GB1415172A - Ac motor control circuits - Google Patents

Abstract

1415172 Control of AC motors GECELLIOTT AUTOMATION Ltd 2 Feb 1973 [2 Feb 1972] 4843/72 Heading H2J A control circuit for a split-phase capacitor motor M comprises a triac T 1 controlling the main motor winding 3 and controlled by another triac T 2 , a clamping circuit 6 and a timing circuit 9, the arrangement being such that the triac T 1 is fired and turned off in each half cycle of the supply voltage to provide phase control of the fundamental component of the main winding voltage independent of its amplitude, and the phase of the fundamental component is controlled in dependence upon a predetermined speed/phase characteristic whereby a predetermined (quadrature) phase relation between the voltages of the main and auxiliary windings 3, 1 is maintained for a range of motor speeds. During each half cycle of the A.C. supply to the motor, the following sequence of the operations occur: (1) the triac T 1 is fired at an angle 8 determined by the circuit 9; (2) the triac T 2 is fired at an angle (#+#) determined by the circuit 9, thus causing a charged capacitor C 1 to turn off the triac T 1 ; (3) the consequent back EMF raises the potential at point X to a value limited by further charging of a capacitor C 2 in the circuit 6 and by the discharge and charge of the capacitor C 1 to reverse its polarity; (4) decrease of the main winding current to zero causes the capacitor C 1 to switch off the triac T 2 . Typical characteristics of main winding voltage and current and the voltage across triac T 1 are discussed, Fig. 5 (not shown) and it is explained how, by a suitable relationship between the firing and turn-off angles 8 and (#+ #) of triac T 1 over a desired speed range, the amplitude and phase of the "slice" of supply voltage applied to the main winding 3 during each half cycle can be independently controlled to achieve speed control whilst maintaining the predetermined quadrature phase relationship between the voltages of windings 1, 3. The relationship between the firing and turn-off angles is produced by the timing circuit 9, which is controlled by a speed input signal derived manually or from a closed loop temperature control system wherein the motor drives a fan. The relationship between the speed signal and the firing and turn-off angles can be calculated or empirically determined using the torque/ speed characteristics for the motor and fan or by running the motor on load at a number of different speeds, each time adjusting the two angles together to obtain the quadrature phase relationship, whereby a desired speed/angle relationship is obtained which is built into the circuit 9. This relationship may be modified over the lower portion of the speed range to meet other operating Tequirements of the circuit, Fig. 3 (not shown). Comparison is made with described conventional speed control circuits, Figs. 1 and 2 (not shown).