How to detect faults in a motor using a clamp-on ammeter
Workers check the motor obstruction, usually with a stylus or multimeter check. The author introduces a few cases with a clamp ammeter to measure the no-load current of three-phase asynchronous motor to find obstacles to the main points.
Example 1 an ore destroying machine, dragging motor 15kW. motor overhaul no-load operation is normal, but not less than with the load, a plus load, the motor overload tripping. After checking, machine, power supply are normal, measuring motor coil DC resistance were 2.4, 3.2 and 2.4 with clamp ammeter measuring three-phase no-load current were 9A, 5A, 8.8A, you can be sure that the motor coil is impeded. Remove the motor end cover, the invention of one phase of the winding of a wire has been loosened, solder melting. The motor is a double line and winding, which is disconnected, the other is still on, so the torque is low, only no-load rotation, but can not take the load.
Example 2 there is a rated power of 13kW motor, coil rewinding good test machine, motor no-load operation speed is normal, with the load, the motor speed is very slow, and even do not turn. Measured power supply voltage and the resistance of each phase are normal, with a clamp meter to measure the three-phase no-load current is balanced, but the current value is small, which concluded that the winding adjoining the wrong. Open the end cover invented the original △ connection method of the motor mistakenly connected to the Y connection method, so that the normal operation of the pitch is small, can not take the load, due to the Y connection method of the torque is one-third of the △ connection method.
Example 3 a machine tool with a 4kW motor, turn on the power, the motor does not turn only buzzing. Remove the motor line, measure the power side of the power, three-phase voltage is also normal, winding DC resistance is also balanced, insulation and qualification, the machine rotates motorized. After the switch on the lower side of the motor leads with a clamp-on ammeter to measure the no-load current, the effect of two-phase current, a phase of no current. Clarification of the conductor on the line pipe is obstructed. Pull out the wire in the steel tube, the invention of a section of the wire has been simply broken, like two pinpoints against the end of the line with white oxidized powder. This is due to wear tube when the tension is too large, the wire is pulled thin and long, permanent energized current in the seemingly broken at the heat oxidation. At this time the energized wire head can still measure the voltage, but not less than through the current.
