Armature is the rotating part of dc machine. The resistance offered by the armature circuit is known as armature resistance and is represented by Ra.It usually includes two components . One is the resistance of armature winding and other is the brush contact resistance. The armature resistance depends upon the type of the machine. Except for small machines it is usually of the order of 1Ω.
The voltage drop over the brush contact resistance when current flows from armature segment to brushes and finally to external load is called brush contact drop.It is equal to the product of the values of current and contact resistance. The voltage drop allowed for all brushes of each polarity shall be 1.0 V for carbon or graphite brushes and 3.0 V for metal carbon brushes.
Speed control of dc motor can be obtained by armature and field control methods. One of the methods of armature control is armature resistance control method.
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