Patent GB-672376-A (GB)
Title: Improvements in and relating to hall effect frequency meters
IPC: G01R23/02
Filed: 1950-02-27
Abstract: 672,376. Non - contact - making relays. BRITISH THOMSON-HOUSTON CO., Ltd. Feb. 27, 1950 [March 9, 1949], No. 4982/50. Class 40 (iv). [Also in Group XXXVI] A frequency measuring apparatus comprises a plate of material exhibiting the Hall effect subjected to an electromagnetic field excited by an input current through the plate, which is derived from an A.C. source of unknown frequency and traverses an energizing circuit comprising two parallel branches tuned to differing frequencies, the Hall effect undirectional output voltage derived from the plate being measured by a D.C. instrument calibrated to indicate frequency. In Fig. 1, Hall effect plates 4, 5 of e.g. germanium are threaded normally to the surface by reinforcing magnetic fields of coils 6a, 6b arranged as shown in Fig. la, wherein the plates are located in the gap of a double-wound electromagnet 3. Input currents derived from a saturable core voltage-regulating transformer 2 energized by A.C. supply 1 of unknown frequency flow axially between two opposed edges of each plate, through parallel circuits respectively containing an inductance 7 in series with plate 4 and an inductance condenser combination 8, 9 in series with plate 5, and are vectorially combined to excite the electromagnet windings in series. Unidirectional Hall effect voltages proportional to the in-phase components of flux and input current appear across the remaining edges of each plate and are connected in series opposition through a frequencycalibrated centre-zero millivoltmeter 11. The impedance elements 7, 8, 9 are adjusted so that at a datum frequency, e.g. 60 c.p.s., the input currents are equal and have equal components in phase with the field flux, when the unidirectional voltages are equal and opposite and the instrument reads zero. An increase in supply frequency decreases the vector amplitude and increases the phase angle of the input current to plate 4 relative to the field flux, while increasing the vector amplitude and decreasing the phase angle of the input current to plate 5, so that the unidirectional output of plate 5 exceeds that of plate 4, and the instrument is deflected to indicate the frequency. A decrease in frequency produces the reverse effect and an opposite deflection. The instrument frequency scale may extend unidirectionally from zero. In modifications, Figs. 5, 6 (not shown), the plate inputs may be supplied in series from the transformer through parallel circuits comprising the exciting field windings and a series condenser and resistance respectively forming frequency responsive circuits of which the field inductances are part; or the input current of each plate may be supplied in series with its field winding and a resistance, capacity and inductance frequency-responsive circuit. In Fig. 7, a single Hall plate 21 is threaded by the vectorial differential field of coils 22, 23 energized in parallel from transformer 2 through resistance, capacity, inductance frequency-sensitive impedances 24, 25. The plate input current is the vector sum of the field currents, and the unidirectional output voltage operates a centre-zero frequency-calibrated instrument 11. At datum frequency the resultant field is 90 degrees out of phase with the resultant input current, and the unidirectional output is zero. Any increase or decrease in frequency diminishes or increases the phase angle so that a positive or negative unidirectional voltage is developed by the plate and deflects the instrument to indicate the frequency. Temperature errors are corrected by a negative temperatureco-efficient resistance in series with the input current to the plate.