3 cases will teach you how to use a clamp meter to detect motor faults
Case 1
Phenomenon: A ore crusher, the traction motor is 15kW. After the motor is overhauled, it runs normally without load, but it cannot carry load. Once the load is added, the motor will overload and trip. After inspection, the mechanical and power supply are normal. The DC resistance of the motor coil is 2.4Ω, 3.2Ω and 2.4Ω respectively; the three-phase no-load current measured by the clamp ammeter is 9A, 5A, and 8.8A respectively. It can be confirmed that the motor coil is faulty.
Analysis: After removing the motor end cover, it was found that one of the wire ends of one phase winding was loose and the solder was melted. The motor is wound in parallel with two wires, one of which is disconnected and the other is still connected, so the torque is reduced and it can only rotate without load, but it cannot carry load.
Case 2
Phenomenon: There is a motor with a rated power of 13kW. The coil is rewound and tested. The motor runs normally when it is running without load. After carrying load, the motor speed is very slow or even does not rotate. The power supply voltage and each phase resistance were measured to be normal. The three-phase no-load current measured with a clamp meter was basically balanced, but the current values were all small.
Analysis: It was concluded that the winding connection was wrong. After opening the end cover, it was found that the motor, which was originally connected in △ mode, was mistakenly connected in Y mode, which made the normal operating torque small and could not carry the load, because the torque of the Y connection mode is one-third of that of the △ connection mode.
Case 3
Phenomenon: A machine tool uses a 4kW motor. After the power is turned on, the motor does not rotate but only makes a buzzing sound. After removing the motor wire, it was found that there was power on the power supply side, the three-phase voltage was normal, the DC resistance of the winding was balanced, the insulation was qualified, and the machine rotated flexibly. Then, the no-load current was measured with a clamp ammeter on the motor lead on the lower side of the switch. The result was that there was current in two phases and no current in one phase.
Analysis: It shows that the wire is faulty in the wire tube. After pulling out the wire in the steel tube, it was found that a section of the wire was basically broken, facing each other like two needles, and there was white oxide powder at the end of the wire. This is because the pulling force is too large when passing through the pipe, the wire is stretched thin and elongated, and the current is heated and oxidized at the point where it seems to be disconnected for a long time. At this time, the voltage can still be measured on the wire head, but no current can pass.
