Patent GB-539334-A (GB)
Title: Improvements in or relating to phase failure and overload protective systems
IPC: H02H7/08
Filed: 1940-03-04
Abstract: 539,334. Protective cut-out arrangements; control of A.C. motors. IGRANIC ELECTRIC CO., Ltd. March 4, 1940, No. 4034. Convention date, March 3, 1939. [Classes 38 (iii) and 38 (v)] A three-phase synchronous motor 4 provided with a secondary starting winding (not shown) and a normal field winding 4b, is protected against prolonged overload and phase-failure during starting, by means of a series of relays 9... 13 energizes from V-connected current transformers 1, 2 in two of the lines the relays having normally-closed contacts, and relays 9, 10, 11 being connected in star to the transformers 1, 2. Closure of start-button 16 makes, over contacts of a stop-button 17 and contacts 12b, 13b, 7b, the circuit of a relay 8 which closes at 8b the circuit to the closing coil 3a of the main switch 3, which causes the motor to run up on its starting winding. Closure of contacts 3f makes a maintaining circuit for relay 8 and hence for coil 3a. Circuits are also made over contacts 8b for timing relay 14 which subsequently energizes the coil 6a for closing the field-switch 6, and for timing relay 7 which, if switch 6 fails to close, opens the circuit of a at 7b and so reopens the switch 3. When 6 closes, it opens the circuit of 7 at 6c. The dashpots of relays 7, 14 are so adjusted that when all the resistance 15 is in series with relay 7, 14 closes first, but with part of 15 shorted 7 closes first. During starting, the closed contacts 6d cause coils 12, 13 to be connected in parallel and, due to the 120 degrees displacement between the line currents, each coil 12, 13 carries half the line-current so that the response-value of each is doubled and therefore no response to the heavy starting current occurs. The contacts of relays 9, 10, 11 are adapted to open at the normal motor-starting current, thus removing a'shortcircuit from part of a resistor 15. The action of relay 7 is therefore retarded and premature disconnection of the motor, due to failure of 6 to close within a given time, prevented. Relays 12, 13 operate on overload in starting or running and open at 12b, 13b the circuit of relay 8, their response-value during running being less than the starting current. If during starting, the current in line L<2> fails, the transformer-circuit becomes a simple circulating-current circuit through contacts 6d, so that relay 11 drops and short-circuits part of resistor 15 to accelerate the operation of relay 7. If either of the other lines fails, the corresponding relay 9 or 10 becomes currentless and the same action occurs. Fig. 2 shows a modification suitable for motors which may be thrown directly on the line, the overload coils 12, 13 controlling contacts in series with a contactor-actuating coil (not shown). During starting, the currents in 29, 30, 31 are high enough to cause them to close their contacts, whereby resistances 32, 33 are connected across the coils 12, 13 so that the latter take only a fraction of the starting current. Further, the coils 12, 13 are connected in parallel by contacts 31b, 31c, as in Fig. 1, so that each takes only half the line current. If either line L<1>, L<3> fails, the corresponding relay 29 or 30 becomes currentless, whereby relay 13 or 12 receives the full line current and operates. If line L<2> fails, relay 31 is de-energized, opens the parallel connection at 51c and so energizes 12, 13 sufficiently to produce operation thereof. The relays 12, 13 are slightly delayed in action so as to ensure that 29, 30, 31 operate first upon initial energization of the circuit.