Patent GB-683056-A (GB)
Title: Improvements in and relating to dynamo-electric machines
IPC: F02N11/08
Filed: 1951-01-12
Abstract: 683,056. Exciting dynamo-electric machines BRITISH THOMSON-HOUSTON CO., Ltd Jan. 12, 1951 [Jan. 12, 1950], No. 940/51. Class 35. In a cross-flux generator for supplying current without surges to aircraft engine starter motors, the cross field excitation connections may be modified to produce either zero voltage, by means of a field connection in which the voltage produced by the cross field opposes that due to the main field, or a steady current build-up to a constant current characteristic during the running up period. The starting motor 11 is fed by the generator 13 which has main poles 19 saturated by notches 19a, unsaturated crosspoles 21, and, if desired, commutating poles with windings 24. The commutator 15 is engaged by main load brushes 16, 17 and an auxiliary brush 18, the field winding 20 of the main poles being connected between brushes 16, 18 while the cross-pole windings may be energized through a relay or manually controlled switch 23a from brushes 16, 18 to produce the zero output voltage condition, or from brushes 16, 17 to produce the constant current characteristic. Since the main poles 19 are saturated the voltage generated by their flux between brushes 16, 18 is independent of the load. When the field winding 22 is connected to brushes 16, 18, the E.M.F. produced between brushes 17, 18 by the resultant flux in poles 21, opposes the voltage between brushes 17, 18 so that the output voltage of the machine at brushes 16, 17 is zero. When the relay 23a is in the load supply position, it connects the winding 22 to brushes 16, 17 in such a direction as to oppose the armature reaction, the flux in poles 21 thus decreasing as the load current increases to produce a constant current characteristic. Thus by means of the relay 23a the machine may be connected to produce zero output voltage for connection of the generator to the starting motor and then to produce a constant current characteristic so that the motor may be energized smoothly and without current surges. A voltage regulator 25 may be included in the circuit of field windings 22 without affecting this characteristic. The current to the motor passes through a reverse current cut-out unit 29 including a normally open relay 35 having a reverse current coil 31 and differential voltage coil 33, to ensure correct generator polarity. Closure of relay 35 energizes coil 36 to close the main contacts 30, a relay 34 having a voltage relay coil 32 preventing the main contact being closed before the generator is up to speed. Closure of a starter switch 37 energizes coil 38 from battery 39 to change relay 23a from the zero output voltage connection to the constant current connection, and also energizes coil 36 of the unit 29, through the normally closed contacts 41 of an over-voltage cut-out relay HVR, to close the main contacts 30. Current from the generator then builds up to the constant value for which it is designed and thus closes the normally open contacts 42 of a low-current relay LCR having a coil 44 responsive to the load current. As the motor speed rises, the back E.M.F. first causes the over-voltage relay HVR to open contact 41 and as it approaches normal speed the self-sustaining torque of the engine causes a reduction of load current which opens contacts 42 of relay LCR, thus opening contacts 30 of unit 29 and cutting off the supply to the motor. Opening the switch 37 or disconnection of the leads to the motor causes the relay 23a to return the generator to the zero voltage condition ready for the next start.