Induction motor stator current spectral content under sinusoidal and PWM excitation

The use of converters to feed motors instead of the direct-on-line supply has increased the need to understand the behaviour of the induction machine in different operating points. The additional losses in frequency converter fed machines are caused by the harmonic content of a PWM voltage excitation. This paper investigates closely the harmonic contents of stator supply voltage and current. Most of the earlier studies have focused on sinusoidal supply or converter supply in only a few specific load points while this study covers most of the frequency-torque plane below nominal values. A 15 kW standard squirrel-cage machine's stator current and voltage time functions and spectra are analysed in 16 different operating points with torque values of 25 %, 50 %, 75 % and 100 % of the motor's rated torque and supply frequency values of 25 %, 50 %, 75 % and 100 % of the motor's rated frequency. Measurements in all 16 operating points are performed with both sinusoidal generator supply and with a frequency converter supply.