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Old 7th September 2012, 09:27 AM   #1
50AE is offline 50AE  Bulgaria
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Default Motor to generator mains regenerator.

I came across this idea - to power an induction motor from the mains which will drive an AC generator giving mains to my system. This should be equal to almost pure AC. Noise of the motor is not a problem, because I can put it far away from my listening place. It will power up my entire system including PC, DAC, amp.

But I think of a main problem - how to control the frequency to be steady (50HZ)? And what if the generator turns off due to a misfunction - this would progressively lower the frequency. Would this damage equipment?
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Old 7th September 2012, 09:47 AM   #2
AndrewT is offline AndrewT  Scotland
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Make it 3phase.
Fit a really big flywheel.
Use an alternator, not a generator.

Needs regulation to control speed and voltage.
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Old 7th September 2012, 01:59 PM   #3
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You are certainly considering a very large project with a number of obstacles to overcome. The concept of 'pure ac' is overrated in my opinion. You should start by asking yourself what portion of my system is suffering due to non-pure ac. I would correct that equipment so that it can operate more robustly in the presence of harmonic distortion, waveform notching, and noise.

After all, most audio equipment operates off dc, so all you end up doing is rectifying the ac, whether it be distorted or not really has little effect. The exception would be ac heating of vacuum tubes, but there are simpler (and better) ways of addressing this.

To answer your question (but I don't condone the implementation) the optimal method would be to run a small motor off an ac drive. The drive controls the frequency (speed) of the motor very well, even in the presence of load changes and line voltage variation. It will eliminate the need for a flywheel, and provides a degree of protection for the motor. Best solution would be a vector drive, which has an encoder connected to the motor shaft. The drive directly controls the speed of the motor shaft in closed loop.

The motor drives a synchronous alternator with the same number of poles as your motor. You control the field of the alternator with a voltage regulator, which senses the output of the alternator. Many choices of regulator, and usually not very cheap, especially if you want good transient performance and solid regulation.

Then you need to consider winding pitch of the alternator. Depending on the pitch, each will provide different amounts and number of harmonics, so don't think for a minute a generator produces 'pure ac', it certainly does not.

All of this is pretty much geared toward three phase operation, but I am sure you can find equipment suitable for single phase.

My suggestion is to go with a ferroresonant transformer and be done with it. Regulation is excellent, and the harmonics produced are benign, even for ac heating vacuum tubes. Common mode rejection is very high; you can almost survive a lightning strike with those units. Zero noise let-through, although it does produce audible ambient noise that must be dealt with.
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Old 7th September 2012, 02:24 PM   #4
Pano is online now Pano  United States
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I have a gas (city gas) powered 12KW generator at home. More than enough for the whole house. Let me tell you, the AC out of that thing is the ugliest, dirtiest power I have ever seen. Not great for audio, but it keeps the lights on and the HVAC running when power is out.

If you choose to do this, as some high end Japanese installations have, you had better choose your alternator very carefully. It will probably be expensive. I have seen military surplus motor generator units that converted 60Hz AC to 400Hz AC. They may be pretty clean. Don't know about 60-60 or 0-60. Maybe. As ZigZag say, there are better, cheaper ways to do this.
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Old 7th September 2012, 02:31 PM   #5
imix500 is offline imix500  United States
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I would look at a surplus online UPS from a server farm. These regenerate the sine wave to a specified level of distortion.
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Old 7th September 2012, 03:14 PM   #6
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I have seen this used in a a large UPS installation. There was an enormous flywheel attached to the three phase motor-generator set - the idea being that when the mains was cut, the generator would keep going long enough for the backup batteries to kick in. These in turn would keep the electrons flowing while the diesel powered generator started up. Lots of control circuitry handled the switch-overs. Great for keeping sensitive laboratory equipment running, but I have no idea how squeaky clean the sine waves coming out the other end were.
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Old 7th September 2012, 07:25 PM   #7
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You could build or buy a (very) large audio amplifier. Drive it with 50/60Hz generated by e.g. Audacity set to loop continuously driving a soundcard or get somebody to write you a program to send the values continuously from a lookup table.

Bear in mind that most people will regard what you propose as a complete waste of money and resources and likely to damage the environment, contribute to the extinction of species and the rise in worldwide temperature and sea level with a corresponding impact on 3rd. world human populations and even 1st. world prosperity, all for no audible result.

Why not just take the money it would cost and give it to charity? I suggest fusion power research. Plenty of time to take up your project again when that engineering challenge is met.
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Old 7th September 2012, 08:23 PM   #8
imix500 is offline imix500  United States
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Actually, that's a great idea. Aquire a used industrial amp like a Techron gradient drive unit, and just feed a sine wave from a good audio generator into it. Tada. Clean power.
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Old 7th September 2012, 08:25 PM   #9
DF96 is offline DF96  England
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Your local mains must be really bad if you think that a small motor-generator set will be better! 50/60Hz to 400Hz sets are probably intended for testing avionics on the ground; in the days before cheap reliable solid-state power electronics this was the best way of getting 'aircraft' power..
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Old 7th September 2012, 08:52 PM   #10
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..or even a large motor-generator set. Recent project of mine involved a well-engineered 1.5MVA backup genset and even the output of that giant lump - specced to back-stop a hospital - is a mess compared with incoming HV.

Though the V-20 quad-turbo engine sounded great spooling-up under load...
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