Different measurement methods for DC resistance testers

A DC resistance tester is generally used to detect the DC resistance of windings such as transformers and motors. Due to the presence of both inductive and/or capacitive reactances in the circuit of such equipment, it is impossible to conduct a high current test unless there is a testing device with a large capacity (such as a battery pack). The DC resistance tester is used in power systems to measure the DC resistance of transformers or inductive loads, check the welding or connection quality of leads in closed-loop circuits, check for inter turn short circuits or open circuits in windings, and check for good contact with tap changers. The DC resistance tester is an essential test item for transformers after handover, major repairs, and changes to tap changers. At present, there are three measurement methods for portable DC resistance testers: bridge method, voltage drop method, and three-phase winding measurement method:

1. Three phase winding simultaneous pressurization method: that is, apply voltage to the three phase winding of the transformer simultaneously and measure the DC resistance of each phase. When voltage is applied to the three-phase winding simultaneously, the current flowing into each phase winding increases from zero. According to the right-hand screw rule, the magnetic flux direction generated by the three-phase current in each iron core column is different, and their effects cancel each other out, resulting in the composite magnetic flux in the iron core being approximately zero.

2. Voltage drop method: Its principle is to apply direct current to the measured resistance, measure the voltage drop on the resistance, and calculate the value of the measured resistance according to Ohm's law. It has high accuracy and accurate measurement. The voltage and current meters are both digital displays, with a voltage resolution of 0.1kV and a current resolution of 0.1uA. The voltage meter on the control box directly displays the voltage value added to the load sample, without the need for an external voltage divider during use, and the wiring is simple. It has the ability to measure leakage current at both high and low voltage ends, and a circular shielded digital meter is used at the high voltage end for display. It is not afraid of discharge shocks and has good anti-interference performance, making it suitable for on-site use.

3. Bridge method: When using the bridge method for measurement, specialized instruments such as single arm bridges and double arm bridges are often used to measure DC resistance. When the measured resistance is above 10 ohms, use a single arm bridge; When the measured resistance is 10 ohms or less, use a double arm bridge. When measuring the resistance of transformer windings using a bridge, due to the large inductance of the windings, it is also necessary to wait for the charging current to stabilize before closing the ammeter switch; Before turning off the power switch after taking the reading, disconnect the ammeter to prevent insulation breakdown between the bridge arm resistors and ground breakdown of the bridge arm resistors due to the reverse electromotive force at the moment of pulling the power.

Weshine Electric Manufacturing Co., Ltd.