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 Post subject: Variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 12:18 pm 
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I have a need for a small variable frequency AC supply with a variable output voltage of at least 0-120Vac that can handle a minimum of 500mA.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 2:50 pm 
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What frequency?

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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 3:10 pm 
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The frequency range of one I use at work is 40Hz-5KHz.

Also it must have an analog frequency control.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 7:25 pm 
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Sounds like an audio freq. generator hooked up to a substantial audio amp.

The only thing like this I ever saw was a power source for "shaker tables."

Image

Rich


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 8:10 pm 
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The simplest, most economical route is an audio signal gen & a big solidstate PA amp.

I use that topology to run reel tape machines and tool motors.

Dennis

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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 8:23 pm 
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Tube Radio wrote:
The frequency range of one I use at work is 40Hz-5KHz.
Also it must have an analog frequency control.
Motor speed controls will do that, perhaps not that wide a frequency range, but certainly 50Hz.

Search for "Variable-Frequency Drive" or VFD.
They're usually a lot larger than half an amp, but some small ones do exist.

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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Mon 29, 2018 9:32 pm 
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Probably a VFD is not the best choice if he wants true sine wave output.

The best supply for this is made by a company called Kepco, who makes just
such a machine, typically called a BOP (bipolar opamp power supply). He might be able to find a surplus one.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 12:02 am 
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https://archive.org/details/73-magazine ... 5/page/n32

Page 30 is a pure sine, variable frequency 100W inverter. Author even talks about those times when you want other than 60Hz.

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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 12:36 am 
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jim rozen wrote:
Probably a VFD is not the best choice if he wants true sine wave output.

The best supply for this is made by a company called Kepco, who makes just
such a machine, typically called a BOP (bipolar opamp power supply). He might be able to find a surplus one.


What I'm talking about is something along these lines.

Image

Basically it is a variable frequency AC power source.

I use them at work to generate 115Vac 400Hz for a couple test sets I operate.

Those are basically like what one would do with an audio generator, power amp and transformer except it is all in one self contained unit.

I don't need that specific model though. That's just an example of what I'm looking for.

Unless the VFD has a small enough frequency adjustment resolution, I would have to use an analog frequency adjustment as the speed must be exactly 33 1/3 and 45 given I am very pitch sensitive.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 5:22 am 
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Elgar Corporation have been a leading brand of variable frequency power supplies. Their lowest power model was the 121. I use one to power an 1896 Westinghouse “Tesla” electric fan, which has ten poles and was made for the 133-cycle lighting systems that Westinghouse had installed in the U.S. under license from Gaulard & Gibbs. My 121B came with a 300 – 500 cycle plug-in oscillator, which I modified to add a second range. There is also a miniature phone jack to permit the use of a signal or function generator.

These power supplies can be found on eBay, and with patience, a listing with a low price can be found. Elgar called these power supplies “A.C. Power Source”. I think that the Models 251 and 501 are the most commonly found of the older models, but you will need one with an analogue wide-range oscillator installed, unless you are willing to use a separate function generator. Later models have numerical entry for frequency and voltage selection.

Note that on many of the older types, the voltage control is essentially a “volume control” on the drive to the power amplifier. This means that the full rated power output is available only at the full voltage setting. In order to obtain the full output at a lower voltage, a variable auto-transformer can be used, up to the frequency limit of the latter. I use this set-up with the Tesla fan, because it requires 100 volts, and I need the full output of the Elgar supply at start-up of the fan motor.

Because these power supplies have large audio transformers, they are heavy and can be expensive to ship.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 1:48 pm 
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Yes those can be expensive to ship.

But I gotta have it unless there's a comparable way to do it using an analog frequency control.


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 Post subject: Re: WTB: variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 3:22 pm 
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W7TFO wrote:
The simplest, most economical route is an audio signal gen & a big solidstate PA amp.
I have a friend who deigned and manufactured equipment used in hospitals worldwide. He used an audio generator connected to a 500 watt Hafler solid state amp to test the equipment powered up on other voltages and frequencies. But I think there was a step-up transformer involved.

Dave


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 3:37 pm 
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A General Radio 1308-A Audio Oscillator and Power Amplifier will do what you need. It'll source up to 200 watts.

http://www.ietlabs.com/pdf/Manuals/GR/1 ... 20Amp..pdf

IET says they will sell you a refurbished one, although I imagine the price would be quite high:
https://www.ietlabs.com/genrad/genradrefurb.html

For only 500 mA, though, the most economical solution is a 60 watt per channel hi-fi amplifier driven by an audio oscillator, with the amplifier driving the 12.6 volt secondary of a filament transformer rated for at least 5 amps on the secondary. And an AC voltmeter, to set the output amplitude.

