How to use clamp ammeter to diagnose motor failure
When checking for motor failure, workers usually use a test pen or multimeter to check. The author introduces several examples of methods of using a clamp ammeter to measure the no-load current of a three-phase asynchronous motor to find faults.
Example 1: An ore crusher, the driving motor is 15kW. After the motor was overhauled, it ran normally without load, but it could not be loaded. Once the load was added, the motor would trip overload. After inspection, the machine and power supply were normal. The DC resistance of the motor coil was measured to be 2.4, 3.2 and 2.4 respectively. Use a clamp ammeter to measure the three-phase no-load current of 9A, 5A and 8.8A respectively. It can be determined that the motor coil is faulty. Remove the motor end cover and find that one wire end of one phase winding has been loosened and the solder has melted. This motor has two wires wound in parallel, one of which is disconnected and the other is still connected, so the torque is low and it can only run without load, but it cannot carry a load.
Example 2: There is a motor with a rated power of 13kW. After the coil is re-wound and the test machine is completed, the motor rotates normally when running without load. When the load is put on, the motor rotates very slowly or even does not rotate. The power supply voltage and the resistance of each phase 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 too small. From this, it was concluded that the winding connection was incorrect. After opening the end cover, it was discovered that the motor with the △ connection was mistakenly connected to the Y connection, causing the normal operating torque to be too small and unable to carry the load, because the torque of the Y connection is one third of that of the △ connection.
Example 3: A machine tool uses a 4kW motor. After the power is turned on, the motor does not rotate and only makes a buzzing sound. Remove the motor wires and measure whether there is electricity on the power supply side, the three-phase voltage is normal, the DC resistance of the windings is balanced, the insulation is qualified, and the machine rotates flexibly. Then use a clamp ammeter to measure the no-load current on the motor lead on the lower side of the switch. It turns out that there is current in two phases and no current in one phase. It means that the wire is damaged in the line tube. Pulling out the wire inside the steel pipe, it was found that a section of the wire was basically broken, facing each other like two needle tips, with white oxidized powder on the end of the wire. This is due to the excessive pulling force when threading the pipe, causing the wire to become thin and elongated, and the current flowing for a long time generates heat and oxidation in the seemingly broken but not broken parts. At this time, the voltage can still be measured on the powered wire, but no current can pass.
