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maitamites16
Newbie

Joined: 28/02/2012
Location: Philippines
Posts: 2

Posted: 04:04pm 29 Feb 2012

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I'd been checking your site for quite a while now and I really like your projects and thats what motivates me to build my own windmill.The motor that I used was a small DC motor out of a small chargeable electric fan. The problem is, when i give it a spin with my finger it generates electricity but when I install propeller in it it doesn't, even if it spins really good.What could be the problem?That really makes me wander.Can anybody help me with this..
Thanks in advance

Max

Don B

Senior Member

Joined: 27/09/2008
Location: Australia
Posts: 190

Posted: 01:02am 07 Mar 2012

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Hi Max, and welcome to the forum.

Apart from the fact that the motor that you are experimenting with is really too small to produce useful amounts of energy, I think that the difficulty that you are experiencing might be in how you are checking that it "generates electricity".

If you connect a voltmeter across the terminals of the motor (with nothing else connected) and spin the shaft, then you should measure some voltage. If you then fit the propellor and let it spin the shaft for you, then you should also measure some voltage. If you are not seeing voltage with the propellor fitted, then something must be different about how you are measuring voltage with the propellor fitted. Are you still measuring directly across the same motor terminals, and is anything else now connected that was not connected before?

If you now connect some sort of electrical load (ie a small low voltage light globe) across the motor terminals, and continue to measure voltage as you spin the shaft, then the voltage will be less, but you will be actually generating electricity

Note that small motors can only produce small amounts of power at best. Maybe this will drive some small lights, but very little more. Note also that fans or propellors do not make efficient turbine blades for extracting power from the wind, as their twist is wrong for this purpose. No problem using them for your experiments though.

Good luck with your experiments, and also take the time to read through some of the other threads in the forum, as you will find a lot of helpful information there.

Regards

Don B

maitamites16
Newbie

Joined: 28/02/2012
Location: Philippines
Posts: 2

Posted: 01:35pm 09 Mar 2012

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thank you very much Don B..I really appreciate it..can ask one more question if you don't mind..Here it is, is there any specific rectifier that I can use for a motor that generate an electricity about 1-5 volts..I'm going to use it in my mini mill project..And does rewinding the motor will help it generate more power...Again thanks in advance...

Don B

Senior Member

Joined: 27/09/2008
Location: Australia
Posts: 190

Posted: 11:20pm 09 Mar 2012

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Hi again Max

If the fan motor that you are using is a commonly used DC motor, you will not need a rectifier as it already produces DC. If you can see inside the motor, then you can confirm this because the motor will have two fixed spring loaded carbon brushes bearing on a commutator, which is a rotary switch made up of strips of copper that rotate with the motor shaft, as do the windings.

The other possibility is that it is a "brushless" motor, where the switching is done externally by an electronic unit. An example is a stepper motor, common in printers, etc. In this kind of motor, the windings would be fixed, rather than rotating on the shaft. These are more complicated to use for learning about generators, as you need to identify how they are wound, and connect to them in a particular way. If you want a DC output, you would also need to use one or more rectifiers.

Regarding rectifiers, the ones that you would commonly find are either silicon or Shottky types. They both do the same job, however, silicon diodes have a forward voltage drop of around 0.7 Volts, while Shottky diodes have a forward voltage drop of only about 0.25 Volts (meaning lower losses). For the low voltages that you are looking at, you could use just about any kind of diodes that you could get your hands on (assuming that you need any diodes at all).

Regarding re-winding the motor to produce more power, the short answer is that it can't be done. The power that it produces is largely a matter of its physical size. You could re-wind it to produce different voltages for the same shaft speed, but, as you put on more turns of thinner wire to increase the voltage, the current that it is able to produce will fall, and the power, which is Volts multiplied by Amps, will remain much the same.

Regards

Don B

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