DC-motors (Part 1)

Part 1, 2, 3

DC-motors are every where in many form factors, many performances and configurations. The aim of this series of posts is not to cover all aspects of DC-motors as there are so many publications available, most of them being of excellent quality.

Firstly, I will concentrate on the most common brushed DC-motors . Next picture illustrates a typical DC-motor of this kind.


And here is a picture of the dismantled motor. From left to right, the  brushes holder flange, the rotor and the cage holding the two magnets


Next is a picture of the rotor showing the collector and the three magnetic poles


You may find these types of motors in many, many applications, from hair dryers, to telephone vibrators, through cordless drills, toys and servo-motors. On one hand these motors are cheap, on the other hand they have poor torque and balance.

Feeding these motors is easy as far as loose specifications are concerned. In most applications, a fixed DC voltage is applied to the motor. Over-current protection is provided by a fuse (on the left), or even a thermal fuse (middle or right in the next picture) or by an electronic function when the driver features a voltage regulator.


Care must be taken with over-currents because the brushes are generally weak and consist in simple copper blades:


If we leave apart DC-motors fitted with a tachometer or an optical or magnetic encoder, controlling the speed of a DC-motor is not so easy. A method of choice consists in using PWM (Pulse Width Modulation) to power the motor. Next diagram illustrates the principle of PWM.


While t is constant, the on/off time ratio is proportional to required rotational speed. Although many publications describe how to wire such a controller, here is the “ultimate” schematic


R1 is 1 kOhm and T1 is an IRF540 N-Channel MOSFET (or one of its numerous cousins). In my case, VCC was 6 V in order to comply with the motors specifications.

As I plan to go in more details in the next posts, I designed and printed a specific test bench as shown below…


… featuring the following components: 2 identical motors, a rubber tubing which acts as a flexible coupling, a barrel which holds a reflective band (for future tests) and the mainframe with its two clips


Both DC motors come from a defective CD player (diameter 24 mm, height 20 mm).

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