PROJECT.pdf

CHAPTER 1

1.1 PROJECT INTRODUCTION The proposed of this project is developed to control the speed of DC (direct current) motor. This project is also consisting of dc converter, power control circuit and also gate circuit. The input signal from AC (alternative current) supply will go through the transformer for stepping down the line voltage to level suitable for the control circuit at the dc converter and also for the dc motor itself the power control circuit will produced the smooth without ripple signal for dc motor input. The power control circuit include with thyristor. The thyristor (SCR) is line-commutated converter; therefore the firing angle must be delivered synchronously with the line voltage. The additional circuit called gate circuit is an external circuit that is needed to generate the firing angle

1.2 project objective The main objective of this project is to design and integrated the power control circuit. At the same time this power control circuit also will control the dc motor speed.

1.3 problem statement The main problem that can be stated here are, where we could not gate the smooth input signal without ripples for the speed of dc motor and at the same time it also could not control the speed range of the dc motor.

1.4 problem solving The input signal from ac supply is sinusoidal as in figure1.1. After the signal go through the dc converter the signal will be converted as in figure 1.2. If this signal go through the dc motor, so the speed range of dc motor will be constant. At the same time the speed of dc motor also cannot be controlled at all. 1

This situation is not good for the dc motor because it will damage the motor itself. The dc motor or any other motors need to be worm up first with a very slow speed before start with heavy load or speed. For solving this problem we need to add power control circuit to control the speed of dc motor and to produce a smooth input signal for dc motor. In this power control circuit there is a thyristor which will firing the angle of control circuit. As we know the speed of DC motor is consists of voltage, current and also angle. Thyristor is the only component that provides a controllable power output by “phase angle control”, so called because the firing angle (a point in time where the thyristor is triggered in to conduction) is synchronized with the phase rotation of the ac power source. If the device is triggered early in half cycle, maximum power is delivered the motor, late triggering in the half cycle provides minimum power. The effect is similar to a very high speed switch, capable of being turned on and “conducted “of at an infinite number of points with in each half cycle. The efficiency of this form of power control is extremely high since a very small amount of triggering energy can enable the thyristor to control a great deal of output power in dc motor. The final signal input at d c motor speed is as in figure 1.3. This power control circuit needs another additional external circuit called gate circuit to generate the firing angle at thyristor. This is because the thyristor itself with three leads, there are anode, cathode and gate. We need the gate circuit to be connected to the gate lead.

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1.5 SCOPE OF STUDY In this paper, it has been tried to control the speed of a single phase DC motor by using thyristor with the help of MATLAB simulation software. The scope is to study the DC motor characteristic curve at the load terminal.

1.6 SIGNIFICANCE SIGNIFICANCE The speed control of dc motor by using thyristor in general is very useful by preventing the previously mentioned problems of classical control techniques. The ultimate result of the study is to have variable speed, in terms of efficiency and serviceability and to increase or decrease the developed torque.

1.7 Outline of project This project consists four chapters. In first chapter, it discusses about the introduction, objective, problem statement, problem solving and scope of this project as long as summary of works. While Chapter 2 will discuss more on theory and literature reviews that have been done. It well discuss about introduction of dc motor, basics of dc motor , features, construction, principle operation, application of dc motor, dc motor control, basics of speed control . In Chapter 3, the discussion will be on the different speed control of dc motor methods using field control, armature control and SCR control and also discuss about matmatical model of dc motor and block diagram of dc motor speed control using thyristor. The result and discussion will be presented in Chapter 4. Last but not least, Chapter 5 discusses the conclusion of this project and future work that can be done.

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CHAPTER 2 2.0 LITERATURE REVIEW 2.1 REVIWE OF CONTROL SPEED MOTOR SYSTEM A.T.Alexandridis (1997) proposed on the speed control on nonlinear firing angel control scheme, which is suitable series and shunt connected dc motor drive system. The design procedure obtains an equivalent linear model of the system by exact state transformation and feedback rather than by linear approximations about particular set points. Therefore, large change of the operating points through the command input or of the external load can be applied the successfully. In case the simulation results verify an excellent performance of the dc motor speed control. M.Nedelikovic (2000) created a new approach to the design of feed forward fast current controller for SCR is described. This type of current control has excellent stability and the fastest possible limited only by the DC side inductor. As in any type of feed forward control, there is a problem of small inaccuracy in estimating circuit parameters and especially especia lly in this case , due to setting the maximum instant value of current instead of preferable average value . This problem can be solved with the outer control loop with the PI controller, which will generate a reference value for maximum instant value of current. For the sake of simplicity, in this paper, although it can easily be extended to rectifier with any number of phase. A new GTO due to converter is proposed by Khan el al. (1988).The two converters which Constitute the dual converter are always operated simultaneously permitting a free flow of current from the load of the converter. The getting pulse pattern is such that no circulating currents flow through the converters at any time.

