Selecting a DC Motor - Michigan State University

Fall 2013

Selecting a DC Motor

Matthew Stephan Henry

Michigan State University College of

Engineering

Fall 2013

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I.

Abstract

The purpose of this paper is to provide analysis on the process of selecting a DC motor.

It will give thorough descriptions of the principle operations of DC motors. By better

understanding how this type of product works, the process of finding a motor should

become simplified. Comparisons and contrasts will be given for many motor features

aiding this process. The application note should provide clarity on what type of

characteristics to look for and how to better make motor selection decisions.

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II. Keywords

Armature ¨C the component of the motor that produces power. It can be located on either

the stator or the rotor.

BLDC ¨C brush-less direct current

Brush ¨C mechanism that conducts current in between moving parts and stationary wires.

Brushed Permanent Magnet Motor - contains permanent magnets inside, which

eliminates the need for external field current. This design allows a smaller, lighter, and

energy-efficient Brush Motor.

Brushed Series Wound Motor - speed varies automatically with the load, increasing as

the load decreases.

Brushed Shunt Wound Motor - run at constant speed regardless of the load.

Commutator ¨C mechanism which reverses the direction of current in certain electric

motors.

Direct Current ¨C electrical charge constantly flows in the same direction. Opposite of an

alternating current, where current periodically switches direction.

Electrical Power ¨C electric circuits transferring electrical power at a given rate.

IPM ¨C Internal Permanent Magnet

PWM ¨C Pulse Width Modulation

Rotor ¨C rotating device in an electric motor which rotates about the Brush DC Motor

generating torque among the rotor¡¯s axis.

Stator ¨C the part of the Brush DC Motor that is stationary.

Torque ¨C the ability of a force to rotate a given object about an axis or fulcrum.

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Table of Contents

I.

Abstract........................................................................................................................ 1

II.

Keywords ................................................................................................................. 2

III.

Introduction .............................................................................................................. 4

IV.

Motor Design ........................................................................................................... 4

a.

What is a DC motor? ................................................................................................... 4

b.

Brushed vs. Brushless DC Motors ............................................................................... 5

c.

Hall Effect Sensors vs. Back EMF .............................................................................. 6

d.

MCU Based Design vs. Standalone Solution .............................................................. 8

e.

Commutation Methods .............................................................................................. 11

V.

Conclusions ............................................................................................................ 12

VI.

References .............................................................................................................. 13

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III. Introduction

The goal of this application note is to allow the reader to make better decisions when

researching the type of motor to select for a project. I will begin by providing a

description of what a DC motor is and comparing Brushed versus Brushless DC motors.

From there I will go into specific features of motors for example their sensor capabilities,

micro-controller use, and commutation method. I will also provide examples of motor

applications. Finally, I will end with a conclusion of what steps to take with the provided

information. Usually, a motor selection involves many more factors than are presented

here. This method, however, provides the designer with a starting point for the selection

of motors for a particular set of design options.

IV. Motor Design

a. What is a DC motor?

Motors are seen everywhere in the world today. Almost every mechanical movement

seen in your daily life is caused thanks to some type of AC or DC motor. A DC motor is

a mechanically commutated electric motor powered from ¡°direct current¡± hence the

¡°DC¡±. There are many types of DC motor architectures but I will explain the basic type

of motor you will see in typical daily applications. These motors are comprised of six

main components; permanent magnets called the stator, a spinning armature called a

rotor, an axle, a commutator, field magnets, and brushes (optional) [1]. The motor uses

magnetism to create rotational motion and spin the axle. By connecting a battery, current

is allowed to flow which will generate an electric field. This is then placed within

another external electric field that is constantly ¡°flipping¡± directions causing the rotation.

When power is applied, the polarities of the energized winding and the stator magnets are

misaligned, and the rotor will rotate to try and get aligned with the stator's field magnets

[2]. The rotation reverses the direction of current through the rotor winding, leading to a

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