Experiment on the effective value method of power consumption measurement

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Experiment on the effective value method of power consumption measurement [Copy link]

Friday, July 28, 2006 20:46:13

Experiment on the effective value method of power consumption measurement Measure test for RMS method of power loss

Abstract: This paper derives the calculation formula for measuring power consumption by the effective value method, and uses the effective value method to actually measure the power consumption of samples, and compares the results with those measured by the product voltmeter method. Keywords: Power consumption Effective value method Measurement 1 Introduction In 2000, the International Electrotechnical Commission first recommended the effective value method for measuring power consumption in the standard IEC62044-3, and gave the test circuit and test method in Appendix B, but did not involve the origin of the measurement principle and calculation formula. In previously published articles, although some people have stated the effective value method for testing power consumption, the derivation process of its principle is not easy to understand. We have done a lot of exploratory experiments on this method of measuring power consumption by effective value, and found that an important problem is that the power consumption measured after adding an air gap to the sample to be tested is significantly lower than that without adding an air gap. This is very different from the results measured by other methods, and no reasonable explanation has been found. 2 Measurement principle In the IEC standard 62044-3 released in 2000, the effective value method was first recommended to measure the magnetic core standard. The measurement circuit diagram given in its Appendix B is shown in Figure 1. To measure power consumption using the circuit shown in Figure 1, three windings are required on the magnetic core, namely N1, N2 and N3. N1 is the excitation winding, N2 is the measurement winding, and N3 is the flux density monitoring winding; Vrms is the effective value voltmeter, and Vav is the average value voltmeter. The excitation source voltage waveform used in this circuit can be sinusoidal, rectangular, pulse or other shapes. In practice, power sources that generate sinusoidal signals are commonly used, so the effective value method for measuring power consumption discussed in this article is only based on a sinusoidal excitation source as a prerequisite, and other excitation waveforms are not involved. The effective value method for measuring power consumption used in this article is shown in Figure 2. In Figure 2, Es is a sinusoidal excitation source. T is a transformer consisting of a magnetic core to be measured and two isolated windings wound on it. Vrms is an effective value voltmeter, R is a current measuring resistor, S2 is a single-pole double-throw switch, S1 is a double-pole double-throw switch, N1 is an excitation winding, N2 is a common winding for measurement and flux density monitoring, N2 is connected between the middle terminal of S2 and the voltmeter Vrms through S1, and the reversal of S1 is equivalent to swapping the two ends of N2. When S2 reverses to I, the voltmeter Vrms only measures the voltage across the winding N2; when S2 reverses to II, the voltmeter Vrms measures the sum of the voltage drop UR on the resistor R and the induced voltage U2 across N2, that is, UR+U2. When S1 is reversed, the phase of U2 is opposite to the original, that is, the phase difference is 180°, which is equivalent to -U2. At this time, the voltmeter Vrms measures UR-U2. As we all know, under the action of alternating sinusoidal excitation magnetic field, the dynamic hysteresis loop of soft magnetic materials quickly becomes elliptical as long as it is not close to saturation, which means that the change of magnetic field H and magnetic flux density B with time is sinusoidal, but B lags behind H by one phase angle. B is sinusoidal, and according to Faraday's law of electromagnetic induction, the induced voltage across N1 and N2 should also be sinusoidal. In addition, because the excitation source Es is sinusoidal, the voltage across R should also be sinusoidal. In Figure 2, the input impedance of Vrms is very large, N2 can be regarded as an open circuit, and T can be replaced by a series circuit during excitation, as shown in Figure 3. In Figure 3, Ls is the series equivalent pure inductance of the magnetic core to be tested, Rm is the series equivalent resistance caused by loss, and Rc is the resistance of the excitation coil itself. During the test, the secondary is treated as an open circuit, and the impact on the primary circuit can be ignored. The voltage U′1 across (Ls+Rm) should be equal to the induced electromotive force generated at both ends of the excitation winding by the change of B in the magnetic core with time. The voltage URC across Rc is equal to the voltage drop across Rc, and the voltage U1 applied across the excitation winding should be equal to the voltage across (Ls+Rm+Rc). Because all voltages on the secondary are induced electromotive forces generated on the secondary winding due to the change of B over time, they are directly related to the change of B, proportional to U′1, and have nothing to do with URC. Assuming that the excitation source Es generates a sinusoidal voltage, the rotating vector diagram of each related voltage in Figure 2 is shown in Figure 4 under the excitation state. In Figure 4, U1 is the total voltage applied across the excitation winding. This voltage is equal to the vector sum of the voltage UL across Ls and the voltage drop URC on Rs (=Rm+Rc), and U′1 is the vector sum of UL and the voltage drop URm on Rm. U′1 is equal to the induced electromotive force generated on the excitation winding, and U2 is the induced electromotive force generated on the test winding, so U2 and U′1 can only be in the same direction or in the opposite direction. The same direction is called U2, and the opposite direction is (-U2). UR is equal to the voltage drop on the resistor R used for current measurement. The vector sum of UR and U2 is M1, and the vector sum of UR and -U2 is M2. During the measurement process, if S1 is turned to position "I", M1 is measured, and if S1 is turned to position "Ⅱ", M2 is measured. In short, the two M measured at positions "I" and "Ⅱ" are called M1 and M2. After measuring M1 and M2, the power loss of the magnetic core can be calculated. The calculation formula is derived as follows: That is , (1) . ( 2) Subtract (2) from (1) to get: . (3) Because , (4) . (5) Substituting (4) and (5) into (3) yields: . (6) (6) In formula, I1, U′1, M1 and M2 should all represent peak values. If both ends are divided by 2 at the same time, I1, U′1, M1 and M2 can be converted into effective values. That is to say, I1, U′1, M1 and M2 in formula (6) can be regarded as effective values at the same time, and the formula still holds. After I1 and U′1 are regarded as effective values, the left side of formula (6) represents the active power input from both ends (Ls+Rm), and the power consumed by Rc is not included, which means that the measured power consumption is purely the power consumed by the core. Therefore, the core power consumption . (7) (7) is exactly the formula given in Appendix B of the IEC62044-3 standard. 