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This disperses with the need for dc choke and avoids over-rating of the thyristor, two major drawbacks of circulating current dual converter, while maintaining good dynamic response and continuity of the load current.

2.2 SUMMRY OF REVIEW From the above authors reviews’ , there are a lot methods to control the speed of DC the motor ,such as dual mode mode inverter controller , microprocessor , SCR , PWM ,by inserting resistance in the armature and field windings .In those methods we can control the speed of the DC motor but the out puts is not balanced. As a conclusion MATLAB/SIMULINK realization of the speed control of the DC motor SCR is used to control the DC motor easily and it shows controlled wave form and the characteristics of speed-torque, current-torque, and power-torque

2.3 REVIEW OF SELECTED COMPONENTES The project proposed uses of DC motor and thyristor to control the speed of dc motor itself.

2.3.1 INTRODUTION OF DC MOTOR: An  electric motor converts electrical energy into mechanical energy. Most electric motors operate through interacting magnetic fields and current-carrying conductors to generate force. Motor Classification:  Direct Current Motors (DC)  Alternating Current Motors (AC)  Asynchronous Induction Motor (ACI)  permanent Magnet Synchronous Motor (PMSM)  Synchronous Brushless DC Motor (BLDC)

Electric motors are found in applications as diverse as industrial fans, blowers and pumps, machine tools, household appliances, power tools, and disk drives. They 6

may be powered by direct current (e.g., a battery powered portable device or motor vehicle), or by alternating current from a central electrical distribution grid.

Fig. 2.1 DC Motor

2.3.1.2 BASICS OF DC MOTOR: A DC motor is designed to run on DC electric power. The most common DC motor types are the brushed and brushless types, which use internal and external commutation respectively to periodically reverse the current in the rotor windings. The three basic types of DC motors are the series motor, the shunt motor, and the compound motor. 7

 The series motor is designed to move large loads with high starting torque

in applications such as a crane motor or lift hoist.  The shunt motor is designed slightly differently, since it is made for

applications such as pumping fluids, where constant-speed characteristics are important.  The compound motor is designed with some of the series motor's

characteristics and some of the shunt motor's characteristics. This allows the compound motor to be used in applications where high starting torque and controlled operating speed are both required.

2.3.1.3 FEATURES:  Internal commutation  Speed proportionate to voltage applied  Easy to control: ON/OFF, Proportional  Low Cost  Electrical sparks due to commutators  Produce significant EMI!  NOT suitable for deployment in combustible environments  Limited speed due to commutators  Mechanical noise due to commutators  Commutators limit life time.

2.3.1.4 CONSTRUCTION: CONSTRUCTION: The basic components of a DC motor include the armature assembly, which includes all rotating parts; the frame assembly, which houses the stationary stationar y field 8

coils; and the end plates, which provide bearings for the motor shaft and a mounting point for the brush rigging. The DC motor has two basic parts: the rotating part that is called the armature and the stationary part that includes coils of wire called the field coils. The stationary part is also called the stator. The armature is made of coils of wire wrapped around the core, and the core has an extended shaft that rotates on bearings .we can also notice that the ends of each coil of wire on the armature are terminated at one end of the armature. The termination points are called the commutator, and this is where the brushes make electrical contact to bring electrical current from the stationary part to the rotating part of the machine .The coils that are mounted inside the stator are called field coils and they may be connected in series or parallel with each other to create changes of torque in the motor. You will find the size of wire in these coils and the number of turns of wire in the coil will depend on the effect that is trying to be achieved.

2.3.1.5 PRINCIPLE OF OPERATION:

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Fig. 2.2. Principle of Operation A direct current (DC) motor is a fairly simple electric motor that uses electricity and a magnetic field to produce torque, which turns the motor. At its most simple, a DC motor requires two magnets of opposite polarity and an electric coil, which acts as an electromagnet. The repellent and attractive electromagnetic forces of the magnets provide the torque that causes the DC motor to turn.

