Final year project

Analysis and simulation of a converter fed dc motor drive by using MATLAB

Introduction

•Converter-fed dc motor drives are extensively used in special heavy duty application

•The ac-dc converters also known as controlled rectifiers are generally used for the speed control of dc motors

•The speed of dc motors changes due to the changes of load torque

•When the torque is increased, the speed of the motor is decreases due to the voltage drop in the armature resistance

•To maintain the constant speed of the motor, the armature voltage should be varied continuously by varying the alpha angle of a ac-dc converters.

•Thus, the controller must be added to control the alpha angle

Objectives

•To develop the transfer function of the dc motor with and without feedback

•To design controller with a speed feedback (closed loop feedback) using PI controller and simulate the drive under different loading and reference speed.

•To conduct experiments to verify results with the simulated responses.

Methodology•Literature Survey – References from library and IEEE

website

- Through guidance from lecturers

•Simulation – Using MATLAB M-file and SimPowerSystems

KEg fI,where

aaga IREV

(1)

(2)

(3)

Substituting all equations above;

2)(

K

TR

K

V

K

RIV

K

Eeaaaaag

lmae TBdt

dJIKT

(4)

Transfer Function

Using Laplace transform;

)())((

)())((

)(22

sTKBsJsLR

sLRsV

KBsJsLR

Ks l

maa

aaa

maa

Then, the transfer function becomes;

)()()(

)()(

22

0)(

1 KBRBLRJsLJs

K

sV

ssG

aaamamsTal

)()()(

)()(

22

0)(

2 KBRBLRJsLJs

RsL

sT

ssG

aaamam

aa

sVla

Transfer functions of:

•Proportional Integral Controller (PI)

s

KsKsD

s

KKsEsU

dtteKteKtu

ip

ip

t

ip

)(

)()(

)()()(0

)(sE )(sU

s

KsK ip

Block Diagram of dc motor

Block diagram of converter

Block Diagram of a converter-fed dc motor

Torque – speed characteristic

The graph shown below is the effect of armature voltage speed control on a separately excited dc motor’s torque-speed characteristic.

Speed ω

Torque Τ

Va1

Va2

Va3

Dc motor with speed-feedback controller

ω

Effect of change the load torque due to the speed of the motor

Time

Change the load torque

Project ResearchUsing Matlab M-File Design requirement with 1 rad/sec step

reference, settling time less than 3 sec and steady-state error less than 2 %.

Using Simulink (Simpowersystem) Block diagram of dc motor drive with and without

feedback with 100 rad/sec speed reference. Simulation of converter fed dc motor under

different loading and speed references.

MOTOR RATINGS FOR SIMULATION USING M-FILE AND

BLOCK DIAGRAM TRANSFER FUNCTION

240 V, 8.3 A, 60 Hz, 5hp Control input voltage = ±10VMaximum current permitted in the motor is 20A.Moment of Inertia (J) = 0.0607 (kg-m2)Machine frictional torque coefficient (Bt) = 0.0869 (N.m/rad/sec)Armature resistance (Ra) = 4 ΩArmature Inductance (La) = 0.072 HKb = 1.26 V/rad/sec

Simulation Using M-File

•Open loop transfer function of dc motorSettling time>3 secSpeed<1rad/sec

Proportional Integral (PI) Controller with Kp=100 and Ki=150

Settling time<3secSpeed=1 rad/sec

Simulation using Simulink

•Block diagram without feedback

•Block diagram of speed controller dc motor drive

Simulation results

•Without feedbackSettling time>3secSpeed>100 rad/sec

•With feedback (speed and current controller)

Settling time<3 secSpeed= 100rad/sec

DC Motor ratings

Simulation using SimPowerSystems

•Without feedback

Result

•With feedback

Results

•Constant speed and torque with Kp=2 and Ki=20

•Constant speed with changing load torque (Kp=2 and Ki=20)

Step speed reference with constant torque (Kp=2and ki-20)

Step speed reference with changing load torque (kp=2 and ki=20)

Comparison results of using different value of Kp and ki.

