There are two types of exciters, static exciter and rotary exciter.purpose of exciter is to supply the excitation dc voltage to the fixed poles of generator.Rotor exciter is an additional small generator mounted on the shaft of main generator. if it is dc generator, it will supply dc to the rotor poles through slip ring and brushes( conventional alternator). if it is an ac exciter, out put of ac exciter is rectified by rotating diodes and supply dc to main fixed poles ac exciter is the ac generator whose field winding are stationary and armature rotates. initial voltage is built up by residual magnetism. It gives the starting torque to the generator.