Keywords: high power inductance measurement error zone measurement under rated conditions inductance is not constant

1. Introduction

      Inductance has the basic functions of filtering, oscillation, delay, notch, voltage lead current, reactive power generated as inductive load, etc., so it is widely used in electronic circuits and industrial analog loads.

      Our company is a professional manufacturer of inductive load cabinets. Many of the equipments are used in military, aerospace, aerospace and other units. These units have higher requirements on various indicators of equipment. In the process of equipment acceptance, the customer encountered multiple problems with the deviation of the inductance value measured by the special inductance measuring instrument and the factory nominal value. This in the end is why? Later, it was found that the special inductance measuring instrument is relatively accurate for measuring the low-power and high-frequency common inductance. However, when measuring the high-power magnetic core inductance, especially the low-frequency high-power magnetic core inductance, the measured value is often low. And with the development of industry, this phenomenon in the industry is becoming more and more common and urgently needs to be solved. In recent years, through the multi-party experiments, some experience in inductance measurement has been summarized and shared with everyone. I hope that I can help you out of the misunderstanding that blindly believe that special inductor measuring instruments can test high-power magnetic core inductance.

2. The characteristics of the inductor and its basic knowledge

       Inductance is a physical quantity that measures the ability of a coil to generate electromagnetic induction. When the coil is connected to an unstable constant current, a varying magnetic field is generated around it. The greater the power supplied to the coil, the higher the strength of the excited magnetic field, and vice versa (before the magnetic induction reaches saturation).

       Inductors are generally divided into air core inductors and core inductors.

       The inductance of the air core inductor is a fixed constant, which is simple to apply and will not be described here.

       Large-scale magnetic core inductors are used more in the industry, and the accuracy of the inductance value is a key issue, which has great significance both in theory and in practical applications.

      Analysis by the formula L = μ × Ae * N2 / lm:

L represents the inductance, μ represents the magnetic permeability of the core, Ae represents the cross-sectional area of the core, N represents the number of turns of the coil, and lm represents the magnetic path length of the core. It can be seen that when a certain inductor is formed, Ae, N, and lm are all fixed values, and the magnetic permeability μ is affected only after the inductance of the inductor is shipped.

       So is μ a constant? Let's study the magnetic law of the inductor through the μ=B/H magnetization curve:

Figure 1 Figure 2

       It can be seen from Fig. 1 that when there is no magnetic field, both H and B are zero, that is, the coordinate origin 0 of the B-H curve in Fig. 1. As the magnetic field strength H increases, B also increases, but there is no linear relationship between the two. When H increases to a certain value, B no longer increases (or increases very slowly), which indicates that the magnetic induction B has reached saturation.

       From this, the magnetic permeability μ=B/H can be approximately determined, which is mathematically referred to as the slope, that is, tan θ=B/H. Because of the nonlinear relationship between B and H, the magnetic permeability μ is not constant, but varies with H, as shown in Fig. 2.

       Therefore, for the same inductive entity, when different powers are applied to excite different magnetic field strengths, the magnetic permeability of the magnetic core is different, and the inductance value is different. The inductance value at this time is only an inductance value corresponding to the strength of the magnetic field (or corresponding to the applied power). It can be seen that the inductance of the core inductor is not a constant.

3.The working principle of general special inductance measuring instrument

      Special inductance measuring instruments generally use the bridge method (AC bridge), resonance method, digital measurement method, and the most commonly used is the AC bridge method. Regardless of the test method, when measuring the inductance, most of the instruments test the signal voltage of the inductor to only 50 mV~2V, and the current-limiting voltage of the 100Ω~400Ω resistor (refer to the existing special inductor on the market). Measuring instrument manual). It is not difficult to see that the power of the magnetic field strength of the excitation inductor is about 0.04W, which is much smaller than the power of the general power core inductor when working normally is more than 300W (even at 100KW). Therefore, the strength of the excited magnetic field is very small (in most cases only in the initial permeability stage), the corresponding permeability value is low, and the resulting inductance value is lower than the nominal value (as far as the low value is caused by The inherent parameters of the core itself are determined).

      Therefore, when a special inductive measuring instrument measures the magnetic core inductance, the measured value is lower than the nominal value. However, in the design, inspection and measurement of the inductor, it is considered according to the actual working conditions of the inductor (such as frequency, power, current), so the deviation between the measured value and the factory nominal value is not difficult to understand and easy to explain. .

4. It is recommended to use the following methods to measure power inductance

(1) Rated power method

      The voltage and current of the rated power at the specified operating frequency are applied to the measured inductance so that it is in an electromagnetic environment similar to the actual operation. The voltage and current meters with true rms measurement function are used to measure the voltage and current RMS at both ends of the inductor. Then, the inductance impedance Z is calculated from Z=U/I. Then calculate the inductance value according to the following formula. (See Note 1, Note 2)

(2) Reference complement method

      After measuring the inductance value with the above rated power or 10% or more power, it is measured by a special inductance measuring instrument, and then the error value is calculated as reference data, and only the special inductance measuring instrument can be used for daily detection.

In this way, we communicated with the relevant quality inspection department at a certain missile research base, and finally solved the problem that the equipment can meet the practical application and is easy to test, and won the praise of the customers.

Remarks:

      Note 1. Where R is the coil DC resistance. Because the wire diameter of the general power inductor is relatively thick, the internal resistance is very small. It has been proved by the actual test that the influence on the calculation result is less than 0.5%, and the requirement is not high, it can be neglected. After ignoring, the accuracy requirements of general inductive loads can be met. at this time,

     Note 2. For inductors with extremely large power and inconvenient to provide test conditions, it can be tested with reference to 10% of its rated power. The actual verification error will not exceed 2%.