CONTROL CIRCUIT

Abstract: A circuit for control of the speed of an electric motor, particularly a shaded pole motor, in accordance with a control voltage includes a ramp generator (1) for generation of a sawtooth voltage, a comparison voltage (4) with two inputs which are supplied with the control voltage and the sawtooth voltage, respectively, a trigger circuit (5) whose input is connected to the output of the comparison circuit (4), and a triac (6) which is connected in series with the electric motor (7), the control electrode of said electric motor being connected with the trigger circuit. The output of the comparison circuit is connected to a control input on the ramp generator (1) which is arranged to give a steep ramp when the comparison circuit is OFF and a slightly less inclined ramp when the comparison circuit is ON. The circuit can thus also control e.g. motors with a greatly varying torque.

Description: Control Circuit The present invention relates to a circuit for con¬ trolling the speed of an electric motor, particularly a shaded pole motor, in accordance with a control voltage and including a ramp generator for generation of a saw- tooth voltage, a comparison circuit with two inputs which are supplied with the control voltage and the sawtooth vol¬ tage, respectively, a trigger circuit the input of which is connected to the output of the comparison circuit and a triac connected in series with the electric motor, the control electrode of which is connected to the trigger circuit. Such a circuit may for instance be used to control a heating element or the speed of a fan in accordance with e.g. a temperature as the said control voltage can thus be formed by means of a voltage divider consisting of a fixed resistor and a temperature-dependent resistor. The comparison circuit controls 0-passage/not-O-passage impulses to the triac by scanning the sawtooth voltage at one input and scanning a temperature-dependent threshold value at the other input. A control circuit of the above-mentioned kind is not suitable for control of shaded pole motors as such motors and other asynchronous motors have a greatly varying torque which is very low at slow speeds and high at optimum motor speed. According to the invention this disadvantage is avoided by the output of the comparison circuit being connected to a control input of the ramp generator, and by the generator being arranged to supply a steep ramp when the comparison circuit is OFF and a slightly less inclined ramp when the comparison circuit is ON. By this embodiment a double ramp system is used, the comparison circuit controlling the O-passage as well as the ramp generator, so that the comparison circuit in OFF supplies a steep ramp and in ON a slightly less in¬ clined ramp. In what follows, the invention is explained in further detail in connection with the drawing where fig. 1 shows schematically a block diagram of a known control circuit, fig. 2 graphic representations of voltages in the known circuit, fig. 3 schematically a block diagram of a control circuit according to the invention, and fig. 4 graphic representations of voltages in the circuit according to fig. 3. The control circuit shown in fig. 1 includes a ramp generator 1 generating a sawtooth voltage, a vol¬ tage divider consisting of a fixed resistor 2 and a temperature-dependent resistor 3 producing a tempera¬ ture-dependent control voltage, a comparison circuit 4 whose two inputs are supplied with the sawtooth voltage and the control voltage, respectively, a trigger circuit 5, the input of which is supplied with the output signal of the comparison circuit 4, and a triac 6 connected in series with a load 7, for instance a heating element to be controlled, the control electrode of the triac being connected to the trigger circuit 5. Fig. 2 is graphic representations of the sawtooth voltage, the control voltage and the voltage across the load at different temperatures. In dotted line is also shown the load power which is lowest when the tempera¬ ture and hence the control voltage is highest. Fig. 3 shows the control circuit of fig. 1 with the modifications which are made according to the pre¬ sent invention. The output signal from the comparison circuit 4 is in this case applied not only the trigger circuit 5 but also the ramp generator 1 which is hereby controlled in such a way that the ramp inclination has a first value when the comparison circuit is OFF and a second value when the comparison circuit is ON. The result is the voltage sequences suggested in fig. 4, and where the ramp inclination is relatively small when the motor 7 is ON, and relatively steep when the motor is OFF. Through a suitable dimension- ing of circuit components the coefficient of inclina¬ tion of the two ramps can be adapted to the individual motor.

Claims: C l a i m s A circuit for controlling the speed of an electric motor, particularly a shaded pole motor, in accordance with a control voltage and including a ramp generator (1) for generation of a sawtooth voltage, a comparison circuit (4) with two inputs which are supplied with the control voltage and the sawtooth voltage, respectively, a trigger circuit (5) whose input is connected to the output of the comparison circuit (4) , and a triac (6) which is connected in series with the electric motor (7),the control electrode of said electric motor being connected with the trigger circuit, c h a r a c t e r ¬ i z e d in that the output of the comparison circuit is connected to a control input on the ramp generator (1) , said generator being arranged to give a steep ramp when the comparison circuit is OFF and a slightly less inclined ramp when the comparison circuit is ON.

Inventors: SEIER WILLIAM, SEIER, WILLIAM
Applicants: SUPPLY TEAM APS, SUPPLY-TEAM APS
Application year: 1986
Main classification: H02P7
IPC codes: H02P25, H02P27