A series wound DC motor has its armature and field connected in a series circuit. These type motors normally drive loads that require high torque and do not require precise speed regulation. Series DC motors are ideal for traction work where the load requires a high breakaway torque. Such are locomotives, hoists, cranes, automobile starters, or oil drilling rig applications.
Starting torque developed in series motors normally ranges between 300% and 375% of full load, but attain 500% of full load torque. These motors deliver this high starting torque because their magnetic field operates below saturation.
An increase in load results in an increase in both armature and field current. As a result, the armature flux and field flux increase simultaneously. Since the torque developed in DC motors is dependent upon the interaction of armature and field flux, torque increases by the square of current increase.
Speed regulation in series motors is inherently less precise than in shunt motors. If motor load diminishes, current flowing in both the armature field circuits reduces as well, effecting a reduction in flux density.
This results in a greater increase in speed than realized in shunt motors. Removal of mechanical load from series motors results in indefinite speed increase whereby centrifugal forces generated by the armature eventually destroy the motor.
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