On-line wideband measurement of induction motor impedance

Induction motors are increasingly operated from harmonic-rich voltage sources such as adjustable speed drives (ASDs). The harmonic voltages produce harmonic winding currents which in turn cause increased motor loss due to circulating currents in the stator winding and rotor. Typical induction motor tests use only the 60 Hz excitation provided by the power system and may not be completely adequate to predict loss when the motor is operated from an ASD or other harmonic-rich voltage source. This paper describes an additional test which measures the wideband impedance of a motor during normal operation. The resistive part of motor impedance is shown to increase for frequencies above 60 Hz and application of this test to calculating motor loss is described. Extensions of this test applications such as motor parameter estimation and online monitoring of motor temperature by the change of resistance technique are suggested