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Hacking a DC motor to make a generator. RPM vs. Voltage weirdness, gear ratio confusion, and general circuit questions? Answered

I'm hacking an ancient 115 Volt DC water boiler motor to try and make a small, portable, "dremel" generator (meaning it does different things with different attachments). Doing a rudimentary test of it, with a small diode hooked to the wires, I found that it only puts out about .01 volts when being cranked by hand. So I figured I'd put my drill to good use, hook it to the rotor, and let it fly. Through this, I found that after 60 RPM, the voltage goes completely zero, even when spinning at around 220 RPM, nothing. I am completely bamboozled by this, which might have something to do with my limited electronics knowledge and my tendency to try and 'run before I can walk' in new things. I was also wondering if the low voltage could be solved by using gears and gear ratios, but my knowledge of those subjects is very limited indeed. If anyone can explain or converse with me on this, please do! I'd greatly appreciate any and all help. 



3 years ago

Is it possible it is a repulsion motor or some other weird type of motor? In repulsion-start-induction-ran motors, there are a set of brushes that connect different parts of the coils on the shaft together, and the exact position of the 4 brushes will start the motor when connected to AC, and once it gets up to speed, a centripetal switch disengage the brushes and the motor continues as an induction motor operating on AC. It is a pretty complicated design, and it is used for where high starting torque is required.

Also, if it is an normal single phase induction motor, it has 2 sets of field coils inside it, an auxiliary coil which is connected to a starting capacitor in series and to a centripetal switch to the mains, and another main field coil which is always on when the motor is powered normally. There are tutorials on youtube you can learn all about these types of motors, and how they work. It is a bit difficult to understand if you have not had experience with how magnetic fields and electromagnetism works, but I was able to learn when I was repairing an unknown motor that so happened to be a repusion-start induction-ran motor.

This thing is so ancient and well constructed, I can't see inside it at all. It does have two lubrication tubes, but I'm not sure how much it needs it has in it still.

I am also a bit confused as to how this is going to be implemented. A dremel is, in its most basic form, a small motor that has a small chuck on it for attachments. Generally, small motors like those found inside of dremels are universal motors, and have field coils which are connected in series with a coils on a rotor.

So what do you mean when you call it a "dremel" generator? What do you mean it will do different things with different attachments?

Also, have you tried to "flash" this motor while being used as a generator while it is running? The reason this sometimes has to be done is that, in order to make electricity from motion, conductors have to be subject to moving or changing magnetic flux. (You can look into Maxwell's equations to completely fry your brain if you want!)

With small DC permanent magnet motors, dynamos, and alternators, this is simple. (Just use permanent magnets and lots of wire on the stator and rotors!) However, due to the costs of building large motors or generators with huge and $$$ magnets, permanent magnets are avoided. A proper generator might have a relatively small 'permanent magnet' in it; just enough to produce a small amount of electricity, and then a small amount of current ends up being 'pushed' through the larger field coils when a circuit is established. These field coils, because they have current going through them, produce much more magnetic flux than some wimpy magnets (or possibly just the iron inside the motor that has previously became magnetized), and with more magnetism surrounding the spinning rotor, much more power can be produced! Sometimes generators need to be flashed to restore that initial magnetic field. I would spin the shaft of the motor up, and tap the output of the motor temporarily to a car battery to establish a magnetic field, while that output is connected to some load. You can also use the cordless drill method, if you prefer.

You're thinking to technically into the dremel idea. Think of it this way: one attachment means I can put it in a window and use it as a wind turbine, while another is a small water wheel and gives me hydroelectric power.

Ok, that makes sense, you mean that you want the shaft to be modular and have various attachments. I am not sure if you already have a chuck or something to make that possible, so I would look into getting one for that.

If what you are working with an induction coil that has no permanent magnets, then making it so it converts RPM and torque into amps and volts will be tricky. You will have to spin the shaft or rotor faster than the synchronous speed to generate the same voltage and frequency as the original rating, and figure out how to initially excite the coils to create a magnetic field. I see some mentions of having capacitors placed across the coils, but I have not done this, and again, I am just going off of what I am learning myself. I myself have a 1/2HP 3 phase induction motor that would be nice to use as a generator when the shaft is connected to a single phase motor that is powered by single phase power. I found the same issue, that when the shaft is turned, the motor produces millivolts.

I'm just worried about making it work before I worry about the chuck. I'm gonna take it apart, try to get some info or maybe numbers from the illegible label, and calculate the synchronous speed. If it's a speed I'm not capable of generating, I'm gonna go straight to a gear train for it, and try to make it faster than the synchronous speed. If I'm lucky everything else will fall into place, if not....more comments on this thread.

In my own personal research, I found this video that does an EXCELLENT job at explaining induction motors. The same thing happens when the motor is used as a generator, and supposedly, the principals are just the inverse, or reversed.


I do not fully understand how to use induction motors as generators, and have a pretty weak understanding of the principles at work. This wiki explains in detail about induction generators, and may have the information in it you need.

Old motors were designed to be serviceable, so it should be reasonably easy to dismantle and peer inside it. Modern motors are not, as culture has moved on to this idea of "replace if broke" rather than "fix if broke." Heck, with old motors and generators, you can even rewind the entire thing by hand! My father used to burn the wire out of old burned out motors, and then rewind them to make them good as new!

I doubt this is a DC motor....there's your problem....Try your meter on the AC volts range and see if you get anything.

You can try Iceng's trick. The only problem is that it won't generally give you a decent output until its spinning at the same sort of speed it would have been as a motor - and of course it generates AC, not the DC you want.

I don't understand what you mean by a Dremel generator - by definition almost, a Dremel is a motor, which you can attach things to. If you want a DC equivalent, just get a small DC motor/

What I mean is that it's like a dremel: different applications with different attachments. I don't mind if it puts it AC, I only wanted it to be a DC motor because I know how to work with those, then I'd convert the current later. I just need to figure out a way to generate electricity from turning an AC motor, but I don't know how. That's my largest concern now.


3 years ago

If you have an oscilloscope, measure the different combinations of the connection points. I would not use a multimeter AC reading because it is not that accurate when the frequency is really high or way too low, like 5Hz. If the frequency is very low when you turn the motor's shaft slowly, expect low freq. and low voltage. As you speed it up, expect higher freq. and higher AC voltages. An ossiloscope is like a analog multimeter on steroids! It can show you how the voltage changes over time, and with a bit more knowledge, you can determine a LOT of things about the output of the motor.

If it is a induction motor that has been sitting around for while, I believe if you use it as a generator, you have to 'flash' it, the same way an electric generator is flashed. I am not sure, I never did any of that, but knowing that electric generators and motors are very similar in construction, I would imagine the same ideas hold up for both.


3 years ago

Never, in half a century, heard of a motor boiling water AC or DC.

Assuming it might be an AC motor, so it wont hurt anything, if you loose that diode and put a true 370VAC 5uF $4 capacitor across the motor leads. This is called exciting the motor. Spin it up again measuring AC and DC voltage..




Yhea it's most likely AC

Try changing you multimeter to the "AC Reading" before you do anything !

Also second the Cap

Could be a couple of things, motor could be worn out or burnt out, or it the wrong type of motor. The easy way to check everything it once, hook it up to 12 volts the motor should spin, Are you sure its a DC motor, it should have brushes and magnets inside. check to see it the brushes are not worn out. still not sure? have a look here