How to use a clamp ammeter to determine motor failure
Example 1: A mineral destruction machine with a motor of 15kW. After major repairs, the motor operated normally without load, but it could not carry the load. When the load was added, the motor tripped due to overload. After inspection, both the machine and power supply are normal. The measured DC resistance of the motor coil is 2.4, 3.2, and 2.4, respectively. The three-phase no-load current measured with a clamp ammeter is 9A, 5A, and 8.8A, respectively. It can be confirmed that there is obstruction in the motor coil. Remove the motor end cover, and the wire end of one phase winding in this invention has been loosened, causing the solder to melt. The motor is double wound, with one disconnected and the other still connected, resulting in low torque and can only rotate without load, but cannot carry the load.
Example 2: There is a motor with a rated power of 13 kW. The coil is rewound and tested. When the motor is running without load, the speed is normal. However, when the load is applied, the speed of the motor is very slow and even does not rotate. The measured power supply voltage and resistance of each phase are normal. The three-phase no-load current is basically balanced when measured with a clamp meter, but the current values are too small. Therefore, it is concluded that the winding connection is incorrect. Opening the end cap, it was discovered that the motor with the △ connection method was mistakenly connected to the Y connection method, resulting in a smaller normal operating torque and inability to carry the load. As the torque of the Y connection method is one-third of that of the △ connection method.
Example 3: A certain machine tool uses a 4kW motor. After connecting the power supply, 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 machine rotates smoothly. Measure the no-load current with a clamp ammeter on the motor lead under the switch, and the effect is that there is current in both phases and no current in one phase. Clarify that there are obstacles to the wires inside the conduit. Pulling out the wire inside the steel pipe, it was discovered that a section of the wire had completely broken, like two needle tips facing each other, with 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, resulting in heating and oxidation of the current passing through the permanent circuit at the seemingly unbroken point. At this time, voltage can still be measured on the wire head, but current cannot pass through.
