Towards a novel direct online speed-torque curve plotter for three phase induction motor

The measurement of induction motor air-gap flux and rotor speed using flux search coils along with stator windings is a known technique used for variable speed operation of three phase induction motor. The use of stator windings as search coils for sensing air-gap flux was not well received in the industry due to difficulties in signal processing of the sensed rotor voltage signals. This paper discusses various components of air-gap flux wave caused by rotating magnetic flux due to the rotor and stator excitation, and the harmonic components due to motor construction and rotor current. This paper proposes a new method for recovering the Electromagnetic Field (EMF) signal induced by the air-gap flux in stator windings to sense the rotor current (I₂) and rotor current frequency (f_R). As an extension, a Digital Signal Processing (DSP) based algorithm for extracting the rotor current and rotor speed information from the recovered signal to calculate torque, speed, slip of the induction motor (IM) online has been discussed. This has been validated by simulation and experimental data through an off-line plotting of torque - speed (slip) characteristics of Induction motor using the proposed method.