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Magnetic yoke It’s a key component of medium-frequency induction furnaces. Professionals in the field often compare medium-frequency induction furnaces to the heart of the system, highlighting its central and pivotal role—truly deserving of the spotlight! Next, let’s take a closer look at the magnetic yoke, a crucial part of the medium-frequency induction furnace.

 

A magnetic yoke typically refers to a soft magnetic material that does not itself generate a magnetic field (magnetic flux lines) but merely serves to transmit magnetic flux lines within a magnetic circuit. Magnetic yokes are commonly made from soft iron, A3 steel, and soft magnetic alloys, which have relatively high permeability. In certain special applications, magnetic yokes may also be fabricated from ferrite materials.

The magnetic yoke is a core made by stacking silicon steel laminations. It is evenly and symmetrically arranged around the induction coil. Its functions include confining the leakage magnetic flux of the induction coil to prevent it from spreading outward, thereby improving the efficiency of induction heating. Additionally, as a magnetic shield, it reduces heat generation in metallic components such as the furnace frame, and also serves to reinforce the induction coil itself.

Medium frequency Steel-shell furnace The function of the magnetic yoke

The medium-frequency steel-shell furnace features a conformal magnetic yoke built into its body. This magnetic yoke’s shielding capability reduces magnetic leakage, prevents the furnace body from overheating, and enhances overall efficiency. At the same time, the magnetic yoke serves to support and secure the induction coil, enabling the furnace body to achieve high strength and low noise levels. The magnetic yoke is a crescent-shaped structure made of cold-rolled silicon steel laminations and stainless steel clamping plates. The mating surface between the iron core and the coil is arc-shaped, and the compression area is a flat surface rather than a single line as in conventional designs. This structural design provides superior compression effectiveness and significantly reduces magnetic leakage. After the silicon steel laminations are stacked, they are securely fastened using specialized clamping plates instead of dedicated through-bolts. This design fully utilizes the magnetic-conducting area of the silicon steel laminations, thereby minimizing the likelihood of localized overheating in the furnace body. Additionally, a specially designed water-cooled heat sink is installed between the magnetic yoke and the stainless steel clamping plates. During operation, this setup ensures that the upper magnetic yoke remains at ambient temperature, preventing deformation caused by excessively high yoke temperatures. As a result, the support provided to the induction coil is strengthened, enhancing the overall structural integrity of the furnace.

 

 

  Medium-frequency steel shell furnace Magnetic Yoke Operation Procedure

1) Magnetic particle inspection personnel must have uncorrected or corrected nearsightedness and far-sightedness no lower than 5.0 (with a decimal record value of 1.0), and must undergo an annual eye examination; color blindness is strictly prohibited.

2) The magnetic particle inspection of welded joints should be scheduled after the welding process has been completed. For materials prone to delayed cracking, magnetic particle inspection shall be performed—at a minimum—24 hours after welding, as required. Unless otherwise specified, magnetic particle inspection of fasteners and forgings should be carried out after the final heat treatment has been completed.

3) The preparation concentration of the non-fluorescent magnetic particle suspension is 10–25 g/L.

4) Before testing, the wetting performance of the magnetic suspension fluid should be verified. Apply the magnetic suspension fluid onto the surface of the test workpiece. If the liquid film formed by the magnetic suspension fluid is uniform and continuous, then the wetting performance of the magnetic suspension fluid is acceptable. However, if the liquid film is interrupted, the wetting performance of the magnetic suspension fluid is deemed unacceptable. Chicheng Electrical Equipment—on the upper part of the magnetic yoke of the medium-frequency furnace, reddening is observed.

You’ll need to switch to a different model of magnetic paste. The magnetic suspension is applied using a spraying method.

5) When using the magnetic yoke with the maximum gap distance, the AC magnetic yoke shall have a lifting force of at least 45 N; the cross magnetic yoke shall have a lifting force of at least 118 N (with a gap of 0.5 mm between the magnetic pole and the surface of the test piece). The lifting force of the magnetic yoke, as indicated by the ammeter on the magnetic particle inspection equipment, shall be verified at least once every six months. After any damage occurs or after maintenance is performed, the verification shall be repeated.

6) The magnetic inspection spacing for the magnetic yoke should be maintained between 75 mm and 200 mm. The effective inspection area is within 50 mm on either side of the line connecting the two poles, and each magnetization zone should have an overlap of at least 15 mm. During inspection, the magnetic yoke should be positioned at the same location and subjected to two perpendicular inspections. In the cross-magnetic-yoke method, the gap between the magnetic poles and the workpiece should not exceed 1.5 mm, and the moving speed should not exceed 4 m/min.

7) During magnetic particle inspection, the A130/100 standard test piece is generally recommended (when the workpiece being inspected is relatively small, the C-15/50 test piece may be used). This test piece is employed to evaluate the overall performance of the magnetic particle inspection equipment, magnetic particles, and magnetic suspension fluid, and to determine whether the effective magnetic field strength and direction on the surface of the workpiece being inspected, the inspection area, and the magnetization method are appropriate.

8) When using standard test specimens, the side of the specimen that has no artificial defects should face outward. To ensure good contact between the specimen and the surface being inspected, use transparent adhesive tape to firmly affix the specimen to the surface. Be careful not to allow the tape to cover any artificial defects on the specimen. The surface of the standard test specimen must be free from rust, wrinkles, or any changes in magnetic properties; if any of these conditions are present, the specimen should no longer be used.

9) To enhance contrast, you can use a contrast enhancer.

10) The visible illuminance on the surface shall be greater than or equal to 1000 LX; when conditions are limited and this requirement cannot be met, the illuminance may be appropriately reduced, but it must not fall below 500 LX.

11) The application of magnetic powder and the observation of magnetic particle indications shall both be completed within the magnetization energizing time. After energizing for 1–3 seconds, stop applying the magnetic suspension for at least 1 second before terminating the magnetization. To ensure optimal magnetization results, repeat the magnetization process at least twice. When identifying fine cracks, a magnifying glass with a magnification factor of 2 to 10 can be used for observation.

By gaining an understanding of the principle and operational methods of the magnetic yoke in a medium-frequency induction furnace, we’ve truly come to appreciate why the magnetic yoke is often referred to as the “heart” of the medium-frequency induction furnace.