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Modulation techniques can be used in thyristor circuits to obtain frequency changing that is useful for ac motor speed control. Some of the modulation techniques result in three-phase applied voltage waveforms that contain subharmonics of the supply (carrier) frequency as well as higher harmonics. An analytical technique is developed for predicting the performance of an induction motor when subjected to this form of nonsinusoidal voltages. The technique involves the use of reference frames that rotate at subsynchronous speeds consistent with the known subharmonies of the voltages. A theoretical investigation of many applied voltage modulated waveforms shows that only one particular form of phase modulation is feasible for induction motor speed control.