Motor control apparatus with true rms non sinusoidal negative sequence stator current protection mode

Abstract From the Canadian patent

A microprocessor-controlled motor starter in which the negative sequence component is determined by monitoring the three-phase lines of a motor under starting conditions. The stator winding currents are converted to the respective negative and positive sequence components in the microprocessor so that an RMS indication of the actual instantaneous current flowing in the stator windings may be had. This is true whether the current is sinusoidal or non-sinusoidal. By using an appropriate model for the thermal equivalent circuit for the rotor a predetermined safe value of current may be determined above which the motor starting or running action is aborted.