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A DC motor requires at least one electromagnet. This electromagnet switches the current flow as the motor turns, changing its polarity to keep the motor running. The other magnet or magnets can either be permanent magnets or other electromagnets. Often, the electromagnet is located in the center of the motor and turns within the permanent magnets, but this arrangement is not necessary. Electrical current is supplied to the coils of wire on the wheel within the DC motor. This electrical current causes a magnetic magnetic force. To make the DC motor turn, the wheel must have be negatively charged on the side with the negative permanent magnet and positively charged on the side with the permanent positive magnet. Because like charges repel and opposite charges attract, the wheel will turn so that its negative side rolls around to the right, where the positive permanent magnet magne t is, and the wheel's whee l's positive side s ide will roll ro ll to the left, where the negative permanent magnet is. The magnetic force causes the wheel to turn, and this motion can be used to do work. When the sides of the wheel reach the place of strongest attraction, the electric current is switched, switche d, making the wheel change polarity. The side that was positive becomes negative, and the side that was negative becomes positive. The magnetic forces are out of alignment again, and the wheel keeps rotating. As the DC motor spins, it continually changes the flow of electricity to the inner wheel, so the magnetic forces continue to cause the wheel to rotate.

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2.3.1.6 APPLICATIONS OF A DC MOTOR:  Low power DC motors are particularly useful as speed changes from 0 to 1

are almost instantaneous. Therefore, they can be used successfully in digital systems.  A low speed, low power brushless DC motor can be found in most

turntable devices, particularly precision turntable devices. Devices with such motors are things like personal computers, CD and DVD players.  Arguably the most common and useful motors are High Power DC Motors.

These motors are generally used in open systems, and generally used in systems were torque and power, as well as drive are paramount. Examples of such systems include electric wheelchairs, electric scooters, Segway, hybrid cars, as well as in elevators.  Kitchen appliances Power tools  Door locks (example: train doors)  Automotive applications: windows lift, seat adjust, antenna retractor

2.4 DC MOTOR CONTROL The function of a dc motor which must be controllable for practical use are the speed, the torque delivered, and direction of rotation. Speed is proportional to armature back-emf and inversely proportional to field flux. Torque is proportional to armature current and field flux. Direction of rotation is simply a matter of the relative polarities of the armature and field voltages. It follows that it is necessary to control. The armature voltage; back-emf is component of armature voltage. Thus, assuming the field to be constant, control of armature voltage provides complete control of speed up to the point where the voltage reaches the maximum value of which the armature is designed. Armature current is also a function of armature voltage, so that with in the speed range up to maximum voltage, torque is controlled by voltage also, provided that the field is fully – excited, the availability of maximum torque is normally maintained from zero speed up to armature voltage maximum (base speed).

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2.4.1 Basics of speed s peed control In order to investigate the modern speed control techniques for motors, we should first review what determines the speed of a motor- load system. Let as consider the situation in figure 2.1(a) and (b).it may be noted that two devices are involved: the motor, which produces a torque Tdev, and the load, producing acounter torque Tm.in case the motor has significant rotational loss, we shall combine these with the load for sake of simplicity. Using Newton’s second law, we therefore have Tdev = J.dw/dt Where, Tdev = Electromagnetic Torque developed by the motor, in NM. Tm = Mechanical torque required by the load, plus motor rotational loss, NM. J = Moment of inertia of all rotating parts in kg.m2. W = shaft speed in radian/sec.

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FIG 2.3 motor load system and torque speed characteristics 14

CHAPTER 3 SPEED CONTROLOF DC MOTOR 3.0 INTRODUCTION Motors come in a variety of forms, and the speed controller’s motor drive output will be different dependent on these forms. Direct current (DC) motors have been widely used in many industrial applications such as electric vehicles, steel rolling mills, electric cranes and robotic manipulators due to precise, wide, simple and continuous control characteristics. Traditionally rheostatic armature control method was widely used for the speed control of low power dc motors. However the controllability, cheapness, higher efficiency and higher current carrying capabilities of static power converters brought a major change in the performance of electrical drives. The desired torque – speed characteristic could be achieved by the use of conventional Thyristor controlled methods. The thyristor dc drive remains an important speed-controlled industrial drive, especially where the higher maintenance cost associated with the dc motor brushes is tolerable. The controlled (thyristor) rectifier provides a low impedance adjustable dc voltage for the motor armature, thereby providing speed control. In all variable speed drive systems, power electronic converter acts as an interface which accepts electric power from the existing source and converts it in a controlled manner into a suitable form compatible with the particular load or the process for which it is employed. The main sources for electric power are: 1. Single or three phase 50Hz ac from utility system. 2. Dc from storage batteries or solar cells. 15

