In this winding the working emf is induced by the working flux.
The current which is dependent on the load called current. The windings that handles only exciting current is called field winding. Current in fields windings is always DC. It is also known as fields current of magnetizing current. It produces only a working magnetic flux. It is independent to load. Armature winding carries only load current.
Before discussing the methods of speed control of DC motor it is important to know about the classification of DC motor. According to excitation of the filed self excited DC motor further divided into three types Dc series motor shunt motor and DC compound motor.
Figure DC Motor Separately excited(b) Shunt (c) Series (d) Compound
The commonly used DC motors are shown in separately excited motor the field and armature voltage can be controlled independent of each other. In a shunt motor field and armature are connected common source. In case of a series motor fields current is some as armature current and therefore fields flux is a functions of armature current. In a compound motor the mmf of the series fields is a functions of armature current and is in the same direction as mmf of the shunt fields.
For separately excited and shunt motors it is equal to the resistance of armature windings and for series and compound motors it is the sum of armature and fields winding resistances.