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 3:52 pm 
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+1 to ideas from Dave and Steve----probably the cheapest and easiest solution

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 5:22 pm 
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I have a Bogen CHS-60 amplifier that I could possibly use. It uses a capacitor to couple the output of the amp to a transformer which then provides the various impedance taps including a 70 volt line.

The CHS-35 (35 watt version) can drive a speaker directly from the output coupling capacitor and given the similar circuitry of the CHS-60 it too should be able to drive a 4 ohm load from the output coupling capacitor.

The 70 volt tap on that amplifier won't put out enough voltage.

I do have a 200 watt 70 volt transformer with several taps and a low impedance output so perhaps I could use that to step up the voltage enough.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 5:33 pm 
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Moderators, is it possible to have this topic merged with this topic viewtopic.php?f=12&t=348551 which is where I was discussing the design of a speed control circuit.

This topic was supposed to only be an ad wanting a power supply, but it turned into a discussion instead.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 6:38 pm 
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Tube Radio wrote:
I do have a 200 watt 70 volt transformer with several taps and a low impedance output so perhaps I could use that to step up the voltage enough.

I think you might be better off with a filament transformer, rather than stressing a 70 volt line transformer with low frequencies and 2X voltage overload. Does the CHS-60 have a 4 ohm tap? Connect a 12 volt filament transformer there.

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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 7:15 pm 
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stevebyan wrote:
Tube Radio wrote:
I do have a 200 watt 70 volt transformer with several taps and a low impedance output so perhaps I could use that to step up the voltage enough.

I think you might be better off with a filament transformer, rather than stressing a 70 volt line transformer with low frequencies and 2X voltage overload. Does the CHS-60 have a 4 ohm tap? Connect a 12 volt filament transformer there.


It does have a 4 ohm tap

But the amp should be able to drive a 4 ohm load directly without the transformer.

At least the CHS-35 can. The CHS-60 is the same basic amplifier design only it can put out 60 watts.

Not sure what voltage the 70 volt 200 watt transformer will put out at its maximum power into the proper load.

I do only need 100 Vac though since the record player can be set for 100-240Vac operation.

Depending on the load perhaps I could parallel the 200 watt transformer with the built in transformer and put the 70 volt taps in series.

I don't have a transformer with 12 volt secondary, but I may have a transformer with a 6.3vac and 5 vac windings which I can put in series for 11.6Vac which should be close enough.

I may try the 200 watt transformer though and use my scope to see what the waveform looks like provided I can find that transformer.

It's a real big beefy transformer. Forget why I got it though.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 7:29 pm 
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Mind sharing what is the application?

I've done this with an audio frequency generator, and a Dynaco ST-70 with both channels working in parallel, fed into a step-up transformer. I used a tube amp mostly because I had it, and I was worried that a solid state amp could lose an output transistor if anything goes wrong with the load. Tubes are very forgiving when it comes to shorts and momentary overload.

I used it to power European turntable motors with 50 Hz. Later, I machined down the motor drive shaft and abandoned the kludge.

After much searching I came to the conclusion that the kudge of an audio amp, transformer, and audio generator is really the cheapest and easiest way to do this. Maybe down the road you could buy an audio generator board off eBay, a power amp board, and a pair of toroid transformers and put it all into a 19" rack enclosure. I think it would still be cheaper than buying the real deal.


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 Post subject: Re: Variable frequency AC power supply
PostPosted: Oct Tue 30, 2018 7:32 pm 
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Echoing Steve's last comment, I advise working through the engineering steps.

First, you are looking for (IIRC) a frequency range of 60-400Hz. That lets out a lot of OPTs..thus the suggestion to use something actually designed for 60Hz.

Second, look at all the voltage options you have out of the amplifier. Your requirement is for 60watts to the final load. Assuming no losses, you need 60 watts out of the amp. Crudely, one would hook it up to an appropriate load and measure the voltage. Once you know the voltage you have, then you know what transformer will work.

Third, do you need 60 watts continuous? If so, is your amplifier rated for that. I think there are a lot of 60-watt amps out there that would go into meltdown if they were delivering a 60 watt sine wave into a load. (Many here will be smarter on this question than I am)

When I first saw the question, I started ruminating about how one might build from scratch an amplifier specifically designed to put 120 vac into a 240 ohm load. How about a bunch of power MOSFETS in parallel, operating as source followers.....all mounted to a nice big heat sink?

FWIW.......

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