Patent GB-630980-A (GB)
Title: Improvements in apparatus for weighing the output of lifting and lowering machines
IPC: G01G11/04
Filed: 1946-11-19
Abstract: 630,980. Power and energy measurements. ANGLO AMERICAN CORPORATION OF SOUTH AFRICA, Ltd. Nov. 19, 1946, No. 34323. Convention date, Nov. 19, 1945. [Class 37] [Also in Group XXXVIII] A load is measured while being raised or lowered, e.g. in a conveyer or mine hoist, by measuring the power taken by the driving motor when the load is moving at uniform speed, and subtracting therefrom the tare power consumed by the motor at no-load. The nett power may be integrated and operate mechanism for recording the weight over a period. Compensation may also be introduced for the increase in tare power loss with increase of load. Fig. 1 shows a conveyer 2 driven continuously by an electric motor 6, the nett power consumption of which is integrated by a standard watthour meter 8. When the motor is driving the conveyer unloaded, the potentiometers 24, 25 are adjusted to produce through transformers 22 a counter-current in the normal meter current coils 10 which cancels out the no-load current therein produced by the current transformers 11, so that the meter remains at rest. When the conveyer is loaded the meter then registers the nett load on a counter 14, calibrated to read weight, the registration being recorded on a constantly-driven tape by a periodicallyoperated platen 18. Since the mechanical and electrical losses when the conveyer is unloaded, i.e. the basic tare losses, increase with load this increase may be allowed for by overcompensation of the meter so that at no-load the meter tends to run backwards, pawl 27 cooperating with the ratchet-toothed rotor 9, however, preventing backward rotation. In a modification, Fig. 2 (not shown), the transformers 22 are dispensed with, the compensating circuit producing the necessary counter torque in the meter 8 by operating on a further pair of meter current coils. In a further modification, Fig. 3, the compensating counter torque on the meter is produced by a generator 31 acting on auxiliary current coils 30 of the meter, the generator being driven by the motor 6 and cophased therewith, its D.C. field being energized by the current transformer 23 and potential transformers 13 through a rectifier 33 and potentiometer 34. This arrangement may be adjusted to compensate automatically for the increase in basic tare losses with load at several different but constant speeds of operation. In the application of the invention to mine hoists, Fig. 4, where the motor 35 raises one cage 40 as the other 41 is lowered, the compensation is effected as in Fig. 3, by a motor-driven generator 69, but in this case, the generator output is directly connected in series opposition with the current transformers 11 to the current coils of the meter 8c. The D.C. generator field is energized as in Fig. 3. Since discrete loads only are concerned, the power taken by the motor 35 is constant during the period when the hoist is running at constant speed. A standard wattmeter 8c is therefore employed. The current coils 45 thereof are normally short-circuited by contacts 63, a contactor 62 being energized to open these contacts just prior to the moment of mid-wind of the hoist by a cam 58 driven synchronously with the hoist movements, e.g. by one of the hoist depth indicators 57. The meter 8c thereupon takes up a position corresponding to the nett power consumption which is recorded at the moment of mid-wind by a further cam 64 energizing contactor 67 which energizes the recording magnet 55. In a modification, Fig. 5 (not shown), the compensating circuit is connected to auxiliary current coils of the watt-meter. The compensating generator is of the permanent magnet type and energizes the auxiliary meter coils direct, a further compensating potential proportional to the motor current being injected into the circuit by transformer 23 to compensate for increased tare losses with load. The generator may be omitted, Fig. 6 (not shown), compensation being effected as in Fig. 2, and in further modifications, Figs. 6, 7 (not shown), the hoist motor is D.C. and Ward-Leonard controlled.