The larger the capacitance of the tested product and the leakage reactance of the test transformer, the more obvious the capacitance rise effect. Therefore, when we carry out the AC withstand voltage test of the series resonance large-capacity test object, it is required to directly measure the voltage at the end of the test object to prevent the test object from being subjected to excessive voltage.

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The overvoltage phenomenon of the series resonance test device is caused by the following factors:

(1) The primary winding is suddenly pressurized instead of gradually increasing from zero voltage.
(2) When there is still a higher voltage, suddenly cut off the power supply, instead of evenly dropping to zero and then cutting off the voltage. It can be seen from the transition process of the transformer winding that the above two situations will cause overvoltage on the tested product, which is not allowed.

For (1) blocking by the control circuit to prevent the non-zero voltage from being suddenly pressurized.
For (2) the correct operation method should be strictly implemented to avoid.
? ? ? ?(3) The test article breaks down suddenly. This is often encountered and inevitable. If the outlet end of the test transformer is directly connected to the tested product, when the tested product breaks down suddenly, the potential of the outlet end of the test transformer is immediately forced to zero, which is equivalent to a sudden action on the outlet end of the test transformer—one wave front is extremely steep The peak value of the impulse voltage wave is equal to the instantaneous value of the instantaneous test voltage of the test object, but the polarity is opposite. This will generate dangerous overvoltages on the longitudinal insulation of the test transformer windings.

The way to prevent it is to connect a protective resistor with an appropriate resistance value in series between the transformer outlet end of the power frequency series resonance test device and the test object, so that the impulse voltage of opposite polarity acts on the series circuit of the protective resistor and the transformer inlet capacitance. For a short period of time, most of the voltage drops on the protection resistor.