In order to generate a rotating magnetic field,a direct current must be supplied to the field winding on the rotor. Since the rotor is rotating,a special arrangement is required. Usually,DC power is supplied from an external DC source to the rotor by means of slip rings and brushes. Slip rings are metal rings completely encircling the shaft of a machine but insulated from it.Both of the two ends of the rotor winding are connected to the 2 slip rings on the shaft respectively.And two brushes ride on two slip rings respectively.If the positive end of a DC voltage source is connected to one brush and negative end is connected to the other,the DC voltage will be applied to the rotor winding(field winding)at all times regardless of the angular position or speed of the rotor.