Ever wondered what’s inside a LEGO MINDSTORMS EV3 Medium Motor? Look here then, and avoid bothering to open your own motor. In fact, it could be permanently damaged during the disassembly process.
The EV3 Medium Motor stays together without screws. To open it, there are some tight tabs to be unlocked. (Don’t try this with your new motor!) The yellow thingy that looks like a capacitor is a temperature sensor that protects the motor (and probably the EV3 motor drivers) if the motor overheats, in case of stall. The motor includes a rotation sensor (quadrature encoder) that is hidden in the gray plastic. I didn’t open it further because I didn’t want to desolder the motor terminals. Notice that the motor itself would be of the same length as the LEGO Power Functions motor, but the presence of the LEGO MINDSTORMS female connector makes it more than two LEGO units longer.
While the rotation sensor is directly connected to the motor shaft coming out from the motor back, the output shaft is coupled to the DC motor shaft by a two-stage planetary gear train, just like the LEGO Power Functions motors. The adoption of the planetary gear train allows this motor to have almost no backlash on the output shaft. In comparison, the gear train of the EV3 Large Motor is really poor, causing 12 degrees of backlash on the output shaft. Backlash (or play) is caused by the gap between the gears’ teeth, and can be seen by gently moving the red motor shaft by hand back and forth, without making the inner DC motor rotate. This makes the EV3 Medium Motor preferable to drive precise low-torque mechanisms.