Patent GB-632572-A (GB)
Title: Improvements in or relating to alternating current motor drives and control
IPC: H02P5/747
Filed: 1945-09-24
Abstract: 632,572. Control of two A.C. motors. IGRANIC ELECTRIC CO., Ltd. Sept. 24, 1945, No. 24747. Convention date, Sept. 23, 1944. [Class 38 (iii)] A lift hoist drive has main and auxiliary' A.C. motors coupled to the hoist and control means whereby the main motor always exerts its torque in the hoisting direction but the auxiliary motor assists it while hoisting and opposes it when lowering. The auxiliary motor may drive at a speed greater than the main motor and either motor may operate alone in the hoisting direction. The auxiliary motor may be energized alone for lowering light loads. The main induction motor M is connected to the 3-phase supply by the main switch MS (Fig. 1). The higher speed auxiliary induction motor M1 is also subject to the switch MS but can be reversed under the control of the hoisting switch H and the lowering switch L. Resistances Rl to R4 can be cut out of the rotor circuit of motor M by the energization of switches 1A, 2L, 2A, 3A and 4A, partly under the control of the series relay SR, which opens when the rotor current exceeds a certain value, and of the load relay HLR ; the latter, which is preferably of the kind according to Specification 632,571, remains closed by the rotor current until the load is zero or very close to zero with current reversed. Resistance R6 can be cut out of the rotor circuit of motor M1 by energizing the switch 5A. Extra resistance R can be inserted in the rotor circuit of motor M by energizing switch 1A and this motor can be stopped by opening its rotor circuit by energizing switch HS, partly under the control of the load relay LLR which closes when the rotor current of motor M1 exceeds a certain value, and of the voltage relay VR which is connected across the rotor of motor M in parallel with condenser 15 and in series with rectifier 14 and which opens when the rotor voltage falls below a certain value. As shown in Fig. 2 the timing relays 1CT, 2CT and 3CT, each of which has its coil in parallel with a resistor and condenser such as rand 31 and has a time delay when de-energized, are normally energized through rectifier 30. Movement of the drum controller D through its hoisting or lowering steps energizes the brake relay BR (through contacts 19) and takes off the brake B. On the first hoisting step switch MS is energized (through contacts 17 and the hoist limit switch HLS) and motor M is switched on. On the second step switch 2A is energized (through contacts 23 and the now energized relay HLR) to cut out some rotor resistance; relay ICT is de-energized. On the fourth step, switch 3A is energized (through contacts 21 and relay ICT if this has released), and relays 2CT and 3CT are de-energized. On the fifth step switch 4A is energized (through contacts 22 and relay 2CT if this has released) and the motor M is in full speed condition. If, however, the load is heavy, the push PB should be pressed when the controller D is on its third and subsequent hoisting steps. Then on the third step the switch H is energized (through contacts 20 and push PB) to switch on motor M1 to assist motor M. On the fifth step resistance switch 5A is energized (through contacts 27). On the fourth step also the switches HS and 1A are energized (through contacts 21 and 25) and this-if the push be pressed on light load when relay HLR has dropped and de-energized switches 3A and 4A-will open the rotor circuit of motor M, leaving motor M1 hoisting alone. On the first and all lowering steps switch L is energized (through contacts 18) and this energizes switch MS; both motors are switched on, with motor M1 opposing motor M and helping the load to go down. Switch 5A is also energized. On the first step, switches 2L and 2A are also energized (through switches 23 and 24). The torque of motor M1 decreases with speed whereas the torque of motor M-opposing the lowering-increases with speed; the speedtorque relationships for various conditions are given graphically in Fig. 4 (not shown). On the second step 2A is de-energized, on the third step 2L is de-energized, and on the fourth step switch 1A is energized, thus progressively increasing the rotor resistance of motor M. With a light load going down, the rotor current of motor M1 will be sufficient to close relay LLR; pushbutton PB can then energize switch HS through contacts 26 and relay LLR, to leave motor M1 acting alone to lower the load. Switch HS maintains itself when relay LLR subsequently opens, but if the lowering speed becomes too great the rotor voltage of motor M energizes relay VR to de-energize switch HS. In a modification the push-button is omitted; switch H is always energized to start motor M1 on the third hoisting step and switch HS is energized on the fourth step if the load is light and relay HLR is open. Switch 5A is initially energized by switch H to open its contacts and is de-energized to cut out rotor resistance. The contacts 23 for energizing switch 2A are also closed when the drum switch is in the offposition and further contacts also closed in this position maintain switch L closed for electric braking until the series relay SR drops out. In each arrangement, the relay BR and the switches MS, H and L are subject to the usual no-volt relay UV which is initially energized by the contacts 16 when the controller D is in the off position and thereafter maintains itself, and are also subject to the overload relays OL1- OL4.