News

Analysis of Noise Sources in Medium-Frequency Induction Furnaces

 

1 Induction coil Excited by alternating electromagnetic forces

 

Multiple current-carrying cylindrical tubes are used to simulate eddy currents within the workpiece. The diameters of these cylindrical tubes are related to the penetration depth of the eddy currents in the workpiece, and the tubes are concentric with the induction coil. Since an alternating current flows through the induction coil, according to Lenz's law, the direction of the eddy currents in the workpiece is constantly changing. Consequently, the induction coil experiences electromagnetic forces both radially and axially, and the magnitude and direction of these forces vary periodically in synchrony with the current. The vibration frequency of the coil corresponds to the fundamental frequency of the current.

 

2. The induction coil is a good resonant sound emitter.

 

The induction coils are all made by bending copper tubes into circular rings. They are formed through two or three spot welds, with relatively thin tube walls—remaining at about 3 mm thick. The damping coefficient of copper is approximately 0.02. It can be regarded as a thin-walled cylinder, possessing a thin-walled cylindrical structure with thickness h.

 

When the cylindrical resonant structural mode dominates as the primary sound source, certain modes establish standing waves both in the axial and circumferential directions, and their radiation efficiency is higher than that of other modes. When the speed of bending waves equals the speed of sound, the radiation efficiency approaches 1 in all cases. Resonance mode All are highly effective incident waves. At lower frequencies, lower-order circumferential modes are more effective than higher-order circumferential modes.

 

 

From a theoretical perspective: Noise can be reduced by adjusting circuit parameters to decrease electromagnetic forces. For the sensing coil, one can increase its damping loss factor or redesign its structure to lower its admittance and increase its average thickness. In fact, all these measures are highly practical.

 

Governance Measures and Their Effects

 

1

Adjusting the turns ratio of the intermediate-frequency transformer—keeping the 750kW intermediate-frequency furnace’s turns ratio at a long-term value of 8:1—is not the optimal choice. Based on the actual conditions on site, pay attention to using multiple induction coils. By adjusting the ratio to 6:1 to the left, tests show that the coil current has dropped to 900 A. This adjustment still achieves the desired effect of heating the workpiece, but noise levels have been reduced by 0 to 12 dB(A).

 

2

The constraint damping layer induction coil, when adhered to the outer side of the induction coil, serves as the primary vibration-induced sound source. Figure 3 illustrates the noise reduction effect achieved after damping material is attached to the surface of this thin-walled component.

Since the induction coil is internally cooled by circulating tap water, it effectively removes both the heat generated by eddy currents and the heat radiated from the workpiece. The outer-wall temperature must not exceed 35°C, which makes it possible to apply damping materials. We selected a high-strength 602 adhesive with a wide temperature range (-40°C to 110°C). On the outer wall, we applied a polyether urethane/epoxy resin material with a low damping coefficient, with a thickness of 5 mm. Subsequently, we used an aluminum sheet with a thickness of 1.2 mm as a constraint layer. To further enhance the damping effect, we employed two constrained damping layers. Tests conducted on induction coils with inner diameters ranging from 750 to 1100 millimeters showed that, when the medium-frequency power supply was operating, the sound level reduction reached over 14 dB(A).

 

3

Medium-frequency electric furnace Flexible oil pipes are used in conjunction with oil pumps. Oil tanks located near electric furnaces also serve as significant noise sources. By replacing the original rigid connecting pipes with flexible oil pipes, the vibration-induced excitation of the oil tanks has been reduced. The noise reduction exceeds 10 dB(A).

 

4

New-style induction coil design. The air gap has been reduced, improving efficiency. All over 20 induction coils used in the 750 kW medium-frequency induction furnace are custom-made by the factory for convenient installation of workpieces. Typically, the air gap between the induction coil and the workpiece is around 40 mm—whereas the standard gap usually hovers around 10 mm. This inevitably leads to significant magnetic leakage, resulting in unnecessary power losses, increased working current, and heightened noise levels. To address these issues, a series of open-loop clamping-type induction coils have been designed, enabling the air gap to be reduced to just 15 mm. These coils feature inner wall thicknesses of 4–5 mm made from high-purity copper.

As a current path, the outer copper tube serves as a double-layer cooling channel (and also functions as a reinforcing rib). Compared to existing induction coils, it features higher stiffness, lower electrical conductivity, and a thickness that is much closer to the target value. Moreover, it exhibits greater stiffness, lower admittance, and a thickness that is more closely aligned with smaller dimensions.