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Chapter 1INTRODUCTION TO PLC

1.1 Function of a PLC A PLC is a microprocessor-based controller with multiple inputs and outputs. It uses a programmable memory to store instructions and carry out functions to control machines and processes. The PLC performs the logic functions of relays, timers, counters and sequencers. It has the following advantages: Low cost Reliability Reprogram ability Program PLC Inputs Outputs

Fig 1: A programmable logic controller 1.2 Inputs and Outputs The PLC inputs give it information about the machine or process that it is controlling. These are typically switches and sensors. The switches are connected to an input module that provides the interface between the switches or sensors and the PLC. Input module circuits have opto-isolators to protect the internal PLC circuitry from damage.

LED Photo transistor

Fig 2: An Opto-Isolator The PLC outputs are connected directly or indirectly (e.g. through a relay) to actuator controls. Examples include solenoids on directional control valves, motors, motor contactors, alarms and warning lights. There are three main types of output module:

Relay (volt-free): The signal from the PLC operates a relay within the output module connecting the control voltage to the output port and hence to the actuator.

Internal relay Contact Control Voltage (+)

Common PortOutput Port

Solenoid Fig 3 PLC Relay Output Transistor: A transistor is used to switch on the output. This is faster than a relay output but is only suitable for low power direct current applications. Triac: This solid state device is used for switching alternating current devices. It requires some form of over current protection.

1.3 History of PLCThe Programmable Logic Controller (PLC) was conceived by Dick Morley on January 1, 1968. Mr. Morley's PLC, which at the time was called a PC for Programmable Controller and later, after the introduction of the Personal Computer (PC) changed the name to PLC, was produced by his company called Modicon. Modicon stands for MOdularDigitalCONtroller.The first PLC was designed to meet the specifications layed out by the Hydramatics Division of General Motors (GM). The primary goal of the design was to eliminate the extremely high cost involved with electromechancial relay switching circuits and the lack of flexiblity associated with these 'hardwired' circuits. Anyone who is old enough to remember American cars during the 1950's and 1960's knows that the styles and options of the cars during this era changed with every model year. These changes meant that every year the car manufacturers had to retool their factories and make major modifications to the automated assembly lines. These changes required weeks of work by teams of engineers, technicians and electricians to rewire the relay control logic and possibly weeks more to troubleshoot and eliminate wiring errors; a very costly venture.The specifications that GM layed out were:1. The controller had to be solid-state.2. The system had to have the flexibility of a computer system.3. Controllers had to be capable of surviving an industrial environment.4. Programming had to be easy and be able to be maintained by plant electricians.5. The system had to be reusable.6. Pricing had to be competitive with relay control systems.7. Input and Output (I/O) interfaces had to be easily replaceable.8. A modular design had to be used so that subassemblies could be easily removed, replaced and repaired.9. Manufacturing data needs to be collected and passed on to a central computing system.10. The method of programming should be in the form of relay ladder logic, a method that plant electricians, engineers and technicians were already familiar with.

The first PLC was installed at General Motors Oldsmobile plant and at the Lanids Company in Landis, Pennsylvania in 1970.

The initial intent of the PLC was to be a reusable, reprogrammable relay replacer. In order for the PLC to be accepted into industry it had to be marketed as a new form of mechanical relay, timer and counter. Industry is always concerned with reliability and had the PLC been marketed as a computer industry would have been very reluctant to incorporate them. Presently, the PLC industry is and has been attempting to move away from relay ladder logic programming, but it is finding it difficult to do so as a result of this early marketing strategy.

Since the first PLC was installed in the early '70s they have become much more than simply a relay replacer. Applications for PLCs has extended to a very broad range of manufacturers of all types of products. Users are demanding more features to be added such as: Subroutines being added to the language. Perform complex math functions. include data handling capabilities Programmers should be able to program and handle interrupts. analog I/O needs to be accepted, processed and output. The Proportional Integral Derivative (PID) function needs to be included for process control. Control should be distributed. Communications should be capable between other PLCs, computers and devices on a network.One problem with adding these new functions and features is that PLC manufacturers have always had proprietary designs with proprietary networks making it next to impossible to communicate with equipment manufactured by various vendors and because the networks were proprietary, PLCs from different vendors could not talk to each other.

