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The inverter-fed induction motor drive system may become unstable at low frequencies and light load. Taking the inverter-fed five phase induction motor drive system as an example, the small disturbance mathematic model of whole system is deduced, based on small disturbance model of induction motor and switching model of inverter system, the three-dimensional unstable region is presented and discussed by Hurwitz stability criterion of modern control theory, the results of mathematic analysis, simulation and experiments are identical. The effect of variant induction motor parameters, filter-inverter parameters, and operation conditions on system stability is studied in details. The analysis method and the conclusion obtained in the paper may contribute to the design of parameters and stable operation of inverter-fed multiphase induction motor drive system.