How can Center-tapped Transformer parameters be found?

[Ramazan Duzgun]

TL;DR

I have a center-tapped transformer and need to find its parameters such as winding reactance and leakage reactance. Which tests must be applied ? (Laboratory and equipments are available.)

It is a common center-tap transformer which is electrically demonstrated as in figure (ideal one);

In my opinion, open circuit and short circuit tests cannot be applied directly. How can I place secondary reactances ? And how can I test its all parameters in laboratory ?

 

[Baluncore]

Is it a power transformer or signal transformer?

What primary voltage is it designed for?
What is the design frequency range?, how thick are the laminations or what is the grade of the core?

While no secondary load is present, you can use an impedance bridge to measure the inductance and resistance of the primary. That needs to be done at the design operating frequency.

 

[Ramazan Duzgun]

Actually I don't have entire design data. It says on the transformer " E130-500A 21: 3: 3: ". This is a high frequency power transformer, probably 30 kHz. Its core size 15 cm x 15 cm.

 

[Baluncore]

That is a big ferrite transformer core. How do we interpret the "E130 500A 21:3:3" ?
My guess would be that it is from a switching power converter.
The E130 might be the core material, style, maybe made by, or a clone of EPCOS.
The 500A could be the size specification.

Maybe the 21:3:3 is the turns ratio of the primary : secondary+secondary.
But how do you know which is the primary and which the secondary, it may have push-pull drive to a balanced tapped primary winding, with a bridge rectifier on a single winding secondary? That might do better at preventing saturation of the core.

How thick are the conductors, tapes, wires or the twisted bundles of wire?
Can you count how many turns are on any of the windings?
There are ways to measure turns ratios by threading a temporary driver winding through the core, then measuring the voltages on other windings and looking for integer ratios.
Once the application and general properties are known it will be easier to work out what tests are needed.

 

[Ramazan Duzgun]

21 is the primary, HV side, because the cables are thinner than secondary. Cables at the tap point (mid cable) are really large in comparison to top and bottom terminals cable. But I can not reach its number of turns and cable sizes. Its input rated voltage is 320 V square wave (feeding from H-Bridge), its secondary rated current 500 A.

 

[Baluncore]

Remember that for one turn, the wire must pass once through a toroid, or through both holes of an E core. Thread 5 turns of thin insulated wire through the core as a test winding.

Test / Experiment 1.
You can then apply say 10kHz AC, 1V per turn = 5 Vrms to the test winding. If you don't have a sufficiently powerful audio signal generator, use an audio amplifier to boost the signal.
Measure the primary and the secondary voltage with a high impedance oscilloscope.
Expect it to be a lumpy waveform, but the voltage ratio will give you an idea of the turns ratio. Note the primary voltage for the next test.

Test / Experiment 2.
Drive a primary winding with the appropriate higher 10kHz signal. Measure the voltages on the test winding and on one secondary. There should be some simple ratio of two integers. You might adjust the number of turns on the test winding to make the test winding voltage equal to the secondary voltage. You then know the actual number of turns on the secondary.

 

[Ramazan Duzgun]

Thanks, but I want to apply open circuit and short circuit tests. Turns ratios are known already, 21:3:3. My aim is to find its magnetizing reactances and copper reactances. I am not sure what frequency is required to not saturate the transformer. Probably, feeding by 320 V at grid frequency (50 Hz) would be wrong.

 

[Baluncore]

Can you confirm that it is an E section, ferrite core.

First use an AC bridge with a 1kHz to 10kHz test frequency, to measure the primary real resistance, and reactive inductance. Then you can determine what test voltages to use. Check the primary resistance with an accurate DC digital multimeter. What are your results at different frequencies?

What model will you select for the transformer primary? 1. Series inductance? 2. Series resistance, copper loss? 3. Parallel input capacitance? 4. Switching core losses?

 

[anorlunda]

This IEEE document describes a number of factory test protocols, including ratio tests.

http://sites.ieee.org/houston/files/2016/02/HV-Transformers-Factory-Test-Oct-8-9.pdf

This document focuses more on field testing.

 

[Ramazan Duzgun]

Can you confirm that it is an E section, ferrite core.

First use an AC bridge with a 1kHz to 10kHz test frequency, to measure the primary real resistance, and reactive inductance. Then you can determine what test voltages to use. Check the primary resistance with an accurate DC digital multimeter. What are your results at different frequencies?

What model will you select for the transformer primary? 1. Series inductance? 2. Series resistance, copper loss? 3. Parallel input capacitance? 4. Switching core losses?

Yes it is E section ferrite core. But I am confused with the last thing you said; should I choose a model for primary to apply a test ? Or why I can not use common transformer model which is shown below figure ? And if I apply AC test voltages from 1kHz to 10kHz, according to different frequencies I will get different reactances values, so which one will be correct one? Don't we need to consider the saturation ? if we do, how ?

Thanks for your interest!

 

[Ramazan Duzgun]

This IEEE document describes a number of factory test protocols, including ratio tests.

http://sites.ieee.org/houston/files/2016/02/HV-Transformers-Factory-Test-Oct-8-9.pdf

This document focuses more on field testing.

Thans! it helped.