New Help Finding an Isolation Transformer

[wmgeorge]

I need to look for one like that and you got a good deal. There is really no reason why I can not feed 120 volts into it and get out 240 volts out for my Mitsubishi mini split Heat Pump.

 

[Larry Oswald]

I need to look for one like that and you got a good deal. There is really no reason why I can not feed 120 volts into it and get out 240 volts out for my Mitsubishi mini split Heat Pump.

 

[Zoomyn]

Study the nameplates - eBay etc has deals but most are sourced from industrial processes that has low/no concern for economies. My first thought was temperature rise under load, a wham-bam manufacturing circuit wouldn’t care but a 30 or 40C rise would cook other electronics in the same enclosure.

Also - inrush current.. use slowest reacting circuit breaker to keep false trips from happening, might not be an issue on inverter supply but mains power can strobe 300 or 400% for that first 5 or 8 millisecond.

One search keyword to consider is “Medical” use - there are some slick toroidal units available, get two identical xfmrs and run them in parallel…

 

[Hedges]

Also - inrush current.. use slowest reacting circuit breaker to keep false trips from happening, might not be an issue on inverter supply but mains power can strobe 300 or 400% for that first 5 or 8 millisecond.

I've been studying that a bit. Using sine waves, switch a few times and hit various points in the phase at random. Also, apply CV/CC DC supply, watch with a scope as inductance holds off current for about 20 milliseconds, then current shoots up in saturation.

If someone got a 240/480 to 120/240 transformer and had inrush issues using it as a 120/240V auto-transformer, simple solution is to use the primary windings instead. Applying 120V to 240V winding or 240V to 480V winding, the higher inductance will hold off current longer, so should never enter into saturation.

(The issue with transformer inrush is if power was removed after a positive AC cycle, then reconnected just before a positive AC cycle, magnetizing the core in the same polarity twice brings it closer to saturation. A winding meant for twice the voltage, or a transformer meant for lower frequency, will have more margin.)

For an isolation transformer, could use the 120 + 120 = 240V winding for 120V. Use the 240 + 240 = 480V windings for 120/240V split-phase (or 240V single phase.)

Sorry, don't have a saturation cure for 240V single-phase to 120/240V split-phase using transformer with that voltage configuration.

The other point I've read about transformer saturation is that where one winding is on top of the other, outer winding doesn't saturate as badly because it has more leakage inductance to the air. Would depend on winding configuration; if side by side rather than on top of each other, no "better" winding to select.

 

[Larry Oswald]

@wmgeorge
I like your idea of using a transformer "backwards" as a step up from 1`20VAC. Not sure abut the power required for a heat pump. This one used backwards would output 260VAC. Not sure about that either.

 

[timselectric]

Also when stepping up. You need twice the amps going in, than coming out. This usually makes for a higher than standard amperage 120v circuit.