DIGITAL CONTROL STRATEGY FOR FOUR QUADRANT OPERATION OF THREE PHASE

BLDC MOTOR WITH LOAD VARIATIONS

C. Sheeba Joice, S. R. Paranjothi,and V.Jaeahar Seenthil KumarIEEE TRANSACTIONS ON INDUSTRIAL INFORMATICS, VOL. 9,NO. 2,MAY 2013

974~982

老師 :王明賢學生 :方偉晋

Abstract

Brushless DC (BLDC) motor drives are becoming more popular in industrial, traction application. This paper deals with the digital control of three phase BLDC motor.

The motor is controlled in all the four quadrants without any loss of power; in fact energy is conserved during the regenerative period.

The digital controller dsPIC30F4011, which is very advantageous over other controllers, as it combines the calculation capability of Digital Signal Processor and controlling capability of PIC microcontroller, to achieve control.

Outline

Introduction BLDC motor Four Quadrant Operation PI Controller COMPLETE DRIVE SYSTEM

Introduction

BLDC motor has a rotor with permanent magnets and a stator with windings. The Brushless DC motor is driven by rectangular or trapezoidal voltage strokes coupled with the given rotor position. BLDC motors often incorporate either internal or external position sensors to sense the actual rotor position or its position can also be detected without sensors.

BLDC motor

The numbers shown around the peripheral of the motor diagram in Fig. 1 represent the sensor position code.

Based on the combination of these three Hall sensor signals , the exact sequence of commutation can be determined.

Fig. 1. BLDC Motor Star connected.

BLDC motor

These signals are decoded by combinational logic to provide the firing signals for 120 conduction on each of the three phases.

The rotor position decoder has six outputs which control the upper and lower phase leg MOSFETs of Fig. 2

Fig. 2. Equivalent Circuit of power stage of BLDC motor

Four Quadrant Operation

There are four possible modes or quadrants of operation using a Brushless DC Motor which is depicted in Fig. 3. Fig. 3.

Four Quadrant Operation

When BLDC motor is operating in the first and third quadrant, the supplied voltage is greater than the back emf which is forward motoring and reverse motoring modes respectively, but the direction of current flow differs.

When the motor operates in the second and fourth quadrant the value of the back emf generated by the motor should be greater than the supplied voltage which are the forward braking and reverse braking modes of operation respectively, here again the direction of current flow is reversed.

Four Quadrant Operation

Fig. 4. Operating Modes

PI ControllerPI Controller

The regulation of speed is accomplished with PI Controller. By increasing the proportional gain of the speed controller, the controller’s sensitivity is increased to have faster reaction for small speed regulation errors.

This increased sensitivity also reduces the speed overshooting. The armature current reduces faster, once the desired speed is achieved.

PI ControllerPI Controller

An increase of the integral gain will allow the motor speed to catch up with the speed reference ramp a lot faster during sampling periods.

This will indeed allow a faster reaction to small speed error integral terms that occur when a signal is regulated following a ramp.

PI ControllerPI Controller

The controller will react in order to diminish the speed error integral a lot faster by producing a slightly higher accelerating torque when following an accelerating ramp.

On the other hand, too high increase of the proportional and integral gains can cause instability, and the controller becoming insensitive.

Too high gains may also result in saturation. Tuning process is by trial and error method and the Proportional Constant and Integral Constant are 0.1 and 0.03 respectively.

COMPLETE DRIVE SYSTEM

The position signals obtained from the Hall sensors of the motor are read by the I/O lines of the dsPIC controller.

The Hall sensor inputs give the position of the rotor which is fed to the controller. The controller compares it with the reference speed and generates an error signal.

The required direction of rotation either clockwise or counter clockwise can also be fed to the digital controller.

The PWM module of the controller generates appropriate PWM signals, which are applied to the three phase inverter.

COMPLETE DRIVE SYSTEM

Fig. 5. Closed Loop Drive.

COMPLETE DRIVE SYSTEM

The Hall sensor signals and the phase current (of one phase) of three phase Brushless DC motor are shown in Fig. 6.

The digital storage oscilloscope images shown in Fig. 6 indicate the trapezoidal voltage of phases RY and YB.

Fig. 6. Hall Sensor signals and Phase Current