SPEED CONTROL OF DC MOTOR USING CHOPPER Department of Electrical Engineering

SPEED CONTROL OF DC MOTOR USING CHOPPER  Department of Electrical Engineering
SPEED CONTROL OF DC MOTOR USING CHOPPER
ABHISHEK KUMAR SINHA (109EE0309)
BADAL KUMAR SETHY (109EE0304)
Department of Electrical Engineering
National Institute of Technology Rourkela
SPEED CONTROL OF DC MOTOR USING CHOPPER
A Thesis submitted in partial fulfillment of the requirements for the degree of
Bachelor of Technology in “Electrical Engineering”
By
BADAL KUMAR SETHY (109EE0304)
ABHISHEK KUMAR SINHA (109EE0309)
Under guidance of
Prof. MonalishaPattnaik
Department of Electrical Engineering
National Institute of Technology
Rourkela-769008 (ODISHA)
May-2013
2
DEPARTMENT OF ELECTRICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY, ROURKELA
ODISHA, INDIA-769008
CERTIFICATE
This is to certify that the thesis entitled “Speed Control of DC Motor Using Chopper”,
submitted by Abhishek Kumar Sinha (Roll. No. 109EE0309) and Badal Kumar Sethy
(Roll. No. 109EE0304) and in partial fulfilment of the requirements for the award of Bachelor
of Technology in Electrical Engineering during session 2012-2013 at National Institute of
Technology, Rourkela. A bonafide record of research work carried out by them under my
supervision and guidance.
The candidates have fulfilled all the prescribed requirements.
The Thesis which is based on candidates’ own work, have not submitted elsewhere for a
degree/diploma.
In my opinion, the thesis is of standard required for the award of a bachelor of technology degree
in Electrical Engineering.
Place: Rourkela
Dept. of Electrical Engineering
National institute of Technology
Rourkela-769008
Prof. M. Pattnaik
Assistant Professor
3
ACKNOWLEDGEMENT
We would like to express our heartfelt gratitude and regards to our project guide, Prof.
Monalisha Pattanaik, Department of Electrical Engineering, for being the corner stone of our
project. It was her incessant motivation and guidance during periods of doubts and uncertainties
that has helped us to carry on with this project. We would like to thank Prof. A K Panda, Head
of the Department, and Electrical Engineering for his guidance, support and direction. We are
also obliged to the staff of Electrical Engineering Department for aiding us during the course of
our project. We offer our heartiest thanks to our friends for their help in collection of data
samples whenever necessary. Last but not the least, we want to acknowledge the contributions of
our parents and family members, for their constant and never ending motivation. We are grateful
to The Department of Electrical Engineering for giving us the opportunity to carry out this
project, which is an integral fragment of the curriculum in B. Tech programme at the National
Institute of Technology, Rourkela.
Badal Kumar Sethy
Abhishek Kumar Sinha
B.Tech (Electrical Engineering)
a
Dedicated TO
BhagawanShriShirdiSaiBaba
&
Our beloved Parents
b
ABSTRACT
The speed control of separately excited dc motor is carried out by varying the armature voltage
for below rated speed and by varying field flux to achieve speed above the rated speed. This
thesis presents the speed control methodology by varying armature voltage using chopper by
providing control signal to the switches. Speed can be controlled from below and up to rated
speed .The firing circuit of chopper receives signal from controller and variable voltage is given
to the armature of dc motor according to the desired speed .There are two controllers we are
using here one is speed controller and other is current controller. Both controllers are of
proportional -integral type .The reason behind using PI type controller is it removes the delay
and provide fast control. Now the simulation of model is done and analyzed in MATLAB
(Simulink) under varying speed and torque condition.
i
CONTENTS
Abstract
i
Contents
ii
List of Figures
iv
Abbreviations and Acronyms
v
CHAPTER 1
INTRODUCTION
1.1 Introduction
2
CHAPTER 2
CHOPPER
2.1 Introduction of chopper
3
2.2 Principle of chopper operation
4
CHAPTER-3
SEPARATELY EXCITED DC MOTOR
3.1 Introduction
8
3.2 Mathematical analysis of separately excited DC motor
9
CHAPTER-4
MODELLING OF DC MOTOR FOR DRIVE SYSTEM
4.1 Basic idea
12
ii
CHAPTER-5
MATLAB SIMULATIONS, RESULTS AND ANALYSIS
5.1 Simulation of open loop model of chopper with DC machine
16
5.2 Simulation of chopper with r-l-e load
15
5.3 Simulation of closed loop model of chopper with DC machine
18
(Using IGBT as a switch)
5.4 Simulation of closed loop model of chopper with DC machine
20
(Using GTO as a switch)
5.5 Simulation of boost converter with r - load
22
5.6 Simulation of boost converter with dc machine
24
5.7 Simulation of chopper fed dc drives
26
CHAPTER-6
CONCLUSION
6.1 Conclusions
29
References
31
iii
LIST OF FIGURES
Fig. No
Name of the Figure
Page. No.
