When a bulk superconducting material is used as the magnets of an electric motor, the magnetized superconducting pucks are subjected to a varying magnetic field which has a detrimental effect on the trapped field. This may lead to a long term decay of the magnetization of the bulk superconductors. In this paper, we analyze numerically the demagnetization effect on bulk superconductors subjected to a magnetic field applied orthogonally to the initial direction of the trapped flux. The condition of a superconducting sample being exposed to a transverse field with varying magnitude is simulated and is very close to the operating environment in a synchronous machine. The decay of magnetization is calculated. The mechanism is explained in terms of the distortion of current distribution on the cross section of the sample. The long term effect of the application of a transverse field is predicted by the model.