Design of Electrical Parameter Measurement System for Three Phase AC Motor Based on STM32

Transcription

1 Sensors & Transducers 2014 by IFSA Publising, S. L. ttp:// Design of Electrical Parameter Measurement System for Tree Pase AC Motor Based on STM32 Haiong Xiao, Jiming Luo, Scool of Electrical Engineering & Information Henan Institute of Engineering, Xinzeng Longu, , Cina Tel.: Received: 15 May 2014 /Accepted: 30 June 2014 /Publised: 31 July 2014 Abstract: Electrical parameter online measurement is used to monitor te working state of AC motor, to prevent te voltage being too low or te load being too large, to protect te motor safely. Te paper mainly discusses te configuration and te principle based on Cortex-M3 for measuring te electrical parameter of alternating motor. Te formula is given in te paper tat calculates te AC signal armonics voltage, armonics current, armonics power. Importantly te metod of verification of te error measurement of power is given, wic remarkably increases te verification efficiency of apparatus system. Te paper also provides scematic diagram of main measurement ardware and describes its software design. According to te measurement results, te design acieved certain accuracy requirement, and meets te related tecnical requirements. Copyrigt 2014 IFSA Publising, S. L. Keywords: Electrical parameter, Online measurement, STM32, Tree pases AC motor, Sampling. 1. Introduction Tree pase asyncronous motors overload or voltage is too low or too ig, can lead te motor to overeating [1]. So electrical parameter measurement of motors is very important and te measurement system is being te necessary equipment. Many people were analyzed te testing principle of electrical parameter [3-9], and In te past few years, universal DSP, dedicated DSP, are used in te measure electrical parameters of te motor [10-14]. But universal DSP leads ig-cost, and as no ig embedded degree. Dedicated DSP as fixed-function, and engineers can t applied flexibly. Recently, someone designs te test system based on ARM [15], but ARM7 TDMI wic is fitted to te system as poor performance and as no periperal resources enoug at te same time. ARM9 is too complicated and as too ig-cost. So selecting a CPU is very important to design measurement system for te electrical Parameter of tree pase motor. ARM Cortex-M3, STMicroelectronic Company produced, as muc more advantage, suc as ig performance, low cost, low-power, etc. STM32 MCU brings opportunities and space for te design of electrical parameter measurement system for tree pase AC motor. Te STM32 family of 32-bit Flas microcontrollers based on te ARM Cortex -M processor is designed to offer new degrees of freedom to MCU users. It offers 32-bit product range tat combines ig performance, real-time capabilities, digital signal processing, and low-power, low-voltage operation, wile maintaining full integration and ease of development. ttp:// 205

2 2. Design of te System and Working Principle 2.1. Te Design of te Overall System Te measurement system is mainly composed by voltage sampling circuit, current sampling circuit, A/D conversion circuit, PLL circuit and STM32 system circuit. Te overall block diagram as Fig. 1. Te electric parameters measurement of Treepase AC motor, generally, requesting measure tree-pase wire line voltage, current, power by two components, frequency, power factor, 50 times following armonic content and te imbalance extent analysis of Tree-pase voltage. Te level of accuracy is above 0.5, and te measurement voltage and current is provided by external measurement transformer. N 1 2 T 2π b = y( k )sin( k) N N N K = 0 Harmonic amplitude for times: Pase angle: c = a + b, (2) 2 2 a φ = arctg, (3) b According to above equation for times: U = a + b 2 2 Voltage sampling circuit Current sampling circuit AD / Convert circvuit Level- Sifting circuitry Cortex-m3 STM32 Oter circuits U RMS: I = a + b U 2 2 N /2 = U, (4) = 0 2 Waveform conversion circuit PLL Fig. 1. Overall system structure diagram. Sampling signal of tree-pase voltage and current are sent to AD MAX125 wic is wit 14 bit and 6 cannels A/D. Cortex-M3-STM32F103 demand 5 V power source but MAX125 need 3 V, so if connect tem togeter, a level sift IC named 74LVC4245 is necessary. Sine wave of pase A sould be converted into rectangle wave by te circuit of waveform convert circuit, ten sent to PLL for 128 times frequency, tis signal interrupts STM32 to control AD convert circuit. Tis way, MAX125 do te work sampling 128 point equally spaced during one cycle, ten te voltage and current instantaneous. Every time STM32 analyses tese 128 sampling data, terefore voltage and current RMS, power values, as well as te armonic content are obtained Measurement Principle I RMS: I Power for pase C: N /2 = I, (5) = 0 = 1 2 P U I U I cosϕ Power for pase A: = +, (6) = 1 P U I U I cosϕ = +, (7) P = P + P 总 1 2, (8) Tree-pase unbalance factor: Tree-pase line voltage respectively: a= U, b= U, c= U Ten unbalance factor: ab bc ca Te signal waveform is discretized into [2]: a y t a t b t, (1) N 1 () = 0 + ( 2 k = 0 cosω + sin ω ) N 1 2 T 2π a = y( k )cos( k) N N N K = 0 were 1 (3 6 L) ε =, (9) 1 + (3 6 L) a + b + c L = ( a + b + c )

