Any ferromagnetic material will be magnetized and will be magnetic, switching transformer core is no exception, but because the switch transformer core is selected soft magnetic material, the remanent magnetic field strength is relatively low compared to the magnetic material.
When the method is used to measure the VTmax of the limit voltage of the switching transformer, the magnetization process of the switching transformer core is very different from the actual magnetization process. In actual work, the secondary coil of the switching transformer generates an induced electromotive force and generates a current in the secondary coil. This current will demagnetize the core of the transformer; the transformer of the transformer is tested by the method shown in the figure. In the process, the secondary winding of the switching transformer is generally open circuit, so no current will be generated in the secondary winding of the transformer. Therefore, in the course of testing, the superimposed current must cause the magnetic core of the switching transformer to become magnetized, and When the superimposed current disappears, the magnetic properties of the switching transformer core will not disappear. Therefore, in the process of repeated testing of the switching transformer, it is necessary to demagnetize the switching transformer core.
The best way to demagnetize the transformer core of a switching transformer is to let the primary winding of the switching transformer pass through a damped oscillating current with a magnitude of 3 to 5 times Imax (limit current) in a very short time, that is, to allow the current flowing through the primary coil of the switching transformer to be maximum. The value gradually decreases to zero. For this purpose, a 20~40 ohm demagnetization thermistor can be used in series with the primary winding of the switching transformer, and then it is connected to a 110~220V/50Hz AC power supply, which takes about 20 seconds. After the demagnetization thermistor is fully heated, Demagnetization can be achieved.
RT is a demagnetization thermistor, Lx is the primary coil of the switching transformer. Normally, degaussing thermistors for CRT TVs have relatively small resistances. Therefore, two demagnetizing thermistors can be connected in series, or a degaussing thermistor can be used in series with a high-power resistor instead of 20-40 ohms. Degauss thermistor.
For low-power switching transformers, the current flowing through the primary winding of the switching transformer is allowed to be small, and therefore current limiting measures are required in the degaussing circuit.
The specific method is to use a zero-point farad capacitor (select X2 type safety capacitor) in series with the primary coil of the low-power switching transformer, and then connect it in parallel with a capacitor of a few microfarads (use X2 type safety capacitor), and finally A 20 to 40 ohm degaussing thermistor is connected in series. When demagnetizing, the circuit is connected to a 110-220V/50Hz AC power supply, which takes about 20 seconds. After the demagnetization thermistor is fully heated, demagnetization can be achieved.
RT is a demagnetization thermistor, Lx is the primary coil of the low power switching transformer, C1 is the shunt capacitor, and C2 is the current limiting capacitor. The size of the capacitors C1 and C2 needs to be determined according to the size of the excitation current of the primary coil of the low-power switching transformer. The degaussing current needs to reach 3 to 5 times the limiting current Imax.
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