Using a DC Motor as a Generator

If it is a brushed DC motor, you will get DC from it, with the ratio of voltage to speed about the same as when used as a motor.

(Brushless DC motors would output AC -- but they dont fit your "common 6V DC description.)

You probably wouldn't get AC out of a brushless DC motor - at least not at the terminals. With some ingenuity you could connect a three-phase full-wave bridge at the windings - after disconnecting them from the drive electronics - to get pulsating DC. Same as an alternator. Many BLDC motors are three-phase, although some have four or even six phases.

Good point. I tend to think of BLDC motors as being driven by external controllers, but that's because of my own tunnel vision. I'm sure the vast majority have integrated electronics.

Btw, stepper motors make effective alternators, using an appropriate multiphase bridge to rectify the output. I've gotten quite a jolt out of some of the larger 1.8° and 0.9° steppers.

Hi muld4075. You get DC current. I have been doing this for the past 50 years. When I was 12 years old I constructed my own wind generator by mounting a fan to a DC motor, which I attatched to a 15ft tall post in our garden. It worked well except that when there is only a small breeze the power output was such that it would not even light up my torch bulb. Spencer.

I remember the first attempts by Innocenti of Italy, manufacturer of Lambretta scooters.

One of their models had a combination motor/dynamo built in. It would act as a starter motor to start the engine but turn into a dynamo to charge the battery once the engine was running.

You're right! Brings back fond memories. A friend had a Lambretta, and at the time, electric start on a two wheeler was a new thing.

For quite a while, I had a Cub Cadet lawn tractor, which used its generator as a started motor.

Not relevant to electric motors/generators, but an Air Force sarge I knew ran pure nitro in a lawnmower. For about 20 seconds.

Note: FYI.. A little schooling about DC Motors and generators.

DC motors come in several different flavors.

A DC motor requires a magnetic field. The magnetic field can be electromagnetic (wound wire) or a permanent magnet.

The wound field (electromagnetic type) requires a source of electrical energy for field excitation that must be externally supplied, either from the motors own armature or a external power supply.

There are two basic wound field type DC motor configurations, Shunt and Series. Only the Shunt type is considered workable as a generator.

A Shunt connected motor can become a generator but there is a "which comes first, chicken or egg situation". The armature cannot generate a voltage with out a magnetic field and the source of voltage for excitation of the field is the armature.

With larger machines, there is often enough residual magnetism it the field iron core to boot strap the machine, or by special design a small magnet is sometimes imbedded.

If your motor is the permanent magnet field type. (generally common with small motors of late design, it will operate as a generator or a motor simply by supplying a source of voltage to the armature or mechanical power to the shaft.

Permanent magnets are expensive and are reserved for small, or special, motors.

The low voltage starter motor on your car is a series motor and will not work as as a generator.

Generally a PM motor can be identified by the additional turning force required when the terminals are shorted.

There are generators available on the market that use Rare Earth permanant magnets on the rotor and 3-phase windings for the Field, in my experiance typically providing up to 1KW. The 3-phase output then being fed to a 3-phase rectifier, much the same as a conventional car alternator,however the permanent magnet generator does not require external excitation and this is where the problems start. The voltage output of such a generator (as is a car alternator) is directly related to its RPM so some form of regulation is required to maintain a constant voltage at various engine RPM. With a car alternator this is done by controlling the voltage/current to the excitation winding. For the permanent magnet generator because there is no excitation winding the regulation takes place on the output after the 3-phase diode pack, usually being some form of switching regulator for efficiency. If you want to investigate this further 3-phase brushless motors can and are frequently used in this application (see combined starter/generator applications for automotive apps etc), but obviously those without integral controllers.

Cheers John

Well stated and absolutely correct, but these devices will not run as motors unless you remove the rectifiers and operate them as Synchronous devices with some kind of starter to bring them up to synchronous speed. To operate them from a DC source requires some form of commutation. Maybe we both have strayed from the author's original thread

"Using a DC Motor as a Generator" Will the output be AC or DC ?

See also a thread directly related to this right next door.