GB704977A - Improvements in or relating to the overload protection of electric motors - Google Patents

Abstract

704,977. Control of motors. BOOTH, S. Nov. 22, 1951 [Feb. 23, 1951], No. 4481/51. Class 38(3) An overload relay for a compound wound D.C. motor has a series coil 4 and an auxiliary coil 5 which is connected to a switch 7 controlled by a discriminating relay winding 8. When the speed of the motor increases, with a consequent increase of back-electromotive force, the switch 7 closes to energize the winding 5 so that the trip circuit is opened at a lower overload current than that for the starting period. In a modification, Fig. 2 (not shown), the coil of the discriminating relay is in series with the motor armature, and the auxiliary winding of the overload relay is shunted thereacross. The contact of the discriminating relay is open during the starting period and is closed to bring the auxiliary coil into operation when the current falls as the working speed is reached. In another arrangement two of the main conductors 33, 34, for a three-phase motor incorporates normal overload coils 35, 36, whilst the third conductor 37 includes a winding 38 of a discriminating relay. Auxiliary overload coils 40, 41, are connected in series with a centrifugal switch 42 which is manually closed for starting and automatically opened at a predetermined speed just below the normal running value. An alternative circuit for the coils 40, 41, is completed through resistance 43 and switch 39 which is operated by winding 38. During starting, the coils 40, 41, oppose the coils 35, 36, so that the motor can take a higher starting current. When the centrifugal switch 42 opens, the excitation of the coils 40, 41, is reduced by the resistor 43. Upon further reduction of current, the contact 39 opens to disconnect the coils 40, 41. The switch 42 may, be replaced by a push button which is held closed for starting.