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I'm looking for a variable AC power source for use in some electromagnetics experiments and longer term for testing inductors, transformers, core material frequency response, and that sort of thing.

I'm thinking my dream instrument would have voltage variable between 0-125V, be capable of providing a few amps, maybe 10A at low voltages (say 24V or less), accept an arbitrary waveform from my signal generator to determine the AC out (possibly including a DC offset), and have a frequency response from DC to say 10kHz.

I'm willing to compromise on all of those however, especially to get started. I already have a variable 60Hz AC supply, one of the Variac + Isolation transformer type. So the first big addition would be variable frequency.

I've done a lot of searching around on the web, and such a device seems to be difficult to find. The closest thing I've found is called an AC Power Source and these seem to have a limited frequency range, especially on the lower end, bottoming out around 45 Hz it seems. They also seem to be limited to sinusoidal waveforms. These have great power capabilities, but appear to be somewhat targeted toward mains supply simulation for compliance testing or whatever. At about $2k to get started, they're also a bit more than I wanted to spend for essentially academic/hobbyist purposes. Here's an example of one I found:

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Based on this answer I'm inclined to think a possibly better approach would be to use an audio power amplifier. I think the frequency response would be good enough and taking in an arbitrary waveform is natural for such an amplifier.

I'm not sure about the peak voltage I'd be able to attain with such an amp, I expect it would depend on the source voltage (say 25 VAC in the amp suggested in the answer above). But I'm curious enough about that approach that I'm thinking about hooking up my 40+40W Pyle amp just to see what it can do :)

So I guess my question is this:

  • Is there a standard lab instrument available for this sort of thing and I just haven't found the right search term yet?

  • Or should I really be thinking about a power amplifier for these sorts of purposes and be focusing on how to get the output voltages and currents I want, possibly by adapting such a circuit?

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  • \$\begingroup\$ Ooh, just found a comment on a question that popped up as related after I posted. @DaveTweed proposed there using a step-up transformer on the output of an audio amplifier fed by a signal generator. That looks worth a try for the higher voltages :) \$\endgroup\$ – scanny Jan 3 '16 at 6:06
  • \$\begingroup\$ I wonder if you could butcher a Variac to serve as that output step-up, although frequency response could be troublesome...(you may wish an isolation transformer somewhere in the chain as well) \$\endgroup\$ – ThreePhaseEel Jan 3 '16 at 6:11
  • \$\begingroup\$ @ThreePhaseEel - Hmm, that's an interesting idea. I suppose it would involve adding a winding to the Variac, is that what you were thinking? I actually have one in the lab that is burnt on the last dozen turns or so, that might be just about the right number for a primary that steps 20V or so up to 120. I just love inductors, don't you? If it isn't what you want you just take some wire off or put some more on :) I know it's not quite that simple of course, but they're one of the few electronic devices you can change with your own hands :) \$\endgroup\$ – scanny Jan 3 '16 at 6:18
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    \$\begingroup\$ You might be better off if you approach this as a power stage after a small-signal function generator (like in your second bullet). In the industry, I have seen setups with separate function generator and power amp (up to 10MHz and 100W). Among other things, it was used for characterizing inductors. \$\endgroup\$ – Nick Alexeev Jan 3 '16 at 6:44
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    \$\begingroup\$ Regarding audio amplifiers: for low voltage (0-120ishV) and limited bandwidth (~5Hz-25kHz), a power amp would do wonderfully. However, the load needs to be pretty small (say, 1-32Ω). A big beefy amp like the QSC PLD4.5 will happily push 140V+ into an 8Ω load. However, the DC offset might be a problem. You might need to build your own custom high-power class-D amplifier and feed it from a signal generator. \$\endgroup\$ – uint128_t Jan 3 '16 at 6:46
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Look around for audio amplifiers in the 2kw / 3kw power region. I think you'll be surprised at the low price (I was).

What is advertised as a 3kW amplifier is usually a 1500w stereo into 4ohm loads that's bridgeable into a mono 8ohm load.

If you do the sums, that's a mono output capable of 150 vrms into 8ohms, as long as you are happy with a floating load, as the outputs are bridged. If you parallel the two stereo channels, that would drive a ground referenced 2 ohm load with 75vrms, or over 35Arms.

To buy a tranformer, even in the sub-kw range to convert a lower power amplifier's output up to the 100v region, is quite expensive (at least I was surprised by how much they tend to cost). With a transformer, not only do you have a lower frequency limit with voltage (core volt.seconds), but you also have an upper frequency limit with winding capacitance and leakage inductance, and core losses. I can't see a 'mains' transformer getting to 10kHz.

For your voltage, current and frequency requirements, I'd just go for a single commercially available kw audio amp.

WARNING just because it's audio doesn't mean it isn't dangerous. Even a 1kW amp can develop lethal voltages, and fire-starting currents. Take all the precautions with the output of such an amplifier that you would take with mains.

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As an alternative to the audio amplifier solution, with different strengths and weaknesses, you could consider a variable frequency drive (VFD) designed for AC motors on machine tools.

Strengths :

  • Power levels above 1kw are easy to find and inexpensive.
  • Supply 1 or 3 phase at mains voltages, can be stepped down to other voltages with a mains transformer or possibly a variac.
  • Often have soft start facilities that you can program to generate frequency sweeps

Disadvantages:

  • Waveform purity is not usually a design goal : they may generate a stepped square wave, the motor on your lathe doesn't care. It may be possible to clean up with a filter.
  • Limited range of frequencies : 0.1 to 3x normal speed (e.g. 5-150Hz) is probably all you can expect
  • The usual dangers accompanying these power levels.

I am not suggesting that either of these approaches is universally better.

It may be that you can use both : an audio amplifier for precision, flexibility ( 1Hz to 10kHz) and low voltage testing, augmented by an inexpensive VFD for high voltage tests.

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  • \$\begingroup\$ This is a great idea Brian! I've been looking for an excuse to get a VFD and fool around with 3-phase power; this looks like just the good reason I needed :) After transformers and inductors, I'm sure I'll get around to motor and generator windings before I'm through, so this will likely come in handy :) \$\endgroup\$ – scanny Jan 3 '16 at 23:32
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    \$\begingroup\$ You will also have a hard time since the U/F ratio is often not very programmable. Eg: reducing F will reduce U. \$\endgroup\$ – Jeroen3 Jan 2 '18 at 10:37

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