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The presented routine makes it possible to determine the maximum over-current factor resulting from the demagnetization resistance of the magnets for different structures of the magnetic circuit. The difference between the calculated and the measured values of the over-current factor ranges from 1.5 to 18 % and it is due to the simplifying assumptions, the measuring error and, above all, to the discrepancy between the catalogue and the real parameters of magnets. The application of pole shoes is an effective way of increasing the over-current factor of machines with permanent magnets but it reduces. The increasing of yoke reluctance results in smaller increments of the over-current factor but the reduction of the flux is twice smaller in this case. The significant advantage of the machine with increased yoke reluctance is the smaller deformation in the field distribution which results in the improvement of the commutation conditions.