Alternator excitation system
An excitation system for a brushless synchronous alternator contains an exciter having a stator which has first and second sets of field coils ,respectively coupled to a source of DC power and in series with polyphase AC output current generated by the main generator. Here, The base level of exciter field current is provided by the DC current supplied to the first set of field coils. a second variable amount of field current is provided by the AC output current supplied to the second set of field coils. a rotor structure having first and second sets of armature windings is also present in the exciter. This helps to develop first and second exciter currents in response to movement of the rotor structure within the magnetic fields set up by the first and second sets of exciter field coils. Transient response is improved and overall length and weight of the generator package are decreased By dividing the responsibility for providing exciter field current in this fashion.
The synchronous alternator includes a dual winding exciter structure for developing first and second exciter signals which are combined, rectified and coupled to thefield windings of a main generator. armature windings which develop polyphase AC output signals to drive one or more loads are present in the main generator. a voltage regulator and rectifier coupled to the armature of a permanent magnet generator supplies the base current to the first set of exciter field windings. A second set of exciter field windings is connected in series withthe armature windings of the main generator so that the second set of exciter windings receive the load current. first and second exciter magnetic fields are established by the first and second exciter field windings through which the first andsecond sets of exciter armature windings move. a polyphase rectifier converts the exciter armature currents into a DC current for energizing the main generator field winding. A transient regulation is provided by the AC exciter, the demands placed upon the voltageregulator are diminished. the PMG length and weight may be decreased, since only a base level of exciter current is required to be supplied.

For more details visit:
http://patentstorm.us/patents/4467267/fulltext.html
http://criepi.denken.or.jp/en/e_publicat...eika32.pdf
http://198.170.104.138/jas/2007/1186-1190.pdf