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Problem: I am trying to hobby-build a generator using a (old, working, throw-away gas, yard blower) 30 cc, 2-stroke engine. The 2-stroke engine will spin a motor and I want the motor to generate DC current.

My research:

  • Normal small-fan motor without magnets will NOT work; and if it does, it is extremely inefficient
  • Ideally, one should use a brushed, DC motor to generate the power
  • A DC motor used as a generator typically has permanent magnets
  • Brushed DC motor is preferred to brushless DC motor (not sure why)
  • Brushed motors may be spun/rotated in either direction (CW, CCW); however, one direction (CW) is preferred and slightly more efficient
  • One can generate AC current from a DC source using an inverter/converter
  • Ideal building blocks: (input) gas/oil mix --> run 2-stroke-engine --> spin brushed DC motor --> generate DC power --> charge a 12 volts car battery (12-volt 1ah; 12 DCV) --> 12 VDC to 120 VAC Inverter/Converter --> 120 AC current (output)

What I have tried:

  • I used a small-fan 120-volt AC motor (without a permanent magnet) and realized that it will not work; it generates very little power
  • I did google-searches and found some small cheap brushed DC motors on Amazon [e.g. 12/24V 30W High Speed CW/CCW Permanent Magnet DC Motor For DIY Generator (DC 12V 3000RPM)]. Interestingly, its comments/description on Amazon says “The stator windings of the motor are copper wire…” this confused me since I believed that in a DC motor with permanent magnets, the stator is the permanent magnets; i.e. it is not based on stator windings. Also, the motor’s physical dimensions are 1.97 x 1.18 x 1.97 inches. It seems rather small (physically). Is this unit good enough? Will it overheat?
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  • $\begingroup$ Try a car alternator - controlling the field current controls the output & therefore the load. $\endgroup$
    – Solar Mike
    Commented May 18, 2020 at 16:18
  • $\begingroup$ Thanks Mike. I had thought about it initially since an alternator would convert and directly produce AC power; however, I decided to first check other alternatives as an alternator is a bit costly (minimum ~$60 for used item). I will keep it in mind as I would not (then) need an inverter or battery. Thanks for your suggestion. $\endgroup$
    – user97485
    Commented May 18, 2020 at 18:08

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Here is the cheap way to proceed.

First, find you a battery-powered weedeater that is second-hand, or a discard. disassemble and test the motor for operation. If it operates, now you have a generator that has almost exactly the right power output for your application, because in its previous lifetime it was a motor in the same application as the little gas engine you have in hand.

Then you couple the electric motor shaft to the gas engine shaft and connect the output of the motor to a full-wave bridge rectifier that has the same voltage rating as the generator's output, and enough current capacity to manage the power output of the generator- which will be close to the power consumption of the generator in its previous life.

This sounds like a cool project and I hope you have fun building it!

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  • $\begingroup$ Good idea. Never thought about this novel approach. Thanks. $\endgroup$
    – user97485
    Commented May 18, 2020 at 18:07

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