0
\$\begingroup\$

I have a 220VAC:1200VAC 800VA 60Hz toroidal voltage transformer and I'd like to measure its parameters to simulate a circuit I'm gonna build with it, in order to understand better the transients. The measurements doesn't need to be very accurate. I have a very limited knowledge about transformers. My questions are:

  • Is it OK to measure the primary leakage inductance using and LCR meter @ 120Hz? The secondary should be open or shorted (and why)?
  • Is it OK to measure the secondary leakage inductance using and LCR meter @ 120Hz? The primary should be open or shorted?
  • Is it OK to measure the winding resistances with the LCR?
  • How to measure the magnetizing inductance and resistance related to core losses?

Also, I'm having trouble to simulate transformer models in OrCAD Capture CIS. Would something like this be fine for my analyzis?

enter image description here

\$\endgroup\$
2
  • 1
    \$\begingroup\$ Even if it's a bit long, try to read through this thread (external link). It has a lot of details. As for your equivalent schematic in SPICE, it whould work, but you have a builtin coupling factor, k; maybe it's better suited, maybe not, try it. Or try to see this page, even if it's for LTspice. \$\endgroup\$ – a concerned citizen Aug 9 '20 at 6:07
  • \$\begingroup\$ Thank you. I'll take a look at it. \$\endgroup\$ – Rohde Aug 9 '20 at 18:11
1
\$\begingroup\$

120 Hz as a test frequency is close enough to 60 Hz to get reasonable results.

The problem when measuring transformer leakage inductance is that you cannot avoid measuring the combined primary and secondary leakages of the transformer i.e. you measure a composite number either referred to the primary (when the secondary is shorted) or referred to the secondary when you short the primary.

To do this you have to apply shorts to the winding you are not measuring. This means that the magnetization inductance is largely removed from the measurement. And, when measuring magnetization inductance you run the opposite winding open circuit so that the effects of leakage inductance are largely not considered when making the measurement.

Because the magnetization inductance is usually at least 20 times more that the leakage inductance, shorting the secondary puts the secondary inductance in parallel with the magnetization inductance and, the number you read is therefore mainly the leakage inductance (because it is so much smaller in value): -

enter image description here

So, for a 1:1 transformer (as an example) if the secondary is shorted, (and ignoring copper losses), \$L_S\$ becomes in parallel with \$L_M\$ and, because \$L_S\$ is around +20 times lower in value compared to \$L_M\$, \$L_M\$ is more or less removed from the resulting number.

Additionally, you only need to do one measurement of leakage inductance because, which ever direction you do it in, it will be a combined measurement of primary and secondary. So, choose a measurement winding and then short the other winding out.

I'm having trouble to simulate transformer models in OrCAD

If I am making a transformer equivalent model in a simulator I try and mimic the full circuit shown above. I cannot say why your model isn't working for you.

Measuring core losses is more problematic because you have to apply full supply voltage to the primary and use a watt-meter.

\$\endgroup\$
2
  • \$\begingroup\$ So, just to be sure, considering a = Np/Ns: - If I plug the LCR in the secondary with the primary shorted, I'll be measuring Ls + (Lp//Lm)/(a^2) ? - If I open the primary I'll be measuring Ls + Lm/(a^2) ? - Since Lm is a parallel inductance with large value, is it ok to ignore it in the simulation model? \$\endgroup\$ – Rohde Aug 9 '20 at 18:29
  • 1
    \$\begingroup\$ Yes, the turns ratio squared times the primary parallel inductance of Lm and Lp plus, Ls. And yes for part 2. Ignoring Lm is optional - it all depends on what you are trying to do. Given that you are simulating, you might as well include it. \$\endgroup\$ – Andy aka Aug 9 '20 at 18:36

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.