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

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

Ignoramus26083 fired this volley in news:If6dnStz8dp0vRXPnZ2dnUVZ snipped-for-privacy@giganews.com:

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

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?

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

Ignoramus26083 fired this volley in news:C4SdnS9gS7yCuRXPnZ2dnUVZ snipped-for-privacy@giganews.com:

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

The home power crowd has used big induction motors as alternators for years. One big problem is you have poor/no control of frequency. You spin the motor at somewhat above synchronous speed, and you get a frequency lower than the actual rotor speed, due to slip. It works exactly like the induction motor except everything is reversed. It is called an induction alternator. There area couple other problems with this system. It will not self-excite under load, and if momentarily overloaded output will collapse and you will have to remove all load to get the alternator re-excited. To maintain excitation, you need to put substantial capacitor banks across the terminals. You adjust output voltage by adjusting the resistive load on it, I think, and adjust frequency by varying RPM. If the motor was a 1750 RPM motor, the slip at rated load was

50 RPM, so you'd spin it at 1850 RPM to get something close to 60 Hz out.

If you Google induction alternator and maybe some things related to off-grid power, you should turn up plenty of info.

Jon

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

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.

Short answer , "yes ".

Dan

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

i

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

So, what is a difference between a 3 phase generator and a 3 phase motor?

Ignoramus26083 fired this volley in news:Ho6dnSzazuvkthXPnZ2dnUVZ snipped-for-privacy@giganews.com:

If you excite all three windings, yes.

LLoyd

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

In a way, if I may say, I want to not take NO for an answer, at least for a while.

i

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.

American made systems. Good pricing. Perfect to back up a solar/battery power system.

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

You get a brake. No DC is needed, just the capacitors.

Jon

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