The modern converters are compact, cheap, reliable, flexible and completely controllable. They need reduced maintenance too. For dc motor control, controlled dc power from a constant ac supply is obtained by means of controlled rectifiers (converters) using thyristors and diodes. The control of dc voltage is achieved by varying the phase angle at which the thyristors are fired relative to the applied alternating waveform. This scheme of phase control is known as ‘phase control’. In another control system, known known as ‘integral ‘integral cycle control’, the current is gated to flow from the ac supply for a number of complete cycles and is then quenched further for a few cycles, the process being repeated continuously. Control is applied by adjusting the ratio of on and off durations. This method is suitable for the control of fractional kW output dc motors. Phase controlled converters are simple to operate and less expensive as they do not require additional circuitry for commutation process. In such converters natural commutation is achieved, i.e., when an incoming thyristor is turned on, it immediately reverse biases the outgoing thyristor and turns it off. Thereby obtaining control in both half cycles of ac mains. This rectified voltage is fed to the armature of dc motor. Hence speed of the motor can be controlled in proportion with the voltage.

3.1 Different speed control methods of DC motor Dc machines are generally much more adaptable to adjustable speed service. The ready availability of dc motors to adjustment of their operating speed over wide ranges and by a variety of methods is one of the important reasons for the strong competitive position of dc machinery in modern industrial applications.The speed of a dc motor can be expressed by the following relationship. N α (V-IaRa)/ф, 

where

N- Speed of motor Ia- Armature current

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Ra- Armature resistance Ф- Flux per pole

V- Applied voltage It is obvious that the speed can be controlled by varying •Flux per pole •Resistance of armature circuit and •Applied voltage In other words by •Field control and •Armature control

3.1.1 Field control method: It is the most common method and forms one of the outstanding advantages of shunt motors.The method is also applicable to compound motors. Adjustment of field current and hence the flux and speed by adjustment of the shunt field circuit resistance or with a solid state control when the field is separately excited is accomplished simply, inexpensively and without much change in motor losses.The speed is inversely proportional to the field current.The lowest speed obtainable is that corresponding to maximum field current. The highest speed is limited electrically by the effects of armature reaction under weak field conditions in causing motor instability and poor commutation.

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Merits: •Good working efficiency. • Compact controlling equipment • The speed is not effected by load and speed control can be performed effectively even at light loads. •Relatively inexpensive and simple to accomplish both manually and automatically.

Demerits: •Inability to obtain prefer below the basic speed. •Instability at high speeds because of armature reaction. •Commutation difficulties and possible commutator damage at high speeds.

3.1.2 ARMATURE CONTROL METHODS 3.1.2.1Rheostatic 3.1.2.1Rheostatic control: This method consists of obtaining reduced speeds by the insertion of external series resistance in the armature circuit. It can be used with series, shunt and compound motors.It is common method of speed control for series motors.This method is used when speeds below the no load speed is required. Merits: • The ability to achieve speeds below the basic speed. • Simplicity and ease of connection. •The possibility of combining the functions of motor starting with speed control.

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Demerits: •The relatively high cost of large, convinously rated, variable resistors capable of dissipating large amounts of power. • Poor speed regulation for any given no load speed setting. •Low efficiency resulting in high operating cost. •Difficulty in obtaining stepless control of speed in higher power ratings.

3.1.2.2 Voltage control: When the speed is controlled by regulating the motor terminal voltage while maintaining constant field current, it is called voltage control. With voltage control, the change in speed is almost proportional to the change in voltage. The output varies directly with speed and the torque remains constant. Since the voltage has to be regulated without affecting the field, the application of voltage control is limited to separately excited motors.

Merits: •Speed control over a wide range is possible. •This method eliminates the need for series armature starting resistance. •Uniform acceleration can be obtained. •Speed regulation is good.

Demerits: •Arrangement is costly as two extra machines are required. •The overall efficiency of the system is low, especially at light loads.