3 3. Design of te System Hardware and Software Te system ardware consists of te voltage sampling circuit, te current sampling circuit, display circuit, signal (rate) output circuits, communication circuit, etc. Because communication circuit, display circuit, key circuit using conventional circuits, terefore, witout discourse in tis paper Te Design of Voltage and Current Sampling Circuit Te current sampling circuit sown in Fig. 2, te voltage sampling uses te same circuit, wic convert te voltage to te current firstly. Current transformer wit ig accuracy, wide range of CT. Wen inputs current being 5 A, te current transformer outputs 10 ma. CT transformer in series resistance 100 Ω to get 1 V voltage. Beind te voltage amplification circuit, te input signal to te back of 4 V, make te system able to receive te impact of te current rated 120 %. Te voltage transformer as complex structure, and te price is too ig. So te voltage sampling circuit uses te same te current sampling circuit wit series resistance in te current transformer Te Design of A/D Convert Circuit Two A/D cip named MAX125 are used in te measurement system, te interface circuit wit Cortex-M3 as Fig. 3. Fig. 2. Current sampling circuit. Fig. 3. A/D and Cortex-M3 interface circuit. 207

4 Tree pase voltage and current signals are converted by two MAX125, wic outputs te igest level is 5 V. So an IC cip 74AHCT4245 is necessary to matc te 3.3 V voltage level of STM32. Two 74AHCT245 are used in tis circuit as level sifter unit. 74AHCT4245 output signals and input signals are connected respectively to MAX125 and STM32 wit 14-bit DBUS. Te control signals voltage level is matced just as data signal. Equally spaced signal tat comes from PLL and te signal of A/D conversion completed are sifted by 74AHCT4245. Te signals tat two MAX125 receive respectively are Uab, Ubc, Uac, Ia, Ib, Ic. Wen calculate te unbalance factor, using Uac instead of Uca Te Design of Frequency Output Circuit and te Error Calibration Metod Te Design of Signal Output Circuit Te accuracy of power measurement must be calibrated, usually, te metod is getting te error by comparing measuring data wit te standard data, wic is very inconvenient. So te circuit tat frequency output is designed for calibrating te measurement error conveniently. Te circuit is sown in Fig. 4. In te Fig. 4, D/A converter wit 12 bits is used. STM32 sends te value of power wit 12 bits to D/A, ten te D/A output analog signal wic is proportional to te digital data. Troug te follow-up circuit called U/F sift circuit te analog signal cange to frequency signal, wic is proportional to te analog signal. By sending te frequency signal to Standard Electric Energy Meter, te error of te measurement system is obtained Te Calculation of Power Constant Te debug condition is in te case of power factor being 1, inputting rated voltage and rated current, toug te wole process, te circuit outputs frequency of 5 KHz. Wen calibrate te system, divide te frequency signal to low frequency signal, ten one low frequency signal pulse is a constant value, named power constant C. Fig. 4. Frequency output circuit. 208

