A separately excited DC motor and a DC shunt motor are similar in construction and in all aspects except the way in which the field winding is excited in both the machines.
In a separately excited DC motors, field winding is excited from a separate source, and the armature is excited from another supply. But in a DC shunt motor the armature and field windings connected in parallel across a supply.
In separately excited motors the flux in the filed winding is independent on the load, but in the case of a shunt motor it depends on the load on the motor.
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