When electricians check motor faults, they usually use a measuring pen or multimeter for inspection. Introduce several methods of using a clamp ammeter to measure the no-load current of a three-phase asynchronous motor and find faults.

Case 1

Phenomenon:A mineral crusher with a motor of 15kW. After major repairs, the motor operated normally without load, but could not carry the load. When the load was added, the motor tripped due to overload. After inspection, both the mechanical and power aspects are normal. The measured DC resistance of the motor coil is 2.4 Ω, 3.2 Ω, and 2.4 Ω, respectively; Using a clamp ammeter to measure the three-phase no-load currents of 9A, 5A, and 8.8A, it can be confirmed that there is a fault in the motor coil.

Analysis:After removing the motor end cover, it was found that one of the wire ends of one phase winding had been loosened and the solder had melted. The motor is wound in parallel with two wires, with one wire disconnected and the other wire still connected, resulting in a decrease in torque and can only rotate without load, but cannot carry the load.

Case 2

Phenomenon:There is a motor with a rated power of 13 kW. After rewinding the coil, it was tested and the motor ran at normal speed without load. However, when loaded, the motor speed was very slow and even did not rotate. The measured power supply voltage and resistance of each phase are normal. The three-phase no-load current is basically balanced using a clamp meter, but the current values are relatively small.

Analysis:It can be concluded that there is an error in the winding connection. Upon opening the end cap, it was discovered that the motor, which was originally connected in a delta configuration, had been mistakenly connected in a Y-configuration, resulting in a smaller normal operating torque and inability to carry the load. This is because the torque of the Y-configuration is one-third that of the delta configuration.

Case 3

Phenomenon:A certain machine tool uses a 4kW motor, but when the power is turned on, the motor does not rotate and only makes a buzzing sound. Remove the motor wires, measure that there is electricity on the power supply side, the three-phase voltage is normal, the DC resistance of the winding is balanced, the insulation is qualified, and the mechanical rotation is flexible. Measure the no-load current on the motor lead under the switch using a clamp ammeter, and the results show that there is current in both phases and no current in one phase.

Analysis:There is a fault in the wire inside the conduit. Pulling out the wire inside the steel pipe, it was found that a section of the wire had been basically broken, facing each other like two needle tips, and there was white oxidized powder at the end of the wire. This is due to excessive tension when threading the tube, causing the wire to be stretched and stretched, and prolonged electrical current to generate heat and oxidation at the seemingly unbroken point. At this time, voltage can still be measured on the wire head, but current cannot pass through.