Makings of a generator

Hi All.
To put things into perspective. I didn't "set out" to make a generator. I just found these old motors and thought they would look like a nice scale generator to pair with some engines. There are a few small scale generator sets and kits available but not "medium" sized as I would class these. I saw a few ways to convert them to generators using permanent magnets and so set on this journey to see what, if anything, was possible. I tried the original windings with the PM rotor and that provided low out put. Then I tried with the dishwasher internals inside the old frame. Which is where I am still with no real usable output.
It was really a case of trying to make something that worked out of bits and pieces.
I have very little knowledge of generators and motors and have learned quite a bit so far so all is not wasted. What I have learned is that induction motors make poor generators without high RPM and/or excitation. Permanent magnet motors easily generate electricity but not necessarily useful power. The windings are fairly critical so "any old motor" won't necessarily be able to generate useful power. Maybe that's what Bluejets was alluding to in his post.

What I was hoping for Ken, was something that could perhaps charge a small battery that in turn would supply power for a couple of diodes plus the engine ignition. I imagine that is quite feasible if one used say a small starter generator from a lawnmower or similar, but unlikely from ad hoc 240volt motor similar to the one I was trying to use. Unless there is another way like re-connecting the 9 coils differently.

Hi Tony, Guys,

Putting the windings in series will increase the voltage but not necessarily the current available.

Tesla played about for quite a while whilst coming up with electric power generation.
He standardised on 60 Hz when using Niagara Falls to generate power for north America. Most of the rest of the world uses 50 Hz.

I've one or two Ideas for a usable generator ! Small cheap toy electric motors will generate several volts at a few milliamp. Using a stepper motor is another if the driver has enough power. I'm tempted to have a play with some devices that I have kicking around.

I don't know if this is helpful but this is a stepper motor driven by my V Twin I did a few years back.



Ray

Hi Ray,

Thank you for your post & video.

That is the sort of output, approximately 100 volts, I would expect from a stepper motor used as a generator. What is the bulb wattage ?

There are a lot of stepper motors used in printers, scanners and the like !

Hi Tony,
I hope I have been able to give you a better understanding of what you need...
Some "simple rules"?

• Windings - wire size and length - determine the CURRENT for a motor of generator. A 240V 1 A motor is only good (safe) at 1 A at whatever voltage.
• Speed of pole piece passing a coil generates voltage. e.g. a 500rpm 1 " dia rotor develops 1/3rd of the voltage of the same coil and magnet as a 3" rotor. - which is why "modern" generators tend to be shorter shaft and larger diameter shaped than the earliest designs of dynamos.
• Simply connecting coils in series to develop more power... - wrong - just more volts. 3 phases at 12V and 1A = 3 x 12W = 36 W. Connect coils in series = 1 set at 3 x 12V = 36V x 1A = 36 W. But maybe more volts is more useful? 3 x 4V = 12V for an ignition system?
• More speed is more power - YES! Gear it up to run it faster. = More load on the engine though, so understand the engine's "sweet-spot" where it produces best torque, then decide gearing (belt pulley sizes?) to get the generator at the speed it needs to run at for the power you need..
• For an ignition system, I figure current is important, as many systems are for low voltage DC - from batteries. So when you have generated more than 12V, connect sets of coils in parallel (which is what the diodes are doing on the 3 phase motor conversion) so at 12V (to power ignition) you get max current. 3 x 1A sets of windings in parallel = 3A... But connect sets of windings via a diode bridge, as you have done with the 3-phase motor - so the 12V DC output is connected in parallel. If your 3 phase motor is pumping-out 60V x 1A with each phase being 3 coils, then re-connect coils in parallel (per phase) so you get 20V and 3A across the diodes per phase. Then a cheap 12~24V DC to 12V DC "buck controller" (1 per phase) can handle the voltage conversion from the generator to charge a 12V battery.
• I do think you need to understand Amps are as important as Volts. So High voltage and milliamps is "not useful", as is very low voltage!. But what Voltage and Current does your proposed ignition need?

Hope this helps,
K2

Baron this was several years back and my memory is not so good anymore but the original thread was three pages long and may answer your question.

https://www.homemodelenginemachinist.com/threads/stepper-motor-alternator.34612/

Ray

BaronJ said:

Hi Tony, Guys,

Putting the windings in series will increase the voltage but not necessarily the current available.

Tesla played about for quite a while whilst coming up with electric power generation.
He standardised on 60 Hz when using Niagara Falls to generate power for north America. Most of the rest of the world uses 50 Hz.

