DC Motor as a Generator

[dave12]

Hi,

Apologies if this is in the wrong forum, but anyway heres my question.

Could anyone recommend a suitable DC Motor, that I can use as a Generator to charge a single 12V Car Battery? My plan is to Build a wind Turbine and generate the power that way.

Don't worry I know I will need circuitry to control the voltage and charge, but my first step is to find a suitable Motor. As I understand it, it will preferably be high voltage, low RPM. So maybe 36 - 48V and <1500RPM.

Does anyone know where I can source such a Motor? Many Thanks

 

[Space cat]

Go down to the scrap yard and get a car alternator complete with regulator. It's purpose made for the job.

Your next step is to design the wind turbine itself. This will be the hard part but you can get some good ideas by looking at the ones they sell in caravan shops. A typical caravan wind generator has a blade diameter of about 75cm and will claim to put out over 50 watts. I had one for years and was lucky if I got half of that in the average breeze.

There used to be two types available. One had wide blades and could make the necessary 12 volts at quite low rpm. The other had slim blades and ran much faster. You will want the fast version to drive an alternator.

 

[Softus]

I don't understand why you don't just use a cheap and ordinary car alternator.

 

[jj4091]

Just thought this may help. http://theepicenter.com/tow02077.html

 

[dave12]

As I understand it, the problem with a car Alternator is that it requires a really high rpm >2000. Anything less than this would produce little or no voltage. A DC Motor has permanent magnets in, meaning I don't waste energy from my turbine charging up the windings.

 

[Softus]

As I understand it, the problem with a car Alternator is that it requires a really high rpm >2000.

That's why it has a small pulley and the crankshaft a large one.

 

[big-all]

in my humble opinion an alternator will be to "heavy"

you need the lowest mass possible for the output you require otherwise all the energy will be needed to rotate the coil with little left for generating power

would suggest the most efficient would be a motor that will reach capacity at around 75 percent maximum wind speed
this would mean the minimum mass but best efficiency

i would say somthing like a can motor on a 12 volt drill will probably mannage around 150 to 200w and it has a 2 speed gearbox and chuck if it needs to connect to a 10mm shaft

 

[gbrown100]

This sounds quite interesting, I thought about this before myself but never thought of using a 12v drill motor... I have an old 12v one lying around in the garage.

Anyone have any suggestions on how to regulate the voltage? I know nothing about voltage regulators really, could you use a car one or do you have to match it to the characteristics of the motor more specifically?

 

[Softus]

Anyone have any suggestions on how to regulate the voltage?

You need to create a new topic for your new problem.

 

[Space cat]

Our old caravan wind generator used a simple shunt regulator set to 13.8 volts. This makes sense if you think about it. What else are you going to do with the surplus energy when the batteries are full?

The generator itself had magnets inside the hub and windings on a disc fixed to the shaft, also inside the hub. Result, minimal mechanical losses. This is something you need to think about. Any kind of step-down gearbox used in reverse will incur losses. A commutator and brushes will also incur losses but you will save on the rectifier.

The ideal motor will be one with a permanent magnet that, when powered by 12 volts, turns at a speed suitable for your turbine.

 

[TicklyT]

That got me thinking about old british 'bike electrics.... They often used a simple permanent magnet mounted directly on the crankshaft and surrounded by 6 coils on a stator. This setup generated about 6 volts 50 Watts at about 2,000 RPM.
They were often converted to generate 12 volts by wiring the stator coils in series instead of parallel.

A bridge rectifier and a nice chunky 13.8 Volt Zener diode mounted on a heat sink and connected straight across the output completed the ensemble.
http://www.british-motorcycle-parts.com/12v_single_phase_alternator.htm

http://homepage.ntlworld.com/bondi.beech/Projects.htm

 

[sm1thson]

A bridge rectifier and a nice chunky 13.8 Volt Zener diode mounted on a heat sink and connected straight across the output completed the ensemble.

a big zener on a heatsink may be sufficient to clamp the maximum voltage the alternator will give when attached to an engine, but there you know the maximum rpm. on a windmill the rpm could go through the roof, i would say it needs something else to disconnect it when things get too windy.

 

[Space cat]

--- on a windmill the rpm could go through the roof, i would say it needs something else to disconnect it when things get too windy.

Sorry but that's wrong. It was a common misconception with caravan wind generators too. If you leave the load connected it will act as a brake on the turbine. Some people had been advised to switch theirs 'off' if it got really windy and the site warden, who had fitted them, had provided a switch for this purpose. You could tell which ones they were because the blades were screaming round, even in quite moderate winds.

If you're expecting some really high winds, the answer is a bigger heat sink.

 

[sm1thson]

indeed the through the roof part wasnt accurate (unless it dismounts, lol) but the point i was trying to make was more wind would equal more power for the zener to disipate and there is a limit to how much it can dissipate, on a car or bike this is calculated, it may be exceeded on a wind generator, a bigger heatsink is one answer two (or more) in parallel is another, the point is it shouldnt be put on hoping it will work fine, by all means put it on and monitor the heat of the zener to test that its heatsinking is sufficient.

if it were mine i would probably put 2 heatsunk zeners in parallel with a thermal cut out on the heatsink then let it run and see how it goes.

Industrial wind turbines cease to generate when the wind speeds get two high.

 

[TicklyT]

Mount the zener and heatsink on the mast head, so the zener gets the benefit of a cooling breeze when it needs it most.
The output of those 'bike alternators is designed to level out above about 5-6000RPM. I think the output rises about 10 watts per 1000RPM above that sort of speed.

IIRC some more sophisticated permanent magnet 'bike alternators featured a winding on the rotor opposing the permanent magnet field to govern the output at high RPM.

How about a simple centrifugal brake to limit the maximum turbine speed?

Another route to consider may be a variable pitch turbine, something akin to a constant speed aircraft propellor. A simple flyball centrifugal governor controlling the pitch of the blades would limit the maximum rotor speed.

One stage further may be to 'feather' the rotor when the load is removed, or even an electromagnetic pitch control setup optimising the turbine speed for differing wind and load conditions.