A New Generation VLSI Approach for V/F Control of Three-Phase Induction Motor

Transcription

1 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November 21 7 A New Generation VLSI Approach for V/F Control of Three-Phase Induction Motor M.S.Aspalli 1, Veerendra.D 2 and P.V.Hunagund 3 1 Dept of Electrical & Electronics Engg, Poojya Doddappa Appa College of Engg, Gulbarga.INDIA. 2 Dept of Electrical & Electronics Engg, Poojya Doddappa Appa College of Engg, Gulbarga.INDIA. 3 Department of Applied Electronics, Gulbarga University,Gulbarga. Karnataka.INDIA Abstract AC induction motor (ACIM) is the workhorse of industrial and residential motor applications due to its durability, robustness and reliability. In today s energy conscious world, driving and controlling the induction motor efficiently are prime concerns. In the proposed scheme the v/f control of 3-phase induction motor uses the advanced VLSI technology that is a unique VLSI chip called Digital Signal Controller (DSC), which has features of both microcontroller and DSP. In this work, a v/f controlled three phase induction motor controller is developed using dspic30f20 chip. The output voltage is filtered using c-type output filter. A 1HP, 3-phase, 415V, 50Hz, 1440rpm, induction motor is used as load for the inverter. Digital Storage Oscilloscope Textronix TDS24B is used to record and analyze the various waveforms. The experimental results for v/f control of 3-PH induction motor using dspic30f20 chip clearly shows constant volts per hertz and stable inverter line to line output voltage. Keywords: DSC, constant volts per hertz, VLSI, PWM inverter, ACIM. 1. Introduction As far as the machine efficiency, robustness, reliability, durability, power factor, ripples, stable output voltage and torque are concerned, three- phase induction motor stands at the a top of the order. Motor control is a significant, but often ignored portion of embedded applications. Motor control applications span everything from residential washing machines, fans to hand-held power tools, and automotive window lift, traction control systems and various industrial drives [1-2]. All most in all the applications there is a drastic move away from analog motor control to precision digital control of motors using different processors. Digital control of induction motors results in much more efficient operation of the motor, resulting in longer life, lower power dissipation. Although various induction motor control techniques are in practice today, the most popular control technique is by generating variable frequency supply, which has constant voltage to frequency ratio. This technique is popularly known as V/F control [4]. Introduction of VLSI technology for the induction motor control are getting more and more important and popular now a days. Both microcontroller and DSP are presently used in motor control. In this proposed scheme v/f control of three-phase induction motor is developed by using digital signal controller dspic30f20. A Digital Signal Controller (DSC) is a single-chip, embedded controller that seamlessly integrates the control attributes of a Microcontroller (MCU) with the computation and throughput capabilities of a Digital Signal Processor (DSP) in a single core. Microchip s dspic DSC offers everything what we would expect from a powerful 16-bit MCU. By skillfully adding DSP capability to a high-performance 16-bit MCU, Microchip s dspic30f families of DSCs achieve the best of both worlds and mark the beginning of a new era in embedded control of three-phase induction motor [5]. Figure 1. Torque-speed characteristics of the induction motor

