Traction and Normal Forces in the Linear Induction Motor

The single-sided stator and the double-sided stator configurations of the infinitely long linear induction motor are analyzed from electromagnetic field theory, and the traction force and the normal force equations are derived using the Maxwell magnetic stress tensor. The force normal to the air-gap plane, which has not been quantitatively evaluated or understood before, is found to be highly repulsive for slips greater than St = 1/Rm (Rm magnetic Reynolds number). The engineering use of this large repulsive force to provide frictionless contact-free means for levitation, suspension, and mechanical stabilization of high-speed transport at speeds of 300 mi/h is considered. A physical explanation is given for the existence of the normal force components. The equivalent circuit is derived for the linear induction motor so that the forces and the design parameters obtained from field theory can be understood in terms of traditional induction motor theory. The performance characteristics are investigated, and it is found that the two key design parameters are 1) the magnetic Reynolds number Rm and 2) the air-gap-wavelength ratio ßg. The single-sided stator configuration is considered for levitation, and the criterion for a self-levitating vehicle is given. In the double-sided stator configuration, the net normal force is a restoring force which is considered for keeping the vehicle aligned on track against lateral perturbations. In this case, the equivalent electromagnetic spring constant Ck and the natural frequency of oscillation fm are derived and their performance characteristics investigated.