Assessment of a NPC frequency inverter with low switching frequency modulation for a high speed rating operation of an induction motor

Abstract

This paper brings an assessment of a three level NPC inverter with low frequency modulation commands for high speed operation of induction motors. The work is mainly devoted to medium voltage and high power and high-speed systems. It will be approached a very low switching frequencies, reducing the incidence of dv/dt on electric motors and eliminating the 5th and 7th harmonics. It is possible reach good sinusoidal waveforms, with semiconductors switching at 360 Hz, which is very low frequency, being able to preserve motors and semiconductor elements. The 5th and 7th have high amplitude and low frequency, and can cause high losses to the drive. The dv/dts in its turn, damage the electric motors and loads and its reduction contributes to minimizing problems on motors windings, damaged insulation and electrostatic currents on bearings that cause motors degradation. These problems tend to increase for the motor operation in field weakening. Another concern is currents asymmetries that can induce pulsating torques on motors, undesired DC voltages per phase and displacement of neutral voltage of three-phase loads, which in turn can influence common-mode current flows. Considering low frequency modulations, PD-PWM, POD- PWM and the selective harmonic elimination, SHE-PWM, will be studied through simulation and compared with experimental results.