Application of Power Quality Analyzer in Accurate Detection of Inductance

Inductors are important components for the production of passive power filters. The accuracy of their inductance (L value) directly affects the filtering effect of passive power filters. When the inductance L of the inductor reaches the millihenry level and the rated current reaches tens of amperes, higher requirements are put forward for how to accurately detect the inductance (L value) and determine the linearity of the inductance (L value). Traditional measurement methods and measuring instruments can no longer meet such requirements, and more accurate measurement methods and measuring instruments are needed. This article focuses on the method of accurately measuring the inductance (L value) using the 800 series power quality analyzer of Ideal Industries, USA.

1. Traditional method of measuring inductance (L value)

General inductor manufacturers detect inductance (L value) in this way: select a group of AC currents I passing through the inductor from small to large, measure the voltage U across the inductor for each current, and then calculate the impedance Z (U/I=Z) based on each group of U and I to obtain a group of Z values. If the influence of the coil DC resistance R is ignored, it is assumed that Z=XL (the inductive reactance of the coil), or Z=2πfL (f is the frequency of the alternating current). Obviously, a set of L can be calculated from a set of Z values, and the inductance (L value) and the linearity of the L value can be estimated based on the changing trend of this set of L values ​​to see whether they meet the requirements.

2. Method for accurately measuring inductance (L value) (assuming that the inductor consists of a coil and a pure resistor in series)

If the influence of the coil DC resistance R is considered, the relationship between Z, R, and XL is as follows:
Z2=R2+XL2
Z2=R2+（2πfL）2
In the above formula, π, R, and f are all constants. Obviously, Z is a quadratic function of L. After sorting,

The above formula is a standard hyperbolic equation, and the function graph is as follows:

It can be seen from the first quadrant image that Z and L are nonlinear. The traditional method of calculating the L value based on Z=2πfL is only an approximate algorithm, which obviously cannot reflect the true value.
The precise algorithm is: measure the active power P consumed by the inductor, the AC current I of the inductor, and the voltage U across the inductor.

Calculation of the coil DC resistance R: R= P/ I2
Calculation of inductor impedance: Z= U/I
Calculation of inductance:

3. Traditional measuring instruments

The DC resistance R of an inductor with an inductance L of millihenry and a rated current of tens of amperes is generally less than 1 ohm, which is not easy to measure directly and accurately. The indirect measurement method requires an AC voltmeter, an ammeter, and a power meter (i.e., the three-meter method). The three-meter method is troublesome to measure and is greatly affected by the accuracy of the instrument. There are also errors in meter reading and entry errors during table analysis.

4. Advanced measuring instruments

The 800 series power quality analyzer of Ideal Industries of the United States can simultaneously measure the active power P consumed by the inductor, the AC current I of the inductor, and the voltage U across the inductor on one meter, and directly store, list, and analyze the data through software, reducing errors in many intermediate links. The voltage clamp and current clamp of the 800 series power quality analyzer are convenient and fast to operate, with a friendly human-machine interface, and at the same time reflect the advantages of high precision of the digital display meter.
The wiring diagram is as follows:

According to the storage files provided by the 800 series power quality analyzer, 7 sets of P, U, and I data can be obtained in a very short time. P is the active power consumed by the inductor. According to P=I2R, 7 R values ​​can be calculated; according to U/I=Z, 7 corresponding Z values ​​can be calculated; and according to Z2=R2+（2πfL）2, 7 corresponding L values ​​can be calculated. In this way, we can accurately obtain a set of inductance, which meets the requirements of accurately measuring the inductance (L value) and analyzing the linearity of the L value.