Losses in grid and inverter supplied induction machine drives

Inverter supply is known to create additional power losses in an induction machine over those occurring with grid supply. The extent and nature of that increase are examined here for a switching frequency representative of current practice. Data are obtained from a time-stepped finite element analysis (FEA) model of the machine, coupled to circuit equations that include a model of the switching action of the inverter. A variable time-step algorithm is used to ensure accurate representation of the switching waveforms at reasonable computational effort. Further improvement in speed is achieved by not performing a full FEA update of elemental reluctivities at every time-step but instead only when required by a change in magnetic flux. The power loss found in simulation is verified through experimental tests for both the machine and inverter. The examination of losses categorises those losses by cause, location and frequency band. The causes modelled are motor ohmic and iron losses and inverter conduction and switching losses. Iron losses are further divided into hysteresis, classic eddy-current and anomalous losses. Pulse-width modulation (PWM) at 5 kHz gave an identifiable but relatively unimportant increase in ohmic loss in the machine. In contrast, iron losses increased significantly. Eddy current loss in the rotor increased significantly (by up to 150% near the head of the bar) while in the stator, a significant increase in iron loss was caused by the phase-band leakage flux generated at the switching frequency.