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3.2 SCR POWER CONTROLLERS Since the development of SCR power controllers in the late 1950's, the power handling capabilities of SCR's (silicon controlled rectifiers) have advanced from a few hundred watts to many megawatts. As a result, the use of SCR power controllers in industrial applications has increased dramatically and they are now used in almost every major industry.SCR power controllers provide a relatively economical means of power control. SCR power controllers cost less and are more efficient than saturable core reactors and variable transformers. Compared to contactors, SCR power controllers offer a much finer degree of control and do not suffer from the maintenance problems of mechanical devices.

3.2.1Features and benefits of SCR power controllers over other forms of control include: •High reliability: Because the SCR power controller is a solid-state device, there are no inherent wear-out modes. Thus, they provide virtually limitless and trouble free operation.

•Infinite resolution: Power, current or voltage can be controlled from zero to 100% with infinite resolution. This capability allows extremely accurate, step less control of the process.

•Extremely fast response: The SCR controller can switch load power on and off extremely fast providing the means to respond rapidly to command changes, load changes and power supply changes. This feature allows the control of fast responding loads and eliminates the negative effects of variations in load or supply voltages that can occur with other types of control 20

•Selectable control parameters: The SCR power controller can control the average load voltage, the RMS value of the load voltage, the RMS or the average load current or load power. It can also provide useful features such as current and voltage limiting. The ability to control the desired parameter as a function of a command signal and to incorporate limiting features is not normally available with other types of control.

•Minimum Maintenance: Because they are solid state there are no moving parts to wear out or replace. Therefore, the routine replacement required in some forms of control is eliminated.

3.3 Model of Separately Excited DC motor Figure 3.1 shows a model of separately excited DC motor. When a separately excited motor is excited by a field current of I f and an armature current of Ia flows in the circuit, the motor develops a back EMF and a torque to balance the load torque at a particular speed. The If is independent of the Ia. Each winding are supplied separately. Any change in the armature current has has no effect on the field current. The If is normally normally much less than the Ia. The relationship of the field and armature are shown in Equation 3.1.

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Figure 3.1 Model of separately excited DC motor Instantaneous field current:

……………………..3.1 Where Rf and Lf are the field resistor and inductor respectively. Instantaneous armature current:

…………………..3.2 Where Ra and La are the armature resistor and inductor respectively. The motor back EMF which is also known as speed voltage is expressed as …………….3.3 Where Kv is the motor constant (in V/A-rad/s) and w is the motor speed (rad/s). The torque developed by the motor is …………….3.4 Where (K t =Kv) is the torque constant (in V/A-rad/s).

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Sometimes it is written as: ………………….3.5 For normal operation, the developed torque must be equal to the load torque plus the friction and inertia, i.e.:

………………..3.6

Where B = viscous friction constant (N.m/rad/s) T L = load torque (N.m) J = inertia of the motor (kg.m^ 2) Under steady-state operations, a time time derivative is zero. Assuming the motor is not saturated.

For field circuit, …………3.7 The back EMF is given by: …………….3.8 The armature circuit, ………….3.9 The motor speed can be easily derived:

……………………..3.10 23

If Ra is a small value (which is usual), or when the motor is lightly loaded, i.e. Ia is small,

…………………3.11 That is if the field current is kept constant, the speed motor speed depends on the supply voltage. These observation leads to the application of variable DC voltage to control the speed and torque of DC motor.

3.4 DC motor Speed Controller For precise speed control of servo system, closed-loop control is normally used. Basically, the block diagram and the flow chart of the speed control are shown in Figure 3.2 and Figure 3.3 respectively. respectivel y. The speed, which is sensed by analog sensing devices (e.g., tachometer) is compared with the reference speed to generate the error signal and to vary the armature voltage of the motor.

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Fig 3.2 Basic block diagram for DC Motor speed control

Fig 3.3 Basic flow chart of DC motor speed control

There are several controllers that can used to control the speed of the motor such as by using thyristor, phase-locked-loop control, chopper circuit, Fuzzy Logic Controller and etc. Here, we will discuss only at the speed control system by using thyristor, phase-locked loop and PWM technique.

