Method for detection of broken bars in induction motors

A method and apparatus for detection of broken bars in the rotor of a squirrel cage induction motor are described. The test method is easy to implement, no electrical connections need be made to the rotor, and it is not necessary to turn the rotor. The effect of the broken bar is modeled as an image current in a direction opposite to the current if it were intact and a corresponding returning image current in the other bars. Sense coils encompassing 180 degrees are examined, the inner coil is wound in the slots near the gap and the outer coil is on the outside of the stator. The main signal induced in the sense coils is due to the image currents associated with the broken bar. The signal induced in the coils is greatest when the broken bar is near the edge of the sense coils. A test apparatus with 8 poles and 24 slots was constructed and results of measurements on a rotor with 17 intact and one broken bar are presented, The inner and outer sense coils yield complementary information on the state of the rotor.