Nickel Hydride No. 5 battery.In an AC circuit, the entire arc extinguishing process of the inductor and resistance is analyzed. How to measure the coil resistance?

　　Electronic enthusiasts provide you with an analysis of the entire arc extinguishing process of the inductor and resistor in the AC circuit. How to measure the coil resistance? The purpose of measuring the DC resistance of the transformer winding is: to check the welding quality of the winding joints and whether there is an inter-turn short circuit in the winding; whether the contact at each position of the voltage tap switch is good and whether the actual position of the tap is consistent; whether the lead wire is broken, Check whether there are any broken strands in the multi-strand wire winding. When the transformer is overhauled or after the tap position is changed, or after the outlet fault is short-circuited, the DC resistance of the winding together with the bushing needs to be measured. The arc extinguishing process when a purely resistive or purely inductive AC circuit is disconnected:

　　The arc extinguishing process when a purely resistive AC circuit is disconnected

　　For a purely resistive AC circuit, the supply voltage μ is in the same phase as the arc current i. When the current i crosses zero, the arc is temporarily extinguished. At this time, the voltage u also passes through zero. At this moment, the insulation strength of the arc gap is called the initial insulation strength of the arc gap.

　　Assuming that the resistance in the circuit is R, the power supply voltage u is equal to the sum of the voltage drop UR on the resistor and the arc voltage Uh. When the current crosses zero, the supply voltage is also zero. After the power supply voltage u crosses zero, it rises along a sinusoidal curve. If the arc does not reignite, the current is still zero and the voltage drop UR on the resistor is also zero. Then the waveform of the arc gap voltage Uh coincides with the power supply voltage u, that is, Uh is equal to u. If the arc gap voltage Uh reaches or exceeds the arc gap insulation breakdown voltage strength, arc gap breakdown occurs. When the current crosses zero for the second time, the initial insulation strength of the arc gap is greater than the previous time. This is because the distance between the moving and stationary contacts of the switch is getting larger and larger, and the initial insulation strength of the arc gap is exactly related to the length of the arc gap. The longer the arc gap, the greater the insulation strength. If the maximum value of the arc gap recovery voltage does not reach the value of the arc gap insulation breakdown strength, the arc gap will no longer breakdown and the arc will eventually be extinguished.

　　Arc extinguishing process when purely inductive AC circuit is disconnected

　　The difference between a purely inductive AC circuit and a purely resistive AC circuit is that there is a 90° phase difference between current and voltage. When the current crosses zero, the voltage has a maximum value. Moreover, when the current crosses zero, the current change rate is the largest and the induced electromotive force on the inductor is also the largest. Under the superposition of these two voltages, the arc gap recovery voltage uhf may rise rapidly, causing gap breakdown and arc re-ignition. Therefore, it is much more difficult to disconnect a purely inductive AC circuit than a purely resistive AC circuit.

　　How to measure the DC resistance of a transformer coil?

　　The purpose of measuring the DC resistance of the transformer winding is: to check the welding quality of the winding joints and whether there is an inter-turn short circuit in the winding; whether the contact at each position of the voltage tap changer is good and whether the actual position of the tap is consistent; whether the lead wire is broken or not. Check whether there are any broken strands in the stranded wire and winding. When the transformer is overhauled or after the tap position is changed, or after the outlet fault is short-circuited, the DC resistance of the winding together with the bushing needs to be measured. The measurement method is as follows.

　　(1) Current and voltmeter method. Also known as the voltage drop method, its principle is to pass a DC current through the resistor under test, measure the voltage drop across the resistor, and calculate the resistance value under test according to Ohm's law. Since the internal resistance of ammeters and voltmeters will affect the measurement results, the way they are connected to the measurement circuit should be carefully considered.

　　(2) Balanced bridge method. It is a method that uses the principle of bridge balance to measure DC resistance. Commonly used balanced bridges include single-arm and double-arm bridges. When measuring the DC resistance of the transformer, it should be done after the transformer is powered off and the high-voltage lead is removed. For large and large-capacity power transformers, because the charging time constant τ of the RL series circuit is very large, each measurement requires a long time to wait for the current and voltmeter indications to stabilize, so the work efficiency is very low, and special instruments (such as constant current) are often used. power supply) instead of the power supply under test, which can greatly shorten the test time. The standard for measuring the DC resistance of transformer coils is: for transformers above 1600kVA, the difference between the resistances of the windings of each phase should not be greater than 2% of the three-phase average value. For windings without neutral point lead wires, the difference between the wires should not be greater than the three-phase resistance. 1% of the average value. For transformers of 1600kVA and below, the phase-to-phase difference is generally not greater than 4% of the three-phase average value, and the line-to-line difference is generally not greater than 2% of the three-phase average value. Compared with the previous values measured at the same location, the The change should not be greater than 2%.

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