3.4.1 Speed Control by using thyristor Figure 3.4 shows the block diagram of DC motor speed control by using thyristor. The thyristor is used to supply a variable DC voltage to motor, thus it can control the speed of motor (Equation 3.11). The average output of voltage is given by

…………………..3.12 25

Where Vm = peak voltage of voltage supply of thyristor and  thyristor α = firing angle of  thyristor

Figure 3.4 Block diagram of DC Motor speed control by using thyristor From Equation 3.12 , by controlling the firing angle, α, the average of output DC voltage can be varied. If the motor speed is low, the speed sensor frequency will be below the reference frequency. The frequency difference produces a change in the firing circuit that causes the thyristor, SCR to fire sooner (firing angle, α is

reduced). There is a resulting increase in motor speed which brings the output speed back up to the value which is equal to the reference signal. Conversely, if the speed sensor output frequency is above the reference, then the firing circuit will be modified to allow the SCR to conduct for a shorter period of time, the decrease in conduction reduces the DC motor speed.

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CHAPTER FOUR SIMULATION AND RESULTS 4.0

INTRODUCTION

SimPowerSystems software and other products of the Physical Modeling product family work together with Simulink software to model electrical, mechanical, and control systems. SimPowerSystems software operates in the Simulink environment. Therefore, before starting this user's guide, make yourself familiar with Simulink documentation. Or, if you perform signal processing and communications tasks (as opposed to control system design tasks), see the Signal Processing Block set documentation.

The Role of Simulation in Design Electrical power systems are combinations of electrical circuits and electromechanical devices like motors and generators. Engineers working in this discipline are constantly improving the performance of the systems. Requirements for drastically increased efficiency have forced power system designers to use power electronic devices and sophisticated control system concepts that tax traditional analysis tools and techniques. Further complicating the analyst's role is the fact that the system is often so nonlinear that the only way to understand it is through simulation. SimPowerSystems software is a modern design tool that allows scientists and engineers to rapidly and easily build models that simulate power systems. It uses the Simulink environment, allowing you to build a model using simple click and drag procedures. Not only can you draw the circuit topology rapidly, but your analysis of the circuit can include its interactions with mechanical, thermal,

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control, and other disciplines. This is possible because all the electrical parts of the simulation interact with the extensive Simulink modeling library. Since Simulink uses the MATLAB computational engine, designers can also use MATLAB toolboxes and Simulink block sets. SimPowerSystems software belongs to the Physical Modeling product family and uses similar block and connection line interface.

SimPowerSystem SimPowerSystem Libraries SimPowerSystems libraries contain models of typical power equipment such as transformers, lines, machines, and power electronics. The capabilities of SimPowerSystems software for modeling a typical electrical system are illustrated in demonstration files. And for users who want to refresh their knowledge of power system theory, there are also self-learning case studies. The SimPowerSystems main library, powerlib, organizes its blocks into libraries according to their behavior. The powerlib library window displays the block library icons and names. Double-click a library icon to open the library and access the blocks. The main powerlib library window also contains the Powergui block that opens a graphical user interface for the steady-state analysis of electrical circuits.

Nonlinear Simulink Blocks for SimPowerSystems Models The nonlinear Simulink blocks of the powerlib library are stored in a special block library named powerlib_models. These masked Simulink models are used by SimPowerSystems software to build the equivalent Simulink model of your circuit.

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fig 4.1 connection diagram

4.1 DC Machine Description

The DC Machine block implements a wound-field or permanent magnet DC machine. For the wound-field DC machine, an access is provided to the field terminals (F+, F−) so that the machine model can be used as a shunt -connected or a series-connected DC machine. The torque applied to the shaft is provided at the Simulink input TL.

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The armature circuit (A+, A −) consists of an inductor La and resistor Ra in series with a counter-electromotive force (CEMF) E.The CEMF is proportional to the machine speed.

KE is the voltage constant and ω is the machine speed. In a separately excited

DC machine model, the voltage constant KE is proportional to the field current If

Where Laf is the field-armature mutual inductance. The electromechanical torque developed by the DC machine is proportional to the armature current Ia. Where Kt is the torque constant. The sign convention for Te and TL is

The torque constant is equal to the voltage constant.

The armature circuit is connected between the A+ and A − ports of the DC Machine block. It is represented by a series Ra La branch in series with a Controlled Voltage Source and a Current Measurement block. Mechanical part: In the wound-field DC machine model, the field circuit is represented by an RL circuit. It is connected between the F+ and F − ports of the DC Machine block. In the permanent magnet DC machine model, there is no field current as the excitation flux is established by the magnets. KE and KT are constants. The mechanical part computes the speed of the DC machine from the net torque applied to the rotor. The speed is used to implement the CEMF voltage E of the armature circuit.

