Patent GB-1438538-A (GB)
Title: Motor speed control
IPC: H02K17/30
Filed: 1973-04-24
Abstract: 1438538 Automatic control of speed J D VERGARA R GALVAN and F LEAL 24 April 1973 [25 Oct 1972] 19269/73 Heading G3R [Also in Division H2] A system for controlling the speed of a multiphase induction motor, having a rotor assembly including at least three rotor windings and an inversely related speed and induced rotor winding voltage characteristic, comprises a plurality of controlled rectifiers connected to the rotor windings and arranged to be selectively made conductive to short-circuit the rotor windings, and circuit means energizable by the induced rotor winding voltage and connected to provide a D.C. signal of one polarity to the gate of each controlled rectifier, the D.C. signal determining the amplitude of forward voltage required across the rectifiers to render them conductive, the D.C. signal varying in amplitude in response to changes in rotor winding voltage, and the induced rotor winding voltage varying in response to changes in the motor speed whereby the instants at which the rectifiers are made conductive are varied to enable the maintenance of the motor speed substantially at a selected set value. The rotor windings may be delta or star-connected and Fig. 1 shows the latter configuration for a three phase motor 10, whose rotor windings are connected to silicon controlled rectifiers 22, 24, 34, 36 receiving firing signals from a master control circuit 66, details of which are shown in Fig. 2. Terminals 80, 82, 84 receive rotor winding voltage signals from slip rings 68, 70, 72, which signals are applied to a rectifier and resistor circuit 86, 88, 90, 92, 94, 96, 98, whose positive output is applied to the control electrodes of the SCR's 22, 24, 34, 36 through a variable resistance 100, diodes 102, 104, 106, 108, output terminals 58, 60, 62, 64 and slip rings 50, 52, 54, 56. Resistor 100 is adjusted to cause the SCR's to conduct at a certain point and hence determine a selected speed for a corresponding motor load. Load and speed changes cause corresponding variation of the rotor winding voltages, which in turn vary the firing current in circuit 66 and the voltages applied to the SCR's, whose instants of conduction are thus correspondingly varied to provide a self-regulating action maintaining the speed at the selected value. Resistor 100 may be replaced by a resistance of positive or negative temp. coefficeint, a tunnel diode, an electronic photo-cell, a resistivity-varying electrode or a magnetic control with a rectifier bridge. The SCR characteristic of firing current/forward breakdown voltage is also discussed. In a modification, Figs. 3 and 4 (not shown), the circuit 66 has only one output (64) to one slip ring (56) connected to the four diodes 102, 104, 106, 108. The embodiment of Fig. 5 uses a star-connected arrangement of diodes 112, 118, 120 and SCR's 110, 114, 116 to control the rotor windings, the star point 122 and diodes 126, 128, 130, connected to the control electrodes of the SCR's being connected through slip rings 124, 132 to a variable resistor 134 for selecting the motor speed. In a modification similar to Fig. 5, slip rings are omitted and members 122, 126, 128, 130 are directly connected to the respective ends of a light-responsive resistor or photo-cell (136), which is adjustable by a light beam to set the desired motor speed, Fig. 6 (not shown). A typical operation of the embodiments of Figs. 1 and 3 is described, Figs. 8, 9 and 10 (not shown).