AN2372 Datasheet, PDF (1/13 Pages) STMicroelectronics – Low cost sinusoidal control of BLDC motors with Hall sensors using ST7FMC

AN2372
Application note
Low cost sinusoidal control of BLDC motors with
Hall sensors using ST7FMC
Introduction
BLDC motors are the workhorses in most light industrial applications. In some cases, Hall-
effect position sensors are used to simplify control logics for controlling these motors. BLDC
motors, by the nature of currents through them, are somewhat noisy and a little less
efficient. These disadvantages plus the cost of sensors are an integral part of these drive
systems. However, if the motor can be driven with sinusoidal currents, preferably with only
one Hall-effect sensor, these drawbacks can be greatly reduced.
A 3-phase Permanent Magnet Synchronous Motor (PMSM) has permanent magnets on the
rotor and current-carrying windings on the stator. There are two modes of control:
■ as a BLDC motor, where, based on rotor position, only two windings carry current at any
given time (reducing winding utility by 33%)
■ as a three phase AC motor, where three-phase sinusoidal voltages are applied on all
three windings and all three windings carry current at all times
The comparison chart below shows the advantages of controlling the PMSM motor like an
AC motor instead of a BLDC motor.
AC motor
BLDC motor
Currents are sinusoidal
Currents are rectangular
Current harmonics in switching frequency range
Rectangular currents have harmonics in odd
multiples of fundamental frequency (which are in
audible range) plus switching frequency
harmonics.
Lower audible noise
Higher audible noise
Lower core losses in motor
Higher core losses
Current peak value lesser, power circuit
dimensioning can be optimized
Current peak value higher. Higher dimensioning
of power circuit
Phase rms current lower
Phase rms current higher
Electric torque developed is flat
Torque has commutation ripples
Higher switching losses in inverter as all switches Switching losses minimal because only one of the
take PWM
switches take PWM
Implementation is little complex
Implementation is simple
Implementation of this scheme with an ST7FMC can give additional advantages such as
load angle control to help optimize the motor current, and voltage foldback current protection
to help limit motor currents by reducing the applied voltage to implement current limit
control.
July 2006
Rev 1
1/13
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