A dummy coil is a copper coil present in the armature of a DC machine which is used to reduce the effects of armature reaction on the air gap flux. This coil is not connected to the armature of the machine. The only purpose of the this coils is to provide mechanical stability to rotor or armature.
Also, what is the function of compensating winding in DC machine?
In order to neutralize the cross magnetizing effect of armature reaction, a compensating winding is used. The compensating windings consist of a series of coils embedded in slots in the pole faces. These coils are connected in series with the armature.
What is armature reaction in DC generator?
In a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.