The reduction in field causes the motor to speed up, and in extreme cases the motor can even destroy itself, although this is much less of a problem in fan-cooled motors (with self-driven fans). This can be a problem with railway motors in the event of a loss of adhesion since, unless quickly brought under control, the motors can reach speeds far higher than they would do under normal circumstances.
A reduction in field causes the motor to accelerate, and under extreme circumstances, it could potentially destroy itself. Fan-cooled motors are less prone to this issue compared to railway motors, which can experience excessive speed if adhesion is lost and not managed promptly.