1
Chopper circuit diagram and its voltage and current waveform
5
2
Circuit diagram of separately excited DC motor
9
3
Closed loop model for speed control of DC motor
12
4
Simulink model of DC chopper with r-l-e load
15
5
Simulink output of DC chopper with r-l-e load
15
6
Simulink model of open loop model of chopper with dc machine
16
7
Simulink output of open loop model of chopper with dc machine
17
8
Simulink model of closed loop model of chopper with dc machine
18
(Using IGBT as a switch)
9
Simulink output of closed loop model of chopper with dc machine
19
(Using IGBT as a switch)
10
Simulink model of closed loop model of chopper with dc machine
20
(Using GTO as a switch)
11
Simulink output of closed loop model of chopper with dc machine
21
(Using IGBT as a switch)
12
Simulink model of boost converter with r- load
22
13
Simulink output of boost converter with r- load
23
14
Simulink model of boost converter with dc machine
24
15
Simulink output of boost converter with dc machine
25
16
Simulink model of chopper fed dc drive
26
17
Simulink output of chopper fed dc drive
27
18
Simulink output of chopper fed dc drive
iv
28
ABBREVIATIONS AND ACRONYMS
AC
-
Alternating Current
MOSFET
-
Metal Oxide Semiconductor Field Effect Transistor
DC
-
Direct Current
PWM
-
Pulse Width Modulation
MATLAB
-
Matrix Laboratory
BJT
-
Bipolar junction transistor
IGBT
-
Insulated gate bipolar junction transistor
EMF
-
Electromagnetic field
GTO
-
Gate turn off thyristor
PI
-
Proportional -integral
PID
-
Proportional –integral-derivative
v
CHAPTER1
INRODUCTION
1
1.1 Introduction
An electrical drive consists of electric motors, its power controller and energy transmitting
shaft. In modern electric drive system power electronic converters are used as power
controller. Electric drives are mainly of two types: DC drives and AC drives. The two types
differ from each other in that the power supply in DC drives is provided by DC motor and
power supply in AC drives is provided by AC motor.
DC drives are widely used in applications requiring adjustable speed control, frequent
starting, good speed regulation, braking and reversing. Some important applications are paper
mills, rolling mills, mine winders, hoists , printing presses, machine tools, traction, textile
mills, excavators and cranes. Fractional horsepower DC motors are widely used as
servomotors for tracking and positioning. For industrial applications development of high
performance motor drives are very essential. DC drives are less costly and less complex than
AC drives .
DC motors are used extensively in adjustable speed drives and position control system.
The speed of DC motors can be adjusted above or below rated speed. Their speed above rated
speed are controlled by field flux control and speed below rated speed is controlled by
armature voltage. DC motors are widely used in industry because of its low cost, less
complex control structure and wide range of speed and torque. There are various methods of
speed control of DC drives – armature voltage control, field flux control and armature
resistance control.
For controlling the speed and current of DC motor, speed and current controllers are
used. The main work of controller is to minimize the error and the error is calculated by
comparing output value with the set point. This thesis mainly deals with controlling DC
motor speed using Chopper as power converter and PI as speed and current controller.
2
CHAPTER 2
CHOPPER
3
2.1 Introduction of chopper
A chopper is a static power electronic device which converts fixed dc input voltage to a
variable dc output voltage. It can be step up or step down. It is also considered as a dc
equivalent of an ac transformer since they behave in an identical manner. Due to its one stage
conversion, choppers are more efficient and are now being used all over the world for rapid
transit systems, in marine hoist, in trolley cars, in mine haulers and in forklift trucks etc. The
future electric automobiles are likely to use choppers for their speed control and braking.
Chopper systems offer smooth control, high efficiency, faster response and regeneration
facility. The power semiconductor devices used for a chopper circuit can be force
commutated thyristor, BJT, MOSFET, IGBT and GTO. Among above switches IGBT and
GTO are widely used. These devices are generally represented by a switch. When the switch
is OFF, no current will flow. Current flows through the load when switch is ON. The power
semiconductor devices have on-state voltage drop of 0.5V to 2.5V across them. For the sake
of simplicity, this voltage drop across these devices is generally neglected.
