3 cases teach you how to use clamp meter to measure motor fault
Case 1
Phenomenon: An ore crusher with a drive motor of 15kW. After the motor is overhauled, it runs normally without load, but it cannot be loaded. Once the load is added, the motor will trip due to overload. After inspection, the mechanical and power supply are all 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 is certain that the motor coil has Fault.
Analysis: Remove the end cover of the motor, and find that one of the wire ends of one of the phase windings has been loosened, and the solder has melted. The motor is double-wired, 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 the load.
Case 2
Phenomenon: There is a motor with a rated power of 13kW. After the coil is rewound for a test run, the speed of the motor is normal when the motor is running without load. After the load is on, the speed of the motor is very slow, or even does not rotate. The measured power supply voltage and the resistance of each phase are normal, and the three-phase no-load current measured with a clamp meter is basically balanced, but the current values are all small.
Analysis: It is concluded that the winding connection is wrong. Opening the end cover, it was found that the motor with the △ connection was mistakenly connected with the Y connection, which made the normal running torque too small to carry the load, because the torque of the Y connection was one-third of that of the △ connection.
Case 3
Phenomenon: A machine tool uses a 4kW motor. After the power is turned on, the motor does not rotate and only hums. Remove the motor wires, test that there is electricity on the power supply side, the three-phase voltage is also normal, the DC resistance of the winding is also balanced, the insulation is qualified, and the mechanical rotation is flexible. Finally, measure the no-load current with a clamp ammeter on the motor leads on the lower side of the switch. As a result, there is current in two phases and no current in one phase.
Analysis: It indicates that there is a fault in the wire conduit. Pull out the inner wire of the steel pipe, and find that a section of the wire has been basically broken, facing each other like two needle points, and there is white oxidized powder at the end of the wire. This is due to the excessive tension when passing through the pipe, the wire is thinned and elongated, and the long-term energized current heats up and oxidizes at the place that seems to be broken. At this time, the voltage can still be measured on the energized wire head, but the current cannot pass.
