It is seen earlier that motor armature current is decided by the load. On light load or no load, the armature current drawn by the motor is very small.
In case of a d.c. series motor, Φ α Ia and
on no load as Ia is small hence flux produced is also very small.
According to speed equation,
N α 1/Φ as Eb is almost constant.
So on very light load or no load as flux is very small, the motor tries to run at dangerously high speed which may damage the motor mechanically. This can be seen from the speed-armature current and the speed-torque characteristics that on low armature current and low torque condition motor shows a tendency to rotate with dangerously high speed.
This is the reason why series motor should never be started on light loads or no load conditions. Foe this reason it is not selected for belt drives as breaking or slipping of belt causes to throw the entire load off on the motor and made to run motor with no load which is dangerous.