[Elbarto100]

Hello, I have a question regarding using a stepper motor to produce electricity.
Quick back story, I had a motor for an Ebike lying around wondering what to do with it, thought to myself that might make a nice small generator for something, but how to turn it.
So I built some blades which should be more efficient than conventional VAWT blades, then at work saw a phone charging station and thought BINGO, I'll use the motor for the phone charger.
After marrying the motor and the blades, i tested the turbine in front of an industrial ventilator, and hoping for ca 12 volts, at a wind speed of 25km/h, the motor actually produced a disappointing 2-3 volts.
After doing some research, it became blatantly obvious that a planetary motor as used in the ebike motor was the worst type of motor to use in this situation, but in fact a stepper motor is the ideal motor to use.

The motor I got was a Nema23 1.9Nm 5A 2 phase motor, (What other specs should be known).
I built a bridge rectifier for each phase, and wired those in series to convert AC into DC.
The cogs used gave a 2:1 ratio (40 teeth on the turbine side, 20 teeth on the motor side)
At 25km/h produced 4-5 volts, but of course was turning ultra slow.
I then made a cog with ca 500 teeth and the motor spins significantly faster, but it's also significantly harder to spin.
Without the motor connected, the turbines spins freely with little resistance, but with the motor, there is no free spinning.
At this stage, i havnt tested it with the ventilator, just spinning by hand, but I'm fairly confident at 25km/h I'll get 12 volts.
I'm worried that the wind speed to get the turbine spinning will be high.

My question is, in this situation, how big of a role does the torque play in the motor spin easier?
If it does play a role, can anyone recommend a motor that is more suited?
Or is it better to just halve the size of the turbine cog?

Thanks in advance

[joeavaerage]

Hi,
any motor can be used as a generator.

All will in effect have a voltage constant of A_v=nnn Volts/radian/sec or units of that general type. For example the my Delta 750W servos have a voltage constant
of 24.2 mV per rpm (from the Delta spec sheet). So if I spun the servo motor at 1000 rpm I would expect 24.2 V to be generated at the windings of the servo or if I spun it at 3000 rpm then I would get 72.6V

You can see that the effective voltage constant is a very useful specification to have if you want to generate power. While some motors have the voltage constant published in the specs, a lot do not either.
That's OK though, you can measure it. Just put a voltmeter across the windings and spin it at a known speed.

The question is what happens if you have a motor that you wish to use as a generator but you cant spin it fast enough to generate the voltage you need. Lets imagine for example you want 12V to charge a car battery
and you have a motor of 8V/1000rpm. Imagine also you have a turbine that can only go 1000rpm. At 1000rpm the motor would generate only 8V, not enough. You have to either spin it faster OR find another
motor with a higher voltage constant. Either is a drag.....but......there is another way.

Using a buck-boost electronic switching regulator you can increase (or decrease) the voltage from the motor to what you want. This is extremely convenient, because depending on how you control or program the buck/boost
regulator you can use just about any motor to generate any voltage so you don't have to buy yet another motor or have expensive and inefficient gearboxes or belts.

Indeed this is part of how electric cars are controlled. When you want to slow down or are going down a hill you need the motor to be a generator and push power back into the batteries so you can use it later.
If you were very cautiously going down a hill the motor may not be spinning fast enough to have enough voltage to push back into the battery. The control system kicks in and boosts the voltage of the motor so now
it IS pushing power back into the battery and producing the desired braking effect.

A buck/boost regulator is not magic, it does not change the power just the voltage. Lets say you have a motor and spinning as fast as you can and it is producing 5V. If the regulator boosts that
5V up to 10V, and the 10V load is consuming 1 A or 10W, then the 5V generator must produce 2A, or 10W to keep up. So the power is not magically being generated but you can adjust the voltage output
of your generator plant to match the load.

Craig