i THREE PHASE INVERTER FOR INDUCTION MOTOR BY USING PI

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i
THREE PHASE INVERTER FOR INDUCTION MOTOR BY USING PIREPETITIVE CONTROLLER WITH ARDUINO
MOHD NAJIB BIN HUSSIN
A project report submitted in partial
Fulfillment of the requirement for the award of the
Degree of Master Electrical Engineering
Faculty of Electrical and Electronic Engineering
Universiti Tun Hussein Onn Malaysia
JULY 2014
v
ABSTRACT
The proportional integral (PI) repetitive controller that is applied to the three phase
inverter for the three phase induction motor is developed. This project is about to
design a combination of PI-Repetitive controller simulation and fabricating the
hardware to control the speed of a three phase induction motor. The controller
eliminates the periodic errors on the output current due to inverter current nonlinearity
and load disturbances. The controller is implemented on a gate driver, three phase
inverter and filter prototypes which are constructed in laboratory. Open-loop and
closed-loop condition are verified by means of simulation and experiments in order to
investigate the stability of controller. The result of the controller’s performance
between simulation and hardware are explained in this report in terms of the PWM
output and three phase inverter output waveform. The PI-Repetitive controller will
solve the starting problem of the three phase induction motor and minimizes the line
current error to maintain the required steady state condition.
vi
ABSTRAK
Kawalan motor aruhan tiga fasa jenis ‘Proportional Integral Repetitive’ yang
diaplikasikan pada penyongsang tiga fasa dibangunkan. Projek ini adalah untuk
merekabentuk gabungan pengawal PI-berulang secara simulasi dan seterusnya
memfabrikasi perkakasan bagi tujuan ujikaji di makmal. Pengawal ini menghapuskan
gangguan-gangguan berkala ke atas arus keluaran yang disebabkan oleh
ketaklelurusan penyongsang dan gangguan semasa pada beban. Pengawal ini
kemudiannya diaplikasikan pada prototaip yang dibina iaitu di litar ‘gate driver’, litar
penyongsang tiga fasa dan litar penapis di dalam makmal. Kaedah gelung buka dan
keadaan gelung tertutup secara simulasi dan eksperimen digunakan bagi mengkaji
kestabilan pengawal PI-berulang. Hasil ujikaji direkodkan dalam laporan ini dari segi
keluaran bentuk gelombang PWM dan keluaran penyongsang tiga fasa. Pengawal PIberulang akan menyelesaikan masalah motor induksi tiga fasa yang tidak stabil diawal
permulaan pengoperasiannya dan meminimumkan gangguan pada arus semasa untuk
mengekalkan keadaan motor agar lebih stabil.
vii
CONTENTS
TITLE
i
DECLARATION
ii
DEDICATION
iii
ACKNOWLEDGMENTS
iv
ABSTRACT
v
CONTENTS
vii
LIST OF TABLE
viii
LIST OF FIGURES
ix
LIST OF SYMBOL AND ABBREVIATIONS
xii
CHAPTER 1 INTRODUCTION
1
1.1
Project background
1
1.2
Problem Statement:
3
1.3
Objectives:
5
1.4
Scope:
6
CHAPTER 2 LITERATURE REVIEW
7
2.1
Induction motor
7
2.2
Inverter DC to AC
3
2.2.1 Three phase inverter
3
Controller
6
2.3.1 Passive and adaptive controller
7
2.3
2.3.2 Propose controller
10
viii
2.4
Gate driver
12
2.5
Arduino
14
2.6
Other research
15
CHAPTER 3 METHODOLOGY
17
3.1
Hardware flow-chart
17
3.2
Hardware design
18
3.2.1 Gate Driver
18
3.2.2 Inverter
20
3.2.3 Filter
22
3.2.4 Transducer
22
3.2.5 Induction Motor
25
Software flow-chart
26
3.3.1 Kp and Ki analysis
30
3.3
CHAPTER 4 DATA ANALYSIS AND RESULT
34
4.1
Open-Loop Simulation Diagram and Result.
35
4.2
Closed-loop simulation process and analysis.
37
4.2.1 PI-Repetitive Controller
39
4.3
4.4
4.5
4.6
The comparison of PI, Repetitive and PIRepetitive controller.
44
4.3.1 PI-Controller
44
4.3.2 Repetitive-Controller
45
4.3.3 PI-Repetitive Controller
46
Experiment set up for hardware testing
analysis.
46
Open loop hardware analysis
48
4.5.1 Pulsed Width Modulation (PWM)
48
4.5.2 Three phase inverter
50
Closed-loop hardware analysis.
51
ix
4.7
Output result for various input.
