Introduction
Measurement of various magnetic parameters reflecting the magnetic properties of soft magnetic materials. It is one of the contents of magnetic measurement. Soft magnetic materials generally refer to magnetic materials with a coercivity Hc≤1000A/m, mainly low-carbon steel, silicon steel sheet, iron-nickel alloy, and some ferrite materials. The various magnetic properties of soft magnetic materials determine the technical characteristics of magnetic devices or devices made of this material. Therefore, the measurement of soft magnetic materials occupies an important position in the measurement of magnetic quantities.
Soft magnetic materials
There are various curves to characterize the magnetic properties of soft magnetic materials, which can be selected according to the requirements of industrial applications. These curves are mainly: the static magnetic characteristic curve that works under a DC magnetic field and the static magnetic characteristic loop reflecting the hysteresis effect; it works under a changing magnetic field (including periodic alternating magnetic field, pulsed magnetic field, and AC and DC superimposed magnetic fields, etc.) , Dynamic magnetic characteristic curve and dynamic magnetic characteristic loop including eddy current effect. The abscissa of these magnetic characteristic curves is the magnetic field intensity H applied to the material to be tested, and the ordinate is the magnetic flux density B in the material. This way of representation makes these curves only reflect the nature of the material, and has nothing to do with the shape and size of the material. In addition, the dynamic magnetic properties of soft magnetic materials also include complex permeability and iron loss.
Measurement method
Static magnetic characteristic measurement measures the static magnetic characteristic curve and magnetic characteristic loop of the material. The main measurement methods are impact method and integral method.
①Impact method: used to measure the static magnetic characteristic curve. The material sample is made into a bracelet ring, and is wound with a magnetizing coil and a measuring coil. The former is connected to the DC power supply through a commutation switch, an ammeter and a variable resistor that regulates the current; the latter is connected to an impulse galvanometer (see galvanometer). At the beginning of the measurement, the current in the magnetizing coil is adjusted to a certain value by the ammeter, and the magnetic field intensity H value can be calculated from the reading of the ammeter, the number of turns of the magnetizing coil, and the geometric parameters of the magnetic circuit of the material sample. Then, use the commutation switch to quickly change the direction of the current in the magnetizing coil, so that the direction of the magnetic flux density in the material sample suddenly changes, so a pulse electromotive force e is induced in the measuring coil, e makes the pulse current flow through the impact current Count. The maximum throw of the galvanometer is proportional to the electric quantity Q contained in the pulse current, that is, the change in magnetic flux (△φ). △φ is numerically equal to twice the magnetic flux in the material sample. From the reading of the impact galvanometer and the impact constant (Weber/grid), and the equivalent section of the material sample, the corresponding magnetic flux density B value can be calculated. By changing the magnetizing current, all the data required for the static magnetic characteristic curve can be measured. The accuracy of this method is about 1%.
If the magnetizing circuit is modified, the magnetizing current is continuously reduced from a certain maximum value to zero, and then reversed, until the reverse maximum value, the static magnetic characteristic loop can be obtained.
②Integral method: used to obtain the static magnetic characteristic loop. A static magnetic automatic recorder can be used. This kind of instrument is composed of magnetizing current scanning circuit, electronic integrator and X-Y recorder (see pen recorder). The scanning circuit outputs a slowly changing magnetization current with a period between 10 and 40 seconds. The positive and negative amplitudes are equal and can be continuously adjusted. The signal corresponding to the change of the magnetic flux density is taken out from the measuring coil, and the corresponding magnetic flux density B value is obtained through the electronic integrator. Because the magnetic field changes slowly and the influence of eddy current is not considered, the loop automatically recorded by the X-Y recorder can be regarded as the loop of the static magnetic characteristics of the sample material's hysteresis effect. The principle of the static magnetic automatic recorder to measure the magnetic flux density loop is the same as that of the electronic fluxmeter, the difference is that the former replaces the latter's indicating meter with an X-Y recorder. The integrated magnetism of the static magnetic automatic recorder is usually 10-7 Weber/mm, and the accuracy is 2%.
Dynamic magnetic characteristic measurement measures the dynamic magnetic characteristic curve and magnetic characteristic loop of the material. There are 3 main measurement methods.
①Voltmeter-ammeter method: The material to be tested is made into ring-shaped, mouth-shaped and other samples. N1 turns of magnetizing coil and N2 turns of measuring coil are evenly wound on the sample. N1 is connected to the adjustable AC power supply via ammeter A, and N2 is connected to the average voltmeter. According to the reading B of the average voltmeter, the number of turns N2, the frequency f, and the equivalent section S of the sample, the maximum magnetic flux density Bm in the sample section can be calculated. If the magnetizing current I is measured with an effective value ammeter, then The magnetic field intensity of the sample is H=N1I/L, and L is the effective length of the magnetic circuit. Since I is a valid value, H is also a valid value. If you want to find the value of the maximum magnetic field strength Hm at this time, you must replace the ammeter with a combination of mutual inductance M and an average voltmeter. At this time, H is the reading of the average voltmeter. Adjust the voltage of the AC power supply to obtain all the data of the dynamic magnetic characteristic curve. The error of this method is ±(3～10)%.
②Oscilloscope method: used to measure dynamic hysteresis loop. R0 is the sampling resistor, and the signal related to the magnetizing current (ie, magnetic field strength) taken out from this is added to the X axis of the oscilloscope; the magnetic flux density signal taken from the measuring coil is added to the Y axis of the oscilloscope via the integrator. At this time, the dynamic magnetic characteristic loop of the material sample can be displayed on the fluorescent screen of the oscilloscope. This loop reflects the magnetic characteristics when the material has hysteresis and eddy current effects. The measurement error of this method mainly comes from the insufficient accuracy of the reading on the fluorescent screen, and the error is generally ±(5～10)%.
③Electric bridge method: Use some AC bridge circuits to measure the complex permeability and iron loss of magnetic materials. To measure the complex permeability components μ1 and μ2 of a material at audio frequencies, Maxwell bridges are usually used; for iron loss measurement, a modified Hay bridge is usually used (see classic AC bridges). The loop sample is wound with a coil and connected to the bridge circuit. Adjust the bridge to balance it. From the measured equivalent inductance and equivalent resistance of the sample coil and the voltage on the sample coil, the complex magnetism can be calculated Conductivity and iron loss. The error of measuring the dynamic magnetic properties of materials by the bridge method is ±(1～5)%.