The brushed DC motor: Still a very viable option

Brushless DC and stepper motors may get more attention than the classic brushed DC motor, but the latter may still be a better choice in some applications.

Most designers looking to choose a small DC motor – a sub- or fractional-horsepower unit, typically – usually look initially at just two options: the brushless DC (BLDC) motor or the stepper motor. Which one to select is based on the application, as the BDLC is generally better for continuous motion while the stepper motor is a better fit for positioning, back-and-forth, and stop/start motion. Each motor type can deliver the needed performance with the right controller, which can be an IC or module depending on motor size and specifics. These motors can be driven with the “smarts” embedded in dedicated motion-control ICs or a processor with embedded firmware.

But look a little closer at the offerings of the vendors of these BLDC motors, and you’ll see they almost always also offer brushed DC (BDC) motors, which have been around “forever.” This motor arrangement has a long and established place in the history of electrically driven motive power, as it was the first electric motor design of any kind. Tens of millions of these brushed motors are used every year for serious, non-trivial applications such as cars.

The first crude versions of brushed motors were devised in the early 1800s but powering even a small useful motor was challenging. The generators needed to power them had not yet been developed, and the available batteries had limited capacity, large size, and still had to be “replenished” somehow. Eventually, these problems were overcome. By the late 1800s, brushed DC motors ranging into the tens and hundreds of horsepower were installed and in general use; many are still used today.

In contrast, the brushless motor has an array of electromagnetic coils (poles) fixed in place around the housing interior, and high-strength permanent magnets are attached to the rotating shaft (the rotor) (Figure 2). As the poles are energized in sequence by the control electronics (electronic commutation – EC), the magnetic field surrounding the rotor rotates and so attracts/repels the rotor with its fixed magnets, which is compelled to follow the field.

The current driving the BLDC motor poles can be a square wave, but that’s inefficient and induces vibration, so most designs use a ramping waveform with a shape tailored for the desired combination of electrical efficiency and motion precision. Further, the controller can fine-tune the energizing waveform for fast yet smooth starts and stops without overshoot and crisp response to mechanical load transients. Different control profiles and trajectories are available that match motor position and velocity to the application’s needs.

Edited by Lisa