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The mechanical part is represented by Simulink blocks that implement the equation

Where J = inertia, Bm = viscous friction coefficient, and Tf = Coulomb friction torque. Preset model Provides a set of predetermined electrical and mechanical parameters for various DC machine ratings of power (HP), DC voltage (V), rated speed (rpm), and field voltage (V).The preset models are only available for the wound-field DC machine model, that is, when the Field type parameter is set to Wound. Select one of the preset models to load the corresponding electrical and mechanical parameters in the entries of the dialog box. Select No if you do not want to use a preset model, or if you want to modify some of the parameters of a preset model, as described below. When you select a preset model, the electrical and mechanical parameters in the Parameters tab of the dialog box become unmodifiable (grayed out). To start from a given preset model and then modify machine parameters, you have to do the following:

Select the desired preset model to initialize the parameters. Change the Preset model parameter value to No. This will not change the machine parameters. By doing so, you just break the connection with the particular preset model. Modify the machine parameters as you wish, then click Apply.

Mechanical input Allows you to select either the torque applied to the shaft or the rotor speed as the Simulink signal applied to the block's input. Select Torque TL to specify a 31

torque input, in N.m, and change labeling of the block's input to TL. The machine speed is determined by the machine Inertia J and by the difference between the applied mechanical load torque TL and the internal electromagnetic torque Te. The sign convention for the mechanical torque is the following: when the speed is positive, a positive torque signal indicates motor mode and a negative signal indicates generator mode. Select Speed w to specify a speed input, in rad/s, and change labeling of the block's input to w. The machine speed is imposed and the mechanical part of the model (Inertia J) is ignored. Using the speed as the mechanical input allows modeling a mechanical coupling between two machines and interfacing with SimMechanics and SimDriveline models. Field type Allows you to select between the wound-field and the permanent magnet DC machine. Armature resistance and inductance [Ra La] The armature resistance Ra, in ohms, and the armature inductance La, in henries. Field resistance and inductance [Rf Lf] The field resistance Rf, in ohms, and the field inductance Lf, in henries. This parameter is only visible when the Field type parameter on the Configuration tab is set to Wound.

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Field armature mutual inductance Laf The field armature mutual inductance, in henries. This parameter is only visible when the Field type parameter on the Configuration tab is set to Wound. For a permanent magnet DC machine, select the machine constant that you want to specify for block parameterization. The values are Torque constant and Back-emf constant. This parameter is only visible when the Field type parameter on the Configuration tab is set to Permanent magnet. Torque constant The torque per current constant of the permanent magnet DC machine, in N.m/A. This parameter is only visible when the Field type parameter on the Configuration tab is set to Permanent magnet and the Specify parameter above is set to Torque constant. Back-emf constant The voltage per speed constant of the permanent magnet DC machine, in V/rpm. This parameter is only visible when the Field type parameter on the Configuration tab is set to Permanent magnet and the Specify parameter above is set to Back-emf constant. Total inertia J The total inertia of the DC machine, in kg.m2. Viscous friction coefficient Bm The total friction coefficient of the DC machine, in N.m.s. Coulomb friction torque Tf The total Coulomb friction torque constant of the DC machine, in N.m.

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Initial speed Specifies an initial speed for the DC machine, in rad/s, in order to start the simulation with a specific initial speed. To start the simulation in steady state, the initial value of the input torque signal TL must be proportional to the initial speed. Sample time (−1 for inherited) Specifies the sample time used by the block. To inherit the sample time specified in the Powergui block, set this parameter to −1.

Inputs and Outputs TL (Load Torque) The block input is the mechanical load torque, in N.m. W (speed) The alternative block input (depending on the value of the Mechanical input parameter) is the machine speed, in rad/s. M (output) The output of the block is a vector containing four signals. We can demultiplex these signals by using the Bus Selector block provided in the Simulink library.

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This section presents simulation results for the speed control DC motor. The torque-speed curves for the speed control methods are determined using the Simulink models presented in the previous section. For this purpose, a 5-Horse Power (HP) DC motor of 240 V rating 1220 r/min is used in the simulation models. The equivalent circuit parameters of the motor are: Rf=240 Ohm,Lf=120H,Ra=0.6Ohm.

