A novel method for non-contact detection of an I₂ signal is presented using an MI sensor head inside a hole drilled through the shaft of a general-purpose induction motor (IM). An almost pure I₂ signal at the slip frequency is stably detected. The secondary-current, feedback-type torque control system is constructed using a general-purpose V/f inverter controlled by a PC system. Accurate control continues even after long operation that increases the temperature in the IM. An improvement in response speed is obtained using the state-feedback technique.