GB1266691A - - Google Patents

Abstract

1,266,691. Control of D.C. motors. BRITISH DOMESTIC APPLIANCES Ltd. 20 June, 1969 [20 June, 1968], No. 29412/68. Heading H2J. [Also in Division G3] A horizontal clothes-drum is driven by a series motor 1 which is supplied from an A.C. source through thyristor 2, the drum being driven at high and low speeds, and accelerated slowly through a critical speed, at which the centrifugal force on the clothes equals the force of gravity, after which it is accelerated more rapidly until the higher speed is attained. The triggering circuit for the thyristor 2 comprises a transistor 13 which conducts when the charge on capacitor 12 reaches a predetermined value so turning-on a transistor 14, leading to a rapid saturation of both transistors and the production of a firing pulse. The charge on capacitor 12 is controlled by transistor 19 associated with tachometer generator 18 providing negative feedback, and a network 3, 20, 21, 46, which provides positive feedback in accordance with the motor current. The feedback circuits, and the value of a reference voltage from potentiometer P, are chosen to maintain the motor speed at a low value suitable for washing or rinsing. At the end of this period, a timer motor 6 starts a pump to remove free washing liquid from the drum and opens a switch 5 to discharge capacitor 24 through resistor 25. The current through transistor 11 increases slowly to increase the base voltage of transistor 19. This provides slow acceleration of the drum through the critical speed. As the transistor 11 becomes more conductive, transistor 27 starts to conduct and the base voltage of transistor 19 rises rapidly to a maximum. The motor is supplied with maximum power and rapidly accelerates the drum to the higher speed. The spinning operation is terminated by closing timer switch S to switch-off the transistor 19. A switch SS is closed when a shorter spinning time is required a capacitor 31 being charged until transistor 27 becomes conductive. As the capacitor 31 discharges, transistor 33 turns off to prevent the transistor 13 from firing and supplying triggering pulses to the thyristor 2. A switch 30 responds to the drum becoming out of balance and turns on transistor 34 which switches-off the transistors 11, 27, and thereby reduces the base voltage of transistor 19. This lowers the motor speed below the critical value. When the switch 30 opens, capacitor 52 starts to charge and transistor 34 is switched-off to effect controlled acceleration. If the main power supply is restored after failure, capacitor 24 causes transistor 11 to become slowly effective, so avoiding restarting of the motor at full power. Resistors having a negative temperature coefficient protect the motor from over-heating and provide temperature stability for the electronic circuit elements. A transistor 36 is associated with the transistor 19 to afford protection against excessive current. During the washing period motor 1 is periodically reversed by cam-operated switch 29.