Global Journal of Advanced Engineering Technologies ISSN (Online): 2277-6370 & ISSN (Print):2394-0921 Volume 5, Issue 1- 2016 PLC APPLICATION FOR SPEED CONTROL OF INDUCTION MOTORS THROUGH VFD Anshul Tiwari1, Abhay Pratap Singh2, Aditi Dixit3, Debojyoti Sen4 4th Year Student, EN Dept, IMS Engineering College, Ghaziabad, India. 3. 3rd Year Student, EN Dept, IMS Engineering College, Ghaziabad, India. 4 Assistant Professor, EN Dept, IMS Engineering College, Ghaziabad, India. 1,2. Abstract: The paper is about controlling the speed of a three phase induction motor using variable frequency drive (Power Flex 4m) through programmable logic controller(Compact Logix programmed using RSLogix 5000). Programmable logic controller is an industrial controlling device and is used to automate machines and factory assembly lines. A three phase induction motor can also be controlled manually through variable frequency drives. So, the main purpose of this paper is to automate the three phase induction motor by controlling the inputs to variable frequency drive through PLC and therefore as a result the inputs to the induction motor will be changed and thus the speed of induction motor will change accordingly. VFD employed in this experiment work on V/f method of speed control in which flux remains constant. VFD comprises of converter, filter and PWM inverter. The output obtained from these three steps is fed to the induction motor. Obtain speed control up to the range of 0 to 1510 RPM. Keywords: Programmable logic controller, Ladder logic program, Variable frequency drive, V/f method. I. INTRODUCTION Automation has become the heart of industries. Industries are reaching new heights only because of advancement of automation technology. Plant automation is the necessity for the manufacturing industry to survive in the today’s globally competitive market. Growth of Industries leads to the reduction of manual controls day by day. One such contribution to the growing era is made through this paper. This paper is about controlling the speed of induction motor, which is most economical motor, using variable frequency drive (VFD) through programmable logic controller (PLC). VFD has been chosen specifically because they provide the advantages of energy savings, low motor starting current, reduction of thermal and mechanical stresses on motors and belts during starts, simple installation, high power factor and lower KVA. Variable frequency drives (Power Flex 4m) are generally required because in many applications it is not desired to run the motor at same speed all the time www.gjaet.com due to its surrounding circumstances. The revolution per minute of the driven shaft need to be increased or decreased depending on load changes, application requirement or other circumstances. For example, a pump delivering cooling liquid supply may require peak load operation only for a requisite period of time and may require only much less amount during remaining time of the day. VFD will allow the speed of the pump to run at a lower rate in such case thereby enabling energy saving benefits [1]. The Compact Logix PLC is used which is interfaced with the help of a software known as RSLogix 5000. The PLC has been connected to control and monitor a VFD which acts as a go-between the three phase induction motor and the PLC. The PLC processes the inputs according to the ladder logic programming and initiates corresponding output to the VFD. The VFD in turn once again processes the PLC input to it and accordingly controls the speed of three phase induction motor. Ladder logic programming is carried out in RSLogix in the personal computer. A PLC based control system was set up comprising of an Allen Bradley Compact Logix PLC, an Allen Bradley Power Flex 4M variable frequency drive, a three phase induction motor in the Rockwell automation laboratory GLA University Mathura U.P. Speed control methods of three phase induction motors · · Stator voltage control method used have many disadvantages such as it offers control over limited speed range, its use introduces harmonic content and thus reduces the input supply power factor and can be used for only low power drives like fan, blowers, centrifugal pumps etc[6]. Stator frequency control method used have following disadvantages such as at low frequencies reactances will be low leading to high motor currents, more losses and reduced efficiency, after saturation of magnetic circuit, motor parameters will change leading to inaccurate speed-torque characteristics and Page | 30 Global Journal of Advanced Engineering Technologies ISSN (Online): 2277-6370 & ISSN (Print):2394-0921 · · · with increase in frequency flux and torque also get reduced[6]. Stator current control method used have following disadvantages such as low starting torque as compared to stator voltage control and if constant current is provided through current source inverter then there is generation of unwanted harmonics in the system and torque pulsations[6]. Static rotor resistance control method used have following disadvantages such as reduced efficiency at low speeds, speed changes very widely with load variation, unbalances in voltages and currents if rotor circuit resistances are not equal, Speed above normal value is not possible and this method cannot be implemented in squirrel cage induction motor[6]. All the above mentioned methods of speed control have considerable disadvantages therefore V/f method of speed control is adopted which has following advantages starting current decreases, however starting torque becomes more, stable operating region of motor is increased, at base speed, the voltage and frequency achieve their rated values[5,6]. II. V/F METHOD In this method, V/f ratio remains constant which leads to constant flux. Result of constant flux is that the starting torque is inversely proportional to frequency and maximum torque is independent of frequency( except at low frequency). Thus maximum torque is constant in this case. Other advantages are Power factor at starting is improved, therefore power input to motor is more, starting current decreases, however starting torque