CHAPTER 5 CONCLUSION AND RECOMMENDATION
54
57
5.1
Conclusion
57
5.2
Recommendation
58
REFERENCE
59
x
LIST OF TABLES
3.1
List of the components for gate driver circuit
19
3.2
Parameters of Three Phase Induction Motor
26
3.3
Controller parameters for the Ziegler-Nichols frequency
respons method
27
4.1
Experiment’s Parameters
47
4.2
Various parameters of input voltage versus input
current reference
54
xi
LIST OF FIGURES
1.1
Hardware block diagram
2.1
Torque-speed characteristic at constant voltage and
3
frequency
2
2.2
Hex bridge of inverter
4
2.3
Proportional Integral (PI) Controller block diagram
8
2.4
Repetitive control block diagram
11
2.5
PI-Repetitive control block diagram
12
2.6
Arduino board
15
3.1
Hardware process control
17
3.2
One phase Gate Driver schematic circuit
19
3.3
Three phase Gate Driver PCB layout
20
3.5
Current Sensor BB-ACS756
22
3.4
The circuit for the LC filter
22
3.6
Current Sensor BB-ACS756 pin configuration
23
3.7
Simulation block diagram for current converting
25
3.8
Three phase induction motor
25
3.9
Software process control
26
3.10
Step response of PI-Repetitive controller
30
3.11
Root Locus and Bode diagram of PI-Repetitive
controller
31
3.12
PI-Repetitive Controller block diagram
32
3.13
Proportional integral repetitive controller simulation
32
3.14
PWM simulation generator block
33
xii
3.15
PWM simulation
4.1
Three phase inverter by using PI-Repetitive Controller
33
in MATLAB simulation.
34
4.2
PWM generator block prameter setting
35
4.3
PWM block diagram simulation
36
4.4
PWM output waveform of open-loop simulation
36
4.5
Inverter output waveform of open-loop simulation
37
4.6
Closed-loop simulation process.
38
4.7
Three Phase PI-Repetitive Controller for simulation
39
4.8
ADC output waveform From Current Sensor
40
4.9
DAC output waveform of PI-Repetitive Controller
simulation
40
4.10
Simulation of error in controller
41
4.11
Output of PWM from simulation block
42
4.12
Block diagram of the inverter
43
4.13
Output waveform of the inverter (before filtering
process)
4.14
43
Output waveform of the pi-rc system (after filtering
process)
44
4.31
The output voltage waveform of the PI-Controller
45
4.32
The output voltage waveform of the RepetitiveController
4.33
The output voltage waveform of the PI-RepetitiveController
4.15
46
Three phase inverter by using pi-repetitive controller
hardware.
4.16
45
47
PWM output waveform of open-loop hardware injected
to gate driver.
48
4.17
Gate Driver.
49
4.18
Gate Driver output waveform of open-loop simulation
49
4.19
Inverter hardware
50
xiii
4.20
Inverter output waveform of open-loop process.
50
4.21
ACS756 Current Sensor.
51
4.22
The output waveform result of the hardware at current
sensor at 0A.
4.23
52
The output waveform result of the hardware at current
sensor at 0.75A.
52
4.24
The PWM output waveform for phase A and Phase B.
53
4.25
The LC filter
53
4.27
The output waveform of the PWM for input dc voltage
= 20v, input reference (current) = 0.8amp
4.28
The output waveform of the PWM for input dc voltage
= 30v, input reference (current) = 0.8amp
4.29
55
The output waveform of the PWM for input dc voltage
= 20v, input reference (current) = 1amp
4.30
54
55
The output waveform of the PWM for input dc voltage
=30v , input reference (current)= 1amp
56
xiv
LIST OF SYMBOL AND ABBREVIATIONS
ABS -
Automatic Brake System
AC -
Alternating Current
ADC –
Analogue to Digital
DAC -
Digital to Analogue
DC -
Direct Current;
DOL -
Direct On Line
DTC -
Direct Torque Control
DSP -
Digital Signal Processing
FFNN -
Feed Forward Neural Network
FOC -
Field Oriented Control
GA -
Genetic Algorithms
MV -
Manipulated Variable
PCB -
Printed Circuit Board
PD –
Proportional Derivative
PI -
Proportional Integral
PID -
Proportional Integral Derivative;
PWM -
Pulse Width Modulation;
VVVF -
Variable Voltage Variable Frequency
1
CHAPTER 1
INTRODUCTION
1.1
Project background
Induction motor or asynchronous motor is an AC electric motor in which the rotor
torque produced is caused by electromagnetic induction from the stator winding of the
magnetic field. Rotor induction motor can either type of wound or squirrel-cage. Its
are the most widely used motors for appliances, industrial control, and automation,
and often called the workhorse of the motion industry because of its robustness,
reliable, low cost, high efficiency and durable [1, 2]. Three-phase induction motors
are widely used in industrial drives and single-phase induction motors are widely used
for smaller loads especially in domestic appliances, such as washing machines and
fans. Conventionally, mechanical gear system is used to obtain variable speed.
Recently, electronic power and control systems have matured to allow these
components to be used for motor control in place of mechanical gears. These
electronics not only control the motor’s speed, but can improve the motor’s dynamic
and steady state characteristics.
