Motors as Generators

[Ron Reeland]

Hi: Text books on electric motors state that any electric motor can also function as a generator. I have no problem with using permanent magnet D.C. motors as D.C. generators. But A.C. motors are another matter.

I have tried to use small shaded pole motors as generators without sucess.
These are the small sizes as used in cooling fans, old phonographs, etcetera.

I make sure that I am driving them in same direction of rotation as when used as a motor. I have also read that the field coils may need to be "flashed" with DC voltage to set up some residual magnetism in the field coil laminations. I have done that, too. The text books state that they must be started without any load to allow the magnetic field to build up. Apparently, this magnetic field is not being created to any great extent.

The output is extremely feeble; only about .6 volts AC. I guess that any output verifies that the theory of using a motor as a generator is correct, but how can I increase the output to the expected 115 volts AC?

Thanks,
Ron

[Chris Smith]

Open up the books, its all there.

DC generation is a concept of TWO whole fields, not just one.

There is a whole slew of information out there as to how and why these two produce energy, while the shaded pole motor isnt even close to whats needed.

Any AC motor "can be set up to produce", while the shaded pole motor doesnt even come close.

[ecerfoglio]

To use a DC permanent magnet motor as a generator you just spin its axle and it works.

If you have a (DC) motor with a field winding you have to use an external supply to excite it or "flash" to set up some residual magnetism in the field coil laminations and then use part of the generated current to reinforce this magnetism.

Even the sincronous AC motor is easily put to work as a generator. Almost every generator you may see is, in fact, a sincronous generator and may work as a (sincronous) motor.

But most AC motors are asincronous motors (also called induction motors). From the small shaded pole ones up to the big three phase motors.

They can work as a generator, getting mechanical power from their axle and outputting AC power to the grid.... BUT, to do that, they must be connected to an external AC power supply that must provide them with "reactive power", while you spin its axle faster than the seed you can obtain as a motor (ie: a 1700 RPM motor will act as a generator if you spin it faster that 1800 RPM - that is to say about 1900 RPM while it is still connected to the 60 Hz AC source).

The so called asincronous generators are rarely used.

[Chris Smith]

Unfortunately “producingâ€

[rshayes]

Induction motors, such as the shaded pole motor, generate torque by using the intereaction of two magnetic fields. One field is the rotating field peoduced by the stator and the other is a field produced by currents induced in the rotor.

If the rotor is turning at less than synchronous speed, the rotor sees a changing magnetic field at a frequency that is the difference in the rotation rate and synchronous speed. This changing field induces a current in the rotor that creates a second magnetic field. The interreaction of these two fields produces torque.

If the rotor is driven faster than synchronous speed, the motor will generate power. There are generators available that use this effect, but the operation of such a generator is not simple, since the excitation power required to create the field must be applied to the field windings and then manipulated in some way to extract power from the same field windings. It may be necessary to drive the windings above synchronous speed to create the field in the rotor and then reduce the drive frequency below the synchronous speed to extract power.

Shaded pole motors usually use a rotor consisting of copper conductors wound in slots in an iron rotor. The iron will have a small remnant magnetic field. The low output that you are observing is probably this remnant field acting as a weak permanent field. Truly operating it as an AC motor in reverse would require an AC exciting field. The output might be in the form of a reversed polarity current flowing back to the excitation source and this might be hard to observe.

Remember that shaded pole motors are one of the less efficient forms of AC motor and would probably be equally inefficient as an AC generator.

Another form of AC generator is the drag cup tachometer. This uses a rotating copper cup with an exciting winding in one direction and a pickup winding at right angles to the drive winding. AC excitation is applied to the drive winding, and, if the cup is rotating, an AC voltage appears on the pickup winding that is proportional to the rotation speed. I don't know if this method has been used to generate actual power. It may not be efficient enough to be worth the trouble.

[haklesup]

Physics tells us that we must cut through a magnetic field with our wire to generate a current ####2 voltage. So in any AC motor (of which there are many types) you likewise need to establish a magnetic field first. As you would expect, in most instances, you need some sort of external circuit to excite the (stator usually) so that the rotor has a field to move its windings through. Once it gets going, you can rob some of the generated power to keep the field alive but the method and efficiency depend on the type of motor you are trying to use as a generator.

I'm not an expert so I won't go much further but I expect such a circuit to have capacitance to store some charge and some inductance to induce a phase shift in the current waveform (between the rotor output and stator input fields). I would expect only some types of AC motors to be appropriate for power generation and others to be impractical. I'll leave the research up to you.

Like most motor applications, the type and size of the motor must be well matched to the application and load or you will suffer frustrating inefficiencies.

[dacflyer]

easiest form of a AC generator would be to modify a car alternator,,
you have all your windings and you can regulate the armature for your magnetic field, and i hear that theres been cases of making 120 volts easily at high rpms...but i think the frequency might be way off too
but it would be fine for power tools and lights..
i used to have a generator / alternater cookbook here.. cannot find it now..
but it had info on turning motors like washing maching motors into generators,, altho i don't think its practical .. i think a hakked alternator is your best bet..

[Chris Smith]

Car alternators are great for charging, AC generation, and lights.

[Ac or Dc]

They can produce from 90 to 160 volts but at up to 3600 hz,... or just DC if you add in the diode pack.

[haklesup]

Using a generator to make unregulated 115V AC is one thing, doing that at exactly 60Hz or any other specific frequency then regulating that under changing load conditions is entirely more complicated. In this case, the speed of rotation needs to be mechanically controlled and regulated through feedback as is done at say a dam.

The other option is to rectify whatever you get to DC and then electronically reconstruct 115V, 60Hz with an inverter which is the common method on portable generators. In this case the throttle is regulated to maintain constant torque rather than constant speed.

[dacflyer]

haklesup >> speaking of which.. got any ideas on how to governor / regulate engine speed on a engine that has no governor?
i was thinking of some sort of feed back circuit and using a R/C servo but i got no idea how to implement it.. i know i'd need a pickup or sensor to detect rpms,, thats easy,,but the control circuity is my stumbling block,,

[Fran]

Look up <induction generators> All motors can be converted into generators, bit many are useless. For example you could use a starter motor as a generator, but it would burn out after a few minutes of use.

[Chris Smith]

A lot of Starter motors are great generators, however their shaft uses a bushing instead of bearings which wears out quite quickly.

[haklesup]

I'm no expert on small engines but isn't a governor just a maximum limit on the throttle. Depending on the engine, it may not be practical to regulate speed to the precision required. In any case, you would feed back the frequency of the generator output rather than the engine speed directly, afterall, that's the product you wish to have regulated. In most cases engines spin way too fast so you would need a reducing gear somewhere as well.

By taking the first derivitive of the frequency you get a scalar value that can be input to a comparitor to generate a go faster or go slower signal.

Really, an inverter would be a lot easier.

[Chris Smith]

Motors with generators generally have “ACTIVEâ€

[dacflyer]

haklesup >> i have a nice generator here, but the mechanical governor is
trashed,, and parts for the engine are no longer made.
i can hard set the throttle,,but then the power is not constant for the load
no regulation... frequency is not critical, i just need the voltage to be average enough to make it useable. with out the engine bogging down,or stalling.