Why use brushed servo motors?

The idea of brushed servo motors may seem a bit counterintuitive—most of us think of servo motors as high-performance devices used in highly dynamic applications, while brushed DC motors are low-cost solutions for mass-produced consumer devices. And, to a large extent, this is correct. But remember that “servo motor” is a fairly broad term that applies to any motor used in a closed-loop system with feedback. In other words, whether a motor can be classified as a “servo motor” has less to do with the motor type and construction than it does with the type of system the motor is used in.

Brushed DC motors are also referred to as permanent magnet DC (PMDC) motors, because the stator incorporates permanent magnets on its inner diameter. The rotor (also referred to as the armature) consists of a slotted iron core with windings that are attached to a commutator. PMDC motors are mechanically commutated, with brushes that carry current to the windings. This is in contrast to brushless DC (BLDC) motors—a type commonly used in servo applications—which are commutated by electronic means rather than by a commutator and brushes.

While the mechanical commutation of brushed DC motors is often seen as a drawback—the brushes are wear parts which must be maintained and replaced—this design is typically less expensive than that of a BLDC motor. And in terms of performance, brushed servo motors have very smooth speed output (especially at low speeds), a wide speed range, and good speed control, with the added benefit of using regenerated energy to brake the motor. (Alternatively, the regenerated energy can be sent back to the power supply). They can also produce very high torque for startup or acceleration. Compared to other DC motors, brush types are generally smaller and more energy efficient, since there is no field coil.

There are two important characteristics of brushed servo motors that engineers and designers need to keep in mind. First, the rotor is located inside the motor, and the thermal path for heat dissipation from the rotor is not very efficient, so pay close attention to the thermal characteristics of the motor. Second, the permanent magnets on the stator can be partially demagnetized if excessive current is applied.

Manufacturers of PMDC motors—i.e. brushed servo motors—often cite automotive applications such as power seats and windows or windshield wipers, as their main area of use, due to low cost and a simple control method. These characteristics also make them a common choice for household appliances and small consumer electrics. In industrial applications, PMDC motors are commonly used for equipment where torque is only (or primarily) required during acceleration and deceleration.

Feature image credit: Portescap