Before the development of semiconductor power electronics, induction motors
were mainly used in fixed speed applications because it was difficult to vary the
frequency. There are 4 ways to control the speed of three phase induction motor from
stator side which are further classified as V / f control or frequency control [3],
changing the number of stator poles, controlling supply voltage and by adding
rheostat in the stator circuit. The speed controls of three phase induction motor also
can be done from rotor side which are by adding external resistance on rotor side,
2
cascade control method and by injecting slip frequency emf into rotor side. There are
a few methods for controlling the speed such as FOC and DTC. Another way is
VVVF or V/f method which is most suitable for applications without position control
requirements or the need for high accuracy of speed control [4].
There are many types of digital controller like a microprocessor,
microcontroller and DSP are widely used to control algorithm in motor controller.
PID, Fuzzy logic, and neural network are the examples of algorithm techniques used
in induction motor drive applications. Common controller for induction motor is a
Proportional-Integral, but with the more demanding and sophisticated application, a
conventional PI controller is designed accordingly for PWM current controller to
produce a better controller such as PI-Fuzzy controller, PI-Time Delay controller, PINeural Network and PI-Repetitive controller. Repetitive control is an iterative
approach that improves the transient response performance of processes whose tasks
are repetitive or periodic. The concept of repetitive control was first introduced in
[5].
In this project, one method of controller with additional system will be
implemented on a 3 phase 0.75kW, cage type induction motor. An Arduino Uno will
be used to drive the gates at a logic output voltage of 5V. Then the gate driver will
drive the voltage 15V to the inverter. The motor used in this project is rated for 1.5A
so the current will most likely operate in the ranges most likely conducive to IGBTs
implementation. For controller, a PI plus Repetitive controller will be implemented to
control the current since the speed of the motor is proportional to it. The combination
design method PI and Repetitive system controllers possess high performances in
different areas. PI is used in the steady state area because it gives a zero error while
closed-loop hybridization of Repetitive is a robust control strategy that able to track or
reject unknown periodic signals of a known period. Repetitive control resembles what
is called iterative learning control, where the error in tracking a finite-duration
reference signal is reduced by means of correcting the input in each iteration, based
on the error observed in the preceding iterations. For experimental analysis, matlab
simulink version 2013a will be used.
3
DC
Three
Induction
Phase
Motor
Inverter
Current
Gate
Arduino
Sensor
Driver
Figure 0.1: Hardware block diagram
Figure 1.1 shows the whole system of the project. They are three phase inverter,
arduino and gate driver. which are working together with current sensor to make the
induction motor function as needed. The PI-Repetitive controller will be connected to
the Arduino where Arduino UNO is used as interface instrument to communicate
between hardware and software. The computer simulation program, which will be
used to predict the performance of the designed PI-Repetitive controlled system. The
variables of the simulated program such as currents, voltages etc., are observed from
the scope graphic window. The waveforms obtained from the simulation results are
evaluated and compared with the output of experiment in laboratory.
1.2
Problem Statement
Induction motor which is also called asynchronous motor derive their name from the
way the rotor magnetic field is created from the rotating stator magnetic field that
induced currents in the short circuited rotor. The three-phase induction motor is the
most widely used electric motor worldwide in industrial facilities and large buildings.
4
It is the most favourable drive solution in terms of price and quality. The three phase
induction motor are classified into two types which are Squirrel cage induction motor
and Slip ring induction motor or wound induction motor or phase wound induction
motor [6].
Speaking of 3-phase induction motor, the main thing is concerned with how to
control the speed and keep the motor speed maintain to the desired value. The speed
control of induction motor is more important to achieve maximum torque and
efficiency. Generally, to control the speed of three-phase Induction Motor is by using
DOL, variable resister and frequency inverter. When focusing attention on the speed
control segment of the three phase induction motor, conventional controller have
some problems such as less efficiency due to slow respond, steady state error or
sluggish response to the perturbation in reference setting.
The conventional control method such as PI, PD and PID controller is widely
used in motor control system due to the simple control structure and easiness of
design. However tuning the parameters of controller is a difficult task for varieties of
plant parameters and the nonlinear operating condition, fixed gain PI controllers
cannot provide the desired control performance [7]. The control and estimation of ac
drives in general are considerably more complex than those of dc drives, and this
complexity increases if high performances are demanded. The main reasons for this
complexity are the need of variable-frequency, harmonically optimum converter
power supplies, the complex dynamics of ac machines, machine parameter variations,
and difficulties of processing feedback signals in the presence of harmonic PI
controller can never achieve perfect control, that is, keep the speed of induction motor
continuously at the desired set point value in the presence of disturbance or set point
changes [8].
To enhance the capabilities of traditional controller, several approaches such as
GA, fuzzy logic, Time Delay or Repetitive have been suggested to be add to the
system in order to make speed more stable.
