A dc series motor should not be switched on without a load on its shaft because at light or no load, the motor tends to attain dangerously high speed. So, it is also called variable speed motor for this reason. The motor has a high starting torque.
The DC series motor has its field winding and armature winding in the series connection. Therefore, the armature current and the field current is the same.
At light load the field flux is low, and the motor may attain dangerously high speed. Why?
It is because speed of the motor is inversely proportional to the flux.