ST AN2372 Application note

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, Halleffect 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

Rectangular currents have harmonics in odd

Current harmonics in switching frequency range

Lower audible noise Higher audible noise

Lower core losses in motor Higher core losses

Current peak value lesser, power circuit dimensioning can be optimized

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 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.

multiples of fundamental frequency (which are in audible range) plus switching frequency harmonics.

Current peak value higher. Higher dimensioning of power circuit

Switching losses minimal because only one of the switches take PWM

July 2006 Rev 1 1/13

www.st.com

Contents AN2372

Contents

1 Theory of operation and control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Experimental implementation using ST7FMC . . . . . . . . . . . . . . . . . . . . 5

2.1 Speed and absolute position estimation . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2 Current control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Appendix A Phase current comparison between 6 step BLDC drive and sine

BLDC drive for same power output10

Appendix B Test procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

4 Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2/13

AN2372 Theory of operation and control

1 Theory of operation and control

In three-phase AC motors, currents flowing in the stator windings create a magnetic field with a definite magnitude and orientation inside the motor. When a DC current passes though these windings, it produces a static magnetic field. The permanent magnets in a free spinning rotor interact with the stator flux and experience a force of attraction to fall in line with the stator flux and lock with it. If now the stator flux orientation is changed by adjusting the stator currents, the rotor that is already locked with the stator flux, also changes its orientation to take the new position of the stator flux. If the stator is now excited with sinusoidal varying currents, the stator flux inside the motor spins at the frequency of its sinusoidal currents and pulls along the rotor at this frequency.

The ability of the rotor to stay locked with the stator flux depends on the strength of the magnetic fields and the magnitude of load torque disturbances on the rotor. Once the rotor is in motion, if at any time the it falls out of alignment with the stator flux, it cannot spin anymore and comes to a halt. If the stator is still excited with sinusoidal currents, then the rotor experiences pulsating torque in either direction at the frequency of stator currents.

Figure 1. Self control of PMSM

3 Phase

PWM

PMSM

Generator

and Inverter

δθ

Absolute Position Sensor

However, this situation can be overcome if we force the sinusoidal angular values of stator currents to correspond to the angular position of the rotor (plus an offset) as shown in

Figure 1. What this means is that even if the rotor tries to fall out of alignment for any reason,

since the stator current (which determines the stator flux magnitude and orientation) depends only on the angular position of the rotor, it pushes/pulls the stator flux in the direction of the rotor disturbance to maintain alignment, thereby giving improved stability and control.

Under this condition, the PMSM motor acts like a DC motor where commutation is performed by inverter switches and the speed is determined by the magnitude of applied voltage. The frequency of applied sinusoidal voltage varies directly with speed and automatically tracks itself to a value such that it matches with the V/f ratio for the motor. For precise speed maneuvers, load angle tuning can be brought into play. For operations in field weakening mode, applied voltage magnitude can remain at the maximum level and the load angle should be increased appropriately.

3/13

Theory of operation and control AN2372

Even though it acts like a DC motor, it still follows the basic theory of AC synchronous motor control. A single-phase equivalent circuit of the motor and a phasor diagram of motor voltages and current are shown in Figure 2. By adjusting the phase angle between back-emf and applied voltage, and/or the magnitude of applied voltage, the power factor of the machine can be set to unity. This helps to maximize the power output for a given value of phase current and to minimize the inverter rating.

Figure 2. PMSM phasor diagram at UPF

= [Es + IsRs] + j[ωLs]

jωLsI

RsI

To implement this control, knowledge of rotor position is necessary. An absolute position encoder may give incredibly accurate resolution and precision, but its cost is very prohibitive. On the other hand, Hall sensors mounted in BLDC motors give a very course resolution of close to 60° to 180° depending on the number of sensors, but they are inexpensive. They generate rising/falling edges at these positions to indicate the angular value at that point. However, to get intermediate angular positions of the rotor between these edges, additional intelligence is needed by the controller for estimation.

4/13

+ 9 hidden pages