5
1.3
Objectives:
Objectives of this project are listed as follows:
i.
To design a closed-loop control for a three phase induction motor.
ii.
To design a combination of PI-Repetitive controller to control the
speed of a three phase induction motor.
iii.
To develop the hardware of gate driver and three-phase inverter to
control the three phase induction motor.
iv.
To successfully make a communication between computer and three
phase induction motor using Matlab software and Arduino.
6
1.4
Scope:
In this project the scope of work will be undertaken in the following four developmental
stages:
i.
Design the closed-loop control with current feedback for the
0.75kWatt, 1.5A three phase induction motor type squirrel cage.
ii.
Design the PI-Repetitive controller to control the current of a three
phase induction motor. Design the PWM signals which enables the
inverter to control the motor within a speed of 10% - 100% (V/F open
loop control). This implied that the range of frequency used to control
the motor varies between 5 Hz – 50 Hz.
iii.
Build the hardware, which is the rating of three-phase induction motor
squirrel cage types is 0.75kW, 50Hz, 1.5A, 1500 rpm with 4 poles,
gate-driver that generate 15 volt for the inverter and the inverter is
using Hex bridge of IRF530 MOSFET.
iv.
Successfully make a connection between Matlab's Simulink and
Arduino Uno.
7
2
CHAPTER 2
LITERATURE REVIEW
2.1
Induction motor
Three-phase induction motor is a very popular motor and most widely used in the
electrical industry. They run at a constant speed from no load to full load. However,
speed is frequency dependent and so the motor cannot be easily adapted to control the
speed. That’s why it is essentially be known as a constant speed motor and its speed
cannot be changed easily. For this factor, the DC motor is said to be even better when
dealing with the factor of speed variations. However, 3-phase induction motors is
simple, durable, low price, easy to maintain and can be made with features to suit
most requirements of the industry [9]. The most popular type is the 3-phase, squirrelcage AC induction motor. It is a maintenance-free, less noisy and efficient motor. The
stator is supplied by a balanced 3-phase AC power source [10]. The synchronous
speed ns of the motor is calculated by Equation 2.1:
(2.1)
Where;
ns = synchronous speed
fe = the synchronous stator frequency
p = the number of stator poles.
2
The principle of operation of three phase induction motors is based on creating
a rotating magnetic field in the air gap between the stator and rotor. The rotating field
can be obtained by feeding the stator windings from a three phase alternating current
source. Thus, the currents in the windings generate are revolving magnetic field
having the same angular speed of the source frequency in the air gap. Meanwhile, the
continuous change in the stator field induces currents in the rotor's squirrel-cage bars
causing another field in the rotor. Both the rotor and stator fields interact with each
other in the air gap causing the rotor to rotate. The rotor flux lags that of the stator
causing the rotor speed to slip. The ratio of slip and synchronous speeds is called the
motor slip as Equation 2.2 below.
(2.2)
Where;
The motor speed is controlled by variation of a stator frequency with the influence of
the load torque.
Motor Torque
Work Point
Torque
Load Torque
Speed
Motor
Generator
Figure 2.1: Torque-speed characteristic at constant voltage and frequency
3
2.2
Inverter DC to AC
Power converter which is known as inverter is an electrical device that can change
direct current DC to alternating current AC. The converted AC can be at any required
of voltage and frequency usually used in transformer , switching and control circuit. It
also be used from small switching power supplies to large high-voltage electric
equipment applications that transport bulk power and solid-state inverters have no
moving parts. Commonly inverter is used to supply solar panels or batteries and it
perform the opposite function of a rectifier. The electrical inverter is high-power
electronic oscillator because generally AC to DC converter was made to work in
reverse and thus was inverted which is to convert DC to AC.
2.2.1
Three phase inverter
The standard three-phase inverter has as its genesis, the hex-bridge [11]. There are
two kinds of switches that were considered for this range of power applications,
Insulated Gate Bipolar Transistors (IGBTs) or MOSFETs. The hex-bridge takes a
DC voltage and uses six switches (MOSFETS) arranged in three phase legs as shown
in Figure 2.2.
The power circuit consists of six self-commuted semiconductor
switches S1 to S6. The switch pairs (S1, S4), (S3, S6), and (S5, S2) form three legs of
the inverter. The switches in the same leg conduct alternately. Sometime must elapse
before the turn-off of one switch and turn on of another to ensure that both do not
conduct simultaneously.
Their simultaneous operation will cause a short circuit of the dc source
resulting in a very fast rise in current. This fault, known as short-through fault can
only be cleared by fast-acting fuse links [12].
