What Is Magnetic Permeability?

Permeability, English name: magnetic permeability, a physical quantity that characterizes the magnetic properties of a magnetic medium. Represents the resistance of a magnetic flux or its ability to conduct magnetic lines of force in a magnetic field after a current flows through a coil in space or in a core space. Its formula = B / H, where H = magnetic field strength, B = magnetic induction strength, commonly used symbol , is the permeability of the medium, or absolute permeability. ^[1]

Permeability equals

The measurement of permeability is

Speaking of the measurement of the permeability , it seems to be very simple. Just wind a few turns of the coil on the material sample ring, measure its inductance, and find a formula to finish it. In fact, it is not possible to measure different magnetic permeability for the same sample ring by using different instruments, winding different turns, applying different voltages, or using different frequencies. The reason for the huge difference in test results is that not every tester has the energy to figure it out. This article mainly discusses the influence of different test turns and calculation formulas on permeability measurement.

Influence of magnetic permeability calculation formula

As we all know, the method of measuring the permeability is generally to measure the inductance L of the N-turn coil on the sample ring. For a ring core with a small inner diameter, the inner diameter is not as easy to measure as the wall thickness. The path length is taken as the magnetic path length of the magnetic core. The magnetic permeability calculated using them is called the ring permeability of the material. Some people say that the permeability measured with a ring sample is called the ring permeability, and this statement is incorrect. In fact, the permeability of the ring is higher than the true permeability of the material, and the thicker the wall of the sample ring, the greater the error.

For the sample ring, the magnetization field is non-uniform in the radial direction under the same ampere-turn magnetomotive force excitation. The closer to the outer side of the ring wall, the weaker the magnetic field. Under the condition that the permeability of the sample ring is constant, the closer to the outside of the ring wall, the lower the magnetic flux density B of the ring. In order to eliminate the influence of this uneven magnetization on the measurement, we consider the sample ring to consist of an infinite number of thin-walled rings with radius r and infinitely thin wall thickness dr.

If the sample ring is composed of the same material, the calculated permeability is the true permeability of the material. It is slightly lower than its ring permeability.

N Influence of the number of turns N of the magnetic permeability test coil

Since the inductance L is directly proportional to the number of turns N2, the calculated permeability should no longer be related to the number of turns N, but in fact it is often related.

Regarding the measurement of material permeability, the test frequency used is generally not high, and it is often tested at a frequency of 1kHz or 10kHz. The test signal is generally a sinusoidal signal, because the frequency is not high, the resistance part of the coil resistance of the sample surround group can be ignored, and the winding coil is regarded as a pure inductance L connected to the measuring instrument. The test equivalent circuit is shown in the figure. The effective value of the voltage generated by the signal source of the instrument is U, and Ri is the output impedance of the signal source.

When measuring the permeability, the magnetization field strength in the sample ring is related to the number of turns of the test coil. When the number of turns is a certain value, the magnetic field strength will reach the strongest value. The magnetic permeability of the material is closely related to the magnetization field strength, so the measurement of the magnetic permeability is related to the number of test coil turns. The effect of the number of turns on the permeability measurement is discussed in detail with reference to the figure.

U Case of low permeability test voltage U

As mentioned earlier, for high-end instruments, such as the Agilent 4284A precision LCR tester, its test voltage can be adjusted so low that when the variation of the test magnetic field strength with the number of turns reaches the strongest, it still does not exceed the threshold of the magnetic permeability. Starting area. At this time, the initial permeability _{i of the} material is always measured, which is independent of the number of turns N of the test coil. With the same instrument, if the test voltage is adjusted higher, it can no longer be guaranteed that the permeability measured at different turns is the initial permeability, and the measured permeability at this time will be the same as the test coil turns About it.

U When the permeability test voltage U cannot be adjusted

Most of the simple instruments for measuring inductance cannot be flexibly adjusted in test voltage and frequency. Such as 2810 LCR bridge, its test frequency is 100Hz or 1kHz, the test voltage is less than 0.3V. ^[3]