Review of Brushed and Brushless DC Motors

A Quick Review of Brushed and Brushless DC Motors

Brushless DC (BLDC) motors have become extremely popular over their predecessor, the brushed DC motor (see figure below). As the name implies, "brushed" DC motors use brushes, and a commutator, for controlling the movement of the motor's rotor.

Figure 1. Brushed DC motors use brushes and a commutator. Image courtesy of Clemson University.

Again, as implied by its name, brushless DC motors don't utilize brushes; motor movement is controlled by means of carefully designed drive signals. Compared to brushed motors, brushless motors offer improved reliability, longer life, smaller size, and lower weight. BLDC motors have become more popular in applications where efficiency is a critical concern and, generally speaking, a BLDC motor is considered to be a high-performance motor capable of providing large amounts of torque over a wide speed range.

Some BLDC motors use Hall-effect sensors for detecting the position of the motor's rotor with respect to the motor's stator (see Figure 2 below).

Figure 2. BLDC motor using Hall-effect sensors. Image courtesy of Nidec.com.

Other motors don’t have sensors; they’re referred to as sensorless BLDC motors. In place of using Hall-effect sensors for determining the rotor's position and/or speed, a phenomenon called back EMF is employed (see Figure 3 below).

Figure 3. Sensorless BLDC motor control using back EMF. Image courtesy of Microchip (page 4).