5 UI e e C =, (10) 5 Send te low frequency pulse into te electrical energy error calculator, and input te C to te electrical energy error calculator, te measurement error of te system will be obtained just as te Energy Meter s Te Design of te Software Te system software is used to complete te measurement of voltage, current, power, armonic, unbalance factor of tree-pase. Te subroutines of Cortex-M3 function library are applied as armonic calculation program. Te main flow carts of system completes te system initialization, starts A/D sampling function to complete 128 times A/D in one voltage cycle. Te flow cart as sown in Fig Measurement Error In library, te standard electric energy meter named WT 3030 and tree-pase testing power are selected to test te accuracy of tis measurement system. Input 200 V voltage and 5 A current. Te power testing data is as Table 1. Table 1. Power testing data. Voltage (V) Current (5A) Power factor Standard data Testing data % % % % % 0.5L % 0.8C By Table 1, te error of tis system is: δ = 100% = 0.1% Conclusion Tis system is designed for tree-pase motor parameter measurement, te experimental results sow tat te system as ig accuracy, low cost, convenient debug, aving a broad application prospect. Acknowledgements In te design process, Cen Zong Ma and Wang Zen, studying in te Electronic and Information Department, Henan Institute of Engineering, do a lot of work in te ardware and software design. In te tesis writing process, Zou Liuyang and Ge Jingtao gave me a lot of elp, expressing gratitude togeter. References Fig. 5. System main flow cart. [1]. Wang Yiquan, Zang Bingyi, Motor testing tecnology, Science Press, 2008 (in Cinese). [2]. Zang Hong, Power signal syncronous sampling algoritm, Journal of Power Systems and Automation, Vol. 24, No. 3, 2012 (in Cinese). [3]. Liu Jun, Wu Cunua, Huang Jian Ming, Yu Jin Sou, Te parameter measurement of permanent magnet syncronous motor, Power Electronics, Issue 1, 2010, pp (in Cinese). [4]. Zang Yongjun, Harmonic error modifying of electric energy meter considering armonic response, Power System Protection and Control, Vol. 37, No. 22, 2009, pp (in Cinese). [5]. Hui Jin, Yang Honggeng, Harmonic and interarmonic detection metod based on neigboring spectral offsetting and windowed TDA in two stage, 209

6 Power System Protection and Control, Vol. 37, No. 23, 2009, pp (in Cinese). [6]. P. Bordignon, Design Tools of DC EAF Power Supply Systems for Optimum Reduction of Flicker and Inter Harmonic Disturbances, in Proceedings, [7]. Zang Ming, Li Kaiceng, Real-time ig accuracy armonic power measurement using adaptive Fourier linear combiner, Electric Power Automation Equipment, Vol. 29, Issue 10, 2009, pp (in Cinese). [8]. Xiong Jiefeng, Summary of power system armonic measurement, Cina Science and Tecnology Information, Vol. 23, No. 2, 2007, pp [9] S. Corino E. Romero L. F. Mantilla, How te efficiency of induction motor is measured?, in Proceedings of te International Conference on Renewable Energies and Power Quality (ICREPQ '08), Santander, [10]. Gao Yunpeng, Teng Zaoseng, Liu Peng, Wang Yi, Zou Liangzang, Design of tree-pase multifunctional armonic energy meter, Journal of Hunan University (Natural Science), Vol. 35, No. 9, 2009, pp (in Cinese). [11]. Li Xiuying, Te latest Development of International Motor Energy Efficiency Standards, Motor and Control Applications, 34, [12] Zang Hao, Researc of Motor Efficiency Testing Tecnology, Motor and Control Applications, 39, [13]. Zu Yuua, Efficiency of Efficient Motor and te Analysis of Effectiveness, Science and Tecnology Communication, Vol. 7, [14]. Cen Lizou, Electric measurement, Cina Macine Press, Beijing, [15]. Liu Bowen, ARM Cortex-M3 detailed examples of application development, Publising House of Electronics Industry, Copyrigt, International Frequency Sensor Association (IFSA) Publising, S. L. All rigts reserved. (ttp:// 210