I. Introduction
Interior permanent magnet (IPM) motor has been proposed as a strong candidate of the driving source of electric vehicles, not only because of its high torque output and high efficiency, but also because they are easily controlled over a wide range of speeds. IPM motors retain the various shapes of permanent magnets inside the rotor core to increase the magnetic flux density in the air-gap in such a way to utilize both magnetic and reluctance torque. However, the magnetic force acting along the teeth of the stator and the surface of rotor not only generates torque to run the rotor, but also excites the IPM motor, which in turn generates a significant amount of vibration and noise. The vibration characteristics of IPM motors can be investigated through a forced vibration analysis which is the response of the IPM motor due to the excitation of magnetic force.