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Short circuit fault of a motor, due to breakdown of an insulating material of winding, is one of the most probable faults in motor drive systems. Establishment of an easy and effective fault diagnosis method has been strongly required to assure operation with high reliability. This paper proposes a novel diagnosis method for insulation failure of motor windings by a combination of impulse testing and pattern recognition based on Hidden Markov Model (HMM). A voltage waveform across two winding terminals is recorded under application of an impulse voltage. HMM is exploited to distinguish a small difference in voltage waveforms between healthy and faulty winding insulations. Usefulness of the proposed diagnostic method is verified through voltage waveforms experimentally obtained for motors with several kinds of turn-to-turn insulation failures.