Hedges
I See Electromagnetic Fields!
- Joined
- Mar 28, 2020
- Messages
- 20,695
My guess is bad waveform from inverter.
I was thinking for a moment that a 24VDC supply (SMPS, no transformer) would work, but the relays and solenoids were designed for AC.
I have not come up with a simple fix. Possibly a high wattage incandescent bulb in series would protect it from burnout and still allow operation.
To kill a transformer, you can overload the output so it draws too much current, you can apply higher voltage or lower frequency so it goes into saturation and draws excess current, or you can apply DC (frequency = 0) and it will draw lots of current.
Assuming the transformer isn't overloaded, I'm guessing the HF inverter is doing something wrong, maybe asymmetric waveforms. Something my transformer isolated LF inverter isn't capable of (it could damage its own isolation transformer, but not any loads.)
A transformer is a funny thing, an electromagnet. Run current through a winding, and the iron core becomes magnetized. While the core is getting magnetized, it pushes back and resists current increase. Once it is magnetized fully, no resistance to current increase due to the core, just due to inductance of the windings as an air-core inductor.
No problem if you do that with a current source. Apply a voltage source, current rises slowly as it gets fully magnetized, then shoots up to whatever winding resistance works out to.
Reverse polarity of voltage source, process repeats. (a fun experiment with CV/CC supply, transformer, oscilloscope with current probe.)
Not supposed to be a problem with AC. The transformer gets turned into a permanent magnet, Oriented N-S one way, then S-N the other way. But only if waveforms are symmetric. If asymmetric, it "walks" into saturation, then burns out.
The same magnetization of core causes inrush. If turned off after a phase magnetizes it N-S one way, and turned on with same phase, it goes into saturation and current shoots up. Sometimes trips breakers.
I think you've discovered one more thing that inverter designers neglected to consider. I've seen this before, where a "special" inverter had transistors directly driving a transformer. When I had to design such a circuit, relatively low power and much higher frequency, for an instrument, I put capacitors in series to make absolutely sure it could not happen.
I was thinking for a moment that a 24VDC supply (SMPS, no transformer) would work, but the relays and solenoids were designed for AC.
I have not come up with a simple fix. Possibly a high wattage incandescent bulb in series would protect it from burnout and still allow operation.
To kill a transformer, you can overload the output so it draws too much current, you can apply higher voltage or lower frequency so it goes into saturation and draws excess current, or you can apply DC (frequency = 0) and it will draw lots of current.
Assuming the transformer isn't overloaded, I'm guessing the HF inverter is doing something wrong, maybe asymmetric waveforms. Something my transformer isolated LF inverter isn't capable of (it could damage its own isolation transformer, but not any loads.)
A transformer is a funny thing, an electromagnet. Run current through a winding, and the iron core becomes magnetized. While the core is getting magnetized, it pushes back and resists current increase. Once it is magnetized fully, no resistance to current increase due to the core, just due to inductance of the windings as an air-core inductor.
No problem if you do that with a current source. Apply a voltage source, current rises slowly as it gets fully magnetized, then shoots up to whatever winding resistance works out to.
Reverse polarity of voltage source, process repeats. (a fun experiment with CV/CC supply, transformer, oscilloscope with current probe.)
Not supposed to be a problem with AC. The transformer gets turned into a permanent magnet, Oriented N-S one way, then S-N the other way. But only if waveforms are symmetric. If asymmetric, it "walks" into saturation, then burns out.
The same magnetization of core causes inrush. If turned off after a phase magnetizes it N-S one way, and turned on with same phase, it goes into saturation and current shoots up. Sometimes trips breakers.
I think you've discovered one more thing that inverter designers neglected to consider. I've seen this before, where a "special" inverter had transistors directly driving a transformer. When I had to design such a circuit, relatively low power and much higher frequency, for an instrument, I put capacitors in series to make absolutely sure it could not happen.
