Showing papers on "Induction motor published in 1970"
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01 Jul 1970TL;DR: The self-cascaded machine as discussed by the authors is a single-unit version of two separate induction machines connected in cascade, which can be run asynchronously, with resistance control if required, or synchronously without any external connections to the rotor winding.
Abstract: The self-cascaded machine is a single-unit version of two separate induction machines connected in cascade. In construction, however, it closely resembles a conventional induction motor. It may be run asynchronously, with resistance control if required, or synchronously without any external connections to the rotor winding. As a motor it is particularly suited to low-speed duties. With its simple and robust form of rotor construction, and the absence of slip rings or rotating diodes, it is also ideally suited for operation at high speeds as a high-frequency brushless alternator. A general description of the machine, and the underlying principles involved, are presented. A theoretical treatment is included based on doubly stator-fed operation of the machine. General performance equations are derived, and equivalent static networks are obtained for steady-state operating conditions.
215 citations
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TL;DR: In this article, the problem of solving the general three-phase induction motor performance equations is discussed and methods of digital solution for both the threephase motor and the two-axis model are described.
Abstract: The problem of solving the general three-phase induction motor performance equations is discussed and methods of digital solution for both the three-phase motor and the two-axis model are described.
69 citations
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TL;DR: In this paper, the single-sided stator and double-sided stochastic stator configurations of the infinitely long linear induction motor are analyzed from electromagnetic field theory, and the traction force and the normal force equations are derived using the Maxwell magnetic stress tensor.
Abstract: The single-sided stator and the double-sided stator configurations of the infinitely long linear induction motor are analyzed from electromagnetic field theory, and the traction force and the normal force equations are derived using the Maxwell magnetic stress tensor. The force normal to the air-gap plane, which has not been quantitatively evaluated or understood before, is found to be highly repulsive for slips greater than St = 1/Rm (Rm magnetic Reynolds number). The engineering use of this large repulsive force to provide frictionless contact-free means for levitation, suspension, and mechanical stabilization of high-speed transport at speeds of 300 mi/h is considered. A physical explanation is given for the existence of the normal force components. The equivalent circuit is derived for the linear induction motor so that the forces and the design parameters obtained from field theory can be understood in terms of traditional induction motor theory. The performance characteristics are investigated, and it is found that the two key design parameters are 1) the magnetic Reynolds number Rm and 2) the air-gap-wavelength ratio sg. The single-sided stator configuration is considered for levitation, and the criterion for a self-levitating vehicle is given. In the double-sided stator configuration, the net normal force is a restoring force which is considered for keeping the vehicle aligned on track against lateral perturbations. In this case, the equivalent electromagnetic spring constant Ck and the natural frequency of oscillation fm are derived and their performance characteristics investigated.
65 citations
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01 Apr 1970TL;DR: A survey of linear-motion electrical machines is presented in this article, where the main concern of linear induction motors are discussed in considerable detail, based on topological considerations, and their development through the last 70 years is reviewed.
Abstract: A survey of linear-motion electrical machines is presented. Although various types of dc and ac linear machines are briefly mentioned, linear induction motors are the main concern of the paper and they are discussed in considerable detail. Based on topological considerations, a classification of these machines is presented and their development through the last 70 years is reviewed. A brief qualitative description of the newly developed hybrid machine is also included. Analysis and design problems, and some solutions, as unique to linear machines are discussed. Several possible applications of these machines are included.
57 citations
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TL;DR: In this article, a method for incorporating the rotor deep-bar effect in a dynamic simulation of induction machines under dynamic conditions has been presented, based on two-phase models using constant circuit parameters.
Abstract: Simulations of induction machines under dynamic conditions have heretofore been based on two-phase models using constant circuit parameters. Most induction motors in the integral horsepower sizes have rotor bars of sufficient depth that they cannot be accurately represented by a constant rotor resistance under all operating conditions A method for incorporating the rotor deep-bar effect in a dynamic simulation is presented.
55 citations
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07 Oct 1970
TL;DR: In this article, a standby generating system consisting of a generator running in synchronism with the normal AC supply, the generator being used to maintain a supply to a network when the normal DC supply fails during the period whilst the main standby engine driven alternator is being started and connected to the network.
