Patent GB-555382-A (GB)
Title: Improvements in dynamo-electric machines
IPC: H02P7/34
Filed: 1941-11-21
Abstract: 555,382. Automatic control systems for dynamoelectric machines ; transmitting mechanical power; dynamometers. ENGLISH ELECTRIC CO., Ltd., TABB, R. P. E., and BEGENT, H. H. Nov. 21, 1941, Nos. 15045/41, 8382/42 and 9613/42. [Classes 35 and 38 (iv)] [Also in Group XIX] Torque control.-A dynamo-electric machine driven at various speeds has its torque controlled to a predetermined function of speed other than proportionality. It is particularly adapted for dynamometric tests of aeroplane engines where the dynamo is loaded by a motor and the circulating current automatically maintained at a value determined by a regulator responsive to the current. This regulator may be electrically biased in accordance with the desired function and the biasing determined by a small motor driving a facsimile propeller load at speeds varying with engine speed, or by the sum of a succession of adjustable terms each proportional to a different integral power of the engine speed. Alternatively a resistance controlling torque may be adjusted automatically by a speed responsive means ; or the resistance may be controlled by a reversible motor subject to co-operative action and means responsive to speed and torque whereby a graph of the characteristics is followed. As shown in Fig. 2 a machine 1, 1a is driven at various speeds by an engine under test and loaded by a motor 2, 2a which is restrained to constant speed by coupling to a synchronous machine 3 whereby the power may be absorbed by a supply network or power for running-in the engine obtained. The machine 3 drives an exciter 5, an exciter 6, 6a and an amplifying exciter 8, 8a. A vibratory or other regulator 9a, 9b subject to current through a resistance 10 controls the field of exciter 8 whereby the excitation of the exciter 6 for the motor 2 derived from a constant field generator 4, 4a driven at the speed of the machine 1 may be modified to maintain a constant torque. According to the invention this torque is caused to hold a definite relation to the speed of machine 1 by connecting a biasing generator 11, 11a in series with the regulator coil 9a, and exciting it in accordance with the sum of a constant term derived from the exciter 5 through adjustable resistance 32 and terms proportional to the speed and to the square of the speed of machine 1. The last two terms are derived through an adjustable resistance 33 from an exciter 7, 7a driven at the speed of machine 1 and excited by the sum of currents derived respectively through a rheostat 12 from exciter 5 and through rheostat 13 from exciter 4. The resulting speed-torque function is T = K + BN + CN2 where K, B, C are adjustable constants. An N<SP>3</SP> term may be provided by a further exciter driven by machine 1. The biasing generator 11 according to another modification, Fig. 6 (not shown), may be excited by a mixing generator excited by three field windings supplied respectively from exciter 5, exciter 4 and exciter 7 (excited from exciter 4). A function T = ANp, p being about 1.8, and particularly suited for testing aero engines, may be obtained by substituting for exciter 7 the potential drop across a resistance in series with a motor having constant excitation driving a miniature screw and connected across exciter 4. Fig. 3 (not shown). Fig. 4 shows a contact 20 in series with a reversible motor 30 which is normally maintained on an insulating strip 27, conforming to a graph and the torque-speed relation desired by co-operative action of a screw 23, adjusted by a motor 24 governor or tachometer in accordance with speed changes of the machine 1, and a shaft 25 turning in accordance with the torque of the machine. Any departure from the desired relation will close the circuit of motor 30 through brush 28 or 29 to adjust a rheostat 31. This rheostat may be in series with the field of exciter 8, the regulator 9a being omitted or it may be in series with the field of generator 11 which may alone supply the regulator coil 9a. The contact systems and operating means may take a number of equivalent forms which are described. A rheostat may be graduated in accordance with the desired function, actuated by speed responsive means and placed in the field circuit of machine 2 or exciter 6 or 8 so as to maintain the circulating current in desired relation to speed of machine 1 ; alternatively this rheostat may be used to superpose a variable excitation on the steady excitation of generator 11. In testing engines with two or more related shafts two or more machines 1 may be connected in series with a similar number of absorption machines 2 preferably alternating with machines 1 round the circuit, one point on which, preferably adjacent to resistance 10, is earthed. The torque ratio of the machines 1 may be determined by coupled field resistances of these machines which coupling may be arranged so as to obtain the effect of sudden cessation of torque on one shaft. To compensate any difference between the inertia of the dynamometer machine(s) and that of the destined engine load the speedtorque relation may be distorted during speed changes in accordance with rate of change of speed as by transformers or flywheel machines. The stators of the dynamometer machine and their coupled machines may be trunnion mounted. Specification 554,622 is referred to.