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

So, I have been buying, selling and scrapping 3 phase motors in the last couple of years. I even made (and sold) a 15 HP phase converter a
couple of months ago. That worked out great financially.
All of this made me think: what exactly is the difference between a motor and a generator? Can a circuit of some kind be added to a regular 3 phase motor so that it converts mechanical energy input into electrical energy output?
For example, a phase converter is actually a "generator": if two of its "legs" are excited with single phase input, it "generates" third phase that is shifted 120 degrees.
This, obviously, does not qualify as a standalone generator, since it requires snigle phase input on L1-L2.
But can this motor somehow excite itself to generate electricity?
i
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On 11/16/2013 6:50 PM, Ignoramus26083 wrote:

I think that they can, but with some issues. You have to spin them up with externally supplied power, or at least get it biased up, so it generates the stator field. The power factor sucks.
Look up self excited induction generator or self excited squirrel cage generator. A solar energy place I talked to a few years ago was planning on using them. In the process of studying for the interview, it appeared to me that there are several better choices, but they do work.
BobH
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You spin a generator with mechanical power, for example a gasoline engine.

I believe that if I excite one leg with DC, it will statr producing AC. Right?
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Ayup... with lousy efficiency, because it's only the decaying flux in the armature passing AWAY from the excitation and under the next field that would transfer energy to the other fields.
Works... yes... efficient, not.
LLoyd
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Ignoramus26083 wrote:

No, it will burn out. That's DC injection braking, what most VFDs use for the final stage of stopping a motor. It may be necessary to blip some DC on an induction alternator to get it excited, but apparently with a cap back across the windings, they usually will self-excite as long as there's no load on it.
Jon
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So, what is a difference between a 3 phase generator and a 3 phase motor?
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Ignoramus26083 wrote:

Absolutely no difference between an induction motor and an induction alternator, except the cap bank for excitation.
Now, a real 3-phase alternator has a DC winding in the rotor and a regulating device to control that DC excitation based on output voltage and phase angle. Modern portable generators use permanent magnets in the rotor and get rid of the regulator.
An induction AC motor uses the field of the stator to create an induced field in the shorting bars of the rotor. This requires the stator current to always have a lagging power factor. The principle of the induction alternator is to get the same induced field in the rotor by making the stator always have leading current in it. (Everything has to be backwards as the direction of power flow is backward.) Placing capacitors across the line terminals assures a leading power factor, and that will keep the alternator excited. Then, you have to apply a load to the alternator to prevent the voltage from soaring. You may also have to cut capacitors in and out as load varies.
All electrical machine are supposed to be reversible.
Jon
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Thanks. This actually makes sense. It would seem that it is straightforward to build a 3 phase alternator to produce power, as you described.
It may require some initial flashing to get started from black start, but it will work. At the same time, such a system would be difficult to regulate for constant voltage as load varies.
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On Sat, 16 Nov 2013 22:39:47 -0600, Ignoramus26083

But a generator based on an induction motor won't work WELL, and it won't work efficiently. It's a kludge, and you have to play with it every time, and you'll be flashing the field every time going "C'mon baby! One more time..." You can't sell a generator built like that.
If you want to make electricity every time it starts, you can get pre-made generator heads that bolt up to suitable gasoline or diesel engines easily, and there are adaptors available if you want to use a repurposed automobile engine. The tricky part then becomes finding the right bellhousing adapter plate and coupling, a proper governor system and interfacing the engine controls to it.
This is why the Generator Duty small engines have both an internal oil-bath Flyweight Governor driven off the camshaft that holds right at 1800 or 3600 RPM +/- 1% or 2%, and a fan-vane style Overspeed off the cooling fan that cuts off if it overspeeds roughly 125% - enough to keep the engine from grenading.
And the Generator Duty engines also have a taper-shaft with threaded counterbore on the end, facing a 4-bolt flange cast into the engine case face so the generator head bolts right on. And the engine has oversized main bearings because those special generator heads only have one ball bearing on the outboard end.
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Bruce L. Bergman (munged human readable) wrote:

Actually, most of the wind turbines made in the last few years use induction alternators. Now, I can easily believe these were made SPECIFICALLY for this use, so that might make a big difference.
They are now transitioning to permanent magnet alternators with electronic converters for startup and then invert power back to the mains.
Jon
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Y'think?
I stick with my original answer. The short answer is "NO!"
You _can_ generate power with an induction motor, but everything about it is wonky, including the phase angles, the frequency at optimum efficiency, and the amount of power per horsepower you'll get.
It's just not worth it, unless you find a PM armature motor, or an authentic slip-ring, wound-armature type. It's a silly waste of time to make what you can buy for less than your hourly rate.
Lloyd
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Ignoramus26083 wrote:

Ah, finally found a serious reference on this, with actual documented operation. http://www.iea.lth.se/publications/Theses/LTH-IEA-1058.pdf
This was a bit bigger gear than you probably were thinking of, 275 KVA.
Jon
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On Saturday, November 16, 2013 8:04:47 PM UTC-8, Jon Elson wrote:

There's a VERY BIG difference in a standard AC motor (squirrel-cage hysteresis type) and a generator or alternator. The rotating core of the motor is not connected with any kind of wiring to create a field in that core, and the residual magnetism of that core is very low.
A large truly synchronous induction motor, however, DOES have slip rings, or a commutator, and can be made to generate electricity. So can a small synchronous motor (permanent magnet type).

Yes, and no; the problem here is that the core is magnetizable with continuous application of external field (powered by the AC that drives the stator), but becomes demagnetized easily (and if the rotor isn't magnetized, it's symmetric: it has no poles, so no generation of electricity).
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On Sat, 16 Nov 2013 18:57:16 -0700, BobH wrote:

When I was an undergrad induction generators were considered the bee's knees for wind power because they didn't require synchronization.
Technology has marched on since then, by a few decades.
--
Tim Wescott
Control system and signal processing consulting
  Click to see the full signature.
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Short answer, "No." Not unless it was a permanent-magnet motor or had a wound armature fed through slip rings or a commutator. There has to be something to excite the fields initially. Common AC motors use reverse- EMF in the armature to repel the fields, but the armature gets its excitation from the field windings.
So-called "universal" motors can act as generators, so long as they either retain some residual magnetism in their fields, or have a DC supply to provide excitation until they can supply their own through a rectifier.
Lloyd
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PS... Bob's right that they _can_ be made to self-excite with a DC supply to kick them, and DC external circuitry to continue the excitation, but their energy density sucks.
Lloyd
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"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message

They'll self excite just fine from residual magnetism. Single phase C2C induction generators made from standard 3 phase motors are very common in small scale hydro projects.
http://www.who.int/management/InductionGenerators%20forSmallHydroSchemes.pdf
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On 2013-11-17, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

For me, number one question is whether it is possible to get decent AC (1 or 3 phase), with some excitation. Efficiency is a secondary question.
i
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If you excite all three windings, yes.
LLoyd
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On 2013-11-17, Lloyd E. Sponenburgh <lloydspinsidemindspring.com> wrote:

Right. If I connect the motor to the grid, and run it at above the synchronous speed, it will feed energy to the grid and I can get paid for it (or reduce my electric bill).
But what if there is no electrical grid, can I somehow make it excite itself?
i
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