PMDC motor as a generator for low power app.

[mystiquesbs]

Hi,

I am currently undertaking a project where we will be using a PMDC motor as a generator to supply a power of less than 20W to power LED's within a rotating sculpture.

I have found that the type of motor I should be looking for should have a low RPM rating at a high DC voltage e.g. 300RPM @ 30V.

I really seem to be struggling to find any motors of this rating. The closest I got was a motor with 3500rpm @ 24Vdc

which cost $30 NZ and then a motor that was rated 3040rpm @ 48V but it is $267. I am pretty sure I could create a gearing ratio of 2:1 and I only require 12V. So with the 3040rpm @ 48V motor that would be 380 rpm @ 12V which sounds OK.

So I am wondering is it possible to use a geared motor as these are cheaper?

I have seen some geared motors with the following ratings:

max speed = 190rpm, Max Torque = 100nMn, Supply Voltage = 24Vdc and $124.00 with a 20:1 gear ratio. (May Be a little to low RPM?)

No Load Speed = 530rpm, Stall Torque = 110nMn, Supply Voltage = 24Vdc and $149 with a 20:1 gear ratio.

Any help would be greatly appreciated,

Regards

Brent Wilson

mystiquesbs@gmail.com

[Todd a]

I do not care for most PM DC motors as they require a much higher RPM.  Now with your listed as so low it likely has a gear box (planetary gears or something like that).  These will add some resistance and massively increase the torque required to turn it.  I am building my own generator.  The flat magnet plates with wire coils are much better for low RPM power.

[wooferhound]

20 watts of LEDs huh ?

most LEDs only require 3 volts or less and some kind of current regulator to keep from overpowering them. What kind of wind turbine are you going to use, Vertical or Horizontal. Do you actually have enough wind to make 20 watts. Gears only make it harder to spin the generator and make startup imposable in low winds.

[mystiquesbs]

Thanks for your comments So you recommend staying away from PMDC motors and looking at something else such as home built PMA's?

the shaft that the prop is attached to is 100mm in diameter. Do you think it would be viable to create a PMA around this and run the LED's from this with regulator/rectifier circuitry in between?

[mystiquesbs]

Thanks for your reply

Yeh we are still contemplating the PMDC motor and are maybe thinking about building PMA's to fit around the 100mm shaft if we don't find a suitable motor.

We have two different ideas on running the LED's to create a glow effect within the sculpture. One will require more led's than the other and we have still not tested how well each method works. But from some simple calculations we can estimate the power needed from 100 led's and work from there.

We will be connecting 25 parallel connections of 4 LED's in series. This way we can run them at 12V and around 50mA per connection depending on the colour of LED. So all up we will have 1.25A @ 12V = 15W.

If we could get a gearless motor that had a low RPM rating with a voltage of 24V+ do you think it could work? The average windspeed in the location these sculptures are being installed is 22Km/h.

The prop is 500mm and the shaft is 100mm.

Another idea was to build some sort of PMA to run straight from the shaft.

Any thoughts would be much appreciated.

And thanks again for your quick reply

Regards

Brent

[Bruce S]

About those LEDs.

Normal LEDs are at full brightness at 20mA, are you using the larger Lux units?

Also, usually best to limit 3 LEDs in a set and use a current limiting resistor, say about 120 Ohms if you go with smaller set of 2 or around 60 Ohms for sets of 3.

Just a thought

Bruce S

[mystiquesbs]

Cheers Bruce,

We are not too worried about the LED's at this moment. We will be purchasing 12V Led waterproof strips or the 12V halogen style LED's depending on what illuminates the sculpture the best. At the moment we are more worried about how we will generate the power to run the LED's. 15-20W is just an estimate but if we can generate power around 20-30W we will be happy.

Another note is that the power does not need to be constant. As this is for a sculpture it could add a nice effect if the LED's were to become duller and brighter depending on the wind speed, to create a breathing effect. And yes there will be a limiter to make sure the current does not blow the LED's in high winds.

Cheers Brent

[wooferhound]

very important question

Do you plan to have a Vertical or Horizontal turbine ?

a string of 4 standard LEDs will pull .020 amp at 12 volts

25 strings strings of 4 LEDs will pull .500 amps at 12 volts

.500amp times 12v equals 6 watts

I would overrate this to account for regulators and resistors and say that you will need less than 10 watts to power your LEDs

Someone else was building a wind powered lighthouse maybe those stories and comments can help you too...

http://www.fieldlines.com/story/2005/9/27/15317/0994

http://www.fieldlines.com/story/2005/12/26/33628/184

[mystiquesbs]

I just made a quick LED string using 22 sets of 4 x LED's. I ran it at 14V and it was drawing 200mA. So only 2.8W and the LED's that were being used are ultrabrights. It created more than enough light for the needed illumination so I am now thinking I will need a fewer LED's maybe only around 40-50 meaning a lot less power.

The sculpture will be using a horizontal turbine.

Cheers Brent