In series motor field winding is in series to armature winding. In any DC motor the speed depends inversely on the flux and directly proportional to emf, N ~E/flux.
Motor armature current is decided by the load.On no load the armature current drawn by the motor will very small.
For d.c. series motor,
Flux is Directly proportional to Armature current.
and on no load as Ia is small hence flux produced is also very small.
According to speed equation,
Speed is Inversely proportional to Flux i.e ( N α 1/Φ)
as Eb is almost constant.
As from Speed Equation Speed is inversely proportional to flux and for no load or less load the flux produced is also less hence the rotor rotates at very high speed which can damage the motor mechanically.
This is the reason why series motor should never be started on light loads or no load conditions.
When the motor is connected across the supply mains without load, then it will draw a small current from supply mains.
Now question arises why small amount of current and why not high current?
It is because when no load is connected on motor, torque requirement will be low. Now as torque requirement is low, speed will be higher as speed is inversely proportional to main flux and torque is directly proportional to main flux. So as speed will be higher, back emf will be higher i.e higher opposition to the flow of current from supply to motor and thus current will be less on no load condition.
coming back to our question:-
So now as low current will flow through the field winding as well as armature winding ( as it is series motor) so main flux produced will be lesser, as before saturation point current is directly proportional to flux. So as main flux is lower, speed will tend to increase. As speed increases, back ems also increases and thus current drawn from the supply will become more and more lower and thus main flux will also become more and more lower and thus speed will tend to increase more and more and this continues till armature of motor has attained speed which is so high that due to centrifugal forces set up in the rotating parts damages armature.
Therefore as a thumb rule we never run series motor on less than 15% of full load. If accidentally it occurs then quickly disconnect the motor from the supply.
Contact Person: Mr. Larry