0
\$\begingroup\$

How does one choose a proper main output transformer for a power inverter? Is enough to simply use a "standard" transformer "in reverse"? Also, do toroidal transformers convey any special advantages to inverters?

This would be for a North American system with a 120v/60hz output, but the DC input voltage is negotiable, and could be 12v, 24v, or something else.

\$\endgroup\$
5
  • \$\begingroup\$ Welcome to EE.SE! Transformers are bi-directional so using one intended for 120->24 Vac down conversion would work just as well in converting up 24->120 Vac. Please share details about your inverter. \$\endgroup\$
    – winny
    Sep 3 '20 at 13:51
  • \$\begingroup\$ Thank you. I wasn't sure if either running in reverse, or using for an inverter posed any special requirements. i googled for this answer, but came up with nothing. My interest is academic. I have no specific design yet, but I may take a stab at building one. I've seen plenty of information on the issues of drive circuits & modulation, but I have yet to find anyone discuss the issues of the transformer. \$\endgroup\$
    – spinjector
    Sep 3 '20 at 15:02
  • 1
    \$\begingroup\$ If you really want to get into technicalities, inverter transformers need to withstand the common mode DC voltage from most inverters, but at these low voltages, you don’t need to worry. \$\endgroup\$
    – winny
    Sep 3 '20 at 17:44
  • \$\begingroup\$ Would that be what's called "DC offset" in the audio world..? Like when an AC signal doesn't alternate perfectly centered across zero, and is offset slightly above or below..? \$\endgroup\$
    – spinjector
    Sep 3 '20 at 23:48
  • \$\begingroup\$ Only in the CM domain. A differential DC offset would saturate your transformer within miliseconds. But you don't need to worry about this unless you scale it up to kV levels for a train or wind turbine inverter or similar :-) \$\endgroup\$
    – winny
    Sep 4 '20 at 9:31
1
\$\begingroup\$

How does one choose a proper main output transformer for a power inverter? Is enough to simply use a "standard" transformer "in reverse"?

This answer assumes that a transformer with a regulation factor of 10% is used. For bigger transformers (circa 100 VA or beyond) the regulation might be 5% or less.

It would be really nice if a step down transformer producing (say) 12 volts AC from an input of 120 volts AC has a turns ratio of 10:1 in step down. Unfortunately this isn't quite the case because transformers are not ideal and might "sag" the output voltage by 10% on full load. This means that a 120 volts to 12 volts transformer probably has a turns ratio more like 9:1 and, unloaded, the output will be 13.2 volts. On full load it will be 12 volts.

OK, given that knowledge about power transformers, using it as a step up device from an AC source of 12 volts RMS means that the unloaded output will be 109 volts (not 120 volts). And, under full loading conditions the output might drop to 99 volts.

do toroidal transformers convey any special advantages to inverters?

Not particularly, given that you might need to add 20% turns to the high voltage winding and rewinding a toroid (by hand) is a pain compared to a regular transformer (which can usually be dismantled).

\$\endgroup\$
3
  • \$\begingroup\$ Ahhh..! So that's why unregulated "12v" wall-warts always seem to be putting out more than that. I've vaguely wondered that ever since I was a kid. In an inverter, would a possible solution to this be a multi-tap transformer, the kind with a bunch of extra taps at the end of the primary..? Of course it would need a feedback & switching system, probably synchronized to the zero-crossing point..? Or I suppose it could/would be better handled in the duty cycle of the PWM waveform dished out by the switching controller (this said with a pure-since inverter in mind). \$\endgroup\$
    – spinjector
    Sep 3 '20 at 17:00
  • 1
    \$\begingroup\$ Use PWM is my advice. \$\endgroup\$
    – Andy aka
    Sep 3 '20 at 17:02
  • \$\begingroup\$ @spinjector are we done with this question and answer now? \$\endgroup\$
    – Andy aka
    Sep 5 '20 at 13:04
0
\$\begingroup\$

Click here and navigate to conclusions to read about reversing a step down into a step up transformer for your power inverter. There's more detailed explanations given on what are the technical factors to be considered.

Regarding the toroidal transformers, Yes they have a certain advantage because of the way they are coiled, there is reduction of resistive/winding loss, shorter coil span compared to a standard transformer. But this is a power transformers and mostly not popularly used as Inverter Transformer. You can consider looking up the difference between the inverter and power transformer for your benefit by clicking here.

Generally, Toroidal input side takes AC and outputs AC. But inverters take DC into AC. So that should sum it up. Hope this helped.

\$\endgroup\$

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.