Patent GB-625950-A (GB)
Title: Improvements in or relating to alternating current electric motor equipments
IPC: B66B1/12
Filed: 1947-03-11
Abstract: 625,950. Automatic speed control systems. GENERAL ELECTRIC CO., Ltd., and FRIEDLANDER, E. S. March 11, 1947, No. 6765. [Class 38 (iv)] [Also in Groups XXXVII and XXXVIII] An alternating-current electric motor equipment including a wound rotor induction motor, and resistance arranged to be connected with the generating winding during dynamic braking (or with the secondary winding during motoring) is controlled by means responsive to or dependent upon the speed of the motor in such a manner that, with a constant value of current in the . exciting winding during braking (or with constant voltage on the primary winding during motoring), the motor develops a braking (or motoring) torque that is substantially constant. A three-phase induction motor 1, Fig. 2, has a wound rotor and is supplied from alternating-current bus-bars 2 for motoring and reverse power braking operation, and from a direct-current exciter 3 for dynamic braking. A liquid resistor 8, which is adjusted by a hydraulic servomotor 10, is connected in the rotor circuit of the induction motor. A valve controlling the operation of the servomotor is operated by torque motor devices 13, which are of the type described in Specification 613,856. The coils of the device 13 are supplied from an induction motor-driven tachometer generator 15, which may have a two-phase output and is excited with direct-current or three-phase alternating-current, for dynamic braking or motoring operation respectively, similarly to the induction motor 1. The frequency of the output current of the generator 15 is arranged to be equal or proportional to that of the induction motor rotor current. The voltages supplied to the two coils of each unit of the torque devices 13 are phase-displaced by 45 degrees, the current to one coil passing through reactors 17 and lagging its voltage by 90 degrees, the other current passing through reactors 18 and resistors 19 in parallel, or through reactors 20 and resistors 19 in parallel, according to whether the induction motor is being dynamically braked or is motoring. For the torque devices to be in a balanced condition, wherein the valve of the servomotor is closed, the currents in each of the two coils should be 90 degrees out of phase, and this condition is afforded when the value of the resistance 19 corresponds with the value of the impedance of the appropriate reactor 18 or 20. During dynamic braking operation, assuming constant direct-current excitation of the induction motor stator winding, the reactance of the coils 18 is dependent upon the rotor frequency and speed of the induction motor, so that the adjustment of the resistance 19 follows the value of the frequency of the induction motor rotor in such a manner as to keep the device 13 balanced, the corresponding value of the resistor 8 being arranged to afford a maximum braking torque for the existing speed and value of directcurrent excitation of the induction motor. A similar control of the rotor resistance 8 is afforded during motoring operation, the reactors 20 being adjusted by means of a control lever 22 which also controls the connection of the induction motor and tachometer generator to the alternating-current supply. The output of the tachometer generator is now a measure of the slip frequency of the inductor motor. If the control lever is moved rapidly to the full torque position, the inductance of the reactors 20 is reduced to a minimum value and the servomotor follows up to the point at which the resistance 19 equals the minimum reactance of the coils 20 at the existing slip frequency. As the slip frequency decreases, so does the setting of the resistor 8. The exciter 3 is essentially a self-excited machine, but a control voltage is supplied to its field winding 27. The speed of the motor during dynamic braking is held substantially constant for a given position of the lever 22 and the function of the control voltage is to produce maximum direct-current excitation when the speed of the motor is too high and to reduce the excitation to a minimum value when the speed is too low. The speed at which this sharp transition occurs depends upon the current passing from a pilot exciter 26 through a variable potentiometer 28, which is controlled by lever 22, and a split direct-current winding 29 of a saturable reactor 30. The split direct-current winding 32 of a saturable reactor 31 is excited with rectified current derived from the tachometer generator 15. The control voltage for the field winding 27 is represented by the difference of a rectified voltage derived from tappings in the alternating-current winding 34 of the reactor 30 and the voltage of the pilot exciter 26. The alternating-current windings of the reactors are connected in series and a small increase of the direct-current excitation of the reactor 31 produces a relatively large increase of the control voltage derived from the winding 34 so as to increase the braking torque. of the induction motor. Since the winding 32 is excited in dependence upon the motor speed the breaking torque is varied so that the motor is held at the speed determined by the position of the lever 22. An auxiliary direct-current winding 36 of the reactor 31 introduces a bias excitation to assist the winding 32 and thereby maintain the braking torque at low speeds. Owing to the saturation of the motor 1 with large values of excitation current it is necessary to reduce the rotor resistance and increase the excitation for a given speed. This is afforded by appropriate arrangement of the field excitation of the pilot exciter 26, the reactors 18 being designed to become saturated in a similar manner to the motor 1 and so cause the value of the resistance 8 to be lower at high excitation than on low excitation for a given speed. A similar effect may be afforded by changing the reactance of the elements 18 by means of a direct biassing current, the reactors 18 and 20 may then be connected in series with resistors 19. Various modified forms of the control lever 22 are referred to, safeguards against faulty operation being provided. The reactors 20 may be adapted for air-gap control, and the resistors 19 may be of the liquid type. Other modifications relate to alternatives of the tachometer generator 15 and the torque devices 13. The resistance 8 may be of the grid type which is controlled by a step-by-step selector switch. In Fig. 5, the resistance 8 is controlled by an oil servomotor 10 and the valve 11. The tachometer generator 15 is excited with constant alternating - voltage during motoring operation and by a battery 48 during dynamic braking. During motoring the current in a coil 42 controlling the servomotor 10 is constant so that, for a given position of voltage transformer 44 and lever 45, the servomotor controls the resistance 19 so that the part thereof which is in series with coil 43 is proportional to the slip frequency, thus providing constant torque. During braking the rotor frequency is proportional to speed, the resistance 46 being graded so that its value relative to the tapping ratio of the transformer 44 affords adjustment of the resistance 8 to the value providing maximum torque. Stability of operation may be provided by adjusting the resistance 19 so that the torque is slightly below its maximum value. Specification 620,206 and 625,951, [both in Group XXXVII]. also are referred to.