What test items do residual magnetism have an impact on transformers?
1. Measuring voltage ratio: Currently, when measuring voltage ratio, the voltage used is lower than the working voltage of the bridge, and the current applied to the primary winding is also relatively small. The working magnetic flux generated in the iron core is very low, and sometimes it may not be able to offset the influence of residual magnetism, causing the measured voltage ratio deviation to exceed the allowable range. In this case, the double voltmeter method can be used. Apply a higher voltage to the winding to overcome the influence of residual magnetism.
2. Measuring DC resistance: Residual magnetism can affect the inductance value of the charging winding, thereby increasing the measurement time. To reduce the impact of residual magnetism, measurements can be carried out in a certain order.
3. No-load measurement: In general, the residual magnetism in the iron core will not have a significant impact on the measurement of no-load loss at rated voltage. The main reason is that under rated voltage, the magnetic flux generated by the no-load current can overcome the effect of residual magnetism, allowing the residual magnetic flux in the iron core to enter normal operating conditions with the excitation direction of the applied no-load current. However, after conducting zero sequence impedance measurement on large three-phase five column products, due to the fact that the zero sequence magnetic flux can form a circuit by the side of the iron core, its zero sequence impedance is relatively large, which is similar to the positive sequence impedance. After completing the zero sequence impedance test, there is a small amount of magnetic flux (i.e. residual magnetism) left in the iron core. If no-load measurement is carried out at this time, abnormal indications will appear on the three-phase power meter and ammeter at the beginning of pressurization. In this situation, the voltage can be applied for an additional period of time until the ammeter and power meter indicators return to normal before reading.
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