Brushless motors dominate the industrial motion control market, but brushed motors still make sense for some applications, thanks to their linear performance and ease of use.
DC motors can use either mechanical or electronic commutation. Brushed DC motors rely on the mechanical method, with brushes and a commutator, while brushless DC motors use electronic methods to achieve commutation. Although the brushes and commutator are wear parts that are avoided with brushless commutation, brushed DC motors do have some advantages over brushless versions.
Brushed DC motor construction
The stator of a brushed DC motor can have either permanent magnets or electromagnetic windings. The more common type for servo applications (and, therefore, for most industrial motion control applications) is the permanent magnet type — typically referred to as a permanent magnet DC (PMDC) motor. The rotor consists of coils wound around a slotted iron core and attached to the commutator. As the rotor turns, brushes make contact with the commutator and deliver current to the windings.
Alternatively, the motor can be a coreless type, where instead of an iron core, the rotor is made from windings that form a self-supporting, hollow cylinder, held together with epoxy. The permanent magnets of the stator sit inside the rotor and the stator is fixed to the motor housing (completing the electrical circuit). The rotor is supported by bearings and rotates around the stator.
Where brushed motors outperform brushless designs
Despite the drawback of wear parts — in the form of a commutator and brushes — brushed motors do offer several benefits over other types. First, brushed versions exhibit linear performance characteristics, with very high torque and a wide speed range. They produce smooth motion at low speeds and have good speed control. Brushed designs are also typically smaller and more efficient, as well as lower-cost, than brushless motors.
With no iron in the rotor, coreless versions (also referred to as “ironless,” “slotess,” or “air core”) offer the benefit of reduced mass and inertia, which allows higher acceleration and deceleration rates. The absence of iron also means the motor has no iron losses, so they have even higher efficiencies than traditional iron core brushed motors. And when the motor contains iron laminations, magnetic interaction between the permanent magnets of the stator and the laminations causes cogging and torque ripple. Since coreless motors have no laminations, cogging and torque ripple are greatly reduced.
Brushed DC motors are the workhorses of consumer goods, small appliances, and automotive applications such as power seats and windshield wipers. In industrial applications, brushed motors make sense when high torque is required primarily during acceleration and deceleration. A common example is dispensing equipment used in the medical and packaging fields. Coreless designs are also ideal for battery-powered devices, since they draw very little current under no-load conditions. And their ability to produce fast, dynamic moves makes them well-suited for robotic applications.