Abstract: 1,207,934 Standby supply systems P RACANIERE 5 May, 1969 [17 May, 1968; 24 March, 1969], No 22762/69 Heading H2H A standby generating system comprises a generator running in synchronism with the normal AC supply, the generator being used to maintain a supply to a network when the normal AC supply fails during the period whilst the main standby engine driven alternator is being started and connected to the network A two speed asynchronous motor 15 is arranged to drive two flywheels 12, 18 through gearing 16, a variable ratio coupling 14 being placed between the two flywheels and the flywheel 18 being fast on the shaft of an auxiliary alternator 11 To place the apparatus in a standby condition contacts 34 and 38 are closed thereby energizing the low speed (1500 rpm) winding 23 of the motor 15 and at the same time causing maximum energization of the coupling 14 by way of a governor 21 associated with the alternator 11 The motor 15 drives the flywheels 12, 18 and the generator up to speed at which point a synchronizing element 31 functions to synchronize the alternator 11 with the normal AC supply 2 and to connect the alternator in parallel with this supply 2 by closing contacts 32 At this point the energization of the coupling 14 is removed, contact 38 opened and contact 39 closed so that the motor 15 now drives the flywheel 12 up to the high speed (3,000 rpm) of the motor, rotation of the alternator 11 being maintained by virtue of its connection to the AC supply 2 Upon failure of the supply 2, a switch 51 changes over to the emergency power supply unit 3 which includes a prime mover Whilst this unit is being started the supply to a network 1 is maintained by the alternator 11 driven by the energy stored in the flywheels 12, 18 Initially the flywheel 18 maintains rotation of the alternator 11 until the clutch 14 is energized Thereafter the governor 21 varies the energization of the coupling 14 so that the high speed flywheel 12 drives the alternator 11 at its normal speed, whilst the flywheel 12 is slowing down Before the speed of flywheel 12 has dropped to the normal speed of the alternator 11 the unit 3 will normally have been started and connected to the network 1 to maintain the supply thereto In other embodiments a prime mover or a battery driven electric motor may be arranged to drive the motor 15 so that the supply to the network 1 may be maintained by the alternator 11 should difficulty be had in starting the emergency supply unit 3 In yet a further embodiment all the power required from the network 1 is supplied by the generator 11
51 citations
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09 Jul 1970
TL;DR: In this paper, an electromagnetic coupling is connected mechanically with an electric rotating machine having a stator, for instance a synchronous motor, a cage rotor type or wound rotor type induction motor etc.
Abstract: When an electromagnetic coupling is connected mechanically with an electric rotating machine having a stator, for instance a synchronous motor, a cage rotor type or wound rotor type induction motor etc., and the said electromagnetic coupling is connected electrically with the said electric rotating machine through a phase sequence change over switching device, further the electromagnetic coupling combined mechanically with the rotor of the electric rotating machine drives a load, and the other rotor of the electromagnetic coupling is driven by a driving machine, the electric machine arrangement has such advantages as shown by (1) obtaining power transmitting devices with operation similar to reduction gears which transmit outputs of other driving machines to loads; (2) possibility of driving one rotor of electromagnetic coupling in direction opposite to the rotation of the other rotor; and so forth. If the mechanical combination between the rotor of the electromagnetic coupling and the rotor of the electric rotating machine is made by using a rotor in common for the electromagnetic coupling and for the electric rotating machine, the electric machine arrangement becomes compact.
44 citations
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01 Nov 1970
32 citations
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TL;DR: The magnetic field of a linear motor is evenly distributed over the air gap between the primary and secondary members by providing a ladder-like grid structure of conductors at each side, by interleaving the laminations with conductive stampings near the sides, by strips of conductive material on or near the surface of the stator, or by chamfering the edges as mentioned in this paper.
Abstract: The magnetic field of a linear motor is evenly distributed over the airgap between the primary and secondary members by providing a ladderlike grid structure of conductors at each side, by interleaving the laminations with conductive stampings near the sides, by strips of conductive material on or near the surface of the stator, or by chamfering the edges.
30 citations
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01 Aug 1970
TL;DR: In this article, it is shown that a conventional 60° phase-spread winding can be subdivided into two components, one of which is connected in delta and the other in star, the two winding components being connected in parallel to a 3-phase supply.
