Real Time Harmonic Elimination Using a Modified Carrier Hirak Patangia College of Engineering & Information Technology University of Arkansas Little Rock, USA e-mail: hcpatangia@ualr.edu Sri Nikhil Gupta Gourisetti College of Engineering & Information Technology University of Arkansas Little Rock, USA e-mail: gigupta@ualr.edu Abstract— The paper proposes a harmonic elimination technique that is based on comparison modulation with a modified carrier. Typically, selective harmonic elimination method calculates the switching angles offline using iterative computations and programmed into a digital processor. Here we are advancing a modulation-based harmonic elimination technique to remove the unwanted harmonics. It is a novel PWM method where a reference sinusoid with the desired signal frequency is compared against a modified carrier signal with the same time period as the reference. Simulation and laboratory studies are included. I. INTRODUCTION There exists a vast amount of literature related to harmonic elimination in power electronics. Sinusoidal pulse width modulation (SPWM) is a well-known method to eliminate lower order harmonics in modern inverter control [1]. Here a sinusoidal reference is compared against a high frequency triangular carrier to produce the PWM switching signal as shown in Fig. 1a. A frequency ratio of 10:1 or greater virtually eliminates all lower order harmonics as shown in the FFT of Fig. 1b. V tri V control t t V (t) pwm Fig.1a Sine Triangle PWM (SPWM) Fig.1b SPWM Spectra (fc = 10fref, fc = Carrier frequency) __________________________________________________________ The work was partially supported by NSF (Grant #0942327) Selective harmonic elimination (SHE) is another method popular in motor control that results in lower switching frequency [2, 3]. Here the switching times are chosen in such a way that the unwanted harmonics except the fundamental are eliminated. The harmonic elimination problem boils down to solving a set of transcendental equations which are solved off line using iterative computations. The switching times are programmed into a look up table in a digital processor and to be read in real time. To avoid the need for solution of complex transcendental equations offline, recent research activities for harmonic elimination have been focused toward modulation based harmonic elimination [4, 5]. This has the advantage of real time comparison of the reference with a modified carrier to produce the desired PWM signal. The generation of the modified carrier reported in [4] is complex since it requires fast digital computation on the fly and D/A conversion. A digital signal processor is required for its implementation. In the present work, we are formulating a simplified modulation based harmonic elimination technique without the need for ‘on the fly’ signal processing. The method produces a well-defined comparing signal and the resulting comparison with a sinusoidal reference produces the PWM signal similar to sine-triangle PWM with suppression of the undesired harmonics. The fundamental is tunable while suppressing the undesired harmonics. The paper reports the preliminary finding which is novel and appears to have some attractive features. The motivation for the work originated from our original desire to develop low cost switching modulators which have lower switching frequency for RF application [6]. II. SINE-SINE PWM The most common PWM that eliminates lower order harmonics is sine-triangle. As seen in Fig. 1b, the first appreciable harmonic (carrier sideband) appears at f c – 2f o , where f c is carrier switching frequency and f o is reference frequency. If higher f c is acceptable, the sideband components can be pushed out easily. For lower f c , it is necessary to remove the sidebands to allow more baseband space. The Fourier coefficients of the sine-triangle SPWM are complex in nature since it involves Bessel functions [1] and often simulation is used to verify the nulling of the harmonics. For simplicity and ease of harmonic elimination, we begin the proposed method with a sine-sine PWM. Here, 978-1-61284-1325-5/12/$26.00 ©2012 IEEE 273