New rotor-current detection methods for squirrel-cage induction motors are presented using amorphous micro-core magnetometers which detect magnetic fields of 10-5 −50 Oe and of dc −10 kHz. The magnetometer is composed of two amorphous cores 0.5-mm in diameter and 2-mm in length, and two switching transistors constituting a multivibrator bridge circuit with a dc output. Two methods are used for rotor-current detection: (i) detection of the flux due to the end-ring current by setting the cores into a narrow space near the end-ring, and (ii) detection of the rotor axial ripple flux due to the rotor currents through the skewed rotor bars by setting the cores in the stator slots of 0.6-mm depth.