I've one or two Ideas for a usable generator ! Small cheap toy electric motors will generate several volts at a few milliamp. Using a stepper motor is another if the driver has enough power. I'm tempted to have a play with some devices that I have kicking around.

Click to expand...

Don't forget the generators in hand wound flashlights and the like that were floating about a few years back. Those lights didn't last very long, but I don't thimpfk it had anything to do with the little generators in them. You can still find those some places.

Hi Richard,

Yes those hand held generator flashlights ! I still have one kicking around. They used step up gearing to drive a small toy motor. One I had tended to burn out the Led's !
As you say they were all the rage at the time.

BaronJ said:

Hi Richard,

Yes those hand held generator flashlights ! I still have one kicking around. They used step up gearing to drive a small toy motor. One I had tended to burn out the Led's !
As you say they were all the rage at the time.

Click to expand...

Wish I had some now. would keep them till am ready to use.

Also had one to re-charge my first mobile phone... Took an hour of hand pumping! - had hands like a Blacksmith afterwards!

Out of curiosity I just ran up a spare 12V brushed motor I had for a model boat. I don't remember the specs but it is about 45mm Dia 65mm Long. 5mm shaft I think it was spec'd at 900Rpm. I connected to a 12V- 21w bulb. This small motor was pushing out 6V DC @ 2400 rpm and 1.25Amps. Illuminating the bulb fairly brightly

At that size it sound slike it may be a 775 motor. I have been playing with one with the intension to make a dynamo. This is a 12V 3W LED. Now going to squeeze it into the 3D image.

TonyM said:

Out of curiosity I just ran up a spare 12V brushed motor I had for a model boat. I don't remember the specs but it is about 45mm Dia 65mm Long. 5mm shaft I think it was spec'd at 900Rpm. I connected to a 12V- 21w bulb. This small motor was pushing out 6V DC @ 2400 rpm and 1.25Amps. Illuminating the bulb fairly brightly

Click to expand...

That is what I would expect from a small motor like that.

So when using a motor as a generator what governs the voltage and power output. Trying to get a feel for what could be useful as I have access to a variety of motors at different voltage. mostly 12 and 24
For example would a 24V 50W 3000rpm motor running at 1500rpm be better or worse than a 12V 50W 3000rpm motor running at 1500rpm

Hi Tony,

There are many variables involved here. The magnet strength, the number of turns being cut by the magnetic field. Which now involves the rate at which the turns are cut. In addition to the load imposed.

So a higher number of coil turns gives more volts, if the speed is increased the voltage will be greater, then you hit a limit that is dependent upon the impedance of the windings and the load resistance, and rate of change.

Its not really possible to say that two physically similar motors designed for different speeds, one half the other will give half the voltage output for a given rotational speed.

Practically the answer is to actually test them removing as many variables as possible.

You do the Maths:
1500/3000 = half.
You'll get half the voltage from each as a generator at the lower speed. But also the current will be restricted by the winding impedance, and the 24V device will have twice the impedance => half the limiting current.
What voltage and current do you WANT/NEED?
Only you can decide "Better". = Better for your input (engine) and output (load/lights).
K2

Hi Ken,

Why not try it ! Take a motor used as a generator at any speed, measure the voltage and then measure the voltage at half or twice the speed (RPM), with or without a fixed load, and see what you get !

BaronJ said:

Hi Ken,

Why not try it ! Take a motor used as a generator at any speed, measure the voltage and then measure the voltage at half or twice the speed (RPM), with or without a fixed load, and see what you get !

Click to expand...

That's part of why I asked. When I ran the 12V motor way overspeed (I can't remember the exact spec of the motor but I am confident it was not more than 900rpm) I was running it at 2400. I still only got 6Volts. I have no idea if it makes any difference whatsoever but both terminals on the motor have small caps to the frame for radio suppression.

It would also seem that they need a certain number of revs before they start doing anything so simple maths will not apply. Halve the revs and you may get nothing.

Just out of curiosity I ran up a couple more small motors today. Both with the same 21w 12v bulb for load. One was 14v from an old drill(so probably high RPM.) It barely illuminated the bulb and was showing 4.5v the other was marked as 12V showing 10V brightly illuminating the bulb. Very similar physical sized.

A question for the more knowledgeable in the subject. - Brushless motors are rated in Kv which is how many RPM per volt. How might this be applied to using the motor as a generator? Say for example we had two motors that were similar in physical attributes but one was rated at 400Kv and one was rated at 100Kv. My guess is the 100Kv would be higher torque as it is lower speed for the same voltage. Would it be correct in assuming this is therefore the better option.