apply a LATR (laboratory adjustable autotransformer) with ammeter in series, switch any of windings and draw a graph i(o)=f(U). Where the exponential growth will begin, there is the max voltage for this particular winding. Other criterion is that this current must not exceed the some 5...10% of permitted current of that particular wire diameter (where You apply the 3,5 A/mm2). Often the wire diameter is thing what is easy to measure.
Then give this Voltage, let it say 18,8 Volts into the trafo, and with tester measure all combinations to get out wire numbers what gives 220 Volts (or 127 or whatever You have a domestic mains voltage).
So, now switch those newfound wires into the network without of LATR, and use a voltmeter to map all winding voltages.
Max amperage will come out of 3,5 A/mm2 and wire diameter.
 

Janis59

Joined Aug 21, 2017
1,205
RE:""Where are you located?""
Do You mean that the methodology of measuring will sharply change from the State to State??
Its something new for me!
 

Thread Starter

Sparkyrobbin

Joined Feb 26, 2018
4
We can help you if you provide us with a number of vital pieces of information.

Where are you located?
What is the AC mains voltage in your country?
What does the transformer look like? Post a photo.
How heavy is the transformer?
What are the dimensions?

@MaxHeadRoom will soon come along and give you some directions.

We presume that you have already identified the primary winding and secondaries.
Also you have already connected AC mains to the primaries. What was the voltage on the secondaries?
The reason you got no current readings on your meter is likely because the fuse in the meter is blown. Do you know how to use the ammeter to measure current without blowing the fuse?

You will need some high wattage power resistors or a power rheostat in order to present a sizable load on the output. Do you have these?
Thanks, for helping me out, actually I am new in power electronics and I don't have that much experience.

Well the answers for your questions are as follows :
1)In my county the supply voltage is 230VAC
2)I have attached a few photos of it....
3)The weight of the transformer is around 250grams or more.
4)The dimensions are around 6 X 3 X 5 cm.

Yes I know which one is the primary and secondary, the red thick wire is the primary and other 5 different colour wires are of 5 seperate windings having five different voltages.
1) Blue wire gets around 8.8 volts
2) Yellow wire gets around 11 .1 volts
3) Green wire gets around 12.8 volts
4) White wire gets around 23.1 volts
5) Red wire gets around 3.7 volts
(These are all the output voltages when the transformer is given 230VAC as input.)

Also I was using a digital multimeter, in ampmeter selection in series with some high wattage ceramic resistance to test the maximum output power of the transformer but it showed nothing, then I tried measuring a load with my multimeter to check weather it's fuse is blown or not and found that my multimeter is all perfect.

I am really confused about what to do next, as I want to use this transformer for building a variable powersupply circuit, but without verifying the maximum power output I can't design the circuit properly.

Thanks again for helping me,
Will be waiting for a reply from you.
Regards,
Robbin
 

Attachments

Thread Starter

Sparkyrobbin

Joined Feb 26, 2018
4
As a general rule you can go by the core size for (K)Va and the various wire gauge.
Is it a Torioidal or EI type?
Max.
It's an El type small transformer, but I haven't measured the wire gauge and the core size, it's quite small and maybe I have to disassemble the transformer for measuring the core size.
Can you give me some other idea please ?

Regards,
Robbin
 

Thread Starter

Sparkyrobbin

Joined Feb 26, 2018
4
Do you have a sine wave signal generator?
You should be able to find the turns ratio and figure out the secondary voltages. Set the generators output to 1v RMS, 60hz and connect to primary. Then CAREFULLY (by that I mean treat it with respect!) measure the output voltage of each secondary with an RMS voltmeter.
Thanks for your reply, but I don't have a sine wave signal generator, also as I am new in power electronics I am not that much experienced about these things.

Regards,
Robbin
 
Top