The 1980's saw a shift away from the proprietary systems and PLCs, PCs and other equipment started to be capable of working together as a single unit. GM was again a leader in standardization and developed a communications standard called Manufacturing Automation Protocol (MAP). The industry was also reducing the size of PLCs and today some of them are the size of a cigarette pack.A continuation toward standardization was seen in the 1990s when the International Electrical Commission standard IEC 1131-3 was established to standardize the programming languages. The standard was later changed to IEC 61131. Also during this decade sub-networks such as ControlNet, DeviceNet and Foundation Fieldbus were introduced that further extended the distributive control features and allowed for different vendors to coexist on the same sub-network.It is difficult to predict the future of PLCs, but with the advent of Programmable Automation Controllers (PACs) in the 21st century it is easy to imagine that PLC/PACs will be approaching PCs in short order. PACs can be programmed using a variety of languages from relay ladder logic, sequential function chart, block diagram and structured text to C++ and Java or a mixture of all. Internet is fast becoming the network of choice and the Internet/IP protocol is already a standard. Many of the networks are becoming wireless and self-healing. The future is without bound and PLC/PACs development will surely move forward at an ever increasing rate.

1.4 Todays Programmable Controllers:-Many technological advances in the programmable controller industry continue today. And these have led to many hardware (physical components) and software (control program) upgrades.The following list describes some recent hardware and software enhancements:- Faster scan times are being achieved using new, advanced microprocessor and electronic technology. Small, low-cost PLCs, which can replace four to ten relays, now have more power than their Predecessor, the simple relay replacer. Mechanical design improvements have included rugged input/output enclosure and input/output systems that have made the terminal an integral unit. High-density input/output (I/O) systems provide space-efficient interfaces at low cost.

Figure 4 PLC system with high density I/O (64 point nodules)

Intelligent, microprocessor-based I/O interfaces have expanded distributed processing. Typical interfaces include PID (proportional - integral-derivative), network, CANbus, fieldbus, ASCII communication, positioning, host computer, and language modules (e.g., BASIC, Pascal). Special interfaces have allowed certain devices to be connected directly to the controller. Typical interfaces include thermocouples, strain gauges, and fast-response inputs. Small PLCs have been provided with powerful instructions, which extend the area application for these small controllers. High-level languages, such as BASIC and C, have been implemented in some controllers modules to provide greater programming flexibility when communicating with peripheral devices and manipulating data. Advanced functional block instructions have been implemented for ladder diagram instruction sets to provide enhanced software capability using simple programming commands. Diagnostics and fault detection have been expanded from simple system diagnostics, which diagnose controller malfunctions, to include machine diagnostics, which diagnose failures or malfunctions of the controlled machine or process. Floating-point math has made it possible to perform complex calculations in control applications that require gauging, balancing, and statistical computation.

A. LITERATURE REVIEW

1. Lan-Hua Wang, Yao-Ming Chu, and Yung-Dian Wang: The purpose of this study was to introduce an activity of using the computerized ladder diagrams on the learning of programmable logic controller (PLC). A PLC is a user-friendly, microcontroller-based, specialized computational device that can carry out control functions. The advantages of PLC make it the brain of modern industrial control systems. For this reason, learning PLC operations is integral for modern industrial technologists. The goal of the intermediate representation is to help the user gain an in-depth understanding of the problem quickly. In the case of PLCs, the intermediate representation helps identify and classify the system elements, and enables grouping them into sub-problems amenable to solution.2. Sheila Mahapatra, Aman Jain, and Divyanshu Singh: The correct incentive for applying automation is to increase productivity and quality that is possible with current human labour levels so as to realize economies of scale and realize predictable quality levels. This is possible when a PLC is used. The main aim in this process is to apply PLC to design Automation system and all objectives in this project were successfully done as planned. Finally, the basic control system and logic design application can be used as a reference to design other applications of automation system, and can also be used as a teaching material for the Industrial Control subject. Besides using PLC as a controller, the other controller that can be used in future work is microcontroller. However, many factors must be considered like cost, feasibility and others.3. Maha M. Lashin: PLC has the ability to arrange the inputs/outputs. It has low cost compared with microcontroller systems because using PLC in different applications only required to change the software for each application but in case of using microcontroller the hardware components itself must be changed with different applications. Ladder Diagram programming method used for PLCs units which used in these two applications .Two applications had been introduced PLC as a new application of this control system in the sprayings robot and glass washing robot gave a good results. PLC most easier and safe control system for industrial applications like robot.4. Bo LI, Hai ZHAO, and Chun-He SONG: An approach to design and implement the control function of LD (Ladder Diagram) in the hydropower simulation system based on all paths searching algorithm is proposed in this paper. LD is widely used as a programming language for PLC (Programmable Logic Controller), but it doesnt be executed automatically in the hydropower simulation system which is a software system, and there is no compiler or interpretation for LD in it. A method to design an interface for LD is presented with typical class diagrams and the graphical interface, then an algorithm is proposed to implement the control function of LD through transforming LD to an undigraph and search all the paths between LW and NL. The application example and the analysis verify the validity of the algorithm. The running time is shown at last, it proves that the LD system with the algorithm can meet the real-time request in the Hydropower simulation system. 5. Osama Mahfooz: In this research automation of water tank is achieved by using inductive proximity sensors (type PNP) in order to set a low level and high level inside the tank. A PLC is used to automate the process and in order to simplify the programming process it was programmed by Ladder Logic. The process can also be implemented in Industries as Ladder logic is simple and user friendly.

