Disclosed is a method for suppressing a speed ripple occurring during an operation of an AC motor by using a torque compensator based on an activation function. The method includes the steps of calculating a speed error ωerr based on a reference speed ωref and an actual speed ωact; calculating a controller output Trm by using the speed error ωerr as an input of a PI control and an operation of a compensated torque Tcom; and determining a torque variation based on the controller output Trm and a reference torque Tref and operating the torque variation in relation to an anti-windup gain Ka to use torque variation as an input of an integral (I) control. The method suppresses the speed ripple by compensating for the torque ripple through a controller which calculates the compensated torque by taking the signs of the speed error and the differential speed error into consideration.