Generating power with a stepper motor

If you want to generate electrical power from about a tenth of a watt up to several watts, a stepper motor might be worth considering. I’m going to go into detail here on a specific stepper motor that can generate up to a little over ½ watt.

First a few key features:
1. Inexpensive: New this stepper motor costs $2.95 in single quantity. At this price it probably isn’t worth scrounging for one out of an old printer, especially since I’m going give you the performance specifications for this motor when used as a generator.
2. Stepper motors generate power at low RPM. You can get usable power at just a few hundred RPM. For Stirling engines, wind turbines and other low RPM power sources this means you can drive the stepper motor directly from the source without having to use gearing.
3. Efficiency above 40% is possible depending on how you load the motor. While not great (I’d consider above 75% great) it’s pretty decent for these low power levels and considering the low price. Keep in mind that not having to using gearing or belts means you won’t lose more power to friction.
4. Stepper motors are brushless motors so the only wearing parts should be the shaft against the bearings.
5. A stepper motor used as a generator puts out alternating current and must be rectified to produce dc. For some applications, such as driving LEDs, you can use two LEDs so that one conducts in each direction without the need to rectify the output.

This article is written for people with some electronics background. If that doesn’t include you, don’t despair, I’ll provide a future article with more of a cookbook approach for a few circuits that you might want to use with the stepper motor generator.

The following chart shows the voltage output versus RPM for the stepper motor with various load resistors.

You’ll notice the above chart show Vdc. The measurements were made with the circuit shown below. The key components are the diode bridges and the filter capacitor. Although I used Schottky diodes, the stepper motor as generator has sufficient compliance that ordinary silicon diodes will work. You can also use an ordinary bridge rectifier IC instead of individual diodes. For the highest power output the Schottky diodes will provide a slight advantage.

This stepper motor has a 7.5 degree step size so you get 48 steps per revolution. The frequency of full wave rectified pulses should be 48 * RPM/60 for computing your filter capacitor. At 300 RPM you are already at 240 Hz so flicker on LEDs even without filtering is not visible. If you need low ripple for the DC voltage then filter capacitor value required is even less than for working from a standard rectified 50 or 60 Hz AC power.

This next chart provides the same data as the first but in a current vs RPM format.

One property of stepper motors that can be useful in their primary application is detent torque. This torque holds the motor in discrete rotational steps, even when no power is applied. To overcome this torque resisting rotation requires a relatively high drive torque and causes inefficient operation at low power levels.

Even with zero electrical power output, the minimum torque starts at just above 40 g-cm. To achieve reasonably high efficiency you need to operate the stepper motor with a reasonable load.

Also worth noting is the torque dropping rapidly at higher RPM for the 47 ohm load case. I don’t have sufficient expertise in motor/generator design to be sure, but I’m guessing this is a magnetic saturation effect. Note that even though the torque is decreasing, the overall input and output power are still increasing, although they are doing so more slowly.

The following chart shows the efficiency (% efficiency = 100*electrical power out/mechanical power in).

Notice that the maximum efficiency occurs with a 100 ohm load over the full RPM range. The 47 ohm load will result in higher current but lower efficiency. You can see from the chart that higher RPM might continue improving the efficiency (slightly) at loads for 100 ohms and above.

I obtained this stepper motor from Jameco. The part number is 171601. Jameco provides a link to the data sheet for the part; a few of the specifications are:
MABUCHI MOTOR COMPANY
PF35T-48L4
Mounting hole spacing: 1.65 in (42mm)
Shaft diameter: .078 in (2mm)
Motor dia: 1.38 in
Motor depth: .58 in

The motor has a dual shaft and a 10 tooth brass gear on one shaft. The other shaft (actually the other end of the same shaft) is just visible in the photo.