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IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 19, NO. 3, SEPTEMBER 2004 469Design and Implementation of PLC-BasedMonitoring Control System for Induction MotorMaria G. Ioannides, Senior Member, IEEEAbstractThe implementation of a monitoring and controlsystem for the induction motor based on programmable logic con-troller (PLC) technology is described. Also, the implementation ofthe hardware and software for speed control and protection withthe results obtained from tests on induction motor performanceis provided. The PLC correlates the operational parameters tothe speed requested by the user and monitors the system duringnormal operation and under trip conditions. Tests of the inductionmotor system driven by inverter and controlled by PLC prove ahigher accuracy in speed regulation as compared to a conventional86 102 control system. The efficiency of PLC control is increasedat high speeds up to 95% of the synchronous speed. Thus, PLCproves themselves as a very versatile and effective tool in industrialcontrol of electric drives.Index TermsComputer-controlled systems, computerizedmonitoring, electric drives, induction motors, motion control,programmable logic controllers (PLCs), variable-frequencydrives, voltage control.I. INTRODUCTIONSINCE technology for motion control of electric drives be-came available, the use of programmable logic controllers(PLCs) with power electronics in electric machines applicationshas been introduced in the manufacturing automation 1, 2.This use offers advantages such as lower voltage drop whenturned on and the ability to control motors and other equip-ment with a virtually unity power factor 3. Many factoriesuse PLCs in automation processes to diminish production costand to increase quality and reliability 49. Other applica-tions include machine tools with improved precision comput-erized numerical control (CNC) due to the use of PLCs 10.To obtain accurate industrial electric drive systems, it is nec-essary to use PLCs interfaced with power converters, personalcomputers, and other electric equipment 1113. Neverthe-less, this makes the equipment more sophisticated, complex, andexpensive 14, 15.Few papers were published concerning dc machines con-trolled by PLCs. They report both the implementation of thefuzzy method for speed control of a dc motor/generator setusing a PLC to change the armature voltage 16, and theincorporation of an adaptive controller based on the self-tuningregulator technology into an existing industrial PLC 17. Also,other types of machines were interfaced with PLCs. Thereby,an industrial PLC was used for controlling stepper motorsManuscript received December 2, 2002. Paper no. TEC-00004-2002. Thiswork was supported by the National Technical University of Athens.The author is with the Faculty of Electrical and Computer Engineering ofthe National Technical University of Athens, Athens 15773, Greece (e-mail:mioannidece.ntua.gr).Digital Object Identifier 10.1109/TEC.2003.822303in a five-axis rotor position, direction and speed, reducingthe number of circuit components, lowering the cost, andenhancing reliability 18. For switched reluctance motors asa possible alternative to adjustable speed ac and dc drives, asingle chip logic controller for controlling torque and speeduses a PLC to implement the digital logic coupled with a powercontroller 19. Other reported application concerns a linearinduction motor for passenger elevators with a PLC achievingthe control of the drive system and the data acquisition 20.To monitor power quality and identify the disturbances thatdisrupt production of an electric plant, two PLCs were used todetermine the sensitivity of the equipment 21.Only few papers were published in the field of inductionmotors with PLCs. A power factor controller for a three-phaseinduction motor utilizes PLC to improve the power factor andto keep its voltage to frequency ratio constant under the wholecontrol conditions 3. The vector control integrated circuituses a complex programmable logic device (CPLD) and integerarithmetic for the voltage or current regulation of three-phasepulse-width modulation (PWM) inverters 22.Many applications of induction motors require besides themotor control functionality, the handling of several specificanalog and digital I/O signals, home signals, trip signals,on/off/reverse commands. In such cases, a control unit in-volving a PLC must be added to the system structure. Thispaper presents a PLC-based monitoring and control systemfor a three-phase induction motor. It describes the design andimplementation of the configured hardware and software. Thetest results obtained on induction motor performance showimproved efficiency and increased accuracy in variable-loadconstant-speed-controlled operation. Thus, the PLC correlatesand controls the operational parameters to the speed set pointrequested by the user and monitors the induction motor systemduring normal operation and under trip conditions.II. PLC AS SYSTEM CONTROLLERA PLC is a microprocessor-based control system, designedfor automation processes in industrial environments. It uses aprogrammable memory for the internal storage of user-orien-tated instructions for implementing specific functions such asarithmetic, counting, logic, sequencing, and timing 23, 24.A PLC can be programmed to sense, activate, and control in-dustrial equipment and, therefore, incorporates a number of I/Opoints, which allow electrical signals to be interfaced. Input de-vices and output devices of the process are connected to thePLC and the control program is entered into the PLC memory(Fig. 1).0885-8969/04$20.00 2004 IEEE470 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 19, NO. 3, SEPTEMBER 2004Fig. 1. Control action of a PLC.In our application, it controls through analog and digital in-puts and outputs the varying load-constant speed operation ofan induction motor. Also, the PLC continuously monitors theinputs