From all the above review it can be stated that Ladder logic is used in various industrial applications and it is one of the most preferable PLC programming language as it is user friendly. In this project I had used the Ladder logic to programme industrial applications like Continuous bottling system, Elevators etc

Chapter 2PLC PROGRAMMMING

As PLCs have developed and expanded, programming languages have developed with them. Programming languages allow the user to enter a control program into a PLC using an established syntax. Todays advanced languages have new, more versatile instructions, which initiate control program actions. These new instructions provide more computing power for single operations performed by the instruction itself.In addition to new programming instructions, the development of powerful I/O modules has also changed existing instructions. These changes include the ability to send data to and obtain data from modules by addressing the modules locations. For example, PLCs can now read and write data to and from analog modules. All of these advances, in conjunction with projected industry needs, have created a demand for more powerful instructions that allow easier, more compact, function-oriented PLC programs.The three types of programming languages used in PLCs are:- Ladder Boolean GrafcetThe ladder and Boolean languages essentially implement operations in the same way, but they differ in the way their instructions are represented and how they are entered into the PLC. The Grafcet language implements control instructions in a different manner, based on steps and actions in a graphic oriented program.

2.1 PLC Ladder ProgrammingA very commonly used method of programming PLCs is based on the use of ladder diagrams. Writing a program is then equivalent to drawing a switching circuit. The ladder diagram consists of two vertical lines representing the power rails. Circuits are connected as horizontal lines, i.e., the rungs of the ladder, between these two verticals.In drawing a ladder diagram, certain conventions are adopted:1. The vertical lines of the diagram represent the power rails between which circuits are connected. The power flow is taken to be from the left-hand vertical across a rung.2. Each rung on the ladder defines one operation in the control process.3. A ladder diagram is read from left to right and from top to bottom, Figure 2.1 showing the scanning motion employed by the PLC. The top rung is read from left to right. Then the second rung down is read from left to right and so on.

Power flowLeft power railRight power railRung 1Rung 2Rung 3Rung 4End rungEND

Fig 5 Scanning the ladder programWhen the PLC is in its run mode, it goes through the entire ladder program to the end, the end rung of the program being clearly denoted, and then promptly resumes at the start. This procedure of going through all the rungs of the program is termed a cycle. The end rung might be indicated by a block with the word END or RET for return, since the program promptly returns to its beginning.4. Each rung must start with an input or inputs and must end with at least one output. The term input is used for a control action, such as closing the contacts of a switch, used as an input to the PLC. The term output is used for a device connected to the output of a PLC, e.g., a motor.5. Electrical devices are shown in their normal condition. Thus a switch, which is normally open until some object closes it, is shown as open on the ladder diagram. A switch that is normally closed is shown closed.6. A particular device can appear in more than one rung of a ladder. For example, we might have a relay that switches on one or more devices. The same letters and/or numbers are used to label the device in each situation.7. The inputs and outputs are all identified by their addresses, the notation used depending on the PLC manufacturer. This is the address of the input or output in the memory of the PLC.Figure 2.2 shows standard IEC 1131-3 symbols that are used for input and output devices. Some slight variations occur between the symbols when used in semi-graphic form and when in full graphic. Note that inputs are represented by different symbols representing normally open or normally closed contacts. The action of the input is equivalent to opening or closing a switch. Output coils are represented by just one form of symbol.To illustrate the drawing of the rung of a ladder diagram, consider a situation where the energizing of an output device, such as a motor, depends on a normally open start switch being activated by being closed. The input is thus the switch and the output the motor. Figure 2.3a shows the ladder diagram.Starting with the input, we have the normally open symbol jj for the input contacts. There are no other input devices and the line terminates with the output, denoted by the symbol ( ). When the switch is closed, i.e., there is an input, the output of the motor is activated. Only while there is an input to the contacts is there an output. If there had been a normally closed switch j/j with the output (Figure 2.3b), then there would have been an output until that switch was opened. Only while there is no input to the contacts is there an output. Semi-graphic form full graphic form

