Brushed Dc Motor Fundamentals

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AN905
Brushed DC Motor Fundamentals
Author: Reston Condit Microchip Technology Inc.

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).

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.

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.

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.

FIGURE 1:

SIMPLE TWO-POLE BRUSHED DC MOTOR

N
NORTH
Axle

SOUTH

Brushes

Commutator Field Armature

Magnet or Coil

 2004 Microchip Technology Inc.

DS00905A-page 1

AN905
Brushes and Commutator
Unlike other electric motor types (i.e., brushless DC, AC induction), BDC motors do not require a controller to switch current in the motor windings. Instead, the commutation of the windings of a BDC motor is done mechanically. A segmented copper sleeve, called a commutator, resides on the axle of a BDC motor. As the motor turns, carbon brushes slide over the commutator, coming in contact with different segments of the commutator. The segments are attached to different rotor windings, therefore, a dynamic magnetic field is generated inside the motor when a voltage is applied across the brushes of the motor. It is important to note that the brushes and commutator are the parts of a BDC motor that are most prone to wear because they are sliding past each other.

Shunt-Wound
Shunt-wound Brushed DC (SHWDC) motors have the field coil in parallel (shunt) with the armature. The current in the field coil and the armature are independent of one another. As a result, these motors have excellent speed control. SHWDC motors are typically used applications that require five or more horsepower. Loss of magnetism is not an issue in SHWDC motors so they are generally more robust than PMDC motors.

FIGURE 3:

SHUNT-WOUND DC MOTORS
Brush

TYPES OF STEPPING MOTORS
As mentioned earlier, the way the stationary magnetic field is produced in the stator differentiates the various types of BDC motors. This section will discuss the different types of BDC motors and the advantages/ disadvantages of each.

DC Voltage Supply

Shunt Field Armature

Series-Wound
Series-wound Brushed DC (SWDC) motors have the field coil in series with the armature. These motors are ideally suited for high-torque applications because the current in both the stator and armature increases under load. A drawback to SWDC motors is that they do not have precise speed control like PMDC and SHWDC motors have.

Permanent Magnet
Permanent Magnet Brushed DC (PMDC) motors are the most common BDC motors found in the world. These motors use permanent magnets to produce the stator field. PMDC motors are generally used...
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