International Journals of Advanced Research in Computer Science and Software Engineering Research Article June 2017 Closed Loop PI Control of a Single Phase Induction Motor Using SPWM Kuheli Ghosh Goswami EE Department, UCSTA Assistant Professor, BGI-SDET DOI: 10.23956/ijarcsse/V7I6/0169 Amit Ghosh Technology Analyst, Infosys Limited, India Abstract In this paper we discuss the speed control of a single phase induction motor by proportional integral control. The PI controller fuses the properties of the P and I controllers. It shows a maximum overshoot and settling time similar to the P controller but no steady-state error. The output of the inverter has been smoothen by using LC filter. THIPWM techniques have lower total harmonic distortion than the SPWM technique and SVPWM have much lower THD than that of THIPWM. The THIPWM can increase the fundamental output voltage by 15.5% over the SPWM technique and SVPWM more than that. SPWM Simulation results are obtained using MATLAB / Simulink environment for effectiveness of the study. Keywords SPWM, ASD, INVERTER DESIGN, PI CONTROL STRATEGIES, MATLAB. I. INTRODUCTION Variable Speed Drives (VSDs), also known as adjustable speed drives, are large industrial electric motors whose speed can be adjusted by means of an external controller. They are used in process control and help saving energy in plants that use many powerful electric motors.the use of adjustable speed in process control matches the motor speed to the required tasks and may compensate for changes in the process's variables. The use of adjustable speed for saving energy is exemplified by the adjusting the speed of a cooling fan motor to match the temperature of the machinery parts it is cooling.adjustable frequency drives are a specific type of VSDs; they are controlling the rotational speed of an alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the motor.sinusoidal PWM is a type of "carrier-based" pulse width modulation. Carrier based PWM uses pre-defined modulation signals to determine output voltages. In sinusoidal PWM, the modulation signal is sinusoidal, with the peak of the modulating signal ways less than the peak of the carrier signal.here a SPWM inverter is designed and DC voltage is given to it, DC is converted to ac and a single phase induction motor is driven by this ac o/p and speed of this motor is adjusted by closed loop speed control method. Here PI control strategies has been adopted. II. THEORITICAL BACKGROUND Sinusoidal Pulse width modulation (SPWM) generated by comparing amplitude of triangular wave (carrier) and sinusoidal reference wave (modulating) signal. SPWM switching for three phase inverter is similar to that of a single phase inverter. Basically, each switch is controlled by comparing a sinusoidal reference wave with a triangular carrier wave. The fundamental frequency of the output is the same as the reference wave, and the amplitude of the output is determined by relative amplitudes of the reference and carrier waves. Each pair of switches requires a separate sinusoidal reference wave. The three phase reference sinusoidal is 120 apart to produce balanced three phase output. Figure 1: Sinusoidal Pulse Width Modulation www.ijarcsse.com, All Rights Reserved Page 123
Figure 2: Three Phase Voltage Source Inverter Inverter output voltage: When V control>v tri, V dc/ 2 When V control<v tri,-v dc/ 2 Figure 3: pulse width modulation technique. Control of inverter output voltage: SPWM frequency is the same as the frequency of V tri Amplitude is controlled by the peak value of V control Fundamental frequency is controlled by the frequency of V control. Modulation Index (m), Figure 4: waveforms of three phase spwm inverters www.ijarcsse.com, All Rights Reserved Page 124
BENEFITS OF VVFD 1. Controlled starting current. When an ac motor is started across the line, it takes as much as seven-to-eight times the motor full-load current to start the motor and load. But when using an adjustable speed drive, the substantially reduced starting current extends the life of an ac motor. The payback is less wear and tear on the motor, which translates to extended motor life and less motor rewinds. 2. Reduced power line disturbances. Starting an ac motor across the line and the demand for starting current places an enormous drain on the power distribution system connected to the motor. Typically, the supply voltage sags, which can cause sensitive equipment connected on the same distribution system to trip. Items such as computers, sensors, proximity switches and contactors are voltage sensitive and can drop out when a large ac motor is started nearby. Using an adjustable speed drive eliminates this voltage sag. 3. Lower power demand on start. The power needed to start an ac motor across the line is significantly higher than with an adjustable speed drive. This is true only at start, since the power to run the motor at load would be equal regardless if it were fixed speed or variable speed 4. Adjustable operating speed. Using an adjustable speed drive enables process optimization and making changes in a process. It also allows starting at reduced speed and allows remote adjustment of speed by programmable controller or process controller. 5. Controlled stopping. Controlled stopping helps reduce product loss due to breakage or mechanical wear and tear attributed to shocks to the process. 