472,472. Dynamo electric machines. ENGLISH ELECTRIC CO., Ltd., CARNEGIE, H. S., and WILSON, J. C. Feb. 18, 1936, Nos. 4935 and 8893. [Class 35] [Also in Group XXXVII] Exciting.-The excitation of a dynamoelectric machine 11 is varied in accordance with a chosen variable, shown in connection with load current compounding, over a restricted range by means of an exciter 17 supplying excitation current to the machine which exciter generates a voltage varying with the variable and opposed by a biassing voltage in its armature circuit, a rectifier 18 or polarized relay being provided in the circuit so as to reduce the current therein in one direction below that in the reverse direction. The biassing voltage may be an auxiliary source in the armature circuit of the exciter or it may be produced by an opposing biassing field winding 16 on the exciter 17. In place of the rectifier 18 a polarized relay 18a, 18b, Fig. 4, may be used having a polarizing winding 18c or a permanent magnet. In the event of reverse current flow due to failure of the polarized relay a reverse current relay 21a, 21b may be included in the field circuit. The compounding characteristics may be varied by altering the connections of the current directional responsive device 18. In an arrangement, Fig. 7, in which compounding is required only between two values of load current, the generator is provided with a separate' excitation 12 and a winding 10 supplied through a rectifier 28 from an exciter 27 which is provided with a biassing field winding 26 and a winding 25 supplied through a rectifier 18 from a further exciter 17 which has a biassing field winding and a winding 15 carrying load current. The arrangement is such that up to a critical value of load current the exciter 17 supplies no current to the winding 25 and the exciter 27 supplies a steady current to the winding 10. Above this value the current in the winding 25 increases and the output of the exciter 27 decreases to zero and is thereafter inappreciable. The characteristic may be reversed by reversing the valve 18 and field 25. Compounding may be rendered ineffective over an intermediate range by providing two field windings such as 10 on the generator and supplying each through a rectifier from a separate biassed exciter responsive to load current. The critical load currents of the biassed exciters may be made such that their ranges of operation overlap or are separated by an intermediate range of no compounding. The same effect may be obtained with one biassed exciter by arranging saturation effects in the exciter so as to render de-compounding ineffective at low loads or compounding ineffective at high loads. Compounding effects may be enhanced by providing a shunt winding on the generator or a winding on the biassed exciter energized from across the generator. The various co-operating windings on the generator may be replaced by a single winding energized from a further exciter carrying the co-operating windings. Alternatively the additional exciter may be a " braking " exciter in series in the supply from a constant voltage source to the field winding of the generator. In a system in which the compounding is applied to a generator supplying a motor, re-generative braking may be obtained by providing an additional field winding on the generator energized through a rectifier or polarized relay from across a shunt in the load circuit. The additional winding is only effective under regeneration conditions. If the motor is reversible means are provided for reversing the biassing field on the exciter or the series biassing voltage and also the polarity responsive means. Two rectifiers may be provided and connected in circuit selectively through a change-over switch. In a Diesel-electric ship propulsion system, Fig. 16, the drive motor 41 is supplied from a Ward-Leonard generator 11 having a field winding 35 supplied from an exciter 33 having four field windings 10, 12, 31 and 37. One field winding 12 is supplied through a control resistance 44 from constant voltage and controls the motor speed. A second winding 10 is supplied through one of two rectifiers 18, 18a, from an exciter 17 having a biassing field winding 16 connected in parallel to the winding 12, and a winding 15 in series in the load circuit so as to give the required compounding or de-compounding characteristic. A third winding 31 is shunted across the exciter 33 and a fourth winding 37 provides for regenerative compounding by being connected in the load circuits through one of two rectifiers 38, 38a. A single controller governs the speed and the compounding characteristics and the reverse running connections. The invention is also applied to the protection of large alternators from sudden excess voltages, Fig. 17, in which case an exciter 51 for the alternator 48 is provided with an auxiliary field winding 49 which is supplied from an exciter 17 in series with a biassing voltage 19 and a rectifier 18. The voltage of the exciter 17 is made to vary with the alternator voltage so that the winding 49 which is ineffective at normal values of alternator voltage is energized so as to reduce the excitation as an inverse function of the voltage if the voltage rises above a predetermined excessive value. The biassed exciter operation may be varied in this example, in any of the ways above described.