DC components and subharmonics generated by naturally sampled PWM techniques

One possible implementation form of Pulse Width Modulation is the naturally sampled one (NS-PWM). Its favourable properties are that no distortion or delayed response are introduced by it. The paper compares three different realizations of NS-PWM techniques by simulations and experimental measurements when the carrier over reference frequency ratio mf is small number as in inverters supplying ultrahigh speed induction motor (USIM). The three NS-PWM techniques are the Sinusoidal, Sinusoidal and third-harmonic reference injection and the Space Vector Modulation. The paper verifies both by simulation and laboratory results that the three techniques generate DC voltage component in the output voltage of the inverter contrary to many previous publications. In addition subharmonic currents and fluxes with considerable amplitudes are produced as well due to the small mf and very low subharmonic frequency even though the subharmonic voltage component is very small.