Patent GB-1487826-A (GB)
Title: Contactor interlock circuits
IPC: H01H47/32
Filed: 1973-11-23
Abstract: 1487826 Control of D.C. motors SEVCON Ltd 25 Nov 1974 [23 Nov 1973] 54475/73 Heading H2J A contactor interlock circuit, for use with an electromagnetically actuated contactor having an actuating coil and an armature which moves on energization of the actuating coil to close the contactor contacts and which moves on deenergization of the coil to open the contactor contacts and to create an air-gap in the magnetic circuit of the coil, comprises a voltage sensing circuit connectible in parallel with the actuating coil and providing an output signal when the absolute value of the voltage across the actuating coil exceeds a predetermined value, which is lower than the absolute value of the voltage appearing across the coil at the instant at which the air gap in the magnetic circuit of the coil is created, whereby the output signal is maintained for a predetermined time after the contacts open and until the absolute value of the voltage across the coil falls to the predetermined value. When the coil is de-energized, its inductive energy may be dissipated through the interlock circuit or through a voltage transient suppression circuit connected across the coil. Fig. 1 shows the application of the invention to a conventional pulse controller for the D.C. traction motor 14 of a battery-electric vehicle, which controller includes a main thyristor 16 in parallel with a contactor 12, whose actuating coil 20 is energized by closure of a switch 32 when the accelerator pedal is fully depressed and by firing of a thyristor 34, with delay provided by a known time delay circuit if the depression is sudden. When coil 20 is energized the normally-open contacts of a relay 40, in an interlock circuit 10 in parallel with the coil 20, are closed to provide an output signal as the relay coil 42 is energized through resistor 54 and rectifier bridge 48. The relay contacts close before the contactor 12 and the output signal is applied to the control circuit of the pulse controller, to perform the usual inhibiting and commutating capacitor charging operations. When pedal movement opens switch 32 to de-energize coil 20, the voltage induced in the latter is reversed and assumes a lower value, but energization of relay coil 42 is maintained through the rectifier 48 and diode 56. The coil 20 voltage falls as its energy is dissipated through the circuit 10, until the contactor armature (22) moves from a stop (26) to produce an air gap in the magnetic circuit of coil 20 just before the contacts (24) of the contactor open, Fig. 2 (not shown). This causes the coil voltage to increase from the voltage level V corresponding to the instant at which the air gap is created, reach a maximum point (D) and then decrease below the level V to the predetermined value (E) lower than the level V, Fig. 3 (not shown). Thus the contacts of relay 40 remain closed until the predetermined value is reached, corresponding to a pretermined time after contactor contacts (24) open. In a modification, Fig. 4 (not shown), the interlock circuit comprises a rectifier 48 supplying an arrangement of resistors and switching transistors, the last of which provides the output signal. A diode (84) in this modification or the (unreferenced) resistordiode circuit of Fig. 1 may provide the alternative path to dissipate the energy of coil 20 produced when it is de-energized.