Patent GB-601107-A (GB)
Title: Improvements in or relating to the automatic regulation of speed
IPC: H02P23/16
Filed: 1944-07-03
Abstract: 601,107. Automatic speed and temperature controlling systems. BRISTOL AEROPLANE CO., Ltd., and FORTESCUE, P. July 3, 1944, Nos. 12673/44 and 12811/45. [Class 38 (iv)] A method of governing the speed of apparatus such as a gas turbine, involves a Maxwell Bridge network connected across a source of direct current, with one arm of the bridge comprising a condenser which is charged and discharged alternately with a frequency proportional to the speed to be governed, the operation being based on the known condition that, with certain relative values between the condenser capacity and the resistances in the other three arms of the bridge, the bridge is balanced when the condenser is charged at the datum frequency n 1 , and at a different frequency nb the current Ig through the galvanometer connected across the bridge has the value Ig=K (ln 1 /nb), where K is a constant dependent only upon the circuit constants. According to the invention, the current through the bridge is used to control means which operates to restore the speed to the datum value and re-establish balance in the bridge network. An alternative control for regulation of the temperature as well as the speed of apparatus may be provided by combining with the Maxwell bridge network a Wheatstone bridge with one arm comprising a temperaturesensitive resistance, means being provided whereby the output points of both networks may be connected alternatively to common means for producing actuation of the speed and temperature controlling devices. In one example, the system is employed to regulate the speed of a gas turbine power plant by varying the quantity of fuel passing thereto. The compressor of the power plant (not shown) is directly coupled to drive an alternator 10, so that the frequency of the current generated is directly related to the speed of the compressor. A polarized relay 12 connected across the alternator has an armature 13 which vibrates between a pair of contacts 14, 15 arranged so that a condenser 16 in one arm of the Maxwell bridge is charged by direct current from a rectifier 17 when the armature engages contact 15, and is discharged when the armature engages contact 14. The output terminals c, d of the bridge are connected through a filter 23 and across a load resistance comprising two equal resistances 24a, 24b. The potential across these resistances is proportional to the net current flowing across the bridge, and therefore proportional to the difference between the prevailing frequency with which the condenser is being charged and discharged and the frequency with which it should be charged and discharged in order to balance the bridge. This potential is applied directionally to one of the grids of a pair of triodes 26a, 26b in an amplifier unit 25, and the anodes are connected across the ends of a polarized relay coil 33, the midpoint of which is connected to a high-tension source provided by the rectifier 17. The polarized relay 33 controls the operation and direction of rotation of a reversible electric motor 35 which is driven from a battery 51, and the motor is coupled to a fuel valve 36, for example by a worm and worm-wheel and linkage system, so that it regulates the quantity of fuel passing from a pump 37 into the pipe 38 to the power-plant. The motor 35 may have a split-field winding and is preferably provided with an automatic brake and clutch of known construction to prevent hunting. The datum speed of the power plant may be varied by varying the resistance 21 in the bridge network, and the range of the datum speed may be varied by providing a suitable fixed resistance 52 in series with the adjustable resistance 21. In adapting the system for response to temperature variations, a manual or automatically operated two-way switch is provided for introducing in place of the half of the bridge comprising the condenser 16 and resistor 19 a temperature-responsive resistance in series with a fixed resistance, so that the Maxwell bridge is replaced by a Wheatstone bridge, with one half of the bridge common to both. The two-way switch may be controlled by a latch operated by a relay which is connected in the valve amplifier circuit and adjusted so that the normal current through the relay is insufficient to actuate it. A shortcircuit or open circuit in the temperatureresponsive resistance produces an increase in the valve current and operates the relay to lift the latch, whereupon a tensioned spring operates the two-way switch to introduce the Maxwell bridge circuit for speed control.