The function of medium frequency transformer is to convert the power supply voltage of medium frequency generator (such as thyristor medium frequency power supply, medium frequency generator or electron tube oscillator, etc.) into the voltage required for quenching induction coil or other devices.
Medium frequency transformer is generally dry type and water-cooled. When high frequency current passes through the conductor, the current distribution on the conductor cross section is uneven due to the influence of skin effect and proximity effect. The skin depth of current in copper is:
So its primary and secondary windings are wound with rectangular hollow copper tubes, and water is passed through the tubes for cooling. The choice of current density is related to the water flow rate, which can reach + amperes per square millimeter. The winding can be wound into a cylindrical or pancake type. When the secondary current is large, it can also be welded into a cylindrical winding with copper plates. The copper tube should be able to withstand a pressure test of 490kPa (5kgf/cm2) and 5min, and the copper plate welded winding should withstand a pressure test of 196kPa (2kgf/cm2) and 5min without water leakage. When both the primary and secondary windings are pancake-shaped, they are arranged in an overlapping manner; when the cylindrical type is used, they are arranged concentrically.
The core is stacked with 0.2mm or thinner electrical steel sheets. Since the intermediate frequency is much higher than the power frequency, a large eddy current and hysteresis loss will be generated in the core, so the magnetic flux density cannot be too high. When the operating frequency is 2500Hz, the magnetic density is 0.75T or less; when the frequency is 8000Hz, it is 0.3T or less. And the heat sink welded with a rectangular hollow copper tube should be properly placed between the core laminations, and a water-cooled structure should be adopted. The heat sink is made of 1mm thick copper plate, and a square copper tube is welded around it. The thickness of the laminations between two adjacent heat sinks is generally about 15mm.
If a ferrite with high saturation, low unit loss, moisture resistance and heat aging resistance is used as the core material, the temperature rise of the core can be reduced. The new manganese-zinc ferrite Mn-Zn-4000 is one of them.
This ferrite is produced by dry method. In mass production, it can be sintered in a chain sintering furnace. However, in terms of quality, it is better to produce by coprecipitation wet method and sinter in a vacuum furnace.
In order to reduce the excitation current, the core can be stacked and overlapped, and the gaps can be filled with 618 epoxy resin mixed with ferrite powder. Considering the high porosity of ferrite (0.1~10%), the core must be dipped in paint to improve moisture resistance. In this way, compared with the electrical steel sheet core, the no-load loss of the ferrite core of the 1000kVA transformer is reduced by 4.2kW, which is only 7% of the original value, while the excitation current is only increased by 4.2A, which is 1.72 times the original value. The core can meet the temperature rise requirements without water cooling.