2.2 Principle of Chopper Operation
A chopper is a high speed “on" or “off” semiconductor switch. It connects source to load and
load and disconnect the load from source at a fast speed. A constant dc supply of magnitude
is given as input voltage and let its output voltage across load be V o. For the sake of
highlighting the principle of chopper operation, the circuitry used for controlling the on, off
periods is not shown. During the period
voltage . During the period
, chopper is on and load voltage is equal to source
, chopper is off, load voltage is zero. In this manner, a
chopped dc voltage is produced at the load terminals.
4
Figure 1: Chopper circuit diagram and its voltage and current waveform
Average Voltage =
=
=α*
- (i)
Also,
= off-time.
T=
α=
+
= Chopping period.
⁄
,
f =1/T =chopping frequency.
Thus the voltage can be controlled by varying duty cycle α
= f*
*
- (ii)
Let Vo be the average voltage, Ton is ON time, Toff is OFF time, T is chopping period, and
Vs is input voltage and α is duty cycle.The average output voltage Vo can be controlled by
varying duty cycle. There are various control strategies for varying duty cycle. They are time
ratio Control and current-limit control.
5
In time ratio control α is varied by two ways constant frequency method and variable
frequency method. In constant frequency method ‘f’ remains constant and Ton is varied.This
scheme is also called pulse-width-modulation scheme. In variable frequency method ‘f’
varies and either Ton or Toff is kept constant. This method is also called frequency
modulation scheme.
In current-limit control scheme, the switching of chopper circuit is decided by the
previous set values of load current. The two set values should be maximum load current and
minimum load current. When the load current reaches the value more than maximum value of
load current then chopper is switched off and it falls below minimum value, the chopper is
switched on. Here Switching frequency of chopper is controlled by setting maximum and
minimum level of current. Current limit control also involves feedback loop and therefore the
trigger circuit for the chopper becomes more complex .PWM technique is the most common
control strategy for the power control in chopper circuit.
The controller used in a closed loop model of DC motor provides a very easy and
common technique of keeping motor speed at any desired set-point speed under changing
load conditions. This controller can also be used to keep the speed at the set-point value when
the set-point is ramping up or down at a defined rate. In this closed loop speed controller, a
voltage signal obtained from a Tacho-generator attached to the rotor which is proportional to
the motor speed is fed back to the input where signal is subtracted from the set-point speed to
produce an error signal. This error signal is then fed to controller to make the motor run at the
desired set-point speed. If the error speed is negative, this means the motor is running slow so
that the controller output should be increased and vice-versa.
There are different types of controller available and its selection is also an important
work. Some of the controllers which are most widely used are – on–off controller,
proportional controller, integral controller, derivative controller and PID controller. In
6
proportional controller error speed is proportional to the measured output. This controller has
the limited use and can never force the motor to run exactly at the set point speed. Therefore
an improvement is required for correction in the output. In PI controller, the proportional term
does the job of fast correction and the integral term takes finite time to act and makes the
steady state error zero. In derivative approach further refinement is done. This controller will
allow the rate of change of error speed to apply an additional correction to the output drive. It
can be used to give a very fast response to sudden changes in motor speed. In simple PID
controllers it becomes difficult to generate a derivative term in the output that has any
significant effect on motor speed. It can be deployed to reduce the rapid speed oscillation
caused by high proportional gain. However, in many controllers, it is not used. The derivative
action causes the noise (random error) in the main signal to be amplified and reflected in the
controller output. Hence the most suitable controller for speed control is PI type controller.
7
CHAPTER 3
SEPARATELY EXCITED DC MOTOR
8
3.1 Introduction
Figure 2: Circuit diagram of separately excited dc motor
Separately excited dc motor has field and armature winding with separate supply voltage.
Field winding supplies field flux to armature. When DC voltage is applied to motor, current is
fed to the armature winding through brushes and commutator. Since rotor is placed in
magnetic field and it is carrying current also. So motor will develops a back emf and a torque
to balance load torque at particular speed.
3.2 Mathematical analysis of separately Excited DC Motor
When a separately excited dc motor is excited by a field current of
of
and an armature current
flows in the circuit, the motor develops a back EMF and a torque to balance the load
torque at a particular speed. The field current If is independent of the armature current Ia.
Each winding is supplied separately. Any change in the armature current has no effect on the
field current.The
is generally much less than the
9
.
In the above figure suppose
ampere,
is the armature voltage in volt,
is the motor back emf in volt,
is the armature current in
is the armature inductance in Henry,
is the
armature resistance in ohm.
The armature equation is shown below:
- (iii)
Now the torque equation will be given by:
.
