GB925238A - Change-over device for automatic switching over from star to delta connections and vice versa depending upon load in electric motors - Google Patents

Abstract

925,238. Control of A.C. motors. U. BARERA, and R. ROSSO. Oct. 21, 1960 [Oct. 31, 1959], No. 36254/60. Class 38 (3). The windings 1 of a squirrelcage induction motor are automatically connected in star for starting, and thereafter in delta or star according to the load by separate contactors 7 and 8 fed from a step-down transformer 5, a stabilizing device sensitive to the load preventing too frequent changes from delta due to transient load changes. Operation of start button 6 energizes main contactor coil 4 and hence coil 12 to close star contactor 8; a high current relay 14 and a low current one 13 are now in series with each other and with one phase of the motor. " Make " contacts 16 on relay 14 maintain the star contactor closed until the motor current drops below a predetermined level. If it drops to a very low value, e.g. the motor running light, both 13 and 14 are released and star connection is held through reclosure of contacts 15. Otherwise, relay 14 only is released, de-energizing the star contactor and allowing delta-contactor coil 11 to be energized through interlock contacts 19. Delayed-action change-over contacts 23 in this coil circuit now move from a " close delta " to a " close star " position but the delta contactor is maintained closed through contacts 20. These latter are closed by a relay having a winding 21 in series with the motor and a second winding 22 fed from the transformer 5. The circuit of winding 22 is periodically broken by a pulsing thermal switch 26, 27, the element 24 of which is in series with the motor. If the motor current transiently drops below the rating of the winding 21, current through 22 holds the contacts 20 closed ; if the drop persists until the thermal switch opens the relay is released and winding 11 is de-energized, opening delta contactor 7 and allowing the star contactor 8 to close through interlock contacts 17. Relay windings 13 and 14 are again in series with the motor and if the current rises again to surpass the rating of the former (but is not as high as the starting surge which energizes 14) the contacts 15 open to initiate the change back to delta. If the motor runs at or near full load steadily in delta the pulsing thermal switch element 24 is short-circuited by contacts 30 on a thermal switch 29 in all phases of the motor circuit.