Principle of Operation
An external voltage source is applied across the series configuration of field winding and armature. So one end of the voltage source is connected to the winding and the other end is connected to the armature through the brushes.
Initially at the motor start up, with the voltage source connected to the motor, it draws a huge amount of current because both the winding and the armature of the motor, both made up of large conductors, offer minimum resistance to the current path. The large current through the winding yields a strong magnetic field.
This strong magnetic field provides high torque to the armature shaft, thus invoking the spinning action of the armature. Thus the motor starts rotating at its maximum speed in the beginning. The rotating armature in the presence of the magnetic field results in counter EMF, which limits the current build up in the series combination of armature and winding.
Thus series motors once started will offer maximum speed and torque but gradually, with an increase in speed, its torque will come down because of its reduced current. Practically this is what required from the motors. Due to the high torque provided by the armature, the load on the shaft is set to rotate initially. Subsequently lesser torque will keep the load on the move. This further helps in increasing the heat dissipation of the motor. However, the amount of torque generated by motor is directly proportional to the winding current. The higher current demands a higher power supply, too.