Patent GB-612726-A (GB)
Title: Open-phase protective apparatus for polyphase induction motors
IPC: H02H7/09
Filed: 1946-02-18
Abstract: 612,726. Motor control systems; protective systems. WAYGOOD-OTIS, Ltd. Feb. 18, 1946, No. 5054. Convention date, May 28, 1945. [Classes 38 (iii) and 38 (v)] An elevator system operating on a threephase supply, has squirrel - cage induction motor - generator sets to supply D.C. for the hoist motor. The system comprises a three-phase supply SO; a generator-motor GM arranged for star-delta starting, and driving a variable voltage generator GA directly connected to the armature of the hoist motor MA; an exciter-motor EM driving a direct current exciter EA to supply the motor and generator fields, and energize the control gear; a door-motor DM to operate the car, and hoistway doors. A small single-phase transformer TRF energized from the supply feeds the motor starting relays through a rectifier RF. To start the motor generator sets, assuming the main switch ML is closed, the starter switch MGS is turned to energize the generator starting switch K, and exciter switch ER connecting the motor GM, EM to the supply, in star formation and making a holding circuit over the lower contacts of the starter switch MGS which can then be released. As the exciter begins to generate, and current builds up in the motor field relay MC energizes the running relay XKR and running switch KR to insert resistance in the exciter field, connecting the generator motor starter windings GMS1, 2, 3 in delta, and light a pilot lamp PL showing the set is ready for operation. The elevator system is of the type where stopping and starting is determined by an operator through the car switch CSS, and final control to bring the car to an exact landing is by electromagnetic levelling relays ULV, DLV and door-zone relays DZ co-operating with magnetizable plates in the hoistway. The' variable-voltage generator GA has.four control fields: levelling field GLF, separately excited for slow speed and direction; series field GSF ; shunt field GF; and demagnetizing field GDF connected across the generator-controlled armature for slowing down and to wipe out the residual field magnetism on stopping. To start the car in an upward direction, say, the car-switch CS, is turned to bridge car-switch contacts CU1-4 to energize the door motor and up direction relay, making a circuit for the field and brake relay H to energize the levelling field with a polarity for up direction, and energize the brake coil B. After this, assuming the car switch is right over to bridge CU4, resistance is cut out of the levelling field circuit, the short is removed from the series field GSF -and the shunt field GF is connected across the generator armature, so that the car is gradually brought up to full speed. If the load on the motor exceeds a certain value, resistance is cut out of the generator shunt field circuit by relay XQ. To stop the car, switch CS is centred when the car is in the levelling zone below the landing, and up levelling relay ULV maintains a circuit for the field and brake switch H, thus retaining the levelling field GLF. First slow speed switch F, upon dropping out, connects the demagnetizing field GDF and voltage relay VR across the armature. Field GDF opposes the shunt field, reducing the generator voltage, to slow the car up, until VR drops out causing both the shunt and demagnetizing fields to be disconnected. As the car nears the landing door-zone switch DZ operates to energize the door motor, and levelling relay ULV drops out to stop the motor. If the car overshoots the landing, down levelling relay DLV operates to return it. The car is moved in the down direction similarly by bridging contacts CD1-4 on the car-switch. The usual safety switches are provided, such as door contacts, hoistway upper and lower limits switches LS, and limit stopping switch SS, overspeed switch, emergency stop switch ES in the car, exciter potential switch C, &c. In addition, when a latched relay PT is operated by single-phasing of one, or all, the motor-generator sets as described below, the contact PT1 is bridged by the door-zone relay contact DZR1 if the car is moving, but contact PT2 breaks the circuit from the switch segment CU4 causing the car to slow down, as described above, and run on the levelling field only until a landing is reached. When the door-zone relay DZR drops out, the supply from transformer TRF is cut .off, shutting down the motorgenerator sets. Protective systems.-To protect the induction motors in the event of single-phasing, a relay PT, which latches in the operated position, is connected between the neutral of the exciter motor EN and the neutral of the supply ES. Artificial neutrals may be used, formed by three star-connected impedances, preferably condensers. If a break occurs in the common supply line, at FU2 say, the motor GM will continue to run as a single-phase motor, and supply the motors EM, DM, as a three-phase induction generator, and also supply the transformer TRF. With light load on the generator there is insufficient out-of-balance voltage to operate the relay, but a heavy load causes an unbalanced supply from motor GM to operate the relay. A break in the circuit to the exciter motor EM causes immediate outof-balance to operate the relay PT. Similarly, blowing of one of the fuses in the circuit of the door motor DM, during operation, causes a difference in potential between the neutrals DM, EM to operate the relay. Specification 21732/00, [Class 38], is referred to.