A horizontal link along which power can flowInterconnection of horizontal and vertical power flowsLeft-hand power connection of a ladder rungRight hand power connection of a ladder rungNormally open contactNormally closed contactOutput coil: if the power flow to it is on then the coil state is onFig 6 Basic symbols

InputInputOutputOutputInputOutput)a(OutputInput(b)

Fig 7 A ladder rungIn drawing ladder diagrams the names of the associated variable or addresses of each element are appended to its symbol. Thus Figure 11.6 shows how the ladder diagram of Figure 2.3a would appear using (a) Mitsubishi, (b) Siemens, (c) Allen-Bradley,(d) Telemecanique notations for the addresses. Thus, Figure 11.6a indicates that this rung of the ladder program has an input from address X400 and an output to address Y430. When wiring up the inputs and outputs to the PLC, the relevant ones must be connected to the input and output terminals with these addresses. Input Output Input Output X400 Y430 I0.0 Q2.0(a) (b)InputOutputInput I0,0OutputO0,0

I:001/01O:010/01

(c) (d)Fig 8 Notation: (a) Mitsubishi (b) Siemens (c) Allen-Bradley (d) Telemecanique2.2 Ladder SymbolsOne method of entering the program into the programming terminal involves using a keypad having keys with symbols depicting the various elements of the ladder diagram and keying them in so that the ladder diagram appears on the screen of the programming terminal. For example, to enter a pair of contacts the key marked:

might be used, followed by its address being keyed in. To enter an output the key marked:

might be used, followed by its address. To indicate the start of a junction:

might be pressed; to indicate the end of a junction path:

To indicate horizontal circuit links, the following key might be used:

The terminal then translates the program drawn on the screen into machine language.Computers can be used to draw up a ladder program. These involve loading the computer with the relevant software, e.g., RSLogix from Rockwell Automation Inc. for Allen-Bradley PLCs, MELSOFT GX Developer for Mitsubishi PLCs, STEP 7 Micro/WIN V4 for Siemens PLCs. The software operates on the Windows operating system and involves selecting items, in the usual Windows manner, from pull-down menus on the screen.

Chapter 3INDUSTRIAL APPLICATIONS OF LADDER LOGIC3.1. Continuous bottle filling system This is one of the important application of PLC in the bottle filling industry where we want our bottles, which are moving on the conveyor belt, to be automatically detected at the appropriate position and get it filled by any desired liquid and also after getting filled the queued bottle gets chance to be filled. If this whole process is carried out manually it will really take a long time and also the quantities will be quite lesser. So PLC becomes requisite controller for these types of industry. Here also just a small demonstration of the process was performed with the help of PLC where a ladder diagram was created to control the process and the ladder diagram was run the PLC trainer kit to see its justification. ObjectiveWe will implement a control program that detects the position of a bottle via a limit switch then waits for 0.5 secs, and then fills the bottle until a photodetector detects the filled condition of the bottle. After the bottle is filled ,the buzzer sounds and the control program will again wait for 0.7 secs. before moving to the next bottle .Until the limit switch signals ,the feed motor,M1 runs while there are fixed rollers which carries the filled bottles. Motor,M2 keeps running after the process has been started. Fig 9 Bottle filling system Inputs address

