Patent GB-477584-A (GB)
Title: Improvements in or relating to electrical devices for measuring the speed of moving vehicles and rotating bodies
IPC: G01P3/46
Filed: 1936-06-30
Abstract: 477,584. Inductor alternators. ELLIOTT BROS. (LONDON), Ltd., and PIZZEY, C. F. June 30, 1936, No. 18142. [Class 35] [Also in Group XXXVIII] An electrical device for measuring the speed of a vehicle or rotating body comprises an electric generator of the permanent magnet inductor type having its rotor system mounted directly on the rotating body or a rotating part of the vehicle, such as a wheel axle, a direct current measuring instrument connected to the generator windings and rectifying means, for example, a dry rectifier, connected between the generator and the instrument. The generator comprises a divided annulus 1, 2 of non- magnetic material, e.g. aluminium, provided with lugs 3 whereby the two halves can be bolted to an axle 4 ; the annulus has a number of spaced recesses 5 containing laminated inductors 6 which co-operate with one or more stationary permanent magnets 7. Each magnet is joined to the yoke of a U-shaped core 8 of silicon steel carrying generating windings 9 connected in series or in parallel. The permanent magnets may be of nickel-aluminiumsteel or nickel - aluminium - cobalt - steel. As shown, two stator systems are provided, located at opposite ends of a diameter, and the coils of the two systems are connected in series to compensate for variation of the air gap as the result of wear in the bearings or axle. The air gaps are preferably large, of the order of one tenth of an inch, and the magnetic system preferably has a high leakage factor. The magnets 7 and cores 8 may be arranged with their ends on a line transverse to the periphery to co-operate with axially displaced rows of staggered inductors or the ends of the limbs may be on a radial line to co-operate with a face of the annulus carrying two concentric rows of inductors arranged in staggered relation. The stator systems may be carried by an axle-box. Fig. 4 shows a mounting for use on a truck consisting of frame 16 pivoted at 17 to the framework of the truck and carrying at its other end the magnet system 7, 8, 9 which is maintained in position relatively to the axle 4 by means of two idle wheels 18 carried by the frame and bearing against the axle surface at diametrically opposite points. The measuring instrument comprising a direct current voltmeter calibrated in miles per hour is supplied, for example, through a copper oxide rectifier and may include a calibrating device comprising a sharply tuned reed energized by a magnet supplied with alternating current from the alternator. Since the effective number of pairs of poles of the generator is is fixed, with a given vehicle speed the frequency will be determined by the diameter of the road wheel and the scale of the instrument is marked with red lines at the speeds corresponding to different diameters of road wheel which give the reed frequency. Accordingly, when the reed indicates that the selected frequency has been reached, the pointer of the voltmeter is adjusted to the red line corresponding to the diameter of the wheel. Adjustment may be effected by means of a variable rheostat or magnetic shunt which can be sealed in the adjusted position.