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1、Electric Measurements with LabVIEW VALENTIN DOGARU ULIERU Faculty of Electrical Engineering, University Valahia of Targoviste 18-20 Blv. Unirii, Targoviste, Dambovita ROMANIA Abstract: The paper presents a data acq
2、uisition system which consists in Hall effect sensors, a PCI 6023(National Instruments) data acquisition board, Lab VIEW graphical programming environment and the experimental results achieved by the authors concerning
3、 the behavior of ac electrical circuits. The instruments used in the measurement technique were developed as computer data base equipments, using well determined functions (the acquisition of parameters, signal process
4、ing/adapting) with the communication possibility on a serial interface or on a parallel port. Today, data acquisition boards are used and can be assembled directly into the computer, having the operation possibility of
5、 an oscilloscope. The appearance of the LabVIEW environment was motivated by the research automation activity and by the application development, based on a hierarchical instrument structure, which is composed by the u
6、ser's interface and the visual programming elements. Keywords: data acquisition, graphical programming, Hall effect, electric power 1 Introduction The use of the LabVIEW graphical programming environment ensur
7、es the analysis and study of power measurement methods in single-phase and three-phase alternative current circuits [3, 6, 7]. The evolution in both electric measurement technique, in the electronic field and in the
8、area of data acquisition systems, arguments the opportunity and justification of designing new instruments in order to improve the research activity in this area [2, 3]. 2. Power Measurement in Single- Phase
9、 AC Circuits 2.1 Powers in AC Circuits The instantaneous power [1,2,3,4] to an electric dipole is defined as the product of the instantaneous values of the voltage (u) to the terminal of the dipole and the current (
10、i) that flows through the dipole: p ui =(1) The instantaneous power can be classified into input and output power, depending on the association of the voltage (u) and the current (i), which respects the rule of recei
11、vers and generators. In a sine-wave steady-state with the T period, the active power (P) can be defined as the average value of the instantaneous power, considering a natural number of periods: P ui nT pdttt nT = =+
12、∫ 111(2) For a single-phase circuit which functions under a sine-wave permanent rate, in which the voltage and current have the following expressions: ( ) u t U t = 2 sinω (3) ( ) ( ) i t I t = 2 sin ω ? ?(4) it resul
13、ts: - the active power: P = UIcos? - the reactive power: Q = UIsin?(5) - the apparent power: S = UI The complex apparent power (S) is defined into the simplified complex representation as the product between the
14、complex voltage (U) and the conjugate complex current (I*): jQ P jUI UI UI S + = + = = ? ? ? sin cos(6) The real part of the complex power (S) is the active power (P), the imaginary part is the reactive power (Q), th
15、e module is the apparent power (S) and the argument is equal to the phase displacement (?) of the circuit: { } S P Re = ; { } S Q Im = ; S S =(7) For a single-phase circuit which does not function in sine-wave rate
16、 [4] and has the terminal voltage u(t): Proc. of the 8th WSEAS Int. Conf. on Mathematical Methods and Computational Techniques in Electrical Engineering, Bucharest, October 16-17, 2006 197galvanic isolation of t
17、he measuring system, but it introduces ratio and angle errors and realizes an inadequate perturbation transfer. The adopted solution was to use current and voltage transducers based on the Hall effect. The block dia
18、gram of the acquisition system is presented in fig.2 and fig.3 presents the experimental results. Remark: The voltage values and the parameters of the consumers in fig. 3a, were introduced into the application realiz
19、ed for simulation (fig.1). 3. Power Measurement in Three- Phase AC Circuits For a random receiver (Z), consisting in linear impedances, forming a system with n nodes which is alimented through a circuit with n conduc
20、tors [1], the total complex apparent power (S) transmitted to the receiver is: S V I V I V I V I k k n n = + + + + + 1 1 2 2 * * * ... ... * (13) By expressing the potentials of the nodes using the potential diffe
21、rences reported to a point N having a random potential, the expression (3.1) becomes: S U I U I U I U I N N kN k nN = + + + + 1 1 2 2 * * * ... ... n *(14) The definitions of active and reactive power give the followin
22、g results: ∑ = = + + + + + =++ + == + + = =nk k n kn nN n nNN N N Nn nN N NP P P P P PI U I UI U I U I U I UI U I U I U S P1 2 12 2 2 2 1 1 1 1* * 2 2* 1 1... ...) , cos() , cos( ) , cos(} Re{ } Re{(15) ) , sin( ... ) s
23、in() , sin( }Im{ } Im{2 2 2 21 1 1 1** 2 2* 1 1n nN n nN N NN N n nNN NI U I U I U I UI U I U I UI U I U S Q+ + ++ = ++ ? ? ? ? + + = =∑ = = + + + + + =nk k n k Q Q Q Q Q Q1 2 1 ... ...(16) The total active power P (resp
24、ectively the reactive power – Q) consumed by a random receiver with n phases and alimented through a line of n conductors, is equal to the sum of n active single- phase powers (or reactive single-phase powers) which
25、are given by the I k line currents, with the U kN voltages between the n conductors and the N point. The alternative three-phase circuits have the following voltage system: ) 32 sin( 2) 32 sin( 2sin 2321π ωπ ωω+ =? =
26、=t U ut U ut U u(17) If the voltage system supplies a three-phase balanced receiver, the current system will be: ) 32 sin( 2) 32 sin( 2) sin( 2321? π ω? π ω? ω? + =? ? =? =t I it I it I i(18) If the phase impedances ar
27、e different, the receiver is not balanced and the absorbed currents from the source can be calculated with methods that are related to star connected three-phase balanced receivers, it results: 0 3 2 1 I I I I = + +(
28、19) Regarding the impedance value of the neutral conductor (Z0), the voltage value will be: 0 0 0 I Z U ? = . (20) The block diagram of the data acquisition system is presented in fig.4. The measuring methods for act
29、ive/reactive power in three-phase ac circuits, depend on the type of the consumer and the number of conductors in the electric energy supply system. I1/I2/I3 – LA55P; IN – LA25NP; U10/U20/U30 – LV25P Fig.4. Data Acq
30、uisition System – block diagram For three-phase circuits with neutral conductor (n=4), the generalized theorem becomes: { } P S U I kN k k = = ? ? ?? ?? ? ?? ??= ∑ Re Re14(21) Proc. of the 8th WSEAS Int. Conf. on Mathem
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