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ccdubs (Electrical) (OP)
7 Jul 02 20:34
Hi,

I am wanting to drive a DC motor as a generator to charge a battery for some data logging equipment.

The system will be 12 Vdc.  I intend to generate with the motor at higher than 12 Vdc and feed this directly to the battery. This may mean a 15 V motor or I may run a 12 V motor at higher than rated speed.

I will put a diode in series with the generator and battery so that I do not try to motor the generator.  The data logging equipment is all rated between 10 - 24 Vdc so I do not intend to regulate the load side of the battery.

Apart from the load capacity and battery size considerations is there anything I should consider with respect to using a DC motor for this application. ie., what sort of load does the battery represent to the generator, do I need to somehow current limit the generator?

Any help is appreciated.
azzmi (Electrical)
8 Jul 02 8:24
hi,
i like to know about the kind of mechanical power source to excite your generator? is it wind or stream etc.?


Azmi Demirel
azzmi@elk.itu.edu.tr
http://www.elk.itu.edu.tr/~azzmi

Lewish (Electrical)
8 Jul 02 14:12
Rather than using a DC motor, why don't you use an automotive alternator with built-in regulator.  They are real cheap at the auto wrecking yards.  Much cheaper than a motor and control circuit would be.  Using a motor would mean you have to design a battery charging and control circuit.  And, yes, you have to current limit the battery as the charge level comes up.
ccdubs (Electrical) (OP)
8 Jul 02 16:33
This is a design for a wind turbine where we have strain guages on the blades (rotating) and need to transfer this data to the logging PC.  What looks like the best solution is a radio transmitter, slip rings wont work as there is nowhere to fit them, beleive me I have looked.

I plan to fit a dc motor/generator with a rubber wheel on the rotor.  There is a place where I can mount the generator near a fixed reference (that supplies a 600:1 gear ratio) that the wheel can run off.  The rotor will turn at nearly constant speed.

I don't expect the power consumption of the unit to be huge so a car alternator is probably overkill and too large.

Am I oversimplifying the issue?  I thought that I could put higher than 12V onto the battery and it would charge.  My concern was the current draw from the generator (i.e., does a battery represent a high load when it has a low charge?)  If so I would need to current limit the generator.

Then on the output I was thinking about using a zener to regulate the 12V but only if this is necessary as the equipment has a 10V to 24V range.
Lewish (Electrical)
8 Jul 02 17:42
OK, let's go back to the DC motor/generator concept.  You should not put more than 14.8 volts into a standard lead-acid battery.  It might blow up!
As you indicated, you will need a series diode to isolate the motor when its output is lower than the battery voltage.  Otherwise the motor will act like a motor.
A discharged battery looks like a low resistance load, while a fully charged battery looks like a high resistance load.
Thus, you will need to both current limit and voltage limit the output of the generator.  These are reciprocal terms.  In a typical battery charger, the charging voltage starts out at something around 14.4 volts and drops to about 13.6 volts when the battery is fully charged.  You will need current limiting only to protect your motor from burning out.
There are numerous battery charger designs on various I.C. manufacturers websites.  I am partial to Linear Tech: http://www.linear-tech.com   Check their app notes for such circuits.
Cubrilo (Electrical)
3 Aug 02 9:30
You’re right that if you put higher than 12 V (actually about 13 V) onto the battery it would charge, but if you put too large voltage (say 15-16 V) the electrolyte will boil! So you gotta have some voltage regulation. That can be achieved by setting rotor speed or by setting field current of motor. So you have to check your actual data: what will be rotor speed, is it really constant, what kind of DC motor you use?
tiki (Electrical)
5 Aug 02 15:49
Hi,
I agree with Lewish, that you should regulate/limit the current and voltage (a current source, supplied by a constant voltage source), additionally in dependence of the temperature. Look at the websites of lead acid battery manufacturers for the T/V-characteristics. Keep also in mind, that the capacity regularly decreases with lower temperatures. For a low level application, oversize your sealed lead acid accumulator, limit the current to a level, that does not overheat your generator and limit the charging voltage to a level inside the tightest temperature characteristic of the used cells (13.5V@50°C versa 15.6V@-20°C). This would be the electrical safe solution.
But I'm very sceptical, concerning the mechanical solution. Look at bicycles: do you find any rubber wheel dynamo, reliable functioning in winter? I did not. And what about the tolerances, vibrations, bending by wind force e.t.c.?
Will the motor survive the ca. 7200rpm continuously with a high and varying radial load, coming from the rubber wheel?
If the curcuit draws only mA current, calculate a sufficient battery for the times between maintenances. You may switch off the circuit between the measuring intervals. I would also think about a photovoltaic solution, but this has to be oversized by a factor of 5 to 10 to guarantee a reliable and suficient power also in wintertime.
tiki
ccdubs (Electrical) (OP)
5 Aug 02 16:56
Thanks all for your contributions.  I have reviewed my options and have decided to go with a slip ring solution instead.  I believe that this will be much simpler.  It will be a home made job but it will save me hassle.
jarvis (Electrical)
22 Aug 02 17:16
How about solar power?

Could you mount a solar panel to your transmitter enclosure and get adaquate sunlight during full rotation?

How about using a gear motor with only a weight attached to the shaft, then you could put it all into an enclosure to keep the elements out.

Just some thoughts...

Jarvis

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