Variable speed generator

Abstract

A variable speed generator for producing AC electrical power includes an alternator to generate a first AC current, and a first rectifier which rectifies the first AC current from the alternator; a main exciter having a first field winding which receives the rectified first AC current and a first armature which produces a second AC current in response; a second rectifier which rectifies the second AC current from the first armature; a main generator having a second field winding which receives the rectified second AC current and a second armature which produces an output AC current in response; and a control arrangement for activating a selected pole configuration. The second field winding provides a plurality of selectively activatable pole configurations which differ in the number of their poles, such that the frequency of the output AC current can be varied by switching between the pole configurations.

1 . A variable speed generator for producing AC electrical power, the variable speed generator including:
 an alternator powerable by rotational action to generate a first AC current,   a first rectifier which rectifies the first AC current from the alternator,   a main exciter having a first field winding which receives the rectified first AC current, and having a first armature which produces in response a second AC current,   a second rectifier which, rectifies the second AC current from the first armature, and   a main generator having a second field winding which, receives the rectified second AC current, and having a second armature which produces in response an output AC current;   wherein:
 the second field winding is configured to provide a plurality of selectively activatable pole configurations which differ in the number of their poles, such that the frequency of the output AC current can be varied by switching between the pole configurations, and 
 the variable speed generator further includes a control arrangement for activating the selected pole configuration. 
   
     
     
         2 . A variable speed generator according to  claim 1 , wherein the second field winding includes a plurality of pole windings, the selectively activatable pole configurations being produced by switching on or off and/or reversing the polarities of selected of the pole windings. 
     
     
         3 . A variable speed generator according to  claim 2 , wherein the control arrangement includes:
 a control switch for changing the frequency of the distribution current,   a secondary exciter having a third field winding which on operation of the control switch receives the rectified first AC current, and a third armature which produces in response a third AC current,   a third rectifier which rectifies the third AC current from the third armature, and   a switching circuit which is activated by the rectified third AC current to switch on or off and/or reverse the polarities of selected of the pole windings of the second field winding and thereby activates a different pole configuration.   
     
     
         4 . A variable speed generator according to  claim 3 , wherein the switching circuit includes a plurality of field effect transistors, the rectified third AC current providing gate currents for the transistors, and the conducting channels between the sources and drains of the transistors transmitting the rectified second AC current from the second rectifier to the second field winding. 
     
     
         5 . A variable speed generator according to  claim 4 , wherein the polarity of each selected pole winding is changed by an arrangement of depletion mode and enhancement mode field effect transistors in the switching circuit. 
     
     
         6 . A variable speed generator according to  claim 1 , wherein the second field winding includes first and second independently activatable winding formations, the first winding formation providing a pole configuration having a first number of poles, and the second winding formation providing a pole configuration having a different second number of poles, such that the frequency of the output AC current can be varied by switching between the first and second winding formations. 
     
     
         7 . A variable speed generator according to  claim 6 , wherein the variable speed generator further includes:
 a second main exciter having a third field winding which receives the rectified first AC current, and having a third armature which produces in response a third AC current, and   a third rectifier which rectifies the third AC current from the third armature; and   wherein:
 the first winding formation receives the rectified second AC current, 
 the second winding formation receives the rectified third AC current, and 
 the control arrangement includes a control switch which directs the rectified first AC current to either the first field winding or the third field winding to change the frequency of the distribution current. 
   
     
     
         8 . A variable speed generator according  claim 1 , which is an aircraft engine variable frequency starter generator, the alternator being powerable by rotational action extracted from the aircraft engine, and the output current being for use within the aircraft.