Start I0:15

Stop I1:15

Limit switch(LS) I2:15

Photo detector(PE) I3:15

Outputs address

Feed motor(M1) O0:15

Outfeed motor(M2) O1:15

Solenoid valve(S1) O2:15

Light(L1) O3:15

Buzzer(B1) O4:15

TABLE: Inputs and outputs employed Ladder diagram Fig 10 ladder diagram for bottle filling system ObservationOnce the start button is pressed the green light (L1) turns ON and remains ON until stop button is pressed. As light turns ON out feed motor(M2) starts running. After M2 runs and if either limit switch (LS) has not signaled or filled bottle condition is fulfilled motor(M1) starts. After limit switch has signalled timer,T1 gets activated. After T1 gives done (DN) signal and photo eye detector (PE) is disabled, solenoid valve gets in operation. As PE signals solenoid stops and buzzer (B1) sounds after which timer,T2 gets enabled which stops the process for 0.7 seconds. Once the filled bottle condition is activated the cycle starts again. The ladder diagram was successfully checked in the PLC simulator and all the prescribed conditions were observed completely.

3.2. Batch mixing system This is another commonly applied application of PLC where two liquids are mixed in required proportion to form a batch .Rate of the flow is already fixed. We only control the time of the flow. Level of the liquids in the tank are sensed by the level sensor switches Objective We try a simple blending of water and acid in a container where we only have three level sensors(L1,L2, and L3) and two liquids flowing in through two solenoid valves, solenoid a(water control) and solenoid b(acid control)and draining out through solenoid c(blend outflow).The batch is to be controlled by timer. After required level of blend is sensed (by L1)the mixer runs for 3 mins. by the motor. They are mixed in ratio of 3:2. The process initiates with the drain valve open, water and acid valves closed, mixer motor is off, and the tank is empty. Fig 11 Batch mixing system Ladder diagram Fig 12 Ladder diagram for batch mixing systemObservationWhen start button is pressed water is filled up to L2 and it ends as L2 is closed. First of all as start is pressed output O: 0/15 turns ON and remains ON until tank is emptied. Rung 2 closes normally open drain valve, before timer T: 4 activates. Rung 3 energises solenoid a until L2 doesnt signal, once it signals solenoid a gets de-energised. Then motor is turned ON and mix it for 3 mins. Similarly acid is filled upto L3 by solenoid b.as level gets detected by L3 solenoid b de-energises .And then mixer gets started and it runs for 3 minutes. After time delay of 3 mins solenoid c opens and the blend gets drained out .Once the blend gets out completely, the process cycle restarts. The ladder diagram was successfully checked in the PLC simulator and all the prescribed conditions were observed completely.

3.3. 3 -stage air conditioning system A simple air conditioner consists of a single air compressor motor which gets switched off when temperature of the space being controlled falls below the setting on the thermostat. Thermostats are provided with a differential setting to avoid on and off of the compressor motor. The three stage air conditioning system helps in conservation of electrical power. ObjectiveThere are two motors compressors in the system. One is of low horsepower and other one is of high horsepower rating. These motors are designated as C1 and C2 in the case. The system is installed in a hall to maintain the temperature between 200C-240C depending on the number of viewers in the hall and the atmospheric temperature. The motors of C1 and C2 are run on three conditions of the thermostats. The three conditions described below are also the control requirements of the air conditioning system:- 1. Compressor 1 and compressor 2 should turn on when the temp of the hall is above 280C. 2. Only compressor 2 should turned on when the temp of the hall is above 240C and below 280C. 3. Only compressor 1 is turned on when the temp. is above 200C and below 240C. A pre- condition for running any compressor is that chilling water flow switch FS1 should be closed. Chilling water flow necessary to take away heat from the compressed cooling water. Three thermostat with different settings are used for the control of compressor motor running in three different stages described above. The three thermostats T1, T2, T3, are set at temp 200C, 240C and 280C respectively. The control of three stages with three the thermostats and Flow switch (FS1) of air conditioning system can be understood from the control circuit shown in fig.4.5. The start push button, stop push button and overload contacts for compressor motor have not been shown in the circuit for sake of simplicity.

