Patent GB-661068-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B66B1/44
Filed: 1949-05-25
Abstract: 661,068. Control of motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. May 25, 1949. [June 19, 1948] No. 13980/49. Class 38 (iii). A system for controlling the movement of a body disposed for movement in a predetermined path by electric motor driving means, said path having at least. one station at which the body* is to be brought accurately to stop, comprises stopping means effective, when a stopping operation is initiated, to stop the body adjacent the station, an energy storage device, means for storing energy in the storage device in accordance with a function of the load represented by the movable body for varying in accordance with a function of the energy stored in the energy storing device, the time when the stopping means become effective after initiation of the stopping operation. A lift system serving three floors is shown, the driving motor being of the polyphase induction type and having phase windings 11, 12, 13. The lift-car is provided with pushbuttons 1, 2, 3, each of which is actuated for movement to the appropriate floor. The pushbuttons, together with a number of hatchway switches, control the energisation of direction relays UR, DR. If the car is below the third floor, for example, it may be sent to that floor by actuation of push-button 3. Up-direction relay UR is energised through call-registering relay 3R and hatchway switch 3 UP, contacts 3RI being closed to shunt the push-button until the call is answered. When the car reaches the third floor, hatchway switch 3UP opens to de-energise relay-coils 3R, UR. Direction switches U, D, and main switch M are controlled by contacts UR2, DR2, of the direction relays to regulate energisation of the motor windings 11, 12, 13, for reversible operation of the lift-car. A brake coil BC is connected across motor windings 11, 12, to release the brake simultaneously with the energisation of the motor. When the car is to be stopped, a capacitor CA is connected through adjustable resistor RI across the coil of a relay SR which is then maintained energised for a period dependent upon the loading of the car so as to obtain accurate levelling operation. With contacts UR2 or DR2 opened to initiate stopping action, the direction switch U or D and'the main switch M remain energised until relay SR drops to open contacts SRI, SR2. Means for charging the capacitor CA in accordance with the loading of the car comprise a current transformer Tl having its primary winding connected in series with motor winding 13, the primary winding of a transformer T3 being connected across supply-lines L1, L2. A voltage proportional to motor current is developed across resistor R7, and one substantially equal in magnitude but opposite in phase to the component of the first voltage which represents the magnetising current of the motor is developed across resistor R9, the resultant voltage, which appears between trip 53 and the negative terminal of a rectifier RE, indicating the load upon the motor. The loadmeasuring voltage is amplified by means of valve 55, the anode circuit of which comprises the primary winding of a transformer T5. The secondary voltage of this transformer is rectified by valve 61 and is then applied through a filter to the capacitor CA. A resistor R5 absorbs energy stored in the capacitor as a result of momentary applications of over-voltage. Resistor R3 provides time-delay in the operation of relay SR to ensure that closure of contacts SRI, SR2 does not cause false operation of either of the direction switches. The relay SR may be omitted, the brake coil being connected across the capacitor, Fig. 4 (not shown). When the car reaches a pre-determined distance from a floor the capacitor is disconnected from the energy-storage means and is connected to the brake solenoid which is held energised for such a period that the lift is correctly levelled despite fluctuations of load. In order to control the charging of the capacitor CA in accordance with the loading a tachometer generator may be coupled to the motor shaft, Fig. 5 (not shown). As the load increases, the voltage output of this generator decreases to lessen a negative bias voltage component applied to the grid of a pentode valve, so increasing its output Which is amplified and rectified to charge the capacitor. The generator may be replaced by the wattmetric system of Fig. 6. Pentode valves 127, 129, have an input derived from the motor current through transformer T13. A voltage input to the pentodes is derived from supplylines L1, L3, through transformer T15 the secondary winding of which is connected across a phase-shifting circuit comprising capacitor 149 and resistor 151. The phase-shifter is adjusted to bring the voltage between tap 147 and earth substantially into phase with the current derived from transformer T13, the output of transformer 141 then being proportional to the real power supplied to the motor. A direct voltage appears acrosss resistor 121 for conditioning the capacitor CA for levelling operation as described above.