Double-arm bridge (Kelvin bridge)
The double-arm bridge is a commonly used DC resistance tester measuring instrument for measuring low resistance of 10^(-6)~10^2Ω, and the measurement accuracy is high.
DC dual-arm bridges are commonly used to measure the resistivity of metal materials, DC resistance of motors, transformer winding sets, low-resistance coil resistance, cable resistance, switch contact resistance, and DC shunt resistors. By measuring the DC resistance of equipment such as transformers and motors, it is possible to check the conductive loops of the equipment for defects such as poor contact, poor soldering, coil failure and wiring errors.
The measurement steps are as follows:
The structure of the stator winding of a 100kW three-phase asynchronous motor is measured by the QJ103 type double bridge as an example.
1. Open the lock of the galvanometer. If the pointer is not at the zero position, turn the mechanical zero adjustment device to make the pointer zero, and then tighten the lock.
2. Note that the voltage terminals P1 and P2 must be connected to the inside of the current terminals C1 and C2.
3. Estimate the approximate range of resistance of the following stator windings. For the resistance of the stator winding of 100KW motor, it is assumed to be between 0.05~0.2Ω. The magnification of table 1-4 should be X10 with an error of ±2%.
4. Turn the proportional arm R1/R2 (magnification knob) to X10, adjust the comparison arm Rn (standard resistance reading plate) to about 0.015Ω, loosen the galvanometer lock knob B and lock it, then use the left hand to check The flow meter button GA, at this time if the galvanometer pointer points to the "+" end of the scale selection, the resistance value of the comparison arm should be increased. If the deflection to the "-" end should reduce the resistance value of the comparison arm. Adjust Rn repeatedly until the galvanometer points to zero. At this time, if the turntable pointer of the table is up to 0.0127, it is according to the formula: Rx=Rn2 R2/R2 = 10*0.0127 = 0.127Ω.
Precautions for using DC dual-arm bridge
1. Strictly observe electrical safety regulations and equipment test procedures
2. The potential of the DC resistance bridge should be close to the resistance to be measured, and the current pile should be connected above the potential pile head.
3. When the double-arm bridge measures the resistance, the working current is large, the power supply capacity is large, and the battery can be used as an external power supply. The measurement speed should be fast when measuring, and the power should be turned off immediately after the measurement is completed.
Precautions in DC resistance measuring meter test
When measuring the DC resistance test, the equipment should be in a cold state, and the temperature of the test object and the surrounding guest gas should be measured at the same time, and there should be no significant difference.
Selection of measuring instruments: current size, measuring range, and properties of the test object.
The measurement needs to be stopped and fully discharged before measurement.
The equipment under test should be disconnected from other equipment and cannot be grounded.
The four-terminal wiring method should be error-free when measuring small resistance.
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