Setting up a Generator to Motor circuit.

Hi All - First time poster question around an electronic
circuit set up and the correct components to use.
Being new to anything of this sort I need some help from
more knowledgeable minds.
I want to use a very small magnet generator I have to
set up a drive source for a small 12 Volt BLDC motor.
Other than advice - Parts in circuit that I think I need are:
Generator 3 Phase 12v-24v AC
3 Phase Rectifier to DC output
Magnetic RPM LED counter
Voltage Regulator
12 Volt LOW RPM Brush-less DC motor

My Questions are:
My main unknown is around the switching from 3 phase AC
3a-5a Max 100w being rectified to DC power.
I can’t seem to find a definitive answer as to the correct
type and size of bridge rectifier I should use.

Along with this I am having problems with finding a good
seller of this sort of component - I am here in Australia.

can you guys help me with this please!

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Automotive alternator. Has rectifiers built in.

Why do you need 3 phase?
Cheers Bob

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Hi Trev,

The most important part of the engineering process is problem identification, and a close second is idea generation. Having an insight into what you did in those steps will help us greatly. Let us know what the problem is that you’re trying to solve with this system, and we might have suggestions for simpler avenues you could go down.

To use a popular analogy, if guests at a hotel are waiting too long for an elevator, your first thought may be to put in a second or faster elevator (expensive, hard), but the same effect could be achieved by putting a button down the hall that calls it early so it gets to them by the time they reach it (cheap, easy).

Keen to hear more about this project!

Hi James, many thanks for coming back to me, I’m always wishing I knew and understood electronics - I am a product designer so I know exactly the type
of scenario you are alluding to. I have the very small generator and I have an
rpm counter, and what I want to do is rectify the voltage from the 3 phase output
of the generator to run a 12 volt DC motor. I will need a controller or voltage regulator to enable speed control, what I want the DC motor to do is simply spin
the generator - so it’s a closed circuit experiment. Then by also including a small capicitor in the circuit I figure I could turn the motor on at any time, it starts to turn the little generator and I can try to do this using less power than the generator is
providing. Even allowing for efficiency loss, frictional factors and other factors it
still seems do able - so I thought I’d try it out. But then I couldn’t find a rectifier and
here we are James. Do you have any experiences with this sort of make.?

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Hi Trev
Have I got this right. You want to have the motor turn the generator which in turn powers the motor to turn the generator and so on.

Called perpetual motion. Can’t be done. About 1st or 2nd year of my high school physics or in present terms year 7 or 8.

Convert everything to Watts and you will see why. You can never overcome the losses or in other words you can’t get something for nothing.

Sorry bout that.
Cheers Bob


That part is doable - a rectifier only requires diodes arranged appropriately, and perhaps some capacitors for smoothing. See, for instance:
What is 3 Phase Rectifier ? - 3 Phase Half Wave, Full Wave & Bridge Rectifier - Electronics Coach
(I am sure there are many other references)

The remainder of your circuit won’t work as you have described it. Somewhere in that system you need to input some energy to make up for the conversion losses.

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Hi Trev,

Good idea to experiment with motors and generators, but you’ll likely run into efficiency losses everywhere (friction in bearings, heat loss in BLDC drivers and motors, and the setup will quickly stop spinning.

Can you think of anywhere else in your life that could do with a decoupled mechanical connection like this (just not in a loop)? I’d see if you could charge a 12V battery or something like that with the generator, maybe make some kind of miniature renewable energy setup?

Hi James

You should change that “you’ll likely” to “you will”. Like I said. Trev has pretty much described perpetual motion. Can’t happen. Not at this time anyway.
Cheers Bob

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Hey Bob, I wasn’t looking necessarily at perpetual motion, but rather to balance
offset the possibilities. The presence of drain or power loss is unavoidable in
any system with the only possibility available being to minimise loss where ever possible.
I will get back to James now as well and pose something you may like to jump in on as well.
cheers trev

Hi trev

That is what you described. This would work if you had a battery inserted between genertor and motor to make up the losses but the battery would run flat without another external charging arrangement.
Cheers Bob

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Hi James, I just touched in with Bob re the efficiency question around power systems. I do understand the losses inherent in frictional movement and heat costing even more power, and for example if using ceramic bearings we can
reduce the shaft friction by another 30% give or take, as for minimising heat
that too is possible although not completely to some degree anyway.
Here’s the thing I don’t get though, OK so if we generate power to run a motor
and that motor has say an 80% efficiency, and so we generate another 20% to
feed the motor constantly knowing this. The we factor for the other losses within
the system friction, wear, heat and even gravity by additional power available to
our motor of another 10% for example. So now the power is there losses and all.
So if the system combined loss equation is covered by an over availability of supply at all times with the motor running at a moderate rpm say maybe 150rpm
providing this readily for consumption IF needed - why would it then stop?
I can’t get my head around that! I don’t mean to be thick but to me it should run.

It wouldn’t, but you didn’t mention that somewhere in that arrangement you were supplying more power.

And it’s not ‘IF needed’ - the additional power input would be required at all times because there will always be some losses.

Hi trev.
The important thing to remember here is that you can NEVER get any more power out of a device than what you put IN less losses (eficiency).

Lets try with some hypothetical numbers. We can understand better I think if we work in Watts.

Assume the generator losses are 1W. We expect to get out of that generator 10W so we have to input 11W. OK so far?

So to do this we have to provide 11W from the motor.

BUT we only have 10W from the generator and with motor losses (1W) we can only get 9W from the motor

To keep things going we need 11W.

So you can see we are 2W short and for this hypothertical situation we have to provide the missing 2W somehow from an external source to keep everything spinning or have everything operating at 100% or more efficiency which is just not going to happen or feasible.
Hope this simple hypothesis helps clear this up.
Cheers Bob

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Hey Bob, Thanks for that - I see now where the issue is in the power losses.
What if we started out with a small motor that uses far less when running than the generation capacity of the generator when running off said motor.
Basically the overkill in available power if you like, so that the watts available are
always in excess to requirement inclusive of losses.
Maybe the motor uses 20 amps to drive the generator which is capable from this
at providing 100amps - just for some numbers, does the loss effect ever catch up?

Hi Trev
I am not going to repeat it all. Just forget about Amps for a bit and use watts.
I will repeat this bit once and for all.
Back to 1st or 2nd year high school physics. Current jargon year 7 or 8.
Or maybe we had a better physics teacher.
Cheers Bob

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All I can suggest is try it.
Get a small DC motor, get a larger one to use as a generator and connect the shafts and measure the input and output.

Someone more knowledgeable in physics and engineering would be able to explain the reasons why it wont work. If the small motor is even able to turn the larger one the output of the larger one will always be less than the input to the small one. Think I may have tried this once.

The way motors work using electromagnetic fields is never 100% efficient. If the small motor is small enough it will not overcome the rolling resistance of the larger one let alone the magnetic fields and such.

What @Robert93820 has said is absolutely correct, I just don’t have enough physics and engineering to explain it better other than try an experiment.


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Hi Jim, Bob and Jeff, I just wanted to say thanks for the time you guys took to answer my query, I am pretty clear on that now, maybe best if I try something
else altogether since my first idea is a non starter. It cost too much these days
to try things that are bound to fail, one needs at least a fair chance of success.
thanks to you all again,
best regards Trev

Hi trev
No problems. Just remember the golden rule. You can only get out what you put in minus losses (efficiency). This applies to just about everything. The shortage has to be made up for anything to work.
In other words you can get nothing for nothing.
Best of luck with any future experiments
Cheers Bob