Patent GB-1096601-A (GB)
Title: Improvements relating to electrical motor control equipment
Inventor: HIGGS GEORGE ROBERT
IPC: B60L9/26
Filed: 1963-06-27
Abstract: 1,096,601. Control of D.C. motors; switching systems for consuming devices; rectifying. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. Sept. 24, 1964 [June 27, 1963; Oct. 3, 1963], Nos. 25683/63 and 39038/63. Headings H2F, H2H and H2J. [Also in Division G3] A load is supplied from an A.C. source through rectifiers which are controlled so that the load is connected alternately to supplypoints having different voltages during trains of cycles or half-cycles, the relative lengths of which trains are variable so as to control the average voltage applied to the load. A tapped transformer ATR has movable contacts MC1, MC2 connected respectively through rectifiers L1, L2 and R1, R2, to the ends of reactor LR whose mid-point is connected to the primary winding of a step-down transformer SDT. The latter has secondary windings SW1- SW4, the winding SW1 being connected through rectifiers A1-A4 to a D.C. traction motor M. When transferring from tap 23 to tap 24, rectifiers L1, R2 are fired simultaneously for a period, so that reactor LR is connected across the tap-section 23-24. Rectifiers L1, L2 are then blocked and the ensuing cycles are passed through rectifiers R1, R2. The tap-changer may then be moved from tap 23 to tap 25. While working on tap 23, it is possible to raise the circuit voltage by 20% of the intertap voltage, for example. The firing control of the rectifiers is modified so that, out of each successive five cycles four are performed with rectifiers L1, L2, conductive, and one with rectifiers R1, R2 conductive. This sequence can be maintained indefinitely, the traction motor operating as if the voltage supply were constant at the average value throughout. In a circuit for controlling the firing of the rectifiers to provide continuous control on an eightcycle basis, an alternating voltage 1 at mains frequency is applied to squaring stage A, the output of which is differentiated at B to produce a negative-going pulse waveform at 3. The square wave from stage A is inverted in stage C and the output differentiated in stage D to produce a further negative pulse waveform at 4. These waveforms are interlaced in time and occur on respective half-cycles of waveform 1 at the zero voltage cross-over points. Waveform 1 is also applied through phase-shifter E to squaring stage F. The outputs from stages G and J are represented by pulse waveforms at 5, 6 which are interlaced in time and are phase-shifted 90 degrees with respect to the waveforms at 3 and 4. Waveform 5 is applied to stage K which is the first stage of binary counting stages K, M, N, which change their states each time a negative-going pulse is received from a preceding stage. The outputs from stages K, M, N are combined in AND gate P to produce waveform 10 in which the potential is negative only when the potentials of waveforms 7, 8, 9 are negative together. Waveforms 10 and 6 are combined in AND gate S to produce an input to flip-flop V. Waveform 10 is inverted by stage T to produce an input to gate U which is also associated with flipflop V. When the latter is in its unstable state, gates Y and Z pass pulses to rectifiers R1, R2. Similarly, gates W, X pass pulses to rectifiers L1, L2 when the stage V is in its stable state. When a control potential applied to the stage V is reduced, the rectifiers R1, R2 are triggered more frequently in each series of eight cycles so that the effective tapping-point is shifted upwardly from tap 23 towards tap 24. The control potential may be obtained from a potentiometer which is driven in response to an error signal representing deviations of the output voltage of the rectifier set or of the motor current from fixed references.