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4.2 DC Motor Rated Values WITH OUT RECTIFRE Induced EMF: Eo =va-Ia*Ra = 240-16.2*0.6 = 230.3 V Pe =Eo*Ia = 230.3*16.2 = 3731 W = 5.0 HP Field current: If = 240/240 =1 A Eo = w*Laf*If from this we can find speed as w = (Eo/La*If) speed w = 230.3/1.8 = 127.7 rad/s = 1220 r/min Nominal torque: Te = Pe/w = 3731/127.7 = 29.2 N.m\ CALCULATION OF MOTOR CHARACTERSTICS BY USING RECTIFIRE From the rectifier out put

For alpha=0 Vav=(2*2^0.5*240)*cos 0/3.14 =216.18v

Induced EMF: Eo =va-Ia*Ra = 216.1-16.2*0.6 = 206.46 V Eo = w*Laf*If from this we can can find speed as

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w = (Eo/La*If) speed w = 206.46/1.8 = 114.7 rad/s = 1095.330 r/min For alpha=30 Vav=(2*2^0.5*240)*cos 30/3.14 =187.12v

Induced EMF: Eo =va-Ia*Ra = 187.12-16.2*0.6 = 177.407V Eo = w*Laf*If from this we can can find speed as w = (Eo/La*If) speed w = 177.407/1.8 = 98.55rad/s = 941.17r/min For alpha=60

Vav=(2*2^0.5*240)*cos 60/3.14 =108.03v

Induced EMF: Eo =va-Ia*Ra = 108.03-16.2*0.6 = 98.3179v Eo = w*Laf*If from this we can find speed as w = (Eo/La*If) speed w = 8.317/1.8 = 54.62rad/s = 521.592r/min For the armature voltage control, simulations are performed using the model shown in Figure 4.1 for three different armature voltages, Va=108.03, 187.127, 216.075and 240V while the voltage applied to the field circuit is kept constant at its nominal value 240 V. From the calculation part shown we can find found the wave forms fordefernt parameters of DC motor as shown below 37

Output voltage wave form of thyristor from 240v, 50H frequency

Output speed wave form the dc motor for higher voltage and low voltage respectively

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Output torque (Te) wave form the dc motor for higher voltage and low voltage respectively

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We thus conclude that speed variation can be adjusted by the method of speed control using thyristors.

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CHAPTER FIVE CONCLUSION AND RECOMMENDATION 5.1Recomendation To achieve the desired speed of dc motor, it is necessary to control the output voltage voltage of the Thyristor Bridge. Bridge. To do this, it is necessary to design control circuit and pulse generator for triggering the Thyristor Bridge and it requires carful understanding.

5.2CONCLUSION Motor speed should be independent of load. Differential drive platform need to synchronize wheel speed to go in a straight line. Controllability, cheapness, higher efficiency, current carrying capabilities of static power converters brought a major change to performance of electrical drives .Excellent control of DC motor drive is designed using rectifier circuit for for formative the optimal optimal speed control. Armature voltage of dc motor is varied and speed is controlled. If the current is limited however the t he torque must also be limited, at the value coincident with the limited current. The effect that this has on the torque speed must also be considered. As the load torque increases the speed drops .The motor torque always equals the load torque when the motor is running at constant speed (this follows from Newton’s first law-“An object in motion tends to stay in motion in the same speed and in same direction unless acted upon by unbalanced force”. The motor torque and load torque must be balanced out if the speed is not changed).Universal motor generally run at high speed making them useful for appliances where high RPM operation is desirable. We successfully controlled speed of DC motor using Thyristor Bridge in useful range to increase efficiency and deduce losses.

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REFERENCE 1) SIMULINK, Model-based and system-based design, using Simulink, MathWorks Inc., Natick, MA, 2000. 2) SimPowerSystems for use with Simulink, user’s guide, MathWorks Inc., Natick, MA, 2002 4) P.C. Sen, principles of Electrical Machines and power Electronics, John Wiley & Sons. New York, 1989

5) www.electronics-tutorials.ws/transistor/tran_7.html 6) M. H. Rashid, Power Electronics: Circuits, Devices, and Applications Prentice Hall, New Jersey, 1988.

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Appendix Dc =direct current AC =alternating current SCR =silicon control rectifier GTO = gate transistor on PWM = pulse width modulation EMI = electromagnetic interface RPM = revolution per minute α = firing angle of thyristor thyristor

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