Working of the control circuit:- (1) When chilled water flow is maintained, flow switch FS1 will actuate and close its contacts. Closing of contact FS1 causes application of high logic signal to terminal 2 of all gates. Fig 13 logic control circuit for 3 stage air conditioning system (2) When the temperature in the cinema hall will be above 280C the contacts of all the thermostat will be closed. Closed contact of T1 will give a high input to terminal 1 of AND1 but closed contact of T2 will give low logic to terminal 3 of AND1 as there is a not gate in series. The output of AND1, therefore becomes low. (3) Closed contact of T3 will give a high input to terminal 1 of AND2 while the other terminal 2 is already high due to closure of FS1 AND2 thus gives a high output which is applied to both OR1 and OR2,as each terminal of OR1andOR2 is now high, their output is also high. Output from OR1 leads to energisation of contactor C1 and output from OR2 leads to energisation of contactor C2 through their respective amplifiers. Thus compressor 1 and 2 will run when temp in the hall will above 280C. (4) When the hall temp. is below 280C and above 240C, contact of thermostat T3 will open, While contacts of T 1andT 2 are closed. Due to open contact of T3 there is low signal at terminal 1 of AND2 and therefore its output is low. Output of AND1 is also low as closed Contact of T2 gives a low signal at terminal 3 due to a not gate in series, in this case AND3 Will have a high output as its input terminal1 has high signal from the closed contact of Thermostat T2,it is to be noted that supply to terminal1 of AND3 is taken prior to the NOT Gate. High output from AND3 goes to terminal 2 of OR2 which then gives a high output. This output energises contactor C2 through the amplifier. Thus compressor 2 will run only when the temperature is above 240C but below 280C. (5) When the temp. falls below 240C contact of thermostat T2 opens and output of AND3 will Go low due to a low signal in its input terminal the open contact of T2 will however give a high signal to terminal 3 of AND 1 (due to not gate in series), It will get switched on as its terminal 1 and2 are already high. The high output from and1 then goes to terminal 1 of or1 which then gives a high output to energise contactor C1.thus one compressor will run when temp. Is below 240C but above 200C. When temp. Falls below 200C, contact of thermostat T1 also opens and terminal 1 of AND1 goes low and it is switched off .Thus Compressor 1 also stops when temp. Falls below 200C. (6) Compressor 1 continues to run if temp. In the hall remains 200C -240C, if due to more viewers in the hall, compressor 1 is unable to maintain, temp. Below 240C. Compressor 1 will be switched off. If the load is still more and compressor 2 alone cannot cope up and temp. Goes above 280C then compressor 1 will also start to bring down the temperature. Ladder diagram:- Fig 14 Ladder diagram for 3 stage air conditioning systemObservation:- The ladder diagram was satisfactorily realized in the lab and all the conditions were tested .The outputs are same as expected

Chapter 4CONCLUSION

The implementation of the PLC was carried out effectively for various industrial applications. It proves to be one of the important controller in industries for its simplicity and robustness and is used all over the world. For any control design approach understandings of the desired control system and how to use the ladder diagram to translate the machine sequence of operation are the most important parts, because it has direct effect on the system performance. PLCs are very good for controlling outputs based on the inputs. They are amazingly robust and are able to withstand all sorts of difficult conditions such as extreme temperature or dust in the air. They even last for a very long period. They dont have contacts that wear out, like relays do. They also can switch fairly quickly without much heating in direct contrast to relays. For any application we need not to change the whole structure only different program has to be embedded as like any other programmable devices. Compared to relays PLCs are almost always a better choice.On the downside it could be observed that PLCs are not very good at handling large amount of data, or complex data. Computers are better for those tasks.PLCs are also not very good with databases or displaying data. Lack of standardization is also one of the major disadvantage of the PLC. This causes much confusion if the PLC used for an application is replaced by one from a different manufacturer, or if a PLC programmer is replaced by a person with a different understanding of PLC programming. In the bottle filling system, only one limit switch was used to detect the position of the bottle. This process has become quite obsolete, instead IR sensor can be used. It is better to add more sensors in this system like a flow sensor to detect water flow or use level sensor to detect water level. Thus, the system will be more sensitive as there will be more sensing points Besides using PLC as controller, the other controller can be used in this future work is like Microcontroller. However, many factors must be considered like cost, practically and others. Talking about the control of dc motor, it is important for the machine designer to be very familiar with various methods of controlling ac and dc motor. These range from the simple motor starter to the sophisticated pulse width modulated (PWM) dc motor controls. The pulse width modulator (PWM) system is capable of efficiently controlling the speed of a dc motor by controlling the average armature voltage of the motor.

REFERENCES

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