becomes more, stable operating region of motor is increased, at base speed, the voltage and frequency achieve their rated values and acceleration and deceleration of the motor can be controlled by controlling the change of supply frequency to the motor with respect to time [5]. A) Variable frequency drive A variable frequency drive is a motor controller that drives an electric motor connected to it by changing the frequency and voltage supplied to it. Working of VFD can be understood in three stages. First stage of VFD is the converter, second stage is to get rid of the AC www.gjaet.com Volume 5, Issue 1- 2016 ripples on a DC bus by using filter and the last stage is passing the DC obtained through inverter after which the output is a supply of variable frequency and variable voltage which is fed to the induction motor. The VFD used in this experiment is Allen Bradley Power Flex 4m. b) Why VFD? Variable frequency drives are used extensively in industries because they reduce energy consumption and energy costs, extend equipment life and reduce maintenance cost and provide a tight process control. III. PROGRAMMABLE LOGIC CONTROLLER A PLC is a solid state, industrial computer that performs discrete or sequential logic in a factory environment. it was originally developed to replace mechanical relays, timer, and counters. A sequence of instructions is programmed by the user of PLC memory. Its purpose is to monitor crucial process parameters and adjust process operations accordingly. PLC saves Material cost, Energy, installation cost and ease of maintenance, trouble shooting, labour cost by reduced wiring and associated errors, less space, no moving parts-rugged and possibility of reprogramming. Some value added benefits greater life and reliability, tremendous flexibility, advanced function, communication, shorter project time, diagnostics and easier storage, archiving and documentation. Application: Packaging, material handling, power generation, security system, paint lines, bottling and canning, machining, HVAC/building automation, automated PLC used in the experiment is Allen Bradly Compact Logix programmed by RSLogix 5000. IV. EXPERIMENT A) Objective: The main purpose of this experiment is to control the speed of a three phase induction motor using VFD through PLC. B) Experimental set-up The Allen Bradley Power Flex 4M AC drive is the most cost effective and smallest drives which in a compact and space saving design provide a powerful motor speed control. Its specifications are shown in the table below. Page | 31 Global Journal of Advanced Engineering Technologies ISSN (Online): 2277-6370 & ISSN (Print):2394-0921 Volume 5, Issue 1- 2016 TABLE 1: SPECIFICATIONS OF VFD DESCRIPTION Dimensions HxWxD Weight Number of I/O Power supply voltage Power supply inrush current Power consumption DETAILS 90x180x87 mm 0.9 Kg 2 digital & 2 analog I/O cards 24V DC 15A for 20 ms 50W The PLC used in this experiment is Allen Bradley Compact Logix series. The basic parts of RSLogix 5000 series are Power supply, analog and digital I/O module TABLE 2: SPECIFICATION OF PLC Rated Output 0.75kW (1Hp) Rated Voltage 240V AC, 1Rated Current 4.2A Rated torque 3.5kg C) Ladder logic programming This experiment was conducted based on ladder logic programming which is done on a software installed on a personal computer(RSLogix 5000) according to which the PLC takes the inputs, processes them according to the program and gives the output to the VFD which again processes this input within the drive and finally controls the speed of the three phase induction motor. Figure 1: Ladder Logic programming www.gjaet.com Figure 2: Plot of frequency vs speed Above graph shows that the graph of frequency vs speed comes out to be linear except at lower frequency showing linear variation of speed with frequency. Figure 3: Plot of V/f ratio vs. frequency Above graph shows the variation of V/f ratio vs frequency. So, this proves that VFD works on the principle of V/f method keeping flux constant. Figure 4 : Plot of voltage vs. frequency Above graph shows the variation of voltage with frequency so this also proves the linear variation of voltage with frequency. Page | 32 Global Journal of Advanced Engineering Technologies ISSN (Online): 2277-6370 & ISSN (Print):2394-0921 D) Another application The result obtained above is used for a simplified approach to control the speed of exhaust fans used in cooling towers in Power Plants and the Ladder logic programming used is as shown under Volume 5, Issue 1- 2016 Computer Science and Software Engineering, ISSN: 2277 128X, Volume 3, Issue 12, December 2013. [4] S. Takiyar and B. K. Chauhan, Hybrid Method for Customized Control of Induction Motor, International Journal of Computer and Electrical Engineering, Vol. 5, No. 4, August 2013. [5] Z. W. Ma and T. Zeng, “Stability improvement of V/Hz controlled PWM inverter fed Induction motors Drive,” IEEE industrial Electronics and Applications’ 1st conference, Singapore, pp. 1-4, May 2006. [6] Power Electronics by P.S. BHIMRA, third edition, khanna publication, ISBN No. 81-7409-056. V. CONCLUSION Being an economical motor, three phase induction motors are used extensively in industries today. So, if speed of three phase induction motor can be controlled then wherever there is a use of three phase induction motor its speed can be controlled. Therefore, the control method discussed in this paper can be applied to everywhere were three phase induction motor is used. REFERENCES [1] Rinchen Geongmit Dorjee, Monitoring and Control of a Variable Frequency Drive Using PLC and SCADA .ISSN: 2321-8169, International Journal on Recent and Innovation Trends in Computing and Communication, Volume: 2 Issue: 10. [2] YASAR BIRBIR, H. SELCUK NOGAY, Design and Implementation of PLC-Based Monitoring Control System for three-Phase Induction Motors Fed by PWM Inverter, International Journal of Systems Applications, ENGINEERING & DEVELOPMENT Issue 3, Volume 2, 2008. [3] Madivalappa.B, M. S. Aspalli, Speed Control of Three Phase Induction Motor by Variable Frequency Drive, International Journal of Advanced Research in www.gjaet.com Page | 33