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Brushed DC Motor Fundamentals

INTRODUCTION Brushed DC motors are widely used in applications ranging from toys to push-button adjustable car seats. Brushed DC (BDC) motors are inexpensive, easy to drive, and are readily available in all sizes and shapes. This application note will discuss how a BDC motor works, how to drive a BDC motor, and how a drive circuit can be interfaced to a PIC® microcontroller.

PRINCIPLES OF OPERATION The construction of a simple BDC motor is shown in Figure 1. All BDC motors are made of the same basic components: a stator, rotor, brushes and a commutator. The following paragraphs will explain each component in greater detail.

Stator The stator generates a stationary magnetic field that surrounds the rotor. This field is generated by either permanent magnets or electromagnetic windings. The different types of BDC motors are distinguished by the construction of the stator or the way the electromagnetic windings are connected to the power source. (See Types of Stepping Motors for the different BDC motor types).

Rotor The rotor, also called the armature, is made up of one or more windings. When these windings are energized they produce a magnetic field. The magnetic poles of this rotor field will be attracted to the opposite poles generated by the stator, causing the rotor to turn. As the motor turns, the windings are constantly being energized in a different sequence so that the magnetic poles generated by the rotor do not overrun the poles generated in the stator. This switching of the field in the rotor windings is called commutation.