Detection of induction motor squirrel cage asymmetry using dynamic torque spectrum analysis

In the paper, an alternative method of induction motor squirrel cage asymmetry detection is presented. The detection is made possible by a modern measurement system and effective computer software, which is specially designed for induction motor torque characteristic measurements using run-up test. Precise and accurate torque monitoring gives us the ability to calculate and further on to evaluate the dynamic torque spectrum, which provides more complex insight into the motor operation and discloses possible malfunctions. The paper contains a theoretical establishment of the consequences of the rotor asymmetry on the motor operation. The theory is based on the observation of additional stator current harmonic components, generated in the case of a damaged rotor, which reflect out as the additional torque components, speed variation and in common as a reduced motor performance. In order to evaluate the proposed detection method a set of induction motors with deliberately damaged squirrel cages was made. The calculated dynamic torque spectrum for different rates of the rotor asymmetry clearly reveals the additional low frequency components that confirm the theoretical statements.