and activates the outputs according to the control pro-gram. This PLC system is of modular type composed of spe-cific hardware building blocks (modules), which plug directlyinto a proprietary bus: a central processor unit (CPU), a powersupply unit, input-output modules I/O, and a program terminal.Such a modular approach has the advantage that the initial con-figuration can be expanded for other future applications such asmultimachine systems or computer linking.III. CONTROL SYSTEM OF INDUCTION MOTORIn Fig. 2, the block diagram of the experimental system isillustrated. The following configurations can be obtained fromthis setup.a) A closed-loop control system for constant speed opera-tion, configured with speed feedback and load currentfeedback. The induction motor drives a variable load, isfed by an inverter, and the PLC controls the inverteroutput.b) An open-loop control system for variable speed operation.The induction motor drives a variable load and is fed byan inverter in constant control mode. The PLC isinactivated.c) The standard variable speed operation. The inductionmotor drives a variable load and is fed by a constantvoltage-constant frequency standard three-phase supply.The open-loop configuration b) can be obtained from theclosed-loop configuration a) by removing the speed and loadfeedback. On the other hand, operation c) results if the entirecontrol system is bypassed.IV. HARDWARE DESCRIPTIONThe control system is implemented and tested for a woundrotor induction motor, having the technical specifications givenin Table I. The induction motor drives a dc generator, which sup-plies a variable load. The three-phase power supply is con-nected to a three-phase main switch and then to a three-phasethermal overload relay, which provides protection against cur-rent overloads. The relay output is connected to the rectifier,which rectifies the three-phase voltage and gives a dc input tothe insulated gate bipolar transistor (IGBT) inverter. Its tech-nical specifications 25 are summarized in Table II. The IGBTFig. 2. Electrical diagram of experimental system.TABLE IINDUCTION MOTOR TECHNICAL SPECIFICATIONSIOANNIDES: DESIGN AND IMPLEMENTATION OF PLC-BASED MONITORING CONTROL SYSTEM FOR INDUCTION MOTOR 471TABLE IIINVERTER TECHNICAL SPECIFICATIONSinverter converts the dc voltage input to three-phase voltageoutput, which is supplied to the stator of the induction motor.On the other hand, the inverter is interfaced to the PLC-basedcontroller.This controller is implemented on a PLC modular system5, 2628. The PLC architecture refers to its internal hard-ware and software. As a microprocessor-based system, the PLCsystem hardware is designed and built up with the followingmodules 2937:central processor unit (CPU);discrete output module (DOM);discrete input module (DIM);analog outputs module (AOM)analog inputs module (AIM)power supply.Other details of the PLC configuration are shown in Tables IIIand IV.A speed sensor is used for the speed feedback, a currentsensor is used for the load current feedback, and a secondcurrent sensor is connected to the stator circuits 32. Thus,the two feedback loops of the closed-loop system are setup byusing the load current sensor, the speed sensor, and the AIM.A tachogenerator (permanent magnet dc motor) is used forspeed sensing. The induction machine drives its shaft mechani-cally and an output voltage is produced, the magnitude of whichis proportional to the speed of rotation. Polarity depends on thedirection of rotation. The voltage signal from the tachogener-ator must match the specified voltage range of the AIM (05Vdc and 200-k internal resistance). Other PLC external controlcircuits are designed using a low-voltage supply of 24 V dc.For the manual control, the scheme is equipped with start,stop, and trip push buttons, as well as with a forward and back-ward direction selector switch. As shown in Fig. 2, all of thedescribed components: a main switch, an automatic three-phaseswitch, an automatic single phase switch, a three-phase thermaloverload relay, a load automatic switch, signal lamps (forward,backward, start, stop, trip), push buttons (start, stop, trip), a se-lector switch (for the forward/backward direction of rotation), aspeed selector, a gain selector, as well as the PLC modules andthe rectifier-inverter are installed in a control panel. The pro-gram is downloaded into the PLC from a personal computer PCand an RS232 serial interface.V. S OFTWARE DESCRIPTIONPLCs programming is based on the logic demands of inputdevices and the programs implemented are predominantly log-Fig. 3. Flowchart of the main program.TABLE IIIPLC CONFIGURATIONical rather than numerical computational algorithms. Most ofthe programmed operations work on a straightforward two-state“on or off” basis and these alternate possibilities correspond to“true or false” (logical form) and “1or0” (binary form), respec-tively. Thus, PLCs offer a flexible programmable alternative toelectrical circuit relay-based control systems built using analogdevices.The programming method used is the ladder diagrammethod. The PLC system provides a design environment in theform of software tools running on a host computer terminalwhich allows ladder diagrams to be developed, verified, tested,and diagnosed. First, the high-level program is written in ladderdiagrams, 33, 34. Then, the ladder diagram is convertedinto binary instruction codes so that they can be stored inrandom-access memory (RAM) or erasable programmableread-only memory (EPROM). Each successive instruction isdecoded and executed by the CPU. The function of the CPUis to control the operation of memory and I/O devices and toprocess data according to the program. Each input and outputconnection point on a PLC has an address used to identifythe I/O bit. The method for the direct representation of dataassociated with the inputs, outputs, and memory is based on thefact that the PLC memory is organized into three regions: input472 IEEE TRANSACTIONS