DC motor as generator

[Ric]

I want to build a prop shaft generator for my 32ft sloop. I cannot use an alternator because it will not be possible to fit a large enough pulley on the propshaft required to spin the alternator at a sufficently high speed to generate anything useful. I therefore want to use a 12v DC motor, drive it in reverse from the prop, and connect it via diodes to the battery, which I hoped would produce a few amps.

I got a second-hand 12v DC motor from a lawn-mower which seemed perfect at first glance. It works fine in "motor" mode, spinning merrily and using a few amps from a 12v battery, so I expected it to produce some volts and useful current when spun (by a 220v drill) in reverse mode. But it only produce microvolts. What's up?

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[Heckler]

the reason d'etre for alternators in cars were that they didnt need to spin v fast to produce more amps. the lucas dynamos we all used to love were in effect what you are trying to do now. if you havent enough space to fit a drive pulley for an alternator you certainly aint got the room for a dc motor drive!
stu

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[bedouin]

I think there are problems with using a motor as a generator (otherwise known as a dynamo of course).

The biggest problem is that the voltage output from a dynamo is proportional to the speed of rotation, so at low speed you don't get enough out to charge the battery, and at high speed you have to waste a lot of power.


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[eggy]

ric i have experimented with very small electric motors spun by a cordless drill and produces some useful voltage (powering LEDS) i seem to remember reading something years ago there are 2 types of electric motors ones better than the other, then i could be wrong. Saying that i watched Science Shack on the telly the other day and they ran a desk fan and a telly from some sort of generator connected to an exercise bike! it looked a very simple set up. also remember that all alternators are AC then converted to DC via the on board diode pack . try contacting the centre for alternative technology on the web (address unknown) type CAT in to a search engine. Let me know how you get on mate!

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[GARYOFFSHORE]

Don't get side tracked the charging amps issue. Contrary to beleif it is actually the volts that do the work!!
If your battery is 12v you need to be putting in 13-15v. As you already found a dyno-motor cannot do this because of the fluxuating coltage.
So whichever option you take try to ensure that you can constantly put in more volts than the battery is rated at otherwise the battery will drain to the same level as it's charging source.
Hope this helps.

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[VicS]

There are, I think, two possible explanations for your motor not working as a generator.

Firstly it may not retain sufficient magnetism to give a field strong enough to start generating. Car dynamos do (did), in fact 'flashing' the field windings across a battery one way or the other will determine the polarity of the output. Once up and running the thing is then able to supply its own field curent.

The second possibility is that the motor is series connected, ie the field coils are in series with the brushes, commutator and armature. If this is the case while no current is being drawn there will be no current in the field coils and hence no magetic field and nothing, but a few millivolts generated.

If you succeed in getting the thing to generate you will need some form of control system like old car dynamos had. I guess you could use a diode instead of an electomagnetic cutout but you will still need a votage regulator.

Generally, though, alternators generate at slower speeds than dynamos did so I doubt wheter there is any point in persuing the idea.

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[HenryB]

Ric,

I've been thinking of doing something similar but have discovered that you need a 24 or 32V motor designed for slow running and fitted with permanent magnets. Then, when you drive them even slower they will generate 16 or so volts for battery charging. These motors are expensive.
Car alternators run faster than dynamos, it's just that they are geared up by the pulley ratios so that they start charging at low engine speeds. It is possible to modify a car alternator to produce a charge at low shaft speeds. The "standard" way is to replace the field windings in the rotor with permanent magnets and to rewind the stator with more windings (but thinner wire so that it all fits into the available space). Then the alternator will produce more volts at a given speed but less amps.
For a shaft driven unit you may get away without needing to replace the field windings but for a wind or trailed log generator it will be needed. As a previous poster suggested - look at the many sites dealing with home brewed alternative power for tips.
Good luck.


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[northener]

Afraid this wont be too helpful - but is it OK on shaft bearings & seals to have a constant sideways force from belt tension?
(Where do those smiley faces come from?)

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[onenyala]

I have two MDB1 engines, one in the boat and another as a working spare.
Both flywheels are identical
Both engines are fitted with identical Bosch Dynastarters.
The pulley wheels on the dynastarters are of different diameters and as a result the armature speed on one rotates at about 18,000 RPM and the other at about 13,000 RPM. (At fast idling speed)
Does anyone have any idea what the pros and cons of the armatures rotating at these quite different speeds is ?

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[MIKE_MCKIE]

Why not go down the scrappy & get a car generator from an older vehicle, before alternators became de rigeur? Sure they will have one & at least it is designed as a generator, not trying to make a silk purse out of a sows ear so to speak! They may also have a voltage regulator for a few bob which may help, altho I'm sure someone on this forum will put you straight on that.
Hope you succeed.
Cheers
Mike

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[DavidTocher]

The problem of using a DC motor as a generator or vv is the position of the brushes relative to the magnetic circuit. One would expect, from the dinky diagrams in books, that the segments of the cummutator are aligned with the stator poles. In practice the brushes need to lead or lag from this position to get the optimum performance for the designed mode of operation.

Try using a 12v DC dynamo from a car as a motor and you can stop it by hand yet it should be about 1/2HP motor (for 500W generator)

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