4
S11
S31
S51
S41
S61
S21
Vdc
A
B
C
Figure 2.2: Hex bridge of inverter
The single phase voltage source inverters PWM technique can be used in three-phase
inverters, in which three sine waves phase shifted by 0°, 120°, 240° with the
frequency of the desired output voltage is compared with a very high frequency
carrier triangle, the two signals are mixed in a comparator whose output is high or
wide in time range when the sine wave is in positive side of the triangle and the
comparator output is low or thin in time range when the sine wave signal is in
negative side of the triangle. This phenomenon is shown in Figure 2.3. As is
explained the output voltage from the inverter is not smooth but is a discrete
waveform and so it is more likely than the output wave consists of harmonics, which
are not usually desirable since they deteriorate the performance of the load, to which
these voltages are applied [13].
5
1
0
-1
Time (sec)
1
0
Time (sec)
Figure 2.3: PWM illustration by the sine-triangle comparison method (a) sine triangle
comparison (b) switching pulses
1
(a)
S1
0
1
(b)
S3
0
1
(c)
S5
0
Figure 2.4: Generation of the switching signals for top devices (a) S1 (b) S3 (c) S5
6
1
(a)
S4
0
1
(b)
S6
0
1
(c)
S2
0
Figure 2.5: Generation of the switching signals for bottom devices (a) S4 (b) S6 (c)
S2
The advantage of inverter is to reduce the consumption of power by converting DC to
AC. This alternated power can be maintained in any frequency or voltage with the use
of an appropriate transformers, circuits and switches to support the electrical
equipment at home and office. Disadvantages of inverter are not ideal for inductive
AC and motor loads. It is also sensitive electronics devices can be damaged by poor
waveforms.
2.3
Controller
In order to compete with DC motor in term of controlling the speed, additional system
which is consist of power electronic devices is needed for better output of induction
motor. Induction motor is difficult to maintain a constant speed whenever the load is
changed. Recently, many controllers have been developed and they can be divided
into two classifications which is passive and adaptive power controller. The example
for passive power controller is hysteresis, relay and sliding mode control and for
adaptive power controller is PID, fuzzy, Repetitive and P-resonant controller. Each of
7
them has their own advantages and disadvantages, such as simple structure and low
maintenance cost.
2.3.1
Passive and adaptive controller
Passive controller is something that is built into the system and is always there and
cannot give a good response to any changes of disturbances. A cars brakes for
instance, it is like a car without ABS – when we push the pedal, brakes go on and
there is no computer in the system to make them do ABS things.
Adaptive controller is a controller that can modify its behaviour in response to
changes in the dynamics of the process and the disturbances. A great number of
industrial control loops are under adaptive control. These include a wide range of
applications in aerospace, process control, ship steering, robotics and other industrial
control systems.
2.3.1.1 Proportional- Integral (PI) controller
The most commonly used controller for the speed control of Induction motor is PI
controller. The combination of proportional and integral terms is important to increase
the speed of the response and also to eliminate the steady state error. The PID
controller block is reduced to P and I blocks only as shown in Figure 2.3 below.
8
+
e
P
+
Plant
Y
I
Figure 2.3: Proportional Integral (PI) controller block diagram
The proportional and integral terms is given by Equation 2.3:
(2.3)
Kp and Ki are the tuning knobs, are adjusted to obtain the desired output, e is error
and R is reference currents. However, the PI controller has some disadvantages such
as the high starting overshoot, sensitivity to controller gains and sluggish response
due to sudden disturbance. So, the relatively a new intelligent controller by using
Matlab/Simulink package give better responses than the traditional PI controller for
the speed control. Torque is the fundamental variable of an induction machine, to get
the accurate control of speed and position is by controlling the torque of induction
machines. The performance of intelligent controller for the speed control has been
verified compared with the conventional PI controller performance. It is very good
dynamic performance and robustness during the transient period and during the
sudden load disturbance [14].
9
2.3.1.2 Fuzzy controller
A fuzzy control system is a system based on fuzzy logic. Fuzzy means uncertainty,
fuzzy computes uncertainty by assigning values between 0 and 1 compared to
conventional computation 0 or 1. This fuzzy logic involves computing using
knowledge base and rule base. In fuzzy systems, input variables are assigned with a
membership function. Each membership function is assigned with specified values.
Fuzzy logic comprises of three stages which is fuzzification, fuzzy inference and
defuzzification. The advantages and disadvantages of the fuzzy logic are stated
below:
Advantage of fuzzy logic control include the following :
i.
A large number of system variables can be evaluated.
ii.
It similar to the way humans think because linguistic variable and not
numerical values are used
iii.
It use permits a greater degree of uncertainty because the output is related to
the input, so better control is possible for some types of system, requires little
maintenance.
iv.
It can be used for the solution of previously unsolved problems
v.
It is more robust.
Some of the disadvantages of fuzzy logic include:
i.
Extracting a model from a fuzzy system is difficult.
ii.
Fuzzy systems require finer tuning before they are operational.
