View Full Version : can an old AC motor be used as a generator?

03-22-2005, 10:48 AM
i was just wondering. i know a DC generator is basically a DC motor, but can an old 3ph AC motor be used as a generator? the reason i ask is because i have access to falling water and a VERY cheap 15hp 3ph AC motor. i'm no generator expert, but it would be a cool project if it was possible.

andy b.

03-22-2005, 11:06 AM
Yes, this is possible, although the details of the wiring elude me. A few years ago a guy named William Fay, a registered engineer, was doing small-scale hydro projects and that is basically what he did. It's not trivial, the way it is with a permanent magnet DC motor. Something about energizing one set of the AC motor windings from the power line, then by turning the motor with (say) a water turbine, you generate power which can be fed back into the power grid to turn your meter backward.

Aside from the wiring issues, which probably are fairly straigtforward once you know how to do it, the big hassle in doing what Billy Fay did was doing the paperwork to be allowed to feed power into the power grid. It's do-able, if you know the routine, and that was part of the service he was selling--he knew how to do the paperwork.

Forrest Addy
03-22-2005, 11:57 AM
In theory yes. Practically, it can be tricky. An induction motor if overdriven will produce electical power in proportion to its slip in over-speed but to get credit from the power company you have to get them to pay you for the power you generate and install a special meter. If you had a synchronus motor you could get a better deal but the bureaucracy will still be a hassle.

Run your lights and home shop from a home built hydroelectric powerplant independent from the power company? Forget it. You need everthing a full size hydro plant has including the controls that maintain a constant alternator RPM and regulate the voltage in a way that the elements cycling in the clothes dryer won't screw up the computer or the TV.

15 HP? You'd need a 10 fall with a 5 wide x 14" high wier to feed an overshot wheel.

The best use for small scale water power is to generate heat. Unless you burn home grown firewood domestic heat takes a fair percentage of your budget, much more so than for lights and motor power alone. Use the falling water to run a water churn to produce domestic heat. Yes agitating water will heat it and if the heat is contained the process is 100% efficient.

If you have a sufficient fall you can generate 180 degree hot water at 150 GPM (through 4" pipe) to heat your house, your shop, your potable hot water and you can use the outfall to keep your hot tub warm.

[This message has been edited by Forrest Addy (edited 03-22-2005).]

03-22-2005, 12:20 PM
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03-22-2005, 01:56 PM
darn, i can only get about 6' of fall. i kind of figured the key to it all would be controlling the rpms. all i'd be interested in it for would be lights or maybe running the reverse electrolysis bath. the type of things an unregulated DC generator would be fine for. i have no desire to tap it into the power grid.

oh well, someday i'll find a nice cheap big DC motor.

andy b.

03-22-2005, 02:42 PM
A friend of mine does exactly what you are thinking of doing. He uses pumps as turbines and induction motors as generators. I don't know if six feet of head would work. That's pretty low. If you want to get a hold of Ron e-mail me and I can pass along your address. I know he won't mind discussing it.


J Tiers
03-22-2005, 05:40 PM
Power comes in different shapes and sizes.

Nothing wrong with 6' of head, as long as you have lots of flow.

The basic power source value of falling water is in the potential energy of the weight at a height (insert unit system of choice here as desired). It can be converted to anything you want, within reason.

Very high heads tend to use Pelton wheels, which do well with high velocity streams.

Medium tend to use "runners" which are like sqirrel cage fans in appearance.

Low heads tend to use the propeller type Kaplan turbine. Or a water wheel.

An old overshot water wheel uses mostly the falling weight, and not much of the flow energy, aside from filling the buckets. The others use flow. The energy comes from the same source either way.

So the energy of one lb of water falling from 600 feet is like 100lb falling 6 feet. Obviously one involves a lot more flow.

I would suppose it is generally easier to harness the energy of a low head if you are constructing the plant yourself. Lots less trouble guiding the water to your plant. Easier and less fussy methods available for constructing the wheel.

You probably need to "gear it up" to get to a generator speed. But no reason not to do it if you have suitable flow.

03-22-2005, 07:11 PM
Have a look at this site, you may find it interesting. They are using wind which you arn't wanting to do but maybe there method of generating will give you some idea's



[This message has been edited by zl1byz (edited 03-22-2005).]

03-22-2005, 08:51 PM
Big water wheel,made from aluminum with a mag anode,use it to turn a hydraulic pump,pipe fluid(vegtable based oil) up to the shop,use a hydraulic motor to turn lineshaft in shop,or power more hydraulic motors monted to the tools in place of the electric motors.

If you insist on electric then maybe do what a guy near here does.He has a wheel maybe 8' in diameter and 3' wide sticking out the side of his floating dock.It is mounted on a 1-ton truck rearend cutoff(the full floating axle and hub part).It provides the bearings for the wheel to run on and transmits the power from the wheel through the center of the axle tube via the axle shaft.The shaft is supported by a flange bearing on the end.It has a 18" three groove pulley turning a jackshaft with a 2-1/2" pulley.His average speed is 140 rpm on the jackshaft,this he found was fast enough to spin a homemade generator fashioned out of an old golf cart motor.He ends up with 13-14vdc at about 20-25 amps,he can charge batteries and run a inverter which alows him to run the lights and his fuel pump for his boat while he is at the river.It cost him mostly time,much of the material he had in the junkpile.