ON ENERGY CONVERSION, VOL. 19, NO. 3, SEPTEMBER 2004TABLE IVPLC MODULES AND I/O DESIGNATIONimage memory (I), output image memory (Q), and internalmemory (M). Any memory location is referenced directly using%I, %Q, and %M (Table III).The PLC program uses a cyclic scan in the main program loopsuch that periodic checks are made to the input variables (Fig. 3).The program loop starts by scanning the inputs to the systemand storing their states in fixed memory locations (input imagememory I). The ladder program is then executed rung-by-rung.Scanning the program and solving the logic of the various ladderrungs determine the output states. The updated output states arestored in fixed memory locations (output image memory Q).The output values held in memory are then used to set and resetthe physical outputs of the PLC simultaneously at the end of theprogram scan. For the given PLC, the time taken to completeone cycle or the scan time is 0, 18 ms/K (for 1000 steps) andwith a maximum program capacity of 1000 steps.The development system comprises a host computer (PC)connected via an RS232 port to the target PLC. The hostcomputer provides the software environment to perform fileediting, storage, printing, and program operation monitoring.The process of developing the program to run on the PLCconsists of: using an editor to draw the source ladder program,converting the source program to binary object code whichwill run on the PLCs microprocessor and downloading theobject code from the PC to the PLC system via the serialcommunication port. The PLC system is online when it is inactive control of the machine and monitors any data to checkfor correct operation.A. PLC Speed Control SoftwareIn Fig. 4, the flowchart of the speed control software is illus-trated. The software regulates the speed and monitors the con-stant speed control regardless of torque variation. The inverterbeing the power supply for the motor executes this while, at thesame time, it is controlled by PLCs software. The inverter alonecannot keep the speed constant without the control loop withfeedback and PLC.From the control panel, the operator selects the speed setpointand the forward/backward direction of rotation. Then, byFig. 4. Flowchart of speed control software.pushing the manual start pushbutton, the motor begins the ro-tation. If the stop button is pushed, then the rotation stops. Thecorresponding input signals are interfaced to the DIM and theoutput signals to the DOM as shown in Table IV.The AIM receives the trip signal from the stator currentsensor, the speed feedback signal from the tachogenerator, andthe signal from the control panel. In this way, the PLC readsthe requested speed and the actual speed of the motor. The dif-ference between the requested speed by the operator and the ac-tual speed of the motor gives the error signal. If the error signalis not zero, but positive or negative, then the PLC according tothe computations carried out by the CPU decreases or increasesthe of the inverter and, as a result, the speed of the motoris corrected.IOANNIDES: DESIGN AND IMPLEMENTATION OF PLC-BASED MONITORING CONTROL SYSTEM FOR INDUCTION MOTOR 473Fig. 5. Flowchart of monitor and protection software.The implemented control is of proportional and integral (PI)type (i.e., the error signal is multiplied by gain , integrated,and added to the requested speed). As a result, the control signalis sent to the DOM and connected to the digital input of theinverter to control variations. At the beginning, the operatorselects the gain by using a rotary resistor mounted on thecontrol panel (gain adjust) and the AIM receives its voltage dropas controller gain signal (010 V).The requested speed is selected using a rotary resistorand the AIM reads this signal. Its value is sent to the AOM anddisplayed at the control panel (speed set point display). Anotherdisplay of the control panel shows the actual speed computedfrom the speed feedback signal. A third display shows the loadtorque computed from the load current signal in Newton-meters(N m). Their corresponding signals are output to the AOM(Table IV).B. Monitor and Protection SoftwareIn Fig. 5, the flowchart of this software is shown.During motor operation, it is not possible to reverse its direc-tion of rotation by changing the switch position. Before direc-tion reversal, the stop button must be pushed.For motor protection against overloading currents duringstarting and loading, the following commands were pro-grammed into the software.i) Forward/backward signal is input to DIM.ii) Speed setpoint signal , the load current , the statorcurrent , and the speed feedback signal are input toAIM.iii) At no load , if the speed set point is lowerthan 20% or r/min, the motor will not start.iv) At an increased load over 0, 4 N m (40% of ratedtorque), , and a speed setpoint lower than 40%or r/min, the motor will not start.v) If the load is increased more than 1, 0 N m (rated torque)and if the speed set point exceeds 100% orr/min, the motor enters the cutoff procedure.vi) In all other situations, the motor enters in the speed con-trol mode and the speed control software is executed asdescribed in Subsection A.C. Cutoff and Restart Motor SoftwareIn Fig. 6, the flowchart of this software is shown.In overloading situations, the motor is cut off and the triplamp (yellow) is lit. The operator must release the thermalrelays and then must turn off the trip lamp by pushing tripor stop button. The thermal relays are set to the motor ratedcurrent 1, 5 A. Following this, the motor can be startedagain.The motor can be cut off by the operator pushing the stopbutton: the display of the actual speed is set to zero, thestart lamp (green) turns off, and the stop lamp (red) turnson and remains lit for 3 s.The load must be disconnected immediately after themotor cuts off and before the drive system is restarted.The motor will not start before 3 s after cutoff even if thestart button is pushed.VI. RESULTSThe system w