Can a 3 phase motor be somehow used as a generator?



In a way, if I may say, I want to not take NO for an answer, at least for a while.
i
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On Saturday, November 16, 2013 10:12:00 PM UTC-5, Ignoramus26083 wrote:

As an engineering student I was on a tour of one of the Roseton NY power plants. Got to look through a small window at the astonishingly small alternator at the end of the turbine shaft.
Stuck onto the end of the alternator shaft was an even smaller excitation generator.
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It is possible to a asynchronous generator/motor auto-excite utilizing the same VA rating of generation with capacitors. but the voltage and frequency is hard to control this way. with no eletrical grid it is better to utilize a permanent magnetic generator. with a electronic inverter or an generator/motor with rotor field coil, with the last it is possible to lock the generator speed same to mains frequency, and control the voltage regulating the field current, very like the car alternator.
but to close circuity with the mains will be necessary to sync the phase.
the permanent magnet has better efficiency.
Regards, Suns

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On Sat, 16 Nov 2013 20:35:37 -0600, Ignoramus26083

Similar to what you're talking about, see these 12v systems.
Look at some of the hand powered generators on YouTube, Ig. Take an old automotive alternator and create backup power from it.
http://www.prestowind.com/ American made systems. Good pricing. Perfect to back up a solar/battery power system.
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On 11/16/2013 7:42 PM, Larry Jaques wrote:

It's been 40 years, so I can't give you any details...but I remember something about the optimum angle between the field and the armature being different whether a device is used as a motor or a generator.
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On Sat, 16 Nov 2013 19:42:20 -0800, Larry Jaques

Not really as the auto "alternator" is just a small A.C. generator complete with a DC excieted rotor which is used to control output. The D.C. output comes from rectifiers.

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Ignoramus26083 wrote:

There's a guy on the Colorado River that is many miles from the grid. But, he has guaranteed water rights to some insane number of cubic feet / second of the river included in his deed. So, he built a micro hydro system. He used a modest 3 phase induction motor, banks of caps to ensure excitation, and a huge tapped resistor grid to regulate the thing. I think there was a flyball governor that controlled RPM by throttling the water flow. He apparently had this running for at least a decade when I read about it in the Mother Earth News back in the '70s. Apparently his regulation was so good electric clocks ran accurately, and only needed resetting every few weeks. he sold excess power to his neighbor. Since the electricity was free, he used it for EVERYTHING, electric resistive heating, etc.
There are neighborhood micro hydro systems in various off-grid areas of the US that have been running for years, although they may not be quite as sophisticated as the Colorado river one.
Jon
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That's interesting. Let's say that RPM of the rotor is a regulatd value. Do you recall how he used resistors and capacitors to regulate AC? It seems like a good project to do and write up about.
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Ignoramus26083 wrote:

I think the deal was that the power output had to be constant. So, if he wasn't using much power in the middle of the night, the resistors had to absorb the maximum power he would use during the day. There are probably more intelligent (efficient) ways to regulate it. But, his energy input was free and constant, so he didn't care.
Jon
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On Saturday, November 16, 2013 9:03:47 PM UTC-5, Lloyd E. Sponenburgh wrote:

Short answer , "yes ".
Dan
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On 2013-11-17, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

And what if I excite one leg with DC (supply DC to L1-L2)?
i
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Ignoramus26083 wrote:

You get a brake. No DC is needed, just the capacitors.
Jon
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On Sat, 16 Nov 2013 20:34:50 -0600, Ignoramus26083

Won't work. If you excite it with AC it WILL work - as long as the speed is right. (synctronous induction generator)
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote:

Better answer, depends on the motor design.
Technically any motor can act as a generator, but efficiency will likely not be optimal.
Locomotives use their traction motors as generators to energize dynamic braking systems for example. If the motors and resistors were 100% efficient, they could stop a train without using brakes, but that isn't the case. They do generate quite a bit of energy though, which is dissipated as heat.
As you say above, something must excite the fields.
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On Sun, 17 Nov 2013 11:18:53 +0100, depucca wrote:

Pretty much any standard 3 phase induction motor can be used as a generator, provided that it has a load with a leading power factor (normally the controller will connect as much capacitance as required for this) or is paralleled with the utility line and driven above synchronous speed. Control is a bit tricky when not line connected, but a search on induction motor generator should lead you to information on how to do it. A wound rotor or permanent magnets are not required, unless you want to be able to generate power without using a complex controller.
You can find a fairly complete mathematical analysis in the chapter on induction generators in "Principles of Alternating-Current Machinery" by Lawrence, 4th edition, 1953.
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http://djm.cc/library/Principles_of_Alternating_Current_Machinery_Lawrence_edited.pdf
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Sorry, that was the 2nd edition.
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On 17/11/2013 2:18 AM, depucca wrote:

Synchronous AC motors work well as generators- and are often used that way in industry-having the advantage of var control. Their excitation comes from the DC field. Induction motors also can be used as motors-however an external excitation source is needed- preferably the power grid. In Some places (e.g Scotland), in remote areas, small streams drive induction generators- once up to speed, they are connected to the grid and feed power into the grid (drawing reactive from the grid). Capacitors can be used once running and the grid connection could be removed- but voltage stability is a problem. An extreme case that I know of was where a farmer with a single phase supply had a large induction motor which he connected mechanically to a smaller single phase motor and excited one phase with the result that he could generate 3 phase which he used to drive another 3 phase motor. It worked well and was cheap because the polyphase motors were surplus units. Sure the energy all came from the single phase supply but it was a lot less expensive than getting 3 phase in a remote area or buying a large single phase machine (and not a great deal less efficient). Nowadays there are electronic drives that do the job better for less money but then, many weird and wonderful arrangements and machines were used. Nightcrawler's reference is quite dated but includes material not generally taught these days (less emphasis, except in machine design texts, on some factors, and circle diagrams went out when or before electronic calculators came in, and transient response was generally ignored). It brings back some memories as much of the same was done in the early 50's. Approaches in texts after the 50's have changed considerably- emphasizing that essentially a motor and a generator are the same except for the direction of the energy flow and taking into account the various advances in control and power electronics.
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Induction motors - that are designed to be motors - make lousy generators, regardless of how well 'tuned' their capacitive excitations circuits are, if they're used for any varying loads, or - for the most part - on any inductive loads.
They're about useless for anything but purely resistive loads like incandescent lights or resistive-limited battery charging.
Lloyd
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On Tuesday, November 19, 2013 7:29:54 PM UTC-5, Lloyd E. Sponenburgh wrote:

You are right if you are talking about using an induction motor for a stand alone generator. But they work well if they are being used to feed power into the electric grid.
Dan
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