Overshoot

Settling time Steady state error

Kp=2

Ki=20

6% 0.385sec 10%

Kp=30

Ki=50

5% 0.175sec 2%

Kp=100

Ki=150

5% oscillates 1.9%

Goodcontroller

Experimental results

PI-controller parameters

Speed (ω) without load (rpm)

Speed (ω) with load (rpm)

Kp=100,Ki=200 1506 1501

Kp=100, Ki=20 1517 1520

Kp=5,Ki=10 1509 1511

Kp=1, Ki=5 1498 1502

The speed with and without load when using PI controller

Conclusion

•A Proportional plus Integral (PI) controller is design to reduce the system errors and improve the dynamics responses.

•The constant KP and KI in PI controller can be changed to meet the acceptable performance.

•This project compares a various study in designing the converter-fed dc motor drive with and without system feedback and simulates drive under different loading and speed references.

•By increasing the constant KP and KI tends to reduce the systems errors and improve the overshoot and settling time. However, large KP and KI will worsen the transients’ stability.

•A good controller will change back the motor speed to the normal value due to the change of load torque. The proposed converter is designed and tested in the lab using the separately excited dc motor.

•The experimental results are shown to be in good agreement with the simulated results.

Future Research

•More extensive follow-up studies can be done so that the proposed approach is always up-to-date.

•Further fine tuning of the speed controller. Example, design the Proportional Integral and Derivative (PID) controller in order to enhance and improved the transients stability. The automatic tuning is also can be done by using the Non-linear Control Design (NCD) in Simulink Blockset.

•Further research in designing the four quadrant three phase rectifier in SimPowerSystems simulation.

References

• Arthur G.O. Mutambara. 1999. Design and analysis of control system.CRC Press.

• A.T. Alexandridis and D.P. Iracleous, “Optimal Nonlinear Firing Angle Control Of A Converter-fed Dc Drive Systems,” IEE Proc-Electr, Power Appl., Vol 145, No.3, May 1998.

• B.H.Khan, Seshagiri R.Doradla and Gopal K.Dubey, “A Three-Phase Ac-dc GTO Thyristor Converter Employing Equal Pulse - Width Modulation (EPWM),”IEEE Transactions on Industry Application, Vol 27, No.2, March/April 1991.

• Charles L. Phillips and Royce D. Harbor. 2000. Control Systems. 4th Edition. Prentice Hall.

• “Dc Machine” http://www.mathworks.com

• “Digital Dc Motor Speed Control with PID control” http://www.engin.umich.edu

•Gene F. Franklin, J. David Powell, and Abbas Emami-Naeini. 1994. Feedback Control of Dynamic Systems. 3rd Edition. Addison-Wesley.

•Katsuhiko Ogata. 2002. Modern Control engineering. 4th Edition. Prentice Hall.

•M Gopal. 2002. Control Systems-principles and design. 3rd Edition. McGraw Hill.

•M Ramamoorty. 1991. An Introduction to Thyristors and their applications. 2nd Edition. East-West Press.

•Muhammad H. Rashid. 2004. Power Electronics-circuits, devices, and applications. 3rd Edition. Prentice Hall.

•Paul C.Krause, Okg Wasynczuk and Scott D. Sudhoff. 2002. Analysis of electric Machinery & Drives Systems. 2nd Edition. IEEE Press and Wiley Interscience.

•Stephen J. Chapman. 2005. Electric Machinery Fundamentals. 4th Edition. McGraw Hill.

•Theodore Wildi. 2006. Electrical Machines, Drives, and Power Systems. 6th Edition. Prentice Hall.

•W. Shepherd, L.N. Hulley, and D.T.W. liang. 2002. Power Electronics and motor control. 2nd Edition. Cambridge University Press.