New here, would like to discuss something about transformers

[Wattif]

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I am working on a real Isolation transformer-based test setup and would like predictions before posting measurements.

For this first question, I am intentionally leaving out the motor and capacitor so the focus stays only on the transformer interaction.

Setup:

AC source ? transformer primary ? load

Transformer secondary is tested in two conditions:

1. Secondary effectively shorted at ~0 ?
2. Secondary shorted through ~1 ?

Question:

What behavior would you predict on the primary/load side when changing from 0 ? to 1 ? on the secondary?

Specifically:
- Would primary/load voltage change?
- Would primary current change?
- Would the effect depend mainly on winding ratio?
- Would you expect the change to be small, large, or negligible?
- Any secondary voltage/current present?

[Wattif]

Simple bench test request:
If anyone has a near 1:1 isolation transformer available, I would be interested in independent measurements using a small resistive load only. No capacitor, no motor.

Setup: AC source ? primary winding in series with a low-watt resistive load. Secondary winding tested two ways:

1. Secondary shorted as close to 0 ? as practical
2. Secondary loaded with approximately 1 ?

Suggested load: small heater, small hair dryer on low/high, incandescent lamp, or other resistive load within safe ratings. No need for high wattage.

Measurements requested:

• Source/load voltage
• Source/load current
• Secondary voltage
• Secondary current
• Secondary resistance used

I am looking for observed data only: what changes between the secondary shorted condition and the ~1 ? condition?
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[Kinetic_Mind]

Hi Wattif,

I am curious as to what your use-case is for this circuit.

In general the isolation transformer can be re-drawn as an equivalent circuit using "Transformer Impedance Reflection" or "Transformer Impedance Referred to Primary" theory. This gives you an equivalent series impedance that your source sees for the transformer portion. This equivalent series circuit will then be in series with your primary's load (motor or otherwise). If you go through the math, which is simple - just algebra, you can determine the answers to all the questions you posted at the end of your first post.

If you would like to measure this you have a few options:

1) Download LT-Spice. It is a free, industry standard simulation tool. It can't take all real-world parameters into effect, unless you know how to accurately model them. But it will give you waveforms and a general idea of how your current will behave.

2) Build it and test it. I am assuming, that since you are asking the question you don't have a lot of experience. If this is wrong, I apologize. I say this because dealing with the voltages and currents in your schematic can be DEADLY. What you could do instead, is get a cheap function generator and do the experiment in the 100kHz range. You can find cheaper transformers (like a common mode choke) that you can use as an isolation transformer. Often new parts have accurate and free Spice models, which you can simulate first before purchasing.

3) If you have the experience and know how to work with DEADLY voltages/currents, you can wire up the schematic you have and measure using appropriate equipment and safety precautions.

-KM

[Wattif]

Thank you for the reply. I have actually built and tested the circuit extensively, including university testing. The reason I asked for predictions is to compare measured behavior with what people expect from transformer theory. One observation that interests me is that changing secondary resistance produces predictable changes in load voltage and secondary voltage. I was curious what specific changes you would predict between a secondary short and approximately 1 ?.