Motor as a generator


Thread Starter


I would like to make an experiment that uses a bike to turn a motor to generate electricity to light a lightbulb. I really do not care whether it is AC or DC, although I have a few old AC motors around. I tried spinning the shafts on these with a drill to see if I get a voltage out, but so far nothing. Would I get more power out just buying a small AC generator?


Robert Scott

Forget about AC. Get a DC motor. It will function just fine as a generator.

Robert Scott
Real-Time Specialties
Embedded Systems Consulting

Michael Griffin

The AC motors are probably induction motors and don't have magnets inside them. It is the magnetic field moving past the wire coils that generates electricity.

Try a conventional (one with brushes) DC motor. A good one for your application would be a motor out of an automobile. You may be able to get one cheaply from an auto wreckers. A motor from a toy would also work, but these are much smaller capacity. They may be enough for a light bulb though.

Curt Wuollet

A car alternator from a junkyard is one of the easiest and cheapest ways. Most PM DC motors would be more expensive. Your run of the mill AC induction motors aren't really useful as you have seen. You would need a small battery to get things started with the alternator. Any permanent magnet brush type DC motor should work as a generator. Most brushless types won't. Treadmills and electric scooters from a garage sale will often yield units capable of considerable power. You might Google wind power for examples of motors converted to generators.

You can not use an Induction Motor as a Generator unless you connect it to grid and run it above synchronous speed.

For your application it will best to use a DC motor.

Phl Corso, PE

Responding to Mark's query... For experimental purposes, yes it can be done! And with an AC motor, but... :

1) The motor must have a source or reactive power. In other words, there must be a capacitor across its terminals.

2) A single-phase, capacitor-start, or split-phase capacitor-run motor will work without extra components. Try a pedestal-type fan motor!

3) A three-phase motor must have at least one capacitor connected across any two terminals!

Phil Corso, PE
Interesting, I am interested in generating three phase power. How would you calculate required capacitance? And would an induction motor work or would it require a brushed motor? Are there any other vital considerations? Would it need any kind of initial power applied for initial excitation, etc.? Thanks in advance for any input or direction to further resources you can give.

I am trying to make a similar setup, and am looking for an appropriate motor to use as a generator. I am currently mostly concerned with the size of the motor. I am on a bit of a budget, so a nice leeson motor is a bit too pricey. I was contemplating using a 1/4HP 12V DC motor from Harbor freight tools as the generator to charge a set of 12V batteries using an MPPT charge controller. If I understand things correctly, i'm pretty sure that even with the MPPT controller, I will still need to crank out at least the battery voltage (10-14V) from the generator to get a charge to go into the batteries.

here's the meat of my question: if the motor is rated at 250W, will I need to put in at least 250W to get out the desired 12V? with all the mechanical inefficiencies, actually getting 250W to the generator will only occur at a sprint, which will make it quite difficult to charge my batteries.

please let me know if i'm right about either my motor size concerns or the issue of voltage necessary to charge the battery. also, if anyone has better suggestions of where to get a motor to use, that'd be greatly appreciated.


Michael Griffin

Is there a reason to not use a car alternator? You can probably get one at a wrecker fairly cheap. 250W is fairly small, so you want a small one. You might actually want one from something like a motorcycle or a snowmobile (I'm not sure what size they use). If you are using ordinary lead-acid batteries, I don't think you need a high tech charge controller either. You would regulate the current in the alternator field coil.

As for power requirements, the *load* determines how much it will try to draw. The load doesn't know how big the alternator (generator) is. You regulate the field excitation to determine how much load the alternator will accept.

You need to regulate the field excitation according to what the prime mover can put out. If you start off with zero excitation, there will be no load (except for friction). As you increase the excitation, the amount of load accepted will increase. When you reach the limit of what your prime mover can put out, stop increasing the excitation. You will need some sort of automatic control on this, but it's hard to say what without knowing much about your system. I'm sure that lots of people have done this before, so you can probably find some good information on this if you do a search.

You will need some diodes in the charging circuit to prevent reverse current flow. You don't want the batteries to drive the generator as a motor.

The output voltage from an alternator will vary a lot under different load conditions. You will want some sort of regulation and protection between the batteries and whatever load you are supplying (TV, lights, etc.).

To add to Michael Griffin. A car alternator is 3 phase. Most of the ones I have worked on would be quite easy to rewind if you need higher voltage.
Another option is the motor from a Fisher and Paykel washing machine, They are multi-pole permanent magnet motors. Very popular for wind power projects.


Ken Emmons Jr.


> Is there a reason to not use a car alternator? You can
> probably get one at a wrecker fairly cheap. 250W is fairly <

I am not an expert in power generation, but from what I've read, the alternator in a vehicle needs to operate at the 1000+RPM range in order to generate electricity. IF you get a DC permanent magnet motor, or an AC servo (also with magnets) these can be used for generators. The ideal scenario might be a large AC servo with a 3 phase rectifier setup. I was going to go down this road, but a friend of mine gave me a Baldor 450W DC motor for free. I'm setting up to run a wind turbine, mostly for fun and to heat hot water or something trivial to start with. Starting with a free motor will get me up and running and to change out to a larger motor will just be couplings etc.

Some people talk about using treadmill motors also. Just don't pay too much money for them, Ebay is now crazed with folks commanding a lot of money for cheap cheap stuff in certain niche items now that Energy is in high demand. Some people are even paying well over $100 for 30 year old DC motors used in old servers (tape drive motors or something). For that money you can buy a used servo and don't have to worry about brush failure.

There are also companies that retrofit car alternators for coils and permanent magnets. Some places even sell as new or New/reconditioned. I don't know how these are, but I would imagine they are decent. Ebay has some of these companies.

Hi Phil,

You mentioned that single phase motors would run as generators without adding any extra components. How about a split-phase resistance start motor? It has no existing capacitor as it uses a coil with high resistance and low inductance to create a slight phase shift for starting. Would I connect the capacitor in the start winding? Thank you!

The capacitor is necessary to supply the inductive requirement of the motor. For the split-phase case, it shoud be connected across the two supply leads!

Phil Corso

Roberto arias

> TRY a cheap orange juicer, the ones that turn any side when you push the orange into it. <

Connect a low power light bulb to the male socket, the push the juicer down (to make contact) and turn. you should light the bulb, I havn't tried it with a filament lamp , but it should work wit a 15 or 20 watts bulb (same voltage as the juicer).

This magnet motors should make a nice generators

helo! friend.

I'm also thinking about the same thing. I am using the motor of toys (mainly the motors of the cars) to generate the current. Generally motors that are used in the toys are dc motors with permanent magnets... and i'm able to generate current in milli amperes. since that motor is not having commutator segments...the output is alternating current .... I think that is negligible. if I'm able to amplify that ac output may i able to light the LED bulbs.....
> You can not use an Induction Motor as a Generator unless you connect it to grid and run it above synchronous speed.

how much power will be generated?