Patent GB-828403-A (GB)
Title: Improvements in electrical decelerating systems particularly for elevator cars
IPC: B66B1/44
Filed: 1958-05-06
Abstract: 828,403. Control of A.C. motors. TURNBULL ELEVATOR CO. Ltd. May 6, 1958 [May 31, 1957], No. 14516/58. Class 38 (3). [Also in Group XXXV] A lift-car is stopped at a floor by returning a car switch CS to the neutral position, a decelerator switch opening at a predetermined distance from the floor to de-energize brake coil BM, and apply an electro-mechanical brake. The brake pressure is applied gradually since the brake coil discharges through rectifiers 34-37 and resistor 32. Relays B and C are de-energized to introduce resistance into the rotor circuit of the motor driving the lift, sliprings 15 of the motor being connected through transformer 41 to a frequency-filter 50. Initially, capacitors 53, 56 are shunted by contacts U2M, D2M, through tap 55C. Capacitors 51, 52, and reactor 54 then constitute a filter circuit which passes all frequencies higher than a predetermined value, so that, as the car speed decreases and the slip-frequency increases a current passes to the brake coil BM which relieves the brake-pressure, whereby the motor speed tends to stabilize at a value corresponding to the critical pass frequency of the filter. At a predetermined distance from the floor, contacts U2M open (assuming upward travel) so that capacitors 53, 56, become operative and the pass-frequency of the filter circuit is increased, so causing the coil BM to be deenergized and full brake-pressure to be restored. The motor then decelerates to a speed such that the slip frequency equals the new critical passfrequency of the filter. Contacts U3M open at a predetermined distance from the floor to disconnect capacitor 56 from the filter circuit which is thereby afforded a still higher critical pass-frequency corresponding to landing speed, the brake pressure being regulated to hold the car at that speed. A short distance before floor level, contacts USL open to de-energize the stator winding of the motor and to remove the slip voltage supplying the brake coil. Contacts TS1 of a timing relay are kept closed for a period so that a governor circuit 60 is brought into operation for maintaining brake current until the car stops, the distance that the car travels past the position of switch USL being substantially the same for all load conditions. Whilst the lift-car is travelling at the landing speed, a voltage is developed across capacitor 68 which is proportional to the brake current passing through resistor 32 and, when the motor is de-energized by the opening of contacts USL, the capacitor is prevented from discharging except through a high resistance 70. Hence, the voltage on capacitor 68 constitutes a record of the magnitude of brake current during the landing period. When the contacts USL open, the brake current begins to decay, causing a reduction in voltage-drop across resistor 32, whereby thyratrons V1, V2 are afforded a positive bias and anode current passes to the brake coil. As soon as the brake current is restored to its original value, the thyratrons cease to conduct. Thus, the brakepressure is held at substantially the same value as was required to keep the motor at a steady landing speed until contacts TS1 open and the brake is fully applied. Automatic re-levelling may be provided, the voltage applied to the brake coil being limited by a resistor 46 for a period during such operation, to prevent overshooting. In a modification, a component of the filter circuit may be continuously adjusted in accordance with time or the motion of the lift-car, so as to provide continuous control of the deceleration instead of the stepped control referred to. The brake may be of the eddycurrent or magnetic powder types.