Detection of rotor bar breakage in squirrel cage motor

Detection of rotor bar breakage in squirrel cage motor

The detection of rotor bar breakage in squirrel cage motors is mainly achieved through the following methods, which are organized based on authority and practicality:

1、 Appearance inspection method
Observe the characteristics of broken bars
The breakage of cast aluminum rotors often occurs near the center of the axial length of the slot, while copper rotors are more common at the welding point between the cage bars and the end rings. If discoloration, overheating, or cracks are found at the junction of the cage bar or end ring, the position of the broken bar can be preliminarily determined.

2、 Power on detection method
Low voltage current test
Connect a low voltage power supply (such as 30-40V) with a rated voltage of 10% to the stator winding, connect an ammeter in series, and manually rotate the rotor. If the pointer of the ammeter oscillates periodically or suddenly drops, it indicates that there is a broken bar fault in the rotor.

Large and medium-sized motors can use specialized detectors to analyze the frequency spectrum of stator current, identify the position of broken bars through software, and achieve non-stop detection.

3、 Iron powder detection method
magnetic field detection
After starting the motor, evenly sprinkle iron powder on the surface of the rotor. If the iron powder is arranged neatly along the direction of the cage bars, it indicates that the rotor is intact; If the arrangement is messy or unable to attach, it indicates the presence of broken bars.

4、 Mutual inductance detector detection
Magnetic flux change detection*
The detector using the principle of mutual inductance, when the rotor bar breaks, the magnetic flux is obstructed, causing the detector reading to abnormally increase. By moving the iron core to observe the changes in the millivolt meter, the slot of the broken bar can be accurately located.

5、 Whole machine testing method
Three phase current monitoring
Apply low voltage electricity (such as 50-60V) to the stator three-phase winding and observe whether the current is balanced when the rotor rotates. If the current varies periodically with the rotation angle, it indicates that the rotor bar is broken.

Attention: Copper bar rotor broken bars need to be repaired or replaced with end rings through welding, while cast aluminum rotor broken bars often need to be replaced as a whole. Non destructive methods (such as iron powder and mutual inductance detectors) should be prioritized during detection, and severe faults should be shut down for disassembly and verification.