GB141870A - Improvements in and relating to the control of electric motors - Google Patents

Abstract

141,870. Brooke, W., and Kerr, E. M. March 22, 1919. Master control systems.-In an automatic control system, a series or accelerating relay acts as an overload relay which remains closed while the current is normal, but which opens under an overload, leaving previously-actuated contactors closed and preventing further actuation of any contactors. In the arrangement shown in Fig. 2, the operating-windings 7a - - 11<a> of the main and resistance contactors are connected with their maintaining-resistances between the master controller and the negative lead l<2>. The relay 14 has a series winding 12 with a fixed core 15 and a movable core 16, the latter being attached to a contact lever 17 pivoted on a pin 18 and held by a spring 20 against a fixed contact 19. The spring is fixed to a cradle 21 which can be moved round its pivot 23 by means of a screw 24 and nut 26 to vary the inclination of the spring to the arm 17, and so to vary the setting of the relay. A scale 28 with a pointer 29 indicates the current strength at which the contacts 17, 19 open. The lower part of the relay consists of a laminated horseshoe magnet 31 with an armature 32 on a hinge 33 and carrying a contact 36 held against the contact 19 by a spring 50. The horseshoe has an energizing-coil 34 which is normally short-circuited by the contacts 17, 19. When the controller 13 is turned to its full-on position, the current in the coil 7a closes its contactor to com. plete the motor circuit. The closure of the auxiliary switch 7b completes the circuit of the actuating-coil 8<a> of the first resistance contactor without the resistance 8<c>. The contactor operates and the remainder of the contactors follow in proper sequence. If at any time the motor current rises to an excessive value, the contacts 17, 19 are opened by the series coil 12 to break the short circuit round the shunt coil 34. The contacts 36, 19 are then opened to insert the resistances 8<c> - - 11<c> in series with the contactor windings. The contactors which are already closed are held closed, but the resistances cut down the currents in the actuating-coils to prevent any more from closing. In a modification, additional auxiliary switches operated by the contactors connect the resistances 8<c> - - 11<c> in maintaining- circuits for the contactors. In another modification, these resistances are replaced by a single resistance connected across the contacts 19, 36. Contactors controlled as described may be used with reversible motors and with alternating-current motors.