Where,
is load torque in Nm,
- (iv)
is the torque developed in Nm, J is moment of inertia in
kg/m², B is friction coefficient of the motor and ω is angular velocity in rad/sec.
Assuming absence (negligible) of friction in rotor of motor, it will yield B=0. Therefore, new
torque equation will be given by:
- (v)
Equation for back emf of motor will be:
- (vi)
Also,
- (vii)
- (viii)
Now, from the above equation it is clear that speed of DC motor depends on applied voltage,
armature current, armature resistance and field flux. So, there are three ways of controlling
speed of DC motor – armature voltage control, armature resistance control and field flux
control.
10
CHAPTER 4
MODELLING OF DC MOTOR FOR
DRIVE SYSTEM
11
4.1 BASIC IDEA
An electrical DC drive is a combination of controller, converter and DC motor. Here we will
use chopper as a converter. The basic principle behind DC motor speed control is that the
output speed of DC motor can be varied by controlling armature voltage keeping field voltage
constant for speed below and up to rated speed . The output speed is compared with the
reference speed and error signal is then fed to speed controller. If there is a difference in the
reference speed and the feedback speed, Controller output will vary. The output of the speed
controller is the control voltage Eg that controls the operation duty cycle of converter. The
converter output gives the required voltage V to bring motor speed back to the desired speed.
The Reference speed is provided through a potential divider because it is linearly related to
the speed of the DC motor. Now the output speed of motor is measured by Tacho-generator.
The tacho voltage we will get from the tacho generator contains ripple and it will not be
perfectly dc. So, we require a filter with a gain to bring Tacho output back to controller level.
The basic block diagram for DC motor speed control is show below :
Figure 3: Closed loop model for speed control of dc motor
The controller used in a closed loop model of DC motor provides a very easy and common
technique of keeping motor speed at any desired set-point speed under changing load
conditions. This controller can also be used to keep the speed at the set-point value when the
12
set-point is ramping up or down at a defined rate. In this closed loop speed controller, a
voltage signal is obtained from the Tacho-generator attached to the rotor which is
proportional to the motor speed is fed back to the input where signal is subtracted from the
set-point speed to produce an error signal. This error signal is then fed to controller to make
the motor run at the desired set-point speed. If the error speed is negative, this means the
motor is running slow so that the controller output should be increased and vice-versa.
There are different types of controller available and its selection is also an important
work. Some of the controllers which are most widely used are – proportional controller ,on–
off controller, integral controller, derivative controller and PID controller. In proportional
controller error speed is proportional to the measured output. This controller has the limited
use and can never force the motor to run exactly at the set point speed. Therefore an
improvement is required for correction in the output. In PI controller, the proportional term
does the job of fast correction and the integral term takes finite time to act and makes the
steady state error zero. In derivative approach further refinement is done. This controller will
allow the rate of change of error speed to apply an additional correction to the output drive. It
can be used to give a very fast response to sudden changes in motor speed. In simple PID
controllers it becomes very difficult to generate a derivative term in the output that has any
significant effect on speed of motor. It can be deployed to reduce the rapid speed oscillation
caused by high proportional gain. Therefore, in many controllers, it is not used. The
derivative action causes the noise (random error) in the main signal to be amplified and
reflected in the controller output. Hence the most suitable controller for speed control is PI
type controller.
13
CHAPTER 5
MATLAB SIMULATIONS, RESULTS
AND ANALYSIS
14
5.1 Simulation of chopper with R-L-E load:
Figure 4: Simulink model of dc chopper with R-L.E load
Rating of the elements used in the above simulation:
Dc voltage source – 100 V
Pulse generator:
Amplitude – 10, period – 2e-3, pulse width (% of period) – 50
Resistance (ohms) – 1, Inductance – 100e-3, E – 50 V
After simulation we are getting a graph of output voltage with respect to time.
Figure 5: Simulation output of dc chopper with R-L.E load
15
5.2 Simulation of open loop model of chopper with dc machine:
Figure 6: Simulink model of open loop model of chopper with dc machine
Rating of the elements used in the above simulation:
Dc voltage source – 280 V
Pulse generator:
Amplitude – 10, period – 2e-3, pulse width (% of period) – 50
Dc motor specification:
Rating – 50 HP, 500 V, 1750 rpm, Field – 300 V
Torque – 5 N-M
After simulation of the above model we are getting a graph of armature voltage, armature
current and armature speed with respect to time.
