Three-phase motor winding DC resistance measurement and precautions

Three-phase motor winding DC resistance measurement and precautions

Three-phase winding DC resistance measurement

The stator of the three-phase asynchronous motor and the synchronous machine, and the rotor of the three-phase wound motor have three-phase symmetrical windings. In the National Motor Standards, methods for measuring the DC resistance of these windings are specified. When measuring, the temperature of the winding to be tested must be measured. If the motor is in the actual cold state, the temperature can be corrected by the ambient temperature. When performing a type test or other test requiring accurate results, measure each phase resistance or each line resistance 3 times, and then take the average of 3 times as the measurement result.

● Direct measurement of phase resistance

When the stator or rotor three-phase winding phase of the motor has 6 terminals at the head and tail, or the three-phase winding is connected to the star (Y-shaped) in the motor, but the neutral line is taken out except the three phase lines ( When the star point is also taken out, the DC resistance of each phase can be measured separately.

●Measure line resistance

When the three-phase winding of the stator of the motor is connected to the star (Y) or the triangle (△) inside the motor, only the resistance of each of the two external ends (ie, the line resistance) can be measured, and then the corresponding phase resistance is converted by the formula.

Conversion of line resistance and phase resistance

If the phase resistance of the winding is R phase and the line resistance is R line, the calculation of the line resistance can be calculated according to the series-parallel principle of the resistor.

●Line resistance by phase resistance

When the motor winding is star-connected, the schematic is shown in Figure 1, and the R line = 2R phase.

When the motor winding is at the corner, the schematic is shown in Figure 2, and the R line = 2R phase / 3.

●The phase resistance is converted by line resistance

The formula for calculating the line resistance from the phase resistance can be mathematically converted under different connections.

When the motor winding is star-connected, the schematic is shown in Figure 1, R phase = 0.5R line

When the motor winding is angled, the schematic is shown in Figure 2, and the R phase = 1.5R line.

● Conversion of resistance values at different temperatures

Generally, the DC resistance of a metal conductor has a fixed relationship with its temperature. This relationship is expressed as:

R1=R2×(K+t1)/(K+t2).........(1)

Where: R1 - DC resistance (Ω) at a temperature of t1 ° C;

R2——DC resistance (Ω) at a temperature of t2°C;

K - constant (reciprocal of the temperature coefficient of the conductor resistance at 0 ° C), for copper windings, K = 235, for aluminum windings, K = 225.

● Resistance standard under different test conditions

In the type test report, the specific resistance value is given according to the different insulation grade of the motor, such as the resistance of the B-class insulated motor at 75 °C, while the F-class insulated motor is the resistance at 95 °C, and the H-class insulated motor is Resistance at 105 °C. In the inspection test evaluation standard, it is customary to give the resistance at 20 ° C. When the temperature deviates, it can be converted according to the formula (1).

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