GB1449600A - Methods and apparatus for speed control of induction motors - Google Patents

Abstract

1449600 Speed control of A.C. Induction motors NATIONAL RESEARCH DEVELOPMENT CORP 13 March 1974 [15 Dec 1972] 58005/72 Heading H2J Method and apparatus for speed control of an A.C. induction motor, preferably a 3-phase squirrel-cage motor, comprises switching each half cycle of an A.C. supply for current flow to the motor during an interval starting at a controlled instant during the half cycle and extending to the next zero crossing of the waveform, and varying the intervals successively in duration substantially according to a similar function and controlling the running speed of the motor by ranging the frequency of the sinusoidal function. Alternatively, the intervals may commence at a controlled instant during one half cycle and extend during a number of successive half cycles. In a 3-phase system Fig. 6, (not shown), the motor has 3-phase windings, each phase winding comprising 3 winding sections. A triac or back-to-back thyristor pair is provided for current control in each winding section, each bi-directional device being controlled from a sine-wave generator arranged to moderate the input A.C. to provide firing pulses indicative of the starts of the intervals. Using a mathematical analysis it is explained how the nine triacs control the respective voltages of the associated winding sections according to nine modulation products given by the products of pairs of voltages Ac Be Cc Am Bm Cm, the first three being the voltages of a 3-phase, balanced, sine wave supply for the motor and the last three being the voltages of modulating envelopes relating a bias voltage amplitude R and a modulating voltage amplitude Q. Equations for the nine modulation products are given. The effect of the control is such that only one of the three frequencies contained in each winding section waveform sets up a rotating motor MMF. This frequency is controlled by the low frequency sine wave generator 4 of the modulator shown in Figure 1, which operates in a point-on-wave manner to control a triac 6 to pass variable portions of each half cycle of an A.C. supply 1, 2. The output of generator 4, as determined by variable frequency control 5, is applied to a transistor amplifier 11 which is combined with members 14, 15, 16 to form a relaxation oscillator. Typical operating waveforms are described, Fig. 2 (not shown). Alternatively a modulator operating in a burstfiring manner may be used, Fig. 3 (not shown), its triac (26) passing one or more complete half cycles of the A.C. supply to the modulator, Fig. 4 (not shown). By varying control 5, the motor speed can be varied from zero to synchronous value in either direction of rotation.