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.
The decrease in field leads to an increase in motor speed, and under extreme conditions, the motor might self-destruct. However, this is less likely in fan-cooled motors (with self-driven fans). This can be a concern with railway motors if adhesion is lost, as they may reach speeds significantly higher than usual without proper control.