© 2005 Microchip Technology Inc.

DS93001A-page 1

GS001

INTRODUCTION

As a means of reducing high energy and maintenance

costs in motor control applications, BLDC motors are

seeing a resurgence in applications where efficiency

and reliability are important. The dsPIC30F motor con-

trol devices are ideally suited to drive and control a

wide range of BLDC motor types, in a large number of

applications. Microchip has developed a number of

solutions using the dsPIC30F and BLDC motors. This

document will help you select an appropriate solution

for your BLDC motor application.

BLDC MOTOR BASICS

DC brush motors have a permanent magnet on the

stator with the motor winding on the rotor. During rota-

tion, the current in the windings is reversed using

mechanical carbon brushes and a commutator located

on the rotor. The BLDC motor has permanent magnets

on the rotor with the electrical windings on the stator.

The first obvious advantage of the BLDC motor is the

elimination

of

the

mechanical

commutator

and

brushes, which significantly improves mechanical

reliability. The commutator and brushes in DC motors

also give rise to sparking, so eliminating these

components means that BLDC motors can operate in a

of a BLDC motor are now on the stator and can be

dissipated very easily. Consequently, efficiency of the

BLDC motor is vastly improved.

There are, however, some challenges when spinning a

BLDC motor. Firstly, a revolving electrical field has to

be created in the windings, which also has to be well

aligned with the magnetic field on the rotor. The

efficiency of the BLDC motor depends largely on the

alignment of the revolving electrical field to the

magnetic field on the rotor. To sense the magnetic field,

Hall sensors are normally used. Based on the signal

presented by the Hall sensors, the windings are appro-

priately excited. As the speed of the rotor increases,

however, there is a certain amount of lag between the

voltage excitation and the current effect on the

windings due to the inductance of the windings. To

overcome this lag, the voltage is initiated a little in

advance. This phenomenon is known as phase

advance and is implemented mainly in software at high

speeds of rotation. The result of phase advance is

better efficiency in the BLDC motor operation.

Sensored BLDC Motor Control

When driving a BLDC motor, it is important to know the

position of the magnetic rotor with reference to the

stator. Most commonly, Hall effect sensors are used to

generate feedback on the rotor position. This type of

control is called sensored BLDC motor control. Most

BLDC motors have three windings. Based on the

position of the magnetic rotor, two windings are ener-

gized at a given time with each phase conducting for

120 electrical revolution degrees, resulting in six

distinct combinations of energization. This type of drive

is called “trapezoidal” or “six-step commutation”.

SIX-STEP COMMUTATION

Figure 1 depicts

a typical six-step commutation

scheme with the Hall sensor output overlay. Six-step

commutation offers a simple, yet efficient, method of

driving a BLDC motor. Hall A (HA), Hall B (HB) and Hall

C (HC) sense the position of the rotor with respect to

the windings, R, Y and B. Depending on the Hall sensor

reading from 1 to 6, an appropriate pair of windings is

driven high and low with the third winding not driven.

Each 360 degree electrical cycle is broken down to six

60 degree electrical sectors, in which one winding is

driven high, a second is driven low and the third is not

driven. Example: In Hall position 6 or sector 1, the R

winding is driven high while the B winding is driven low

and the Y winding is not driven. By reading the Hall

sensors, the six-step commutation algorithm can very

easily be implemented in software.

FIGURE 1:

TYPICAL SIX-STEP

COMMUTATION

Author:

Stan D’Souza

Microchip Technology Inc.

HA

R

HB

Y

HC

B

5

5

0

1

2

3

4

501

4

6

2

3

1

546

Sector

Hall

60°

Getting Started with BLDC Motors and dsPIC30F Devices