Patent GB-439286-A (GB)
Title: Improved method of and means for controlling the performance of electric motors
IPC: H02P5/68
Filed: 1934-03-02
Abstract: 439,286. Control of D.C. motors. BENTLEY, J., and GALLOWAY, H. L., Edenbank, Garngaber Avenue, Lenzie, Dumbartonshire. March 2, 1934, No. 6701. [Class 38 (iii)] Varying E.M.F. ; field control.-In an arrangement in which the speed-torque operating characteristics of a load motor are controlled by the back electromotive force of a regulator motor arranged in circuit with the load motor and driving a load, the speed-torque operating characteristic of the load motor is determined by the pre-arranged torque-speed characteristics of the regulator motor and the device which it drives. In order to limit the load on a serieswound hoisting motor L, Fig. 1, it is connected in series with a regulator motor having a series field B and a shunt field C, the two motors being arranged to give the same power at the same current, and the regulator motor being coupled to a plunger pump which requires a predetermined constant torque to drive it. During normal running of the load motor, the current passing is such that the regulator motor is stalled, but when the load and the current reaches a predetermined limit the regulator motor rotates and its back electromotive force causes the load motor to stall. When the load again returns to normal, the regulator motor stalls and the load motor runs at its normal speed. A circuit-breaker in the main circuit may be operated by a coil, connected across the terminals of the regulator motor, when the voltage of the regulator motor reaches a predetermined value. In applying the invention to steering gear, where it is desired that the speed-torque characteristic of the load motor shall be different from that normally obtained, the regulator motor drives a centrifugal brake. When a switch Z, Fig. 3, is moved from the upper to the lower position, the regulator motor R, connected in series with the load motor L, runs up to a speed which brings the centrifugal brake into action. At various increasing values of current passing through the two motors the speed of the regulator motor increases and its back-electromotive force decreases the speed of the load motor. When the load reaches a predetermined limit the regulator motor causes the load motor to stall. The action of the arrangement is described in the Specification with reference to speed-torque curves, Figs. 5 and 6 (not shown). For large units the centrifugal brake is replaced by a shunt-wound generator connected to the line in parallel with the motors L, R. In order to obtain constant horse power from a serieswound motor L, Fig. 11, the regulator motor R is connected in parallel with the motor field A, and has a series field B, a field D in series with the load motor field A, and a field C carrying the constant line current, the field B opposing the fields C, D. The field windings are so proportioned as to give to the regulator motor a speed-torque characteristic such that with variations of load on the load motor the speed and back-electromotive force of the regulator motor are adjusted to vary the field of the load motor and adjust its speed to give the desired constant horse power. In applying the system to give constant horse power for a shunt-wound motor, two regulator motors R<1>, R<2>, Fig. 13, driving a common mechanical device are provided, the motor R<1> being connected in parallel with the load motor field A in a manner similar to that for the serieswound motor arrangement, and the motor R<2>, in series in the field circuit. When the load motor, in operating a crane or lift, is required to run at a creep speed, it is connected in parallel with the regulator motor R, as shown in Fig. 14, the motor R having a field winding C carrying the load motor armature current, a field D carrying the load motor field current, and a field B in series with its own armature. With full current taken by the motor L, a resistance Y reduces the voltage to give the desired speed, the motor R running without resisting torque from the centrifugal brake. With less than full current the voltage across the resistance Y tends to drop, the field of the motor R being weakened to cause the speed to rise and bring the centrifugal brake into action, and cause the load motor to run at its creep speed. Specification 25449/99, [Class 35], is referred to.