Method of measuring no-load current of three-phase asynchronous motor with clamp meter
Example 1: An ore crusher, the driving motor is 15kW. After the motor was overhauled, it ran normally without load, but it could not carry a load. Once a load was added, the motor overloaded and tripped. After inspection, the machinery and power supply are normal. The DC resistance of the motor coil was measured to be 2.4Ω, 3.2Ω and 2.4Ω respectively. The three-phase no-load current measured with a clamp ammeter was 9A, 5A and 8.8A respectively. It can be confirmed that the motor coil has Fault. Remove the motor end cover and find that one of the wire ends of one of the phase windings 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 open, so the torque is reduced and it can only rotate 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. After putting the load 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 wrong. When I opened the end cover, I found that the motor originally connected in the △ connection was mistakenly connected in the Y connection, causing the normal operating torque to be too small and unable to carry the load, because the torque in the Y connection is one third of that in 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 winding 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. The result is that there is current in two phases and no current in one phase. It means there is a fault in the wire in the conduit. Pulling out the wire inside the steel pipe, I found that a section of the wire was basically broken, facing each other like two needle points, with white oxidized powder on the end of the wire. This is due to the excessive pulling force when threading the pipe, which causes the wire to become thin and elongated, and the long-term current to heat and oxidize the parts that appear to be broken but not broken. At this time, the voltage can still be measured on the powered wire, but no current can pass.
