Method of insulating induction coil of medium frequency furnace
Three commonly used methods for insulating induction coils of medium frequency furnaces
1. For 380V incoming line voltage, the voltage at both ends of the coil is 750V, and the voltage between turns is also tens of volts. If the distance between turns is large enough, the distance between turns can also be used as insulation. This is the early insulation treatment. If steel slag splashes onto the coil, an inter-turn short circuit will form. This method has been eliminated.
2. The currently commonly used insulation treatment process for induction furnace coils is the four-pass insulation treatment method. First, spray insulating paint on the surface of the coil; secondly, wrap a layer of mica tape around the coil that has been sprayed with insulating paint; thirdly, wrap a layer of glass ribbon around the mica tape; finally, spray a layer of insulating paint. This kind of insulation treatment process can ensure that the insulation withstand voltage of the induction furnace coil is as high as 5000V.
3. Another way to insulate induction furnace coils is to directly spray high-temperature insulating paint. Some common insulating paints claim to be able to withstand a temperature of 1800°C. Haishan Mechanical and Electrical Editor believes that this temperature resistance value is still open to question, but spraying high-temperature insulating paint is also a good idea. A simple method. Theoretically speaking, the higher the insulation level of high-temperature insulating paint, the higher the temperature resistance of the insulating paint. The high volume resistivity of high-temperature insulating paint is greater than 1016Ωm at room temperature. High dielectric strength (breakdown strength), greater than 30KV/m. It has good chemical stability, aging resistance, corrosion resistance, and good soothing reactivity. No flash point or fire point, high hardness, greater than 7H. Heat-resistant to 1800°C, it can work under an open flame for a long time.
Common inspection methods for the insulation of induction coils in intermediate frequency furnaces
(1) Observe the voltage and ammeter values
The main thing is to observe whether the indication values of the four small meters in the intermediate frequency furnace cabinet are normal. Among them, the rectifier control voltmeter is 30V, the rectifier pulse ammeter is 130~150mA, the inverter control voltmeter is 12V, and the inverter pulse ammeter is 100~120mA. If the value is within the normal range, it proves that there is no problem with the power supply.
(2) Use a multimeter to detect the resistance value
Use a digital multimeter to check the intermediate frequency furnace rectifier, inverter thyristor anode, cathode resistance and control electrode and cathode resistance values (it is not necessary to remove it from the cabinet for measurement, the tube radiator still has cooling water). The forward and reverse resistance values of the anode and cathode are both ∞, and the resistance values of the control electrode and cathode are 10 to 508. Also, check whether the fuse is blown.
(3) Use an oscilloscope to detect the waveform
Set the transfer switch SA to the check position, use an oscilloscope to check the waveforms of the rectifier and inverter trigger pulses, and check whether the amplitude and time interval of the intermediate frequency furnace are normal. Among them, the rectifier trigger pulse is a double pulse, and the time interval is 3. 33 m s; the inverter trigger pulse is a continuous pulse train, and the amplitude is generally 4 to 6V. The pulses are required to be neat and burr-free. The order of inspection is from the thyristor control electrode to the pulse transformer, then to the rectifier board and inverter board.
(4) Test whether the rectifier board is normal
The inverter board can be unplugged and the transfer switch can be placed in the check position. Press the start button and turn the power adjustment potentiometer to see if the DC voltage can be adjusted to about 500V. If the voltage can be adjusted to 500V, it proves that the rectifier board is normal.
(5) Check the capacitor charging circuit
This refers to checking the capacitor charging circuit in the circuit of starting the intermediate frequency furnace. The method is to still unplug the inverter board and turn on the check gear. After pressing the start button, use a multimeter to measure the voltage at both ends of the capacitor cf. If it can reach about 500V, it is proved The starting capacitor charging circuit is normal.
(6) Check whether the pre-magnetization resistor is burned out and whether the low-pass filter is disconnected.