The general tests of electric motors：2

2. Determination of dc resistance of winding:

1) Significance of dc resistance measurement:

The dc resistance of the windings in cold state is one of the main parameters of three-phase asynchronous motors. By comparing the measured value of the resistance of the windings with the design value, it is possible to check whether the windings have broken wires and short-circuit between turns, whether the welding parts have virtual welding or open welding, whether the contact points have bad contact and other phenomena.

2) Dc resistance measurement method:

Measuring the dc resistance of the motor should be done when the winding is cold.First, place the motor indoors for a period of time, and measure the temperature of the motor winding end or core with a thermometer.At this time, the measured temperature is the actual temperature of the winding in the cold state.If the temperature of the winding end or core cannot be measured, the temperature of the housing may be substituted.For large and medium motors, the thermometer should be placed for at least 15min.

The direct current resistance of the winding is measured by a double - arm or single - arm bridge.Resistance at or below 1 ohm must be measured using a double arm bridge.When using automatic detection device or digital micrometer to measure the resistance of the winding, the test current through the measured winding should not exceed 10% of its normal operating current, the power on time should not exceed 1min.

Open the cover of the junction box for measurement. For the three outgoing windings in the junction box, the dc resistance of l1-l2, l2-l3 and l1-l3 windings shall be measured respectively.For the motor winding with six outgoing terminals, the dc resistance of U1 -- U2, V1 -- V2 and W1 -- W2 phases should be measured.

                     

3)Judgement of dc resistance:

At present, most of us use direct resistance tester to measure the winding dc resistance, making the measurement of dc resistance more convenient and fast.The measured dc resistance value is recorded. The dc resistance of each phase or each branch winding should not be more than 2% of its minimum value after correcting the errors caused by different lead lengths.The relative change should not be more than 2% when compared with the value measured at the same temperature;For stator windings already connected internally, the measured dc resistance imbalance should not exceed 1%.