Back Emf of Magnetic Gearmotors

I tried measuring back emf of Polulu 250:1 Metal Gearmotor 20Dx46L mm 6V CB with Extended Motor Shaft
by using a drill to generate a fixed RPM and then measuring speed and voltage generated using an encoder (Magnetic Encoder Pair Kit for 20D mm Metal Gearmotors, 20 CPR, 2.7-18V). However, this loosened up my shaft and the shaft is freely rotating right now. Are my motors damaged? Are there other easy ways to find back emf of motors?


Those gearmotors are not meant to be back driven, and it sounds like the gearboxes were damaged. I do not have any suggestions for how to measure the back EMF of the gearmotors. You can calculate the back EMF using the electromotive force constant, and the electromotive force constant can be calculated using the specifications we provide on the product page. You can read more about calculating the electromotive force constant in the FAQ on the product page.

Why are you interested in the back EMF?


Thanks a lot for replying. I am interested in back emf because I want to apply controls to it, just like they are described here:, and I need back emf for this.

I want to ultimately apply impedance control, so, I need to modulate current for this.

Kindly let me know if there’s a better way to do this. Also, are there any resources wherein people applied different controls to the same/similar motor?

We have recently started better characterizing our motors to provide more detailed performance curves rather than just the free-run and stall data points, and these might be useful for what you are trying to do. You can see more information about this in the blog post we recently made about the graphs we published for our micro metal gearmotors. We have not finished the 20D gearmotors yet, but we can probably get you some preliminary data on the specific version you are using if you email us.