Contents I. Introduction
An increasing number of electrical motors is supplied from frequency converters. Controlling the rotation speed by varying the supply frequency is essential in process industry and propulsion applications and can in addition allow huge energy savings when applied e.g. to pumps, compressors or extrusion processes. However, the nonsinusoidal supply voltage from the frequency converter causes additional energy losses in the motors. Since the output voltages are formed by controlled switching using thyristors or power transistors, the frequency contents can consist of harmonic components up to tens of kilohertz. Thus the current drawn by the inverter-supplied motor also contains high-frequency harmonics which increase the machine losses due to skin effect in the armature windings, additional hysteresis losses caused by the distorted flux-density waveforms and increased eddy-currents induced into the laminated or solid parts of the machine core. These additional losses depend on the operating point of the motor since they are affected by the magnitude of the armature current and the magnetic saturation of the core.
