How do DC motors work?

In this article, we'll look at how a Direct Current electric motor produces rotational motion.

Principles of operation

When electricity is passed through a wire, it generates a magnetic field. Motors use this phenomenon in conjunction with stationary magnets to produce motion. There are variations in the exact way that motors work. Induction motors, stepper motors and AC motors all use slightly different technologies. Motor design can be a complex topic and some calculations require a higher level of understanding. This article specifically explores the basics of DC motors.

Types of DC Motors

There are two main types of electric motors: the brushed and brush-less motor. All motors have a stator (a stationary part) in which a rotor (a rotational part) runs. Brushed and DC brushless motors vary slightly in the way they use stators and rotors. Brushed Motors
In brushed motors, carbon brushes apply an electric current to the rotor via a commutator. The stator consists of permanent magnets and the rotor consists of many turns of copper wire. As the electricity is passed through the wire, it becomes magnetised and rotor spins. Because the brushes physically touch the rotor, brushed motors are susceptible to friction damage. This includes noise, excessive heat, vibration and brush wear. Brushless Motors
Brushless motors are the opposite of brushed motors. They have electromagnets in the stator and permanent magnets in the rotor. Because this motor doesn't have to overcome the problem of applying a current to a moving part, there are no brushes and hence, less friction. The control of the electromagnets requires relatively complex electrical switching. Therefore, brushless motors have only been possible since the development of transistors in the 1960s. Advantages
As a result, brushless motors can run quietly, at higher speeds and require less cooling. Consequently, they are often used in consumer electronic devices such as DVD players and computer fans. Brushless motors produce less torque at high speeds and so it is uncommon to find these as drive motors in products like the Segway.

Motor Control

Brushed Motor Control
The simplest form of motor control is via switches or relays. However, this does not give any kind of variable speed control and the large current drawn by drive motors often makes this route nonviable. Modern brushed motor controllers work by rapidly switch the power on and off in a process known as pulse width modulation. Brushless Motor Control
To keep the rotor turning, the electromagnets in the stator need to be switched on constantly. This requires a much more complex electric controller. Many brushless controllers use a microprocessor to govern motor rpm. Brushless motors have a higher initial cost but are much more efficient in the long run.