Abstract: The harmonic magnetic field under steady operation is analyzed, with computation methods for harmonic current and ripple torque of the induction motor obtained. Analysis shows that the 3Kth harmonic voltage does not initiate the harmonic current and torque while the (3K-1)th harmonic voltage not only produces the same harmonic current and ripple torque as the (3K-2)th harmonic voltage, but also adds a reverse torque to the fundamental and causes a loss. Experiments proves the correctness and effectiveness of the calculating results. Finally, a solution to restraining the harmonic current and ripple torque is suggested, ie a larger modulation frequency ratio of 3K is to be adopted, as it is used in SPWM.