Method for measuring no-load current of three-phase asynchronous motors using a clamp ammeter
Example 1: A mineral crusher with a driving motor of 15kW. After the motor is overhauled, it operates normally without load, but cannot carry load. Once the load is added, the motor will overload and trip. After inspection, both the mechanical and power supply are normal. The DC resistance of the motor coil is measured to be 2.4 Ω, 3.2 Ω, and 2.4 Ω, respectively; Using a clamp ammeter to measure the three-phase no-load currents of 9A, 5A, and 8.8A respectively, it can be confirmed that the motor coil is faulty. After removing the motor end cover, it was found that one of the wire ends of one phase winding had been loosened and the solder had 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 can only rotate without load, but cannot carry the load.
Exemplo 2: Há está a motor com a rated potência de 13kW. Depois a bobina é rebobinada e testada, o motor gira normalmente quando funcionando em não carga. Depois sendo carregado, o motor gira lentamente ou até faz não gira. O medido potência voltage e resistência de cada fase são normais. O trifásico no-load current é basicamente equilibrado quando medido com a clamp meter, mas o atual valores são relativamente pequenos. Portanto, it is concluiu que o enrolamento conexão está incorreto. Abertura o fim tampa, it foi encontrado que o motor, que foi originalmente conectado em um delta maneira, foi equivocadamente conectado em a Y maneira, resultando em a menor normal operacional torque e inabilidade para carregar a carga, as o torque de o Y maneira é um terço de que da delta maneira.
Example 3: A certain machine tool uses a 4kW motor. When the power is turned on, the motor does not rotate but makes a buzzing sound. Remove the motor wire, measure that there is electricity on the power side, the three-phase voltage is normal, the DC resistance of the winding is balanced, the insulation is qualified, and the mechanical rotation is flexible. Afterwards, a clamp shaped ammeter was used to measure the no-load current on the motor lead under the switch, and the results showed that there was current in both phases and no current in one phase. There is a problem with the wire inside the conduit. Pulling out the wire inside the steel pipe, it was found that a section of the wire had basically broken, facing like two needle tips, and there was white oxide powder at the end of the wire. This is due to the excessive tension during the threading process, resulting in the wire being stretched and stretched, and the prolonged electrification of the current heating and oxidation at the seemingly unbroken point. At this point, voltage can still be measured on the power cord, but current cannot be passed through.
