GB1486559A - Pulse control circuits - Google Patents

Abstract

1486559 Control of DC motors SEVCON Ltd 25 Nov 1974 [23 Nov 1973] 54476/73 Heading H2J A pulse control circuit comprises semiconductor switching means connectible in series with a load and a DC source and operable alternately to connect and disconnect the load to and from the source, a shorting contactor in parallel with the switching means and operable on closure of its contacts to connect the load directly to the DC source, means to establish a predetermined voltage across the contacts when they are open, and a voltage sensing circuit connected in parallel with the contacts and operable to change its state when the voltage across the contacts falls below a predetermined level less than the predetermined voltage. The Figure shows the application of the invention to a control circuit for a D.C. series motor 2 driving a vehicle, the motor being in series with a thyristor 6 which is controlled by a conventional firing and commutation circuit 7 and is shunted by a contactor 8 closed when full power is applied to the motor. The direction of rotation is determined by the position of a change-over switch 14 controlling the coils 10, 12 of contacts connected to the motor field winding 4. The energization circuit for the coils 10, 12 is completed by a thyristor 16, which is fired by a unijunction transistor 18 when capacitors 34, 38 have charged to a predetermined value. The contactor 8 is supplied with a low voltage from a stabilized supply (not shown) or a Zener diode 59, so that when it is open this voltage is applied to a transistor 42 which conducts to render another transistor 52 non-conductive. With contactor 8 closed the conductivities are reversed, whereby transistor 52 conducts to prevent charging of the capacitors 34, 38. Thus the movement of switch 14 from one coil to the other will produce reversal of the motor only if the contactor 8 is open, since thyristor 16 becomes non-conductive when switch 14 de-energizes one coil and is fired by members 18, 34, 38 when switch 14 contacts the other coil. Failure of contactor 8 to open, when its actuating coil 60 is deenergized, prevents firing of thyristor 16 after the above described changeover and hence reenergization of the motor. To prevent the above operation occurring when the contactor 8 is to be kept closed during reversal of the motor to obtain "plug braking", the coil 60 is energized through normally-closed foot switches 13, 72, the switch 14, diodes 76 or 78 and the contacts 68 of a relay whose coil 66 is energized when coil 10 or 12 is energized. Thus during changeover of switch 14, the relay 66, 68 (having an operating time shorter than that of the switch 14 and contactor 8, 60) de-energizes the coil 60 to open the contactor 8, reclosure being effected when changeover has been completed.