for the following reasons
First, the commutator switch of the motor reverses the motor by changing the phase sequence of the power supply to change the direction of the rotating magnetic field. If the melt of a phase of the motor in operation is broken, the motor becomes a single-phase motor, and even if the phase sequence of the power supply is changed, the direction of the rotating magnetic field remains unchanged, so the motor cannot be reversed.
Second, the reversal will cause small damage to the motor, mainly the impact of the power supply and coil bearings, but if the inertia is particularly large motor, and the speed is high, it must stop the reversal, otherwise it will cause the following consequences: the motor shift, the power supply, the bearing, can not be reversed and so on