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Aviation History
1957
1957 - 1744.PDF
834 FLIGHT, 29 November 1957 (Lett) A rotor pack and stator assembly from a hydraulically driven alternator. (Right) One of the new power units on bench test. POWER GENERATION IN MISSILES . . . necessary for both gas- and oil-driven alternators is achieved bythe use of a proportional relay, or transducer. This comprises two permanent magnets with their north and south poles coupledby wound cores, which surround a soft-iron armature. Each core has two windings, which, when fed with current, can affectthe armature position. If the currents in the windings are equal the armature remains in a central position, but if the currentsvary the armature turns accordingly. A drive is taken from the armature pivot, sufficient torque being developed either to operatea throttle directly or through a closed-loop mechanical servo. This mechanism, a Laws relay, is now used extensively by theweapons industry. The type of throttle varies according to application. A simplerotary valve operated directly by the relay proves efficient for a clean, cool gas supply, whereas with hot gas or extremes oftemperature a servo-operated tapered needle is needed. Servo assistance is also necessary for the hydraulic throttles, of whichvarious types have been tried. The most effective has proved to be a "hole and hill" type, which is a rotary valve with two inclinedplanes cut into its top surface. These planes mate with ports in the sleeve so that rotary motion of the valve varies the area ofport uncovered. This system has proved remarkably sensitive. A mechanical feedback is always provided to close themechanical servo loop, generally by a spring which couples the transducer armature to the throttle, although later designs accom-plish this action by a direct linkage mechanism. Mechanical systems such as these have a delay which, when included in thecomplete frequency-control loop, tends to produce instability. To counteract this, the throttle is made to operate an electrical feed-back circuit by moving a wiper arm over a potentiometer. The Controller is divided into two sections, the voltage regula-tor and the frequency governing circuit. In the voltage regulator, the alternator output is fed into the primary winding of a trans-former. The resultant output of the secondary winding is rectified and fed, through a choke, to a neon bridge circuit. Thisis adjusted so that at a given load (approximately mid-range) the control current is fed either to a single or a multi-stage magneticamplifier, which produces the level of field current required to give the nominal output voltage. Variations in this output voltage,due to load changes, then cause changes of error current which in turn alter the field current through the magnetic amplifier, sothat the initial output voltage variation is reduced. The frequency governor controls the operation of the trans-ducer. As in the voltage regulator, the alternator output is fed into a transformer and men into two resonant circuits, one tunedabove the required datum and one below. These circuits are each connected in series with a bridge rectifier so that at the nominalfrequency the output from each rectifier is the same. As the tuned circuits are set one above and one below the datum frequency, theoutput from one increases as the other decreases with an off- speed. The difference in these outputs is then applied through apush-pull magnetic amplifier to the transducer coils. This in turn reacts on the transducer armature, the resulting rotationbeing in the correct direction to reduce the initial frequency deviation. Aircraft Power Units. Once the merit of the early work onalternators for guided weapons had been established, it was a logical step for de Havilland Propellers to design a range of asso-ciated power units for use in aircraft. The unit in this case is of similar basic design to the weapon alternator, but is driven byfluid from the aircraft hydraulic system. It is built to a rigid mechanical specification to achieve continuous rating. The highpower output demands the use of a swash-plate motor rather than a gear motor, and thus two stages of throttling are possible. Finecontrol is achieved by using a rotary valve, while coarse control for large load-changes is obtained by altering the angle of the swashplate and thus reducing the motor capacity. In this waythe considerable amount of heat generated in the oil by a single throttle is obviated, with consequent simplification of the aircraftoil-cooling system. The actuator for the swashplate consists of a damped piston which measures the pressure-drop across thethrottle. If this drop becomes excessive, the piston moves in its housing and, by a simple linkage, alters the angle of the swashplate. As the motor speed is lower than that of the alternator rotora gearbox is necessary. For continuous rating, lubrication is essential and an oil tapping is taken off the motor return line.Oil pressure is maintained at a suitable level by a small valve, and is fed through lubricating jets to a two-stage spur-gear train.Excess oil is scavenged by a small gear pump and returned to the aircraft hydraulic system. A simple lip seal prevents oil fromreaching the alternator. Later developments include a self-con- tained lubrication system which consists of a small tank with feedand scavenge pumps driven off the gearbox layshaft. The relatively heavy rotor shaft is carried on angular contactbearings held together by a spring-loaded piston. Attached to the rotor shaft is a small centrifugal actuator which consists of ahollow piston spring-loaded against two small flyweights. At a given shaft speed (below nominal) the piston slides out andoperates a micro-switch. This switch allows the protective circuits built into the control unit to become operative and to cut offthe hydraulic supply via the on-off valve if the unit should sub- sequently overspeed or underspeed violently. As a protectionagainst run-up to a speed less than the switch setting, a time delay is fitted which causes the switch operation to be overriddenin a finite period and the oil supply to be cut off. It is essential that the alternator coil temperatures do not rise to a value whichwould impair their operation. The techniques employed in their construction give ample scope for cooling to be effected by ramair, despite the high aircraft speeds, and experience has proved that cooling for the controller and condensers is unnecessary formost applications. Where essential, however, provision may be made for a small flow from a cold air unit. The high power developed by the unit demands large con-densers. For convenience of installation, these condensers are fitted into a separate case, the bulkhead of which acts as a junctionbox for the three components which comprise the power unit. A filter is incorporated in this case to improve the output wave-form when running into distorted loads. Three circuits within one case constitute the controller. Theseare the voltage regulator and frequency governor, to which are added a third circuit for protection. As the unit is used in an air-craft for powering delicate components, protection is essential against overspeed, underspeed and low voltage. The charac-teristics of the inductor alternator used are such that protection against high voltage is unnecessary. Newest Developments. The range of alternators already pro-duced by de Havilland Propellers is being used in a number of current aircraft and guided weapons. Little may be revealed ofthe latest developments, but it is permissible to disclose that a low-speed, high-frequency, continuously rated power unit foraircraft use is now on test. This unit is directly driven by a swashplate oil motor, using return oil as the cooling mediurn forheat generated by the alternator. Another development is 3 variable-torque device which replaces the moving throttle onturbo-alternators driven by cordite gas or i.p.n. This has over- come the problem of throttle jamming caused by dirty fuels-Controller circuits are being re-designed to accommodate the latest advances in electronic techniques, and transistors are beingintroduced to reduce space and weight. Comprehensive test facilities have been built up, and thereare now several large laboratories engaged on alternator work- Suitably protected rigs have been built for testing units drivenby each of the fuels in use.
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