2 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November V/F Control of Three-Phase Induction Motor The torque developed by the induction motor is directly proportional to the v/f ratio. If we vary the voltage and frequency, keeping their ratio constant, then the torque produced by induction motor will remain constant for all the speed range. Fig.1 shows the torque-speed characteristics of the induction motor with V/F control. The voltage and frequency reaches the maximum value at the base speed. We can drive the induction motor beyond the base speed. But by doing so only frequency varies but not voltage. Hence the ratio of v/f will no longer remain constant. Since the torque developed by the induction motor is directly proportional to the v/f ratio will not remain constant throughout the speed [4]. Let us consider the sinusoidal ac voltage is applied to the three-phase induction motor [6]. Then we have steady state, V jωφ (1) That is, V ωφ (2) Where V and φ are magnitude of stator voltage and stator flux and V and φ are phasors of stator voltage and stator flux, respectively, Thus, we get φ V V ω 2Πf (3) Hence from the above equation, we can write V φ (4) f The slip for maximum torque is Rr sm = (5) R + β X + X 1/ [( ( ) )] 2 s S r Where S m, R s, R r, X s, X r are Slip for maximum torque, per phase resistance of stator winding, per phase resistance of rotor winding, per phase leakage resistance of stator winding, per phase leakage resistance of rotor winding referred to stator winding respectively. The acceleration and deceleration of the motor can be controlled by controlling the change of the supply frequency to the motor with respect to time [8]. 3. Implementation of V/F Motor Drive 3.1 System overview The basic block schematic of three-phase induction motor drive is shown in Fig.2. It has three-phase full bridge rectifier, three-phase full bridge inverter, control circuit, speed sensing unit and output filter. In the proposed work the three-phase bridge rectifier is designed using IN5408 power diodes. Each power diode is protected from high dv/dt by using metal oxide varistors.the output of rectifier is filtered by 100µF, 450V capacitors. The threephase inverter has FGA25N120ANTD -IGBT switches, with the snubber circuit for each switch. The output of inverter is filtered by c-type filter. The filtered output is applied to the three-phase induction motor. The digital control of motor is achieved by applying gate pulses from the control circuit to each the IGBT s switch through optoisolation. 3.2 Power circuit design The power circuit is designed using 25A, V IGBT. These IGBTs are protected against surge voltages using snubber circuit. The 3-phase induction motor is connected to 3-phase bridge inverter as shown in Figure 3. The power inverter has six IGBT switches that are controlled in order to generate 3-phase AC output from the DC bus. PWM signals, generated from the controller, control these six switches. The amplitude of phase voltage is determined by the duty cycle of the PWM signals While the motor is running, three out of six switches will be on at any given time; either one upper and two lower switches or one lower and two upper switches. The output of the inverter is almost square wave which is having harmonics. Figure 2. Block diagram of complete system A passive filter at the output of power circuit is used to remove the harmonics. The windings of induction motor oppose any sudden change in direction of current flow until all the energy stored in winding is dissipated. A fast recovery diode is connected antiparallel to each IGBT to facilitate this. This diode is known as freewheeling diode.

3 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November 21 9 Hence all the six IGBTS are having six freewheeling diode which is shown in power circuit [3-5]. If the upper and lower switches of the same half bridge are switched on at the same time then this will cause DC bus supply to short. To prevent the DC bus supply from being shorted, certain dead time must be given between switching off the upper switch and switching on the lower switch and vice versa. This ensures that both switches are not conducting at the same time as each one change states. The dead gap is made longer than the maximum turn-on time and turn-off time. Fig.4. shows the gate pulses for firing the IGBTs. 3.3 Control Circuit Figure 3. Three-phase bridge inverter In this work Microchip s dspic30f20 digital signal controller is used. Microchip s dspic30f20 digital signal controllers place unprecedented performance in the hands of 16-bit MCU designers. The dspic DSC has the heart of a 16-bit MCU with robust peripherals and fast interrupt handling capability and the brain of a DSP that manages high computation activities, creating the optimum single-chip solution for embedded control of three-phase induction motor. Figure 4. Gate pulses for the firing IGBTs. It also consists of six opto-coupler for isolating the control and power circuits. Six IGBTs of the power circuit are controlled by the pulse width modulation (PWM) signals generated by the control circuit. These PWM signals are indeed required to derive a varying AC voltage from the power circuit. A dead time of 2 micro second is given between switching off the upper switch and switching on the lower switch and vice versa, to avoid shorting the DC bus. In this proposed scheme a 50% PWM duty cycle is used. Because a 50% PWM duty cycle is normally used as a zero-current reference point [6]. If all the PWM duty cycles are at 50%, then all the phases will have same applied average voltage and there will be zero average current flowing in the motor. If the PWM signals are raised above the 50%, then the positive current will be generated in the winding. If the PWM signals are raised below the 50%, then the negative current will be generated in the winding. 4. Experimental Results and Analysis of Speed Control The proposed scheme is implemented and tested in power electronics laboratory. Various readings are taken for various speed and load. The results are tabulated in table 1, 2, 3, 4, 5 and 6 Fig. 5, 6,7,8,9, and 10 show the various characteristics for the set RPMs 1440,1380,1320, respectively. In this work an accuracy of 98% is achieved in the speed control. Fig 13 shows the complete hard ware experimental set up. Table 1. Results for load Torque vs rpm Load Torque RPM Of V out Current (A) Speed (RPM)

4 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November Figure 5. RPM vs Load Torque for 1440 rpm Speed (RPM) N O Speed (rpm) 1 Table 2. Results for load Torque vs 1380 rpm Load Torque RPM Of V out Current (A) Figure 6. RPM vs Load Torque for 1380 rpm Table 3. Results for load Torque vs 1320 rpm Load Torque RPM 1320 Of V out Current (A) Figure 7. RPM Vs Load Torque for 1320 rpm Table 4. Results for load Torque vs rpm Load Current Torque RPM Of V out (A) Speed (RPM) (Hz) Figure 8. RPM vs Load Torque for rpm Table 5. Experimental results for load 0.5Kg. speed Stator voltage Hz V/F Voltage(volt) Figure 9. Stator voltage magnitude verses frequency Table 6. Experimental results for load 2Kg Stator speed voltage Hz V/F

