Patent GB-660293-A (GB)
Title: Improvements in and relating to electric control systems
IPC: H02P5/68
Filed: 1948-12-23
Abstract: 660,293. Control of D.C. motors. BRITISH THOMSON-HOUSTON CO. Ltd. Dec. 23, 1948. [Dec. 30, 1947] No. 33152/48. Class 38 (iii). [Also in Group XXXVIII] A system for controlling electric motor means driving a vehicle in response to the speed of the vehicle includes an electric tachometer generator producing signals the frequency of which corresponds to the varying speed of the vehicle, and switching means for the motor circuits operated by sensitive relays in response to signals of predetermined frequency derived from the tachometer generator wherein at least one resonant circuit is provided to produce from the tachometer signals on output voltage of steeply rising characteristic and the value of which is less than the value required for operating an associated sensitive relay, at all frequencies below a pre-determined frequency, and is equal to or greater than this value at all frequencies above a higher pre-determined frequency, the relays being adapted to pick up and drop out within the range defined by these two frequencies. A tachometer generator 10 which is driven by an axle of a diesel-electric locomotive may have a rotating permanent magnet field, its output, which is derived from its stator winding, varying in amplitude and frequency in accordance with the locomotive speed. The tachometer generator is adapted to supply a relay 13a through a non-linear resonance circuit and filter network comprising capacitor 15a and a variable reactor 17a which preferably has a closed core. An additional reactor 14a may be provided, the relay-coil being fed through rectifier 16. The relay 13a controls a relay 22 having a contact which renders resistor 19 ineffective when the relay 13a is open. A second control section comprising relay 13b is similar to that described except that the capacitance or inductance values may differ, the relays 13a, 13b, being actuated at the same voltages but at different frequencies. A third control section comprising relay 13c differs from the first in that capacitor 25c is connected in series with reactor 17c and no additional reactor or drop-out tuning resistor is provided. A fourth control section comprises two sensitive relays 26, 29. Main driving motors 1, 2, 3, 4 are supplied from a diesel-driven generator 33, and, initially, are connected in two parallelgroups, each group having two motors connected in series. At a pre-determined speed, relay 13a is closed to energise relay 22 and connect resistors 36 across the series field windings of the motors. At a higher speed, relay 13b close and relay 23 is actuated to connect the motors in parallel, full field excitation being restored. When relay 13c is closed to energise relay 24 the motor field windings are again shunted to provide high speed operation. Closure of relay 26 sounds a warning- device 28, and actuation of relay 29 opens the field circuit of the generator 33, disconnects the generator from the motors, and applies a brake. The relay 23 has contacts 23t, 23t<SP>1</SP>, which de-energise the field winding of the generator 33 for short periods when changing the groupings of the motors. The line 50 (Fig. 2) illustrates the output voltage of the tachometer generator plotted against frequency. Curves 51-54 represent respectively the, voltages applied to relays 13a-13c and 26, 29. 51p-54p and 51d-54d show the pick-up and drop-out points respectively of the relays 13a-26. 54p<SP>1</SP> is the point at which relay 29 picks up for a certain value of resistor 30. Variable reactor 17d may be adjusted to compensate for varying diameters of the wheels attached to the axle from which the tachometer generator is driven.