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

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.

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.

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.

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

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.

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.

If you connect a internal combustion engine to a " phase converter " , and arrange the gearing between the phase converter and the engine so the engin e puts power into the system, you will have a three phase generator. Could be useful if you wanted to run a honking big three phase welder and did no t have an electric outlet that would provide enough power for the welder.

Dan

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.

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.

Sorry, that was the 2nd edition.

On 16 Nov 2013, Ignoramus26083 posted some news:If6dnStz8dp0vRXPnZ2dnUVZ snipped-for-privacy@giganews.com:

With a little help certainly.

See instructions for both 3 PH and 1 PH at link below. It does work although I'd use a DMV in conjunction with his "light bulb test".

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).

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

Jon Elson fired this volley in news:xuqdnWyZ8JykxhTPnZ2dnUVZ snipped-for-privacy@giganews.com:

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

a generator tends to be fed DC power to setup the magnetic field to get things going. Induction motors don't need this. An induction "motor" with a DC winding is an alternator.

yes, under some conditions, but it's as others said, wonky in behavior and not useful outside of experimenting out of curiosity.

You could use an alternator as a motor, but again, it's not optimal and just something you might do for fun.

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.

Thanks. This makes sense.

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