Patent GB-668404-A (GB)
Title: Improvements in or relating to electric motor control systems
IPC: B66B1/30
Filed: 1949-06-24
Abstract: 668,404. Control of D.C. motors. WESTING- HOUSE ELECTRIC INTERNATIONAL CO. June 24, 1949. [Sept. 3, 1948] No. 16810/49. Class 38 (iii). [Also in Group XXXV] A motor control system, preferably for elevators, wherein a motor is energized by a generator having a variable output voltage, comprises a first source of field excitation for the generator, a second source of field excitation for the generator energized from a controlled rectifier, and control circuits connecting the first and second sources for cumulatively exciting the generator during acceleration and full speed operation of the motor, said control circuits connecting the first and second sources differentially during deceleration of the motor. An elevator motor MO is supplied from a constant-speed generator G having a shunt field winding GFI and a separately-excited field winding GF2 which is energized through thyratrons 41, 43. These valves are controlled in accordance with the difference between a reference voltage, derived from a voltage divider 59, and one dependent upon the motorspeed. A pattern voltage suitable for acceleration and full-speed operation is provided when conductor 63 is connected to a tapping 59A, for slow-speed operation an adjustable tap 59B is used. A capacitor 67 and resistor 65 have a time constant which makes gradual the transition of the reference voltage from the high speed value to the low speed value. Voltages respectively representative of the back-electromotive force of the motor and of the resistancedrop in its armature circuit are derived from voltage-dividers 73 and 75, the voltage developed between conductors 69, 71 indicating the speed of the motor. Capacitor 79 and resistor 77 provide damping upon variation of the motor-voltage. A higher proportion of the voltage developed across resistor 75 may be introduced into the control circuit of the valves 41, 43, during landing operation by shifting the connection to the conductor 71 from tap 75A to tap 75B, gradual transition between the two values of derived voltage being provided by resistor 81 and capacitor 83. For upward travel of the car C, a car switch CS is moved to energize up-direction relay UR which in turn energizes a car-running relay M and an updirection switch U. Contacts U1, U2, close to connect the speed-responsive voltage across conductors 69, 71, with proper polarity relative to the reference voltage between conductors 61, 63, and contacts U3, U4, close for appropriate energization of the field winding GF2. The elevator brake is released and a speedcontrol relay GR is energized to connect in circuit the shunt field winding GF1 which acts cumulatively with the regulating winding GF2. The car then accelerates to the desired full speed. To stop the car, the switch CS is moved to de-energize relay UR, a slow-down inductor relay 2L being energized and opens its contacts 2UL to de-energize relay GR when the car reaches an inductor plate which is situated near the desired floor. The field winding GF1 is disconnected from the armature of the generator, the reference voltage derived from the voltage divider 59 being changed as described above so that the motor decelerates. At a pre-determined distance from the floor, a levelling inductor relay IL, previously energized, reaches an inductor plate and opens contacts IDL to de-energize down-levelling relay LD. Contacts LD1 close to shunt resistor 65 thereby anulling its time-delay effect and forcing the elevator car to the desired landing- speed. The generator field winding GF1 is connected to oppose the field winding GF2, the first winding providing approximately twice and the second winding substantially three times the resultant magneto-motive force required, the thyratrons operating at a level to ensure stable operation. At a short distance from the floor, the inductor 1L reaches a second inductor plate and open its contacts 1UL to de-energize relays LU and U. Contacts Ul, U2, U3, U4, open and the brake is applied. If the car over-runs the floor by a pre-determined distance, the relay 1L operates, field winding GF2 is re-connected for downward travel, the brake is released, and the car is lowered until the relay 1L is again actuated to stop the car at the correct level. Operation during movement of the car to a lower floor is similar to that described. The usual safety switches and interlocks are provided. When an operating coil BK is energized to release the brake, a resistor 29 is introduced to reduce the current through the coil to a holding-value. When the brake-coil is de-energized it dissipates its energy in a high discharge resistance 27 until the brake shoe is partially applied, a further resistor 28 then being connected' to decrease the discharge resistance and retard further application of the brake. In the modification of Fig. 2 (not shown), the connection of conductor 71 to tap 75B is made so as to prevent shunting resistor 81 across a portion of the voltage divider 75 when contacts GR3 are closed. A further change, regarding the connections to the voltage divider 59, is made, so that, when the elevator car is to be retarded, the reference voltage is made less than the value required to produce the final landing- speed, a high rate of deceleration being provided for the motor. When the car approaches the desired floor, the reference voltage is increased to a value providing the appropriate landing- speed.