3 Measurement procedure According to the test conditions and the effective cross-sectional area Ae of the measured core, the monitoring voltage E2 during the measurement is formally calculated: (8) In the formula, N2 represents the number of secondary turns, f is the test frequency, and Bm is the peak value of the flux density specified by the test conditions. Put S2 in position "I", adjust Es output so that the voltmeter Vrms reading is E2, immediately turn S2 to "Ⅱ", measure M1 with Vrms, then turn S1 from "I" to "Ⅱ" (or from "Ⅱ" to "I"), read M2 on Vrms, and then use formula (7) to calculate the total power consumption of the core under given conditions. 4 Comparative test of measurement methods We used the effective value method to conduct specific measurement tests on the product. And compared with the measurement data of the product voltmeter method, it was found that some problems could not be convincingly explained. 4.1 Instruments used Product voltmeter method: Use American 2330 VAW METER, effective value method: The voltmeter uses American 2330 VAW METER, and the excitation source Es uses WL3866 broadband power signal generator. 4.2 Test sample A. φ24.62×14.54×7.44 toroidal core; B. A pair of processed EC28 cores (without air gap); C. The same pair of EC28 cores as B (the air gap between the two legs is 0.05×2). 4.3 Test conditions A is 100kHz 60mT 25℃, B and C are 30mT, 100kHz 25℃. 4.4 Measurement method 4.4.1 Single winding product voltmeter method: The same winding N1 is used for excitation, monitoring and measurement. 4.4.2 Double winding product voltmeter method: Winding N1 is used for excitation, N2 is used for monitoring, and the current I1 in N1 and the voltage U2 across N2 are used for measurement. 4.4.3 Effective value method: Using the method described in this article, winding N1 is used for excitation, N2 is used for monitoring, and N2 is used for measurement. 4.5 Measurement results We measured the power consumption of the three samples A, B, and C in Section 4.2 using the method given in Section 4.4. The measurement data is shown in Table 1. 5 Discussion of the problem 5.1 From the test results, for the measurement of power consumption of closed magnetic circuit samples, the power consumption measured by the single winding method is slightly higher than that measured by the double winding method. According to some people, it is sometimes lower and irregular. We believe that the monitoring voltage is the induced voltage used in the double winding method and the effective value method. This induced voltage is completely generated by the change of magnetic induction intensity B over time. Therefore, the voltage monitoring calculated by B meets the B value specified in the test conditions. When using a single winding for product voltmeter measurement, because the measurement and monitoring windings use the same winding, this means that the voltage used for measurement includes the voltage drop on the wire resistance. In fact, the monitoring voltage is lower than the voltage used for measurement. We also found this fact in the experiment. According to the above analysis, the power consumption measured by the single winding method is slightly lower than that of the double winding method, that is, the magnetic flux density B is slightly lower than that of the double winding method, which will reduce the power consumption. If the effects of these two factors on power consumption are exactly offset, the test results of the single winding method and the double winding method will be equal. If they cannot be offset, then it is either too high or too low. 5.2 According to past experience, when the magnetic core is measured, if the end face is not processed well and an air gap appears, the power consumption test result will increase significantly whether it is measured by the parallel resonance method or the product voltmeter (single winding) method. However, when measuring with the effective value method, we found that the power consumption value measured after adding the air gap is significantly reduced compared with the case without adding the air gap. The same phenomenon occurred when measuring with the double winding product voltmeter method. This makes us have to consider a question: after the magnetic core is gapped, when it is running under the same conditions, should its power consumption increase or decrease? Or should it not change? Generally speaking, it is believed in theory that the power consumption density of the magnetic core is related to the temperature, frequency and working magnetic flux density. After adding a small air gap in the magnetic circuit, the total power consumption of the magnetic core should not change because the temperature, frequency and working magnetic flux density have not changed. After the magnetic core is gapped, the effective magnetic permeability decreases, and the impedance of the excitation coil decreases, causing the excitation current to increase significantly, which causes the copper loss of the excitation winding itself to increase, making the total power consumption significantly increased when measuring with the single winding method. This explanation can qualitatively explain why the power consumption increases when measured with a single winding after adding an air gap to the magnetic core. However, we measured the resistance of the excitation winding itself and the current passing through it during measurement. The power consumed by the excitation winding can be calculated using the measured resistance and current. The calculated power is much smaller than the increased power consumption after adding the air gap. That is to say, there are other reasons that have not been found for the sharp increase in power consumption when measuring with a single winding after the air gap is opened. Whether the power consumption has really increased or there is an illusion in it, it is not clear for the time being. In addition, when measuring the air-gapped magnetic core using the dual winding method and the effective value method, it is found that the power consumption has been greatly reduced. We have not figured out what is going on and cannot find a reasonable explanation. 6 Conclusion Due to time constraints, we have not yet deepened our study and research on the effective value method for measuring power consumption. This article is inevitably incorrect. Readers are welcome to provide valuable suggestions for improvement! This work has received the support and help of Comrade Deng Yuan, a senior engineer at the 9th Institute of Electronics. We would like to express our sincere gratitude!

Article author: Zhang Zhongshi, Chen Wen, Li Wei