In the demagnetization of an electric motor of direct current, the permanent magnets used to create the field on which its operation is based are not permanent, losing its magnetism over time. Another important factor is the current that flows through the windings.
All bodies tend to vibrate with greater intensity at certain frequencies and this holds true for the mechanical parts of an electric motor. A DC motor when rotating freely, without load, electric motors tends to accommodate itself in a rotation in which its mechanical parts hesitate in the resonant frequency, which can cause efforts that affect the integrity of the motor and its yield. What should be done is to employ parts that have different resonant frequencies and turn the other way around to override this effect.
The magnetic field created by the windings acts on the permanent magnet and over time causes its magnetism to decrease to the point where it begins to affect the efficiency of the motor. It is important to note that the current intensity above the value in the motor windings can create a very intense field to completely demagnetize the permanent magnets. Thus, pulses of intense currents should be avoided.