What are the low magnetic field magnetic test conditions for transformer core materials?

The national standard recommends the use of square ring and double yoke permeameter to measure the AC magnetic properties of magnetic materials, but it is only suitable for magnetic properties testing under saturated magnetic field, and cannot measure magnetic induction intensity under low magnetic field. At the same time, the American Society for Testing and Materials standard ASTM and the Russian national standard both mentioned that the toroidal transformer core sample can be used as the standard sample for material magnetic testing. For this reason, we use the wound inner diameter of 120mm as the inner diameter of the standard sample, and refer to the relevant Russian national standards to select the ratio of outer diameter to inner diameter of 1.3, and the height of the sample is set to 40mm and 100mm respectively, that is, the sample is an annular sample with an inner diameter of 120mm, an outer diameter of 156mm, and a height of 40mm and 100mm respectively (five pieces of imported A and B core materials respectively).

For ease of comparison, the representative LM363 and LMH-550 products are selected. The measurement-grade core (imported B core material, the number is five and two pieces respectively).

2.2 Test conditions

In order to make the test data more comparable with the production inspection, the test method similar to the actual production inspection is used for testing, that is, the AC test circuit is used.

2.3 Test method

(1) In the production inspection, the external magnetic field is applied to the iron core by changing the excitation current in the primary coil, and then the induced electromotive force at both ends of the secondary coil is measured, and then the magnetic induction intensity value corresponding to the current magnetic field is obtained by the following formula (1).

Uf=4.44 BSN2 f(1)

Wherein, Uf is the induced electromotive force of the secondary winding (V)

B is the calibrated magnetic induction intensity (Gs)

S is the cross-sectional area of ​​the sample (cm2)

N2 is the number of turns of the secondary winding (turns)

f is the repetition frequency of the AC magnetic field (50Hz)

(2) This paper first takes the magnetic induction intensity B as the independent variable, selects a number of data points between 0 and 20 000Gs, calculates the induced electromotive force Uf value under the current magnetic induction intensity according to formula (1), and then obtains the Uf value by adjusting the current in the primary coil, and records the excitation current value I in the primary coil at this time. Then, H at this time is obtained according to formula (2), and finally the corresponding relationship between H and B is obtained.

H=N1·I/π·Da(2)

Wherein, H is the external magnetic field strength value (A/m)

I is the primary excitation current value (A)

N1 is the number of turns of the primary coil (turns)

Da is the average diameter of the sample (cm)

3 Analysis and discussion

B40—B material height is 40mm sample

B100—B material height is 100mm sample

A40—A material height is 40mm sample

A100—A material with a height of 100mm

550—LMH-550 medium-level measurement core

363—LM363 medium-level measurement core

H—After annealing

Q—Before annealing

3.1　Material low-field magnetic analysis

A and B are imported materials of the same brand. Their low-field magnetic properties are compared as follows:

(1) Before annealing, it can be seen from Figure 2 that the low-field magnetic properties of material B are significantly better than those of A.

(2) After annealing, it can be seen from Figures 3 to 5 that from low magnetic field to medium and high magnetic field, B is significantly better than A.

3.2　Influence of core structure on magnetism

(1) Under the same inner and outer diameters, the magnetism of the material with higher height is significantly better than that of the material with lower height (that is, when the material has high magnetism, its magnetization curve is to the upper left of the magnetization curve of the material with higher magnetism), as shown in Figures 3 and 6.

(2) If the ratio of height to average diameter is used to describe the structural characteristics of the core, then

LM363 core height/average diameter = 0.057

LMH-550 core height/average diameter = 0.181

120/156*40 sample height/average diameter = 0.290

120/156*100 sample height/average diameter = 0.725

① As can be seen from Figure 6 (within the range of 0 to 1 000Gs), for the same material, as the height-to-diameter ratio decreases from high to low, the magnetism decreases in sequence, that is, the curves of the 100-height sample, 40-height sample, and measurement-grade core are distributed from the upper left to the lower right in sequence.

② From the test (curve omitted), after exceeding 1 000Gs, the magnetization curves gradually become similar.

3.3 Comparison of magnetization curves before and after annealing

① The magnetism of the core increases significantly after stress relief annealing after cutting and winding.

② Within the range of 1A/M magnetic field strength (i.e., within the range of 120Gs magnetic induction), the magnetism of B core material before annealing is better than that of A core material after annealing.

When the magnetic field strength is greater than 20A/M, the magnetism of the unannealed core quickly tends to saturation with the increase of magnetic field strength; while after stress relief annealing, its magnetism still changes linearly under higher magnetic fields.

3.4 Comparison of magnetism between product core and sample

(1) Low magnetic field magnetism of LM363L and LMH550 measurement-grade core before annealing.

(2) The low magnetic field magnetism of LM363 and LMH550 cores (wound with B material) after annealing is similar to that of the B material sample, but slightly inferior, indicating that the sample is representative.

3.5 Observation and comparison of the microstructure of the core material

(1) From the substructure of a single grain of the material, it can be seen that the cellular substructure in (B material) is more uniform and oriented than (A material in this test batch);

(2) From the observation of the grain boundary of the material, it can be seen that the orientation consistency of the cellular substructure between grains is obviously better than that in Figure 9(b). Among them, B material, A material in this test batch, satisfies the A material of the measurement-grade core magnetic requirements in LM363 and LMH-550 products.

(3) The microstructures with relatively poor low-field magnetic properties in the same batch of materials show that their common feature is that there are more local grain boundary protrusions and wider areas (diamond-shaped with stripes on them), which affects the uniformity and continuity of the overall organization.

From the above observations and comparisons, it can be seen that in the microstructure of the core material, the cellular substructure with good and uniform orientation consistency and narrow grain boundaries has higher low-field magnetic properties.

4 Conclusions

1. Affected by the manufacturing process and process control, the low magnetic field magnetism of the core materials of the same brand but produced by different manufacturers, and the core materials of the same manufacturer in different periods are not the same;

2. The low magnetic field magnetism of material B is obviously higher than that of material A;

3. Stress relief annealing can improve the magnetism of the core, especially the magnetism of the material under the magnetic field, while the core material without annealing tends to saturate quickly under a higher magnetic field;

4. For the cores with the same inner and outer diameters, the ones with higher height have better magnetic properties;

5. In the microstructure of the core material, the ones with good and uniform cellular substructure orientation and narrow grain boundaries have higher low magnetic field magnetism.