Abstarct: Hysteresis motors have being used in special applications for decades. Hysteresis motor has many advantages such as self-starting, constant torque, low noise and vibration, low start up current and robust construction. Although these benefits make hysteresis motor very convenient to use, the motor has its own problems including relatively low power factor and efficiency. Industries, that use hysteresis motors greatly, have to install huge capacitive bank due to low power factor. Applying this solution is so expensive and it does not improve low efficiency. In other solutions, it is required to modify the motor structure. Overexcitation can improve power factor and efficiency of the motor without any changes in the motor structure. During the overexcitation, the input voltage of a hysteresis motor is increased from its steady state value Vi to nVi continuously, and then is reduced again to Vi. This paper investigates the effects of various scenarios of the input voltage variations on the performance of a hysteresis motor including normal and special kinds of overexcitation. A simulation algorithms is presented for performance prediction of the motor. This algorithm gives the relations between circumferential components of magnetic field intensity and magnetic flux density around the hysteresis disk of motor as elliptical loops. by having these loops, Parameters of the motor equivalent circuit are computed and the motor performance is evaluated for each operation condition. It is also discussed that how overexcitation can make considerable improvement in the hysteresis motor characteristics.