6. Energy savings. Centrifugal fan and pump loads operated with adjustable speed drives reduce energy consumption. If the speed of a fan is cut in half, the horsepower required to run the fan is cut by a factor of eight. The energy savings could relate to a return on investment of less than two years. ADVANTAGES OF SPWM The main advantage of SPWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on,there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. SPWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle. DISADVANTAGES OF SPWM There are two significant drawbacks with sinusoidal PWM. (i) Available output voltage Assuming that the DC voltage is created using a diode rectifier and capacitor dc link, the maximum available DC voltage is given by where VLLS is the line-line supply voltage. The maximum output using sinusoidal PWM (ma=1) is a sinusoidal PWM drive cannot produce a line-line output voltage as high as the line supply. One option to mitigate this discrepancy is to use higher supply voltages (e.g. 480V supply, 460V motor; 600V supply, 575V motor). (ii) Short Pulses If the output is to be truly sinusoidal PWM, it is important to include very small pulses when the peak modulation signal is close to the peak carrier voltage. These small pulses can contribute significantly to inverter losses, while not significantly effecting the output voltage. In addition, small pulses may be impractical due to the time required to switch one device off and another device on. Most industrial drives "drop" small pulses to improve efficiency. III. SOFTWARE IMPLEMENTATION & EXPERIMENTAL RESULTS The main aim of any modulation technique is to obtain variable output having maximum fundamental component with minimum harmonics. The objective of Pulse Width Modulation techniques is enhancement of fundamental output voltage and reduction of harmonic content in Three Phase Voltage Source Inverters. In this paper different PWM techniques are compared in terms of Total Harmonic Distortion (THD). Simulink Models has been developed for Sinusoidal PWM (SPWM), Space vector PWM (SVPWM), and Space vector PWM switching Patterns. Simulation work is carried in MATLAB 13.0/Simulink. The simulation parameters used are: Fundamental frequency 50 Hz Switching frequency 2 khz Amplitude modulation index 1.0 DC voltage 600 Volt ph. Asynchronous machine: 415 V, 0.5 HP. ODE Solver ode23tb www.ijarcsse.com, All Rights Reserved Page 125
Figure 5: Circuit diagram for PI Control of a single phase induction motor Figure 6: Unipolar SPWM output without Filter Figure 7: Unipolar SPWM output with LC Filter www.ijarcsse.com, All Rights Reserved Page 126
Figure 8: SPWM Technique Figure 9: Motor speed and Torque IV. CONCLUSIONS This report introduced adjustable speed drive system of a single phase induction motor. We presented SPWM inverter and control its speed. When we are changing the load, the voltage and frequency are changed, but voltage upon frequency ratio is constant so speed is remaining constant. Thus we have adjusted the speed and we have designed adjustable speed drive system incorporating SPWM Technique. But, while doing the project we got some error, LOSS OF VOLTAGE- 17.26% Actual voltage- 230V Practical value- 190.3V Harmonics are present. Harmonic voltages produce magnetic fields that rotate at speeds corresponding to the harmonic frequencies, resulting in increased losses, motor heating, mechanical vibrations and noise, pulsating torques, increased eddy current and hysteresis losses in stator and rotor windings, reduced efficiency, reduced life, and voltage stress on motor winding insulation. Proper speed control means that we need to vary both the motor voltage and frequency to control slip and keep it in a sweet zone in relation to the actual rotation speed. This is what Variable Speed Drives do, for single phase induction motors this idea does not really work.with just one phase, we get a pulsing voltage only. For this reason single phase motors artificially create a second phase to create the rotation. This second phase can be created in many ways using auxiliary windings and phase shift capacitors or coils.all proper variable speed drive controlled induction motors are 3 phase, as are the outputs of the variable speed drives. 3 phase shifted voltages is the minimum required to create a rotating field. But still for harmonic reduction tuned banks will be needed to totally divert all harmonics away from the system. For better approach PLC, fuzzy logic can be used. www.ijarcsse.com, All Rights Reserved Page 127
ACKNOWLEDGMENT I would like to express my gratitude to my professors for their guidance, support and suggestion. I am extremely grateful to the Applied Physics Dept., University of Calcutta and EE department of Brainware Group of Institutions. I humbly express my sense of gratitude to the faculty members, Laboratory staff, Library staff and administration of the institute for providing me a congenial working environment. REFERENCES [1] B. K Bose, Adjustable Speed AC Drives A Technology, [2] P. C. Sen. PRINCIPLES OF ELECTRICAL MACHINES AND POWER ELECTRONICS, [3] R.KRISNAN, ELECTRIC MOTOR DRIVE MODELINGANALYSIS AND CONTROL. [4] M. H. Rashid Power Electronics circuits, device and Application [5] Ned Mohan. Power Electronics: Converters, Applications, and Design [6] G.K Dubey Power semi conductor control drives [7] Simulation and Analysis of PWM Inverter Fed Induction Motor Drive.International Journal of Science, Engineering and Technology Research (IJSETR). Volume 2, Issue 2, February 2013 [8] Matlab based Simulation & Analysis of Three level SPWM Inverter. International Journal of Soft Computing and Engineering (IJSCE) ISSN: 2231 2307, Volume 2, Issue 1, March 2012 [9] Analysis, Design and Control of Sinusoidal PWM Three Phase Voltage Source Inverter Feeding Balanced Loads at Different Carrier Frequencies Using MATLAB International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering (An ISO 3297: 2007 Certified Organization) Vol. 3, Issue 5, May 2014 [10] Phuong Hue Tran, MATLAB/SIMULINK Implementation and Analysis of Three Pulse Width Modulation(PWM) Techniques, [11] JA Houldsworth and DA Grant, The use of Harmonic Distortion to Increase the Output Voltage of a Three- Phase PWM Inverter, IEEE Transactions on Industry Applications, 1990, IA-20(5), 1124-1128. [12] T. Erfidan, S. Urugun, Y. Karabag and B. Cakir. 2004. New Software implementation of the Space Vector Modulation. Proceedings of IEEE Conference. pp.1113-1115. [13] Control of VSI using spwm strategy for adjustable speed motors. 14th International Research/Expert Conference Trends in the Development of Machinery and Associated Technology TMT 2010, Mediterranean Cruise, 11-18 September 2010. [14] Simulation and comparison of spwm and svpwm control for three-phase RL load. IJREAS Volume 2, Issue 2 (February 2012) ISSN: 2249-3905. [15] Modeling, Simulation and Experimental Analysis of Transient Terminal Overvoltage in PWM Inverter fed Induction Motors (IEEE) www.ijarcsse.com, All Rights Reserved Page 128