Patent GB-1498554-A (GB)
Title: Electrical motor control system
IPC: G05B11/06
Filed: 1976-04-13
Abstract: 1498554 Control of A.C. motors HONEYWELL Inc 13 April 1976 [21 April 1975] 14965/76 Heading H2J An electrical control system comprises a bi-directional electric motor, a three-position switch means consisting of two snap-action switches and providing an intermediate open state rendering the motor inoperative and two closed states each rendering the motor operative in a respective one of two opposite directions, means for limiting the angular movement of the motor in each direction, and a thermally responsive device including a thermally responsive element and an electric heater for heating the element, the system being such that when the heater is energized the element moves causing the switch means to assume one of the closed states, to operate the motor in one of the two directions, movement of the motor causing the switch means to return to the intermediate state. In Fig. 1 a reversible shaded-pole motor is used having a rotor 14 connected to a load 64 such as a valve or airconditioning damper for controlling the temperature of a medium sensed by a thermostat 40. The rotor 14 also actuates limit switches 23, 24 in the circuits of respective shaded pole windings 17, 18, which are short circuited by respective snap-action switches 26, 27 to rotate the motor in one or other direction when stator winding 11 is energized from an A.C. supply. The plungers 51, 52 of switches 26, 27 are actuated by a bimetallic element 46, which is controlled by an electric heater 44 energized from an A.C. supply 41, 48 when the thermostat 40 engages a contact 42. The element 46 has a J-shaped cross section, Fig. 2 (not shown) and is rotatable about pivots 57, 58 by a cam 53, which engages its longer side 56 and is rotated by the rotor 14. Switches 26, 27 form the three-position switch means and are respectively normally open and normally closed, the cam 53 being positioned so that the shorter side 50 of element 46 engages plunger 52 to open switch 27 and thus produce the intermediate open state of the switch means, Fig. 4 (not shown). In a typical operation, energization of heater 44 by thermostat 40 causes element 46 to close switch 26 and so produce rotation of the motor, which rotates cam 53 to permit movement of element 46 to allow switch 26 to open and stop the motor. These operations are repeated to produce stepwise opening of valve 64, until the heat demand by thermostat 40 is satisfied and heater 44 is deenergized. Subsequent natural cooling of the element 46 closes switch 27 to cause a number of reverse motor operations producing stepwise closing of the valve 64 until thermostat 40 closes again to demand more heat. If member 64 is an air conditioning damper, thermostat 40 co-operates with a contact 87 and the system operates in a cooling mode. In a modification Fig. 5 (not shown), the three-position switch means is formed by two normally open snapaction switches (66, 67) actuated by a projection (70) of a coiled bimetallic element (65), which is coupled to the motor rotor so that movement of the latter restores the projection (70) to its intermediate state between the switches.