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Author Topic: Autoformers, anyone?  (Read 1646 times)

John Roberts {JR}

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Re: Autoformers, anyone?
« Reply #20 on: October 19, 2017, 10:20:40 am »

Is autoformer a real word in the US?


Steve.
I always thought it was a word,  :-[ ?

Here's a TMI autoformer story...  Back when Peavey entered the fixed install market where most install power amps use output voltage step up transformers to hit the nominal 70V/100V constant voltage (misnomer) sound distribution. Substituting autoformers for those output transformers would not only reduce significant weight, size, power loss, and cost, but the audio frequency response and distortion would even be better.

While it should have been a win-win-win-win....., it was a lose when the customers rejected them. The primary downside to autoformer outputs is that they are not completely floating. Apparently inadvertent ground shorts to one side or the other of 70V audio lines is pretty common. With a transformer's floating output, the short is ignored and the beat goes on. With an autoformer if the hot side is shorted (50-50 chance), the music stops and a difficult (expensive) service call is required. Further an amp upgrade in an old install from a previous transformer output model might not work as expected, until wiring is fixed, and lastly the install technicians didn't need to pay any attention to which output wire was which.

The install market was (is?) conservative and resistant to change so Peavey had an uphill battle even with no identifiable differences to point to. Since the customer is always right, we re-engineered the entire line to use the larger, heavier, less efficient, transformers. (ouch   :o ). That is a lot of copper and iron that arguable wasn't really needed.

JR
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Kevin Graf

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Re: Autoformers, anyone?
« Reply #21 on: October 20, 2017, 07:28:32 am »

Is autoformer a real word in the US?
Steve.
Nope, the real word is autotransformer.
But in the old days, we would call it an auto-xformer.
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Peter Morris

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Re: Autoformers, anyone?
« Reply #22 on: October 20, 2017, 09:31:52 pm »

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Andrew Broughton

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Re: Autoformers, anyone?
« Reply #23 on: October 21, 2017, 01:16:52 pm »

I used a variable autotransformer in China that worked as a voltage regulator. Hadn't seen that before, seemed rather ingenious. Voltages were all over the place in China so it worked very well keeping the output voltage steady without the "jumps" of a standard autotransformer. You could hear it humming away as the wiper moved slightly one way or the other, keeping the output steady.
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John Roberts {JR}

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Re: Autoformers, anyone?
« Reply #24 on: October 21, 2017, 03:09:42 pm »

I used a variable autotransformer in China that worked as a voltage regulator. Hadn't seen that before, seemed rather ingenious. Voltages were all over the place in China so it worked very well keeping the output voltage steady without the "jumps" of a standard autotransformer. You could hear it humming away as the wiper moved slightly one way or the other, keeping the output steady.
Variable autoformers are AKA "Variacs" (trademarked by General Radio in 1934). But Variacs are generally manual with a big knob and sometimes a voltmeter for the user to vary. These are common on test benches to troubleshoot/repair equipment.

Making it automatic is clever, as I mentioned already utilities use similar bump windings (also automatic) to manage mains voltage at substations as load increases and drops off over the day.

An anecdote I've shared too many times, the bump winding at my local substation got stuck one night (probably a decade or two ago). When I got home from work, I noticed my lights were unusually bright and getting brighter as the night went on... I called the power company and the guy working that night didn't believe me (I measured my wall voltage at 135V and rising). But it was a slow night so he drove out the 25 miles to check it out.

When he confirmed my measurements on the drop coming into my house, he drove the couple miles to the substation and probably whacked the stuck stepping contactor with a big non-conductive hammer.

JR
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Andrew Broughton

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Re: Autoformers, anyone?
« Reply #25 on: October 21, 2017, 03:52:14 pm »

Variable autoformers are AKA "Variacs" (trademarked by General Radio in 1934). But Variacs are generally manual with a big knob and sometimes a voltmeter for the user to vary. These are common on test benches to troubleshoot/repair equipment.

Making it automatic is clever

Yeah, it used a servo motor fed from some voltage measuring circuitry and quickly corrected any voltage changes in the buildings, which happened quite often. Also worked from about 240v down to less than 80v. No protection against spikes, but for what we needed, it was a relatively low-tech solution that worked very well.
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-Andy

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Peter Morris

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Re: Autoformers, anyone?
« Reply #26 on: October 21, 2017, 08:31:24 pm »

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Mike Sokol

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Re: Autoformers, anyone?
« Reply #27 on: October 22, 2017, 09:01:44 am »

I used to have a humongous Variac with a 240-volt input and variable output down to 0 volts. I think it was rated for 30 amps at 240 volts, so 6KW or more) and it easily weighed over 100 pounds, But I stupidly sold it for scrap 20 years ago while cleaning out my garage. Now I really want a BIG Variac for my voltage experiments, and I don't have one.

Never throw anything away!!!

John Roberts {JR}

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Re: Autoformers, anyone?
« Reply #28 on: October 22, 2017, 11:35:27 am »

You can also do this trick - http://www.powerqualityworld.com/2011/04/constant-voltage-ferroresonant.html
I first got exposed to magnetic amplifiers in the 60's (used inside an early switching power supply for current limiting). Pretty old school technology.

As your link notes a ferroresonant transformers is effectively running at (magnetic flux) saturation, so pretty inefficient for variable loads. Clever stuff for the very old days. 

I can't say that I ever saw one in the wild (that I recognized as one).

JR
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Peter Morris

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Re: Autoformers, anyone?
« Reply #29 on: October 22, 2017, 08:23:11 pm »

I first got exposed to magnetic amplifiers in the 60's (used inside an early switching power supply for current limiting). Pretty old school technology.

As your link notes a ferroresonant transformers is effectively running at (magnetic flux) saturation, so pretty inefficient for variable loads. Clever stuff for the very old days. 

I can't say that I ever saw one in the wild (that I recognized as one).

JR

The efficiency is only about 75% to 85% - I often used them in automation projects within factories to supply the PLC's and other critical electronics. The electricity supply within a factory was often subject to voltage variations and noise because of all the large industrial equipment switching on and off.  They provided isolation, voltage regulation and filtering and would ride through short interruptions. They were also simple and robust and failure was rare.
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