Hi, I’m working on a project that is hoping to turn mechanical power into electrical power. My mechanical mechanism can output a minimum force of 15N and operates between the 4 to 10 RPM range.
My search has shown me that my best bet would be to select a DC motor have it act as a generator. However, I’m having trouble selecting which motors would work for this application. Specifically due to the low RPM requirement. Can all motors work as generators? What specs should I be most looking at? Are their specifics about gearboxes that I will need to worry about? Any guidance would be great.
For your application, I believe you would need some sort of gearbox to increase the RPM for the DC motor to properly generate electricity. I believe our Design and Integration Services | DigiKey will be able to assist you in finding the right configuration for your application.
As a rule, a DC brush-type motor will operate as a generator.
You may be interested in DigiKey’s line of continuous rotation servo motors. It’s not a perfect match to your specification, but it may be a good place to start:
A few comments / words of caution:
You will need to modify the servo. Most contain a small brush-type DC motor. You will need to gain access to the motor, remove the controlling circuit board and connect wires to the motor’s brushes.
There will be considerable loss in the mechanical transmission process.
The gears are relatively delicate. Too much torque will destroy one or more gears. Replacements gears may be available, but you may need to search to find and then modify the parts for continuous rotation.
It can be challenging to disconnect the control board without damaging the motor’s brush connections.
Avoid brushless motors. While they could be used with appropriate diodes, there will be high losses at the low voltages implicit in your design.
Since you are prototyping, you may also want to try a cordless drill. See if you can find an older brush type drill. Once again, you will need to open the device, remove the controlling circuitry and connect to the motor’s brushes.
P.S. Do you have access to VEX robotics materials. They make a servo with changeable gear modules that may work in this application - again with modification.
P.P.S. You may also find a reasonable solution in this line of gearmotors. Planetary gearsets are generally better, but often have a high cost. Again, watch out for damaging the gears. Finally, know that high reduction gear sets do not like to be driven backwards. Some transmissions will break before they spin the motor / generator.
Thanks for the detailed information! Are DC brush-type motors are permanent magnet DC motors?
What is the reason for why the servo motor may be a good place to start rather than a gear motor?
I may need a motor that is a bit larger than what you suggested. What are the characteristics of a motor (stall torque, max voltage, back EMF constant, etc.) I should look at to maximize efficiency? Are there gear boxes that are efficient in the reverse direction?
If this is for more than just playing around, I recommend you buy an actual generator instead of using a motor as a generator. The device lifetime and power conversion efficiency will both be much better with a device designed for the purpose.
Yes, DC brush types are generally permanent magnet.
This can be difficult as I don’t always have a clear picture of who you are, and your project needs. Consequently, my answer is rather scattershot providing a range of solutions in the hope that one will catch your interest and move the conversation forward. The servo, gearmotor, and old drill are all potential options.
Years ago, I constructed a pedal generator from an old exercise bicycle. In some ways, this is similar to your application. The first step was to find a suitable generator. The permanent magnet DC motor was a good solution. Several different types were used including a large low-speed high-torque motor as well as a smaller high-speed with a gearbox. Both worked reasonably well.
Tech tip: A large capacitor may be used in place of a mechanical flywheel to smooth out power fluctuations such as those originating from a bicycle crank.
At any rate, I’d recommend you experiment a bit to learn about the limitations. Based on my understanding of your needs, an old battery powered drill would be an excellent starting point.
With regards to your efficiency question, we need to seek maximum power transfer. To do this, we need to match the speed / toque characteristics of your unspecified prime mover to the speed / torque characteristic of the generator. This concept is closely related to pedaling a bicycle. You as the human power source have a particular pedal speed and toque in which you maximize your power. The bicycle gears are used to match your power curve to the prevailing condition, be that uphill or fighting the wind.
Please let us know if we can assist in the future. Also, I’m curious what power source you are attempting to harness.
Thanks! For background, I’m back in school, but am coming from a few years working in industry doing mechanical design and systems integration. However, the motors and mechanisms world is very new to me. The power source my team is attempting to extract the energy from a buoy moving up and down from ocean waves.
I definitely am onboard with your plan of action. The hardest thing I’m finding is just locating a motor that works well at low RPM. It sounds like though, that it will probably have to be a try it, experiment, and find out situation.
You need something larger that is battleship ready.
This is a real design challenge to make something that will operate in calm seas but not be battered to pieces in the storms. Also, rest assured that your mechanism will get wet or at least humid thereby acceleration corrosion.
The Amazon device you mentioned appears to be 500 to 750 RPM range. A gearbox will be needed.
The drill, gearmotor, or continuous rotating servo are still good starting points for a small-scale wave-tank demonstration.
One more idea may be hiding in your institution’s janitor closet or surplus staging area. Look to see if you can get your hands on a battery powered floor burnisher. The rotating head in older designs often featured a slow speed brush type DC motor. It’s not good for open ocean, however it may be a good demonstrator e.g., human powered back-and-forth motion driven by a 5-foot 2 x 4.
Please let us know how the project progresses.
P.S. Research 3-phase bridge diode to explore brushless machines.