5 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November (Hz) Voltage (volt) Figure 10. Stator voltage magnitude verses frequency From the Table 5 and 6 we can observe that, as the voltage is varied, the frequency also varies proportionally. Hence the ration of v/f is almost constant. Figure 11 (a), (b) and (c) shows the gate pulses at 500gm load for the 1440, 1380 and 1320 rpm speed respectively. Figure 11(c) Gate pulses at 500gm load for 1360rpm Fig.ure 12 (a), (b), and (c) shows the waveforms of inverter line to line voltage at and 1360 rpm speed. Figure 12(a) Waveforms of inverter line to line voltage for 1440rpm Figure 11(a) Gate pulses at 500gm load for 1440rpm Figure 12(b) Waveforms of inverter line to line voltage for 1380rpm Figure 11(b) Gate pulses at 500gm load for 1380rpm

6 IJCSI International Journal of Computer Science Issues, Special Issue, ICVCI-21, Vol. 1, Issue 1, November Figure 12(c) Waveforms of inverter line to line voltage for 1320rpm 5. Conclusions Figure 13. Photograph of complete system A new generation VLSI approach for the v/f control of three-phase induction motor has been presented. This complete system is developed and tested in power electronics laboratory. Speed control of motor is acquired with the accuracy of ±10 rpm. Hence in this proposed scheme 98% accuracy of speed control is recorded. With the variation of stator voltage, frequency is also varied proportionally, such that v/f ratio is constant. The inverter line to line voltage recoded is very stable and very smooth compared to single-phase. Hence this three-phase induction motor V/F control by DSC is more stable, efficient and economical. References [1] Alfredo,Thomas A. Lipo and Donald W. Novotny, A New Inductio Motor V/f Control Method Capable of High- Performance Regulation at Low Speeds IEEE Trans. Industry Applications, Vol. 34, No. 4 July/ August [2] Pradeep M Patil, Sanjay L Kurkute, Speed control of three phase induction motor using single phase supply alongwith active power factor correction ACSE Journal, Volume (6), Issue (3), Oct. 6. [3] Speed Control of 3-Phase Induction Motor Using PIC18 Microcontrollers, by Microchip Technology Inc. [4] VF Control of 3-Phase Induction Motors Using PIC16F7X7 Microcontrollers by Microchip Technology Inc. [5] An introduction to AC induction motor control using dspic30f MCU, by Microchip Technology Inc. [6] AC Induction Motor Control Using Constant V/Hz Principle and Space Vector PWM Technique with TMS320C240, by Texas instruments. [7] Ned Mohan Tore. Undeland and William.p.Robbins: Power Electronics:Converter,Applications and Design,John Willey and Sons,1995. [8] Rashid M.H,Power Electronics-Circuits,Devices and Apllications, third edition Printice HallIndia,20. [9] K.Gopakumar,V.TRanganathan and R,Bhat, Three phase induction motor operation from pwm voltage source inverter. IEEE Trans. Industry Applications, Vol. 29, No. 5 September/October [10] Werner Deleroi, Johan B. Woudstra, and Azza fathima, Analysis and Application of Three-phase Induction Motor Controller with Improved Transient Performance. IEEE Trans. Industry Applications, VOL. 25,. 2, MARCHIAPRIL [11] A New Induction Motor V/f Control Method Capable of High- Performance Regulation at Low Speeds Alfredo Mu noz-garc ıa, Thomas A. Lipo, Fellow, IEEE, and Donald W. Novotny, Fellow, IEEE. [12] K.Koga, R.Ueda and T.Sonada, constitution of v/f control for reducing the speed error in induction motor system. In Conf.Rec.IEEE-IAS Annu.Meeting,1990. [13] Robbie F. McElveen, Member, IEEE, and Michael K. Toney, Senior Member, IEEEStarting High-Inertia Loads IEEE Transactions On Industry Applications, Vol. 37, No. 1, January/February 20. [14] M.S.Aspalli,, Vinaya Kumar, P V Hunagund, Development and Analysis of Variable Three Phase Induction Motor Drive, IJ-ETA-ETS, July 10-Dec 10, Vol.3, Issue2:PP