Answer Posted / rathish
In any DC motor the speed depends inversely on the flux.
Consider when starting a series motor the current flowing
will be high, but the flux that the motor produces will be
moderate since the series winding has less number of turns.
If u start the motor without load the speed will increase.
as speed increases the back emf increases. this decreases
the current through the series winding and hence flux
decreases and the speed further increases. Theorerically
speaking the speed becomes infinite. But practically the
motor will accelerate to very high speed , which will damage
the motor. But if you start the motor with the load, you are
actually reducing the starting speed hence the motor runs
safely. Since series motors has this property it is used in
trains to pull high load.
shunt motor starting torque is
marginal and hence it should be started with least load.
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