16
Figure 7: Simulation output of open loop model of chopper with dc machine
17
5.3 Simulation of closed loop model of chopper with dc machine (using IGBT as a
switch):
Figure 8: Simulink model of closed loop model of chopper with dc machine
Rating of the elements used in the above simulation:
Dc voltage source – 280 V
Rating of dc machine – 5 HP, 240 V
Torque – 5 N-M, Field voltage – 240 V
Reference speed – 120 rad/sec
After simulation of the above model we are getting a graph of armature voltage, armature
current and armature speed with respect to time.
18
Figure 9: Simulation output of closed loop model of chopper with dc machine
19
5.4 Simulation of closed loop model of chopper with dc machine (using GTO as a
switch):
Figure 10: Simulink model of closed loop model of chopper with dc machine
Rating of the elements used in the above simulation:
Dc voltage source – 230 V
Rating of dc machine – 5 HP, 240 V
Torque – 5 N-M, Field voltage – 240 V
Reference speed – 120 rad/sec
After simulation of the above model we are getting a graph of armature voltage, armature
current and armature speed with respect to time.
20
Figure 11: Simulation output of closed loop model of chopper with dc machine
21
5.5 Simulation of boost converter with R-load:
Figure 12: Simulation model of boost converter with R-load
Rating of the elements used in the above simulation:
Dc voltage source – 230 V
Load resistance – 10 ohm
After simulation of the above model we are getting a graph of output voltage, and output
current with respect to time
22
Figure 13: Simulation output of boost converter with R load.
23
5.6 Simulation of boost converter with DC machine:
Figure 14: Simulation model of boost converter with dc machine
Rating of the elements used in the above simulation:
Dc voltage source – 230 V
Rating of dc machine – 5 HP, 240 V
Torque – 5N-M, Field voltage – 240 V
Reference speed – 120 rad/sec
After simulation of the above model we are getting a graph of armature voltage, armature
current and armature speed with respect to time.
24
Figure 15: Simulation output of boost converter with dc machine
25
5.7 Simulation of chopper fed DC drive:
Figure 16: Simulink model of chopper fed dc drive
Rating of the elements used in above simulation:
DC input voltage – 240 V
DC machine rating – 5HP, 240 V, 1750rpm
Applied field voltage – 300 V
Torque of 10 N-m is applied @ 1 sec , L – 10mH
After simulation of the above model we are getting a graph of armature speed, armature
current, electrical torque and armature voltage with respect to time.
26
Figure 17: Simulation output of chopper fed dc motor
27
Figure 18: Simulation output of chopper fed dc drive
28
CHAPTER 6
CONCLUSION
29
The speed of a dc motor has been successfully controlled by using Chopper as a converter
and Proportional-Integral type Speed and Current controller based on the closed loop model
of DC motor. Initially a simplified closed loop model for speed control of DC motor is
considered and requirement of current controller is studied. Then a generalized modelling of
dc motor is done.Afterthat a complete layout of DC drive system is obtained. Then designing
of current and speed controller is done. Now the simulation is done in MATLAB under
varying load condition, varying reference speed condition and varying input voltage. The
results are also studied and analyzed under above mentioned conditions. The model shows
good results under all conditions employed during simulation.
Since, the simulation of speed control of DC motor has been done. We can also
implement it in hardware to observe actual feasibility. Here speed control of DC motor is
done for rated and below rated speed. We can also control the speed of DC motor above rated
speed and this can be done by field flux control.
30
REFERENCES
[1] SIMULINK, Model-based and system-based design using Simulink, Maths works, Inc,
Natick, MA, 2000.
[2] MATLAB SIMULINK, version 2009, SimPowerSystem, One quadrant chopper DC drive.
[3] Amir Faizy, Shailendra Kumar, DC motor control using chopper, NIT Rourkela 2011.
[4] Bose B.K., Power electronics and motor drives recent technology advances,” Proceedings
of the IEEE International Symposium on Industrial Electronics,” IEEE, 2002, p 22-25.
[5] Saffet Ayasun, Gultekin Karbeyaz, DC motor speed control methods using
MATLAB/SIMULINK and their integration into undergraduate courses, 2008.
[6] Chinnaiyan V. Kumar, Jerome Joritha, Karpagam, J. S.Sheikh Mohammed, Design and
Implementation of High Power DC-DC converter and speed control of dc motor using TMS
320F240DSP, Proceedings of India International Conference on Power Electronics, 2006.
[7] Rashid, M.H., Power Electronics, Prentice Hall of India, New Delhi, 1993.
[8] Ogata, K., Modern Control Engineering. Englewood Cliffs, NJ: Prentice Hall, 2001.
[9] Dubey, G.K., Fundamentals of Electrical Drives. New Delhi, Narosa Publishing House,
2009
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