Brushed DC Motor Working Principle

DC motors are among the most widely used types of electric motors used in several applications today because of their high starting torque and ability to run at high speeds. These motors come in different types, but the brushed DC motor type remains a viable solution, especially for low-end and cost-effective applications.

However, brush DC motors have unique construction and mode of operation. In addition, there are several things an engineer must know before choosing a brushed DC motor for an application.

Figure 1: Brush DC motors have

unique construction and mode of operation

Source: [artbase/Adobe Stock]

Like most electric motors, a brush DC motor comprises two essential parts: the stator (or stationary part of the motor) and the rotor (or rotating part of the motor). The rotor typically features coils wound around an iron core and surrounded by magnets (contained in the stator, as shown in the Figure below.

Figure 2: Brush DC motor

Source: https://www.researchgate.net/figure/Model-of-a-brush-DC-motor_fig5_288617020

However, the presence of a sliding electrical switch distinguishes brushed DC motors from other DC motors. This switch consists of a commutator and fixed brushes mounted to the rotor.

When DC voltage is applied across the brushes, it passes electric current to the rotor windings. This electric current energizes the rotor coils, creating magnetic poles and causing the rotor windings to behave like a magnet. As a result, the magnetic poles start repelling (and attracting) the poles of the permanent magnet that make up the stator, causing the motor shaft to rotate.

As the rotor turns, it reaches a position where the commutator and brushes are no longer in contact, halting electric current flow in the coil. However, initial momentum causes the rotor to continue rotating, which brings the brushes and commutator back into contact. This restores current flow to the system and ensures the motor shaft continues rotating.

The speed of the brushed DC motor can be obtained using:

Where:

                Eb = back EMF

                A = number of parallel path

               P = number of poles

               Fi = Magnetic flux (Wb)

               Z = Total number of conductors in the armature

The permanent magnet brush DC motors are very popular. And as their name implies, these motors use permanent magnets to create the magnetic field (in the stator) required for the operation of the motor. These motors respond well to changes in input voltage and allow speed control.

Engineers will find them used as starter motors in automobiles, windshield wipers, and toys. 

The shunt-wound brush DC motors are designed to have their field coil connected in parallel with the rotor. As a result, these motors usually produce consistent torque at low speeds, making them ideal in industrial and automotive applications with stringent speed control requirements.

A series-wound brush DC motor features a field coil connected in series with the rotor. The electric current in the stator and rotor increases under load, making these motors ideal for high-torque applications like cranes and winches.

Learn more about DC motors on GlobalSpec.

Brushed DC motors offer several advantages over other electric motors. For instance, brushed DC motors have low overall construction costs and a simple controller. Their design also makes them ideal in harsh operating environments. 

Brushed DC motors are usually noisier than brushless DC motors due to the recurring switching action of the commutators. In addition, the brushes and commutators of a brushed DC motor tend to wear out since they are in physical contact with the rotating shaft.

Brush DC motors can meet the requirement of several applications, so long as engineers correctly size and specify them. For instance, when choosing a brushed DC motor for an application, engineers need to consider the output power, torque, efficiency, power dissipation, shaft configuration, gearing, and ideal operating temperature.