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SENSORLESS BRUSHLESS DC MOTOR REFERENCE DESIGN
1. Introduction
This reference design provides a hardware and Most computer hard disc drives use a sensorless
software solution for Sensorless Brushless dc motors. brushless dc motor. However, these small motor drives
This document includes complete schematics, printed use a linear regulator to control the voltage applied to
circuit board layout, and firmware. The Sensorless the motor. This works well for small motors, but is too
Brushless dc motor reference design may be used as a inefficient to use for motors greater than a few watts.
starting point for motor control system designers using Larger motors require PWM control for efficient
Silicon Laboratories MCUs, significantly reducing the operation. Using PWM control makes the task of
design time and time to market.
measuring the back EMF of the motor more difficult, due
to noise coupled from the active windings.
Brushless dc motors consist of a permanent magnet
rotor with a three-phase stator winding. Brushless dc Sensorless Brushless dc motors are well suited for fans
motors evolved from conventional dc motors where the and rotary pumps from a few watts up to about 1 kW.
function of the brushes is replaced by electronics. Fans and pumps’ loads are predictable and fairly well
Brushless dc motors offer longer life and less behaved. Most Sensorless Brushless dc motors do not
maintenance than conventional brushed dc motors.
provide the same level of dynamic speed control
available from Hall effect controlled BLDC motors or dc
motors. While it is theoretically possible to achieve high-
performance from a sensorless BLDC motor using
sophisticated vector control, most practical sensorless
BLDC implementations address the much simpler fan
and pump applications. This reference design is
targeted for simple fan applications.
Most Brushless dc motor designs historically use Hall
effect sensors to measure the position of the rotor. Hall
effect sensors provide absolute position information
required to commutate the motor. Using Hall effect
sensors provides simple, robust commutation and
performance roughly comparable to brushed dc motors.
One of the major barriers limiting the market penetration
of Brushless dc motors has been the cost of using Hall Sensorless BLDC motor drives often compete against
effect sensors. The Hall effect sensors themselves are ac induction motors for certain applications. A system
not particularly expensive. However, the Hall effect designer must consider the power level, efficiency
assembly adds significant expense to the cost of requirements, and starting requirements when choosing
manufacturing the motor. Hall effect sensors also between BLDC motors and ac induction motors. BLDC
typically require 5 additional wires, adding to the motors are most often used for small motors ranging
installation costs.
from 1 watt to 1 kW. AC induction motors are readily
available from 250 W to 10 kW or more. BLDC motors
offer potentially higher efficiency than ac induction
motors. This is due to the fact that ac motors have high
rotor losses while BLDC motors do not waste energy in
magnetizing the rotor. AC induction motors are much
easier to start than sensorless BLDC motors. The
complexity of starting a fully loaded BLDC motor often
makes the ac induction motor a better choice,
particularly for piston pumps and compressors.
A “Sensorless” Brushless dc motor does not have Hall
effect sensors. Sensorless Brushless dc motors employ
more sophisticated electronics using some alternative
scheme to control the commutation of the motor. The
most common scheme involves measuring the back
EMF of the motor and using this information to control
the commutation of the motor.
Rev. 0.2 9/06
Copyright © 2006 by Silicon Laboratories
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