10
2.3.2
Propose controller
This paper is proposed PI-Repetitive controller used as the controller design for the
system control. The PI control scheme is named after its two correcting terms whose
sum constitutes the manipulated variable (MV) by Equation 2.4,
(2.4)
Proportional term of output (
) and Integral term of output
are the
contributions to the output from the PI controller from each of the two terms. The
proportional term (sometimes called gain) makes a change to the output that is
proportional to the current error value. The proportional response can be adjusted by
multiplying the error by a constant
, called the proportional gain. The proportional
term is given by Equation 2.5:
(2.5)
If the proportional gain is too high, the system can become unstable In contrast, a
small gain results in a small output response to a large input error, and a less
responsive (or sensitive) controller. If the proportional gain is too low, the control
action may be too small when responding to system disturbances. In the absence of
disturbances, pure proportional control will not settle at its target value, but will retain
a steady state error that is a function of the proportional gain and the process gain.
The contribution from the integral term (sometimes called reset) is proportional to
both the magnitude of the error and the duration of the error. Summing the
instantaneous error over time (integrating the error) gives the accumulated offset that
should have been corrected previously. The accumulated error is then multiplied by
the integral gain and added to the controller output as Equation 2.6. The magnitude of
the contribution of the integral term to the overall control action is determined by the
integral gain,
.
(2.6)
11
The integral term (when added to the proportional term) accelerates the movement of
the process towards set point and eliminates the residual steady –state error that
occurs with a proportional only controller. However, since the term is responding to
accumulated errors from the past, it can cause the present value to overshoot the set
point value (cross over the set point and then create deviation in the other direction).
PI controllers are good for tracking or rejecting step signals. But for inverters, the
signals are sinusoidal. In order to have good tracking performance, Repetitive control
system should be added. The basic Repetitive control transform functions is shown in
Equation 2.7.
(2.7)
where
is the resonant frequency equal to the synchronous frequency of the stator
voltage outputs [15, 16]. The general Repetitive control block diagram is in Figure 2.
4.
Ref
P
+
W
S2 + W 2
Figure 2.4: Repetitive Control block diagram
12
Set Point
Output
∑
∑
error
W
S2 + W 2
Figure 2.5: PI-Repetitive control block diagram
In past research, a new control scheme named Repetitive control has been introduced.
The stability conditions and the synthesis algorithms have been investigated for linear
and nonlinear repetitive control systems [17]. In the proposed scheme, the high
accuracy tracking property for periodic reference commands can be achieved by
locating the generator for periodic signals in the closed-loop. The scheme has been
applied to the trajectory control of a three-link manipulator and the simulation has
showed the tracking error could be reduced to low level by repetitive operation [18].
Repetitive control is said to be among the best way to make the system stable in speed
condition because of the application of ‘close-loop’ task that being applied. The use
of memory-loops is beneficial in systems with repetitive disturbances or tasks. In
order to improve the capabilities of repetitive controllers for the cases where the
periodicity is hard to measure and is subject to variation, an extension of repetitive
control is developed [19]. Figure 2.5 is one of modified controller that will be done
where the PI controller is combine with Repetitive controller.
2.4
Gate driver
The gate driver circuit which is consist of IGBT’s HCPL3120 is used to improve the
performance of motor drives because it directly related to the availability of power
control of semiconductor devices. The purpose of using a gate driver is the
13
application of a charge pump circuit to the gate of the MOSFET in inverter [20]. By
doing so, it allows the gate voltage to be lifted and maintain the ON position in the
switch. The circuit for the gate driver was designed to meet the specifications needs
where the gate to source voltage needed for desired operation of the inverter is on a
15V level. In addition, the high side MOSFETs in the hex-bridge do not have the
source connected to ground, so the actual voltage needed to drive the gate depends on
the varying voltage at the source.
14
2.5
Arduino
Arduino is an open-source electronics prototyping platform based on flexible, easy-touse hardware and software. Arduino board can sense the environment by receiving
input from a variety of sensors and can affect its surroundings by controlling lights,
motors, and other actuators. Arduino projects can be stand-alone or they can
communicate with software on running on a computer (e.g. Flash, Processing,
MaxMSP).
It includes 6 to 10-bit analogue inputs and can be used for ultra-inexpensive
data acquisition. The Arduino Uno is based on the ATmega 328 that consist of 14
digital input/output pins (of which 6 can be used as PWM outputs), 6 analogue inputs,
a 16 MHz ceramic resonator, a USB connection, a power jack and a reset button. It
contains everything needed to support the microcontroller where just simply connect
it to a computer with a USB cable or power it with a AC-to-DC adapter or battery to
get started. Additionally, daughter boards can be added to the Uno to allow for many
more functions, such as amplified motor output, battery powered operation, Ethernet
communication and electronics prototyping.
In this project, the analogue to digital converter, ADC is really important
because of this feature is use to get the current feedback from the current sensor [21].
