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This paper reports an adaptive gain sliding mode observer for Brushless DC (BLDC) motor for large variations in speed. BLDC motor based on sliding mode observer exhibits multiple zero crossing in back EMF which leads to commutation problem at low speed. In this paper a modified sliding mode observer incorporating the speed component in the estimation of back EMF is proposed. It is found that after incorporating speed component in the back EMF observer gain, multiple zero at low speeds and phase shift at higher speeds are eliminated. Detailed simulation results are carried out to verify the effectiveness of the proposed scheme.