Brushless DC Motor Fundamentals Application Note

AN047

Brushless DC Motor Fundamentals

Brushless DC Motor Fundamentals Application Note

Prepared by Jian Zhao/Yangwei Yu July 2011

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AN047

Brushless DC Motor Fundamentals

ABSTRACT

This application note provides a general overview of BLDC motors, including their advantages against other commonly-used motors, structure, electromagnetic principles, and mode of operation. This document also examines control principles using Hall sensors for both single-phase and three-phase BLDC motors, and a brief introduction to sensorless control methods using BEMF for a three-phase BLDC motor.

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AN047 ? BRUSHLESS DC MOTOR FUNDAMENTALS

INDEX

ABSTRACT ........................................................................................................................................... 2 1. INTRODUCTION ............................................................................................................................... 4 2. MOTOR FUNDAMENTAL CONCEPTS ............................................................................................. 4

2.1 General principle of motor......................................................................................................... 4 a. Magnetic force..................................................................................................................... 4 b. Left-hand rule ...................................................................................................................... 4 c. Right-hand rule .................................................................................................................... 6 d. Right-hand screw rule.......................................................................................................... 6

2.2 Stator ........................................................................................................................................ 7 2.3 Rotor......................................................................................................................................... 7 2.4 Operation theory of motor ......................................................................................................... 8 3. VARIOUS MOTOR TYPES................................................................................................................ 8 3.1 Various types of motor introduction........................................................................................... 9

a. Brushed DC motor............................................................................................................... 9 b. Brushless DC (BLDC) motor................................................................................................ 9 c. AC induction motor (ACIM) ................................................................................................ 10 d. Permanent magnet synchronous motor (PMSM) ............................................................... 10 e. Stepper motor & Switched reluctance (SR) motor ............................................................. 10 3.2 Comparison for various motor types ....................................................................................... 12 4. BRUSHLESS DC MOTOR CONTROL............................................................................................. 13 4.1 Switch configuration and PWM ............................................................................................... 13 4.2 Electronics commutation principle........................................................................................... 13 a. Single-phase BLDC motor ................................................................................................. 13 b. Three-phase BLDC motor ................................................................................................. 15 c. Sensorless control of BLDC motor..................................................................................... 17 5. SUMMARY ...................................................................................................................................... 18 REFERENCES: ................................................................................................................................... 19

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AN047 ? BRUSHLESS DC MOTOR FUNDAMENTALS

1. INTRODUCTION

The BLDC motor is widely used in applications including appliances, automotive, aerospace, consumer, medical, automated industrial equipment and instrumentation.

The BLDC motor is electrically commutated by power switches instead of brushes. Compared with a brushed DC motor or an induction motor, the BLDC motor has many advantages:[1]

Higher efficiency and reliability Lower acoustic noise Smaller and lighter Greater dynamic response Better speed versus torque characteristics Higher speed range Longer life

This document initially provides a general overview to familiarize the reader with motor control fundamentals, terms and concepts, and applications. The latter portion of this document provides detailed descriptions of motor structure, working principle, characteristics and control methods.

2. MOTOR FUNDAMENTAL CONCEPTS

2.1 General Motor Principles Motors convert electrical energy into mechanical energy using electromagnetic principles. The energy conversion method is fundamentally the same in all electric motors. This document starts with a general overview of basic electromagnetic physics before entering discussing the details of motor operation.

a. Magnetic Force Magnetic poles generate invisible lines of magnetic force flowing from the north pole to the south pole as shown in Figure 1. When magnetic poles of opposite polarity face each other, they generate an attractive force, while like poles generate a repulsive force.

N

S

N

S

S

N

N

S

a) Unlike-pole attraction

(b) Like-pole repulsion

Figure 1--Magnetic Force

b. Left-Hand Rule

Current in a conductor generates a magnetic field. Placing a conductor in the vicinity of a separate magnetic can generate a force that reaches its apex when the conductor is at 90? to the external field. The left-hand rule can help the user determine the direction of the force, as shown in Figure 2(a).

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AN047 ? BRUSHLESS DC MOTOR FUNDAMENTALS

Left-Hand Rule: Extend the left hand with the thumb and four fingers on the same plane with the thumb pointing out. Face the palm towards the north pole of the external magnetic field and the four fingers in the direction of the current; the thumb points in the direction of the force.

N F

i

N

-

+

e

B S

v B

S

(a) Left-Hand Rule

(b) Right-Hand Rule

Figure 2--Left-Hand Rule and Right-Hand Rule

The magnitude of the force can be calculated from the equation below:

=F BIL sin

(1)

Where F is the electromagnetic force, B is the magnetic field density, I is the conductor current, L is the length of the conductor, and is the angular difference between B and I.

Given that a coil usually has two effective conductors: a-b and c-d shown in Figure 3(a), these two conductors induce two forces of opposite direction when current passes through in the magnetic field.

F a

L

b

O

c

i

O'

F d

B F

OO'

r

r F

v a

L

e

b

O? r

c

O'

d e

B v

(a)

(b)

(c)

Figure 3--Coil in a Magnetic Field

The torque is the product of the tangential force acting at a radius with units of force multiplied by length. If there are N continuous coil turns, and based on the parameters in Figure 3(b), the generated torque equals:

= TD 2= rFN 2rB= ILN KTI

(2)

Where:

? TD is the electromagnetic torque (Nm) ? r is the distance between axis OO' and the conductor (m) ? N is the number of winding turns ? KT=2rBLN is the torque constant (Nm/A).

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