The ADC feature of Arduino Uno depends from the ATmega 328 microcontroller
itself. It contains 10-bit successive approximation ADC. The operation of translating
an analogue voltage value into different levels is what we call Analogue to Digital
Conversion. This project needs the signal from Pulse Width Modulation, PWM,
which is a powerful way of controlling analogue circuits and systems, using the
digital outputs of microprocessors. Defining the term, we can say that PWM is the
way we control a digital signal simulating an analogue one, by means of altering its
state and the frequency. Figure 2.6 is the Arduino board that will be used in this
project.
15
Figure 2.6: Arduino board
Some of the hardware provided on the Arduino board is:
2.6
USB (Power)
Power indicator
Voltage regulator
Digital input
Barrel jack (Power)
PWM output
3.3V supply
Analogue Ref input
5V supply
Reset button
Ground
Rx,Tx indicator
analogue input
Main IC
Other research
In another past research [8], hybrid controller produces better performances in terms
of rise time, overshoot, undershoot and settling time. There is no steady-state error in
the speed response during the operation. In terms of rise time and Settling Time, good
torque response is obtained with hybrid controller at all-time instants and speed
response is better than only using PI controllers. The speed response with this
controller has no overshoot and settles faster in comparison with conventional
16
controller. It is also noted that there is no steady-state error in the speed response
during the operation when hybrid controller is activated. Good torque response is
obtained with hybrid controller at all-time instants and speed response is much more
better.
From an International Journal of Innovative Computing, Information and Control
ICIC International [7], the PI controller in DTC of induction motor was tuned by an
adaptive mechanism based on FFNN where the system will approximate mapping the
input output of linear and nonlinear system. The paper is concentrated on the
development of the FFNN tuning of PI controller parameters based on changing speed
of induction motor and load torque of electric ship and the effectiveness of the
complete proposed control scheme is clarified through a matlab/simulink simulation
FFNN was utilized to tune the gain of PI speed controller.
Meanwhile, training data used in this model were taken from the optimum value
of gain PI controller for each rotor speed and load torque conditions. Load torque of
the induction motor applied in the electric propulsion system were depending on the
rotor speed of the induction motor directly connected the propeller. From the results
showed that the rotor speed of induction motor for PI-FFNN controller method
provides small steady state error compared with fixed PI controller. The
electromagnetic torque which is used to accelerate and decelerate the speed of the
induction motor by using PI-FFNN controller was smaller than with using fixed PI
controller. Since the magnitude of the stator current is proportional to the
electromagnetic torque of the induction motor speed, the magnitude of the stator
current of the induction motor using PI-FFNN controller was smaller than with using
fixed PI controller.
17
CHAPTER 3
METHODOLOGY
The whole system developed is based on PI-Repetitive controller with Arduino, threephase Inverter, which is used for Induction motor. Also, the three phase 1 input 2
output Gate Driver is used to produce 15v for the Inverter. The work includes stages
such as designing, simulation, implementation and verification. The designing part
involves using Proteus software for inverter and gate design. The simulations is
conducted by using Matlab / Simulink software version R2013a for controller design,
download and implemented on the Arduino UNO board in the implementation stages.
Lastly, the testing of the controller design using a hardware development is done to
verify its correctness.
3.1
Hardware flow-chart
Start
DC Supply
Voltage
3 Phase Current
Source Inverter
Current Sensors
Gate Driver
Arduino
3 Phase
Induction
Motor
Figure 0.1: Hardware process control
18
From the flow chart in Figure 3.1, the system implemented converts a DC input
voltage at a power rating of 750 W to AC by using a three phase inverter, and a LC
filter which assists in stabilizing the output of AC voltage. The three phase inverter is
droved by the gate driver that is amplified the PWM signals generated by the DSP
board and produce the appropriate gate signals. The hex-bridge uses a dc bus voltage
and the gate drive signals to produce balanced three-phase sinusoidal output which
drives the induction machine. Current sensor will detect the three phase current (Ia, Ib
and Ic) and feed to the Arduino. The PI-Repetitive controller that is developed will
analyse and reduce the error if detected after compared to the reference current and
then will generate the PWM before feeding to gate driver. All of the steps of
minimizing the error and generating the PWM will be done in Arduino. The gate
driver will feed the voltage up to 15V for three phase inverter. The three-phase
induction motor is rated at 0.75kW, 1.5A, 1500 rpm.
3.2
Hardware design
Figure 3.1 shows the hardware process control of the system. They are consists of
gate driver, inverter, filter, current sensor and induction motor. Gate driver, inverter
and filter is fabricated in laboratory while current sensor and ac drives is already
available in laboratory.
3.2.1
Gate Driver
The schematic diagram of the gate driver as Figure 3.2 was designed to meet the
specifications needs where the gate to source voltage needed for desired operation of
the inverter is on a 15 volt level. The input of the gate driver is 5 volt and it will boost
up to around 15 volt practically. Table 3.1 shows the list of components for the
development of gate driver. While the Figure 3.3 below shows the three phase gate
driver PCB layout for it’s fabrication.
