GB1123734A - Electrical motor regulating arrangement - Google Patents

Abstract

1,123,734. Control of D.C. motors. ASSOCIATED ELECTRICAL INDUSTRIES Ltd. 18 May, 1967 [10 June, 19661, No. 26030/66. Heading H2J. [Also in Division G3] An electric motor M is supplied from A.C source L, N, through thyristor Th which is controlled by a circuit comprising transistors TR1, TR2, associated with Zener diode ZD2. The motor rotates an opaque disc D so that the impedance of photo-conductor PC switches between high and low values by the flashes of light from lamp L impinging upon it. When the lamp L is obscured by the disc so that element PC has a high impedance, transistor TR1 conducts at a rate determined by the setting of resistor RV1 to charge capacitor C1 in a linear manner. The transistor TR2 remains non-conductive while the charge on capacitor C1 remains below the emitter potential. As one of the slots in the disc registers with the lamp, the impedance of the photo-conductor is reduced to charge capacitor C4 to a datum level determined by diode ZD1. As the slot moves out of register, capacitor C4 discharges for a fixed period during which transistor TR1 is non-conductive. The charging of capacitor C1 is interrupted and its voltage remains at a steady level. This cycle is repeated whereby the voltage on capacitor Cl follows a stepped waveform rising to the threshold value of transistor TR2. When collector current begins to flow in transistor TR2, the charging rate of capacitor C1 is increased sharply whereby transistor TR2 quickly becomes fully conductive. A pulse is then developed across resistor R3 to trigger the thyristor Th which is held in the conductive state until the end of the half-cycle. The capacitor C1 then discharges, transistor TR2 ceases to conduct, and the system reverts to its initial condition. Capacitor C3 and resistor R5 protect the system against excess voltage. The arrangement tends to maintain the motor speed constant at a value determined by the setting of resistor RV1.