19
S1
S2
S3
U2:A
U1:B
D1
3
S1
1N4748A
1
R1
4.3k
1
U1:C
4
5
7414
3
6
2
4081
+15V1
U2:B
7414
C1
5
4
1nF
6
U1:A
U3
1
N/C1
2
ANODE
3
CATHODE
4
N/C2
4081
2
7414
U2:C
8
VDD
R2
4.3k
10
9
C2
1nF
D2
13
U1:E
12
11
R4
U2:D
10
12
11
1N4748A
7414
7414
13
4081
560
J3
1
2
10
+15V2
G15V1
CONN-H2
HCPL3120
560
4081
U1:D
R3
R5
8
VCC
7
VO1
6
VO2
5
VEE
R6
U4
1
N/C1
2
ANODE
3
CATHODE
4
N/C2
10
8
VCC
7
VO1
6
VO2
5
VEE
J4
1
2
CONN-H2
G15V2
HCPL3120
Figure 0.2: One phase gate driver schematic circuit
Table 0.1: List of the components for gate driver circuit
No
Component
Unit
1.
IL0515S
6
2.
IN4748A
6
3.
Capacitor 1nF
6
4.
Resistor 4.3kΩ
2
5.
Resistor 560Ω
2
6.
Resistor 10Ω
2
7.
Resistor 10kΩ
12
8.
IC 7414
3
9.
IC 4081
3
10.
HCPL3120
6
20
Figure 0.3: Three phase gate driver PCB layout
3.2.2
Inverter
Figure 3.4 shows the schematic diagram while Figure 3.5 shows the PCB layout for
three phase inverter. This circuit consists of power transistor MOSFET
(SPP11N60C3) and a 470µH 100v power capacitor. The general function of inverter
is used to transform DC power into three phase AC power. For inverter operation,
power flow from the DC side to the AC side and able to carry the voltage supply up to
400 volts. The three phase inverter circuit consists of several components as listed in
the Table 3.2.
21
Figure 3.4: Schematic diagram of three phase inverter
Table 0.2 : List of components for three phase inverter
No
Component
Unit
1.
MOSFET SPP11N60C3
6
2.
Capacitor 470µF (450V)
1
Figure 0.5: PCB layout for three phase inverter
22
3.2.3
Filter
The LC filter structure is employed to filter the switching frequency components of
the rectangular voltage pulses generated by the inverter. However it increases the
output impedance of the system and causes distortion due to the harmonics of the load
current [22]. The filter will enable the required frequencies to be passed through the
circuit, while rejecting those that are not needed. In this project, the value of L and C
is choose for L=68mH and C=22F.
Input
Output
Inductor
Capacitor
Figure 0.4: The circuit for the LC filter
3.2.4
Transducer
Figure 0.5: Current Sensor BB-ACS756
Current Sensors as shown in Figure 3.5 and 3.6 are used of controlling, monitoring or
measuring AC currents. The sensors serve as feedback elements between the output
line current of inverter and control circuitry providing accurate regulation of switch
mode power supplies. The BB-ACS756 is used to measure current and provide the
23
feedback signal to the PI-Repetitive controller. It is also can be used for over current
fault protection.
The BB-ACS756 is come with the one set of T-connector and a 3 ways angle
pin header. The ACS756 is power up with 5V and gives output voltage to indicate the
direction and current value. The initial condition (no current) the output voltage is 2.5
volt. When the voltage increase mean the current is flowing the one of the direct and
the voltage is proportional to the current. This also applies in the other direct of the
current if the voltage drop. The output sensitivity of this transducer is 40 milivolt per
ampere and it can measure maximum up to 5A, meaning the maximum output voltage
is 2.7 volt.
Figure 0.6: Current Sensor BB-ACS756 pin configuration
According to datasheet of ACS756, the sensor has sensitivity of 40mV/A. This
explain every rise of 1 A will increase 40mV to VIOUT pin of BB-ACS756. This
sensor can be used in both AC and DC circuit. Initially, the sensor produces output of
2.5V from VIOUT pin for 0 A current reading. Since the sensor can measure up to
±50A, thus the maximum voltage for positive current measurement is 4.5 V (+50A)
and negative current measurement is 0.5 V for (-50A).
24
3.2.4.1 ADC for Current conversion
The output of the sensor is voltage which it needs further calculation to convert it to
current. By taking the ratio and sensitivity of the sensor, it can be written as Equation
3.1:-
=
(0.1)
Change of voltage can be written as follows:-
Change of voltage = (Sensor Reading – Initial Reading) bits
Change of voltage is different between output voltage at the time (sensor reading) and
initial output voltage which is around 2.5V. Since 2.5V give 512 in 10 bits ADC in
Arduino, thus change of voltage can be written as Equation (3.2):-
Change of voltage = (Sensor -512) bits
Combining Equation (3.2) and Equation (3.1) gives:-
I (mA) =
(1000mA)
Hence,
I(mA) = (
Or
)(122)
(0.2)
59
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