Abstract: A new method of connection for 3-phase windings is described. It is shown that a conventional 60° phase-spread winding can be subdivided into two components, one of which is connected in delta and the other in star, the two winding components being connected in parallel to a 3-phase supply. Theoretically, the new principle is only applicable where the number of slots per phase is infinite, but it is demonstrated that entirely satisfactory results can be achieved with reasonably small numbers of armature slots. Simplicity of construction is considered to be very important, and, in each of the windings discussed, all the coils are identical and carry currents of equal magnitude. Theoretical examination shows that the new method gives a higher fundamental spread factor than for a conventional 3-phase winding, and a superior m.m.f. waveform, as well as providing a means of reducing the effective number of turns in series per phase. The theoretical studies were verified by tests on a 3-phase squirrel-cage induction motor.
27 citations
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26 Aug 1970
TL;DR: In this paper, a commutatorless direct current motor with permanent magnet rotor is used to detect the rotation angle of the rotor and the stator windings are sequentially passed with a current of constant phase rotation.
Abstract: A commutatorless type direct current (DC) motor which has a rotor made of permanent magnet. Stator windings are sequentially passed with a current of constant phase rotation and thereby the rotor is initiated to rotate. After starting, counter electromotive forces induced in the stator windings are used for detecting the rotation angle of the rotor. Adequate currents are thereby made to flow in the stator windings and a steady rotation of the rotor is continuously performed.
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09 Mar 1970
TL;DR: In this paper, an electric motor with a wound stator and a permanent magnet rotor is combined with optical commutation to provide improved high speed capability and efficiency, long life, and utility in any atmosphere.
Abstract: An electric motor with a wound stator and a permanent magnet rotor, wherein optical commutation is combined with a ferrite stator core to provide improved high speed capability and efficiency, long life, and utility in any atmosphere. A novel pole format of six poles or multiple thereof combined with electronic pulsing gives the motor a stepping capability enabling it to follow incremental tape storage, computer, or master-slave commands, as well as providing further torque and speed potential. Optional use of a ferrite rotor provides extreme longevity to the motor.
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01 May 1970TL;DR: In this paper, the steady-state performance of a slip-energy-recovery scheme incorporating an induction motor and a static frequency changer is described, where a considerable improvement in the low-speed efficiency may be obtained at the expense of a poor power factor.
Abstract: The steady-state performance of a slip-energy-recovery scheme incorporating an induction motor and a static frequency changer is described. A considerable improvement in the low-speed efficiency may be obtained at the expense of a poor power factor. Power-factor correction by the use of primary-side capacitance is found to increase slightly the 5th-harmonic component of the supply current and to double the torque/supply-current ratio. Compensation by the use of fixed secondary capacitance is found to give increased torque at low speeds, increased torque/supply-current ratio, higher efficiency at some speeds, better speed regulation, much improved step response to changes of control signal and good power-factor correction at low speeds. The closed-loop steady-state performance is that of a good stable variable-speed drive, with speed regulation of the order of within 1?3% over the entire speed range.
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01 Jul 1970TL;DR: In this article, a method for synthetising the equivalent-circuit parameters of double-cage induction motors to attain specified levels of starting, accelerating, breakdown torque and running performance is presented.
Abstract: The paper presents a method for synthetising the equivalent-circuit parameters of double-cage induction motors to attain specified levels of starting, accelerating, breakdown torque and running performance. New equations are derived, avoiding certain approximations which have commonly been made hitherto, and these equations are used to relate the equivalent-circuit parameters to the motor performance. A new synthesis procedure is then developed, based on these equations and relationships. The procedure includes a number of sections which are essentially iterative, and it is consequently most conveniently embodied in a digital-computer program. Since the reduction of leakage reactances owing to magnetic saturation is an important factor in motors with arduous starting duties, a method is proposed for taking this effect into account. Using, as a starting point, an approximate representation of the shape of typical curves of leakage reactance against current, equations are derived to represent the saturation effect in a form which isconveniently incorporated in the computer program. Only two additional items of input data are required to specify the saturation effect, these being the current at which saturation commences and the ratio of saturated to unsaturated starting currents. The successful operation of the synthesis procedure is demonstrated by a number of examples, including some in which the computed design is compared with a prior manual design to the same specification.
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Avco1
TL;DR: In this paper, the authors propose a system for supplying electric power to and automatically controlling the speed of driverless, electrically driven, wheeled vehicles on a guide way, which includes a power distributing track divided into a plurality of power segments and a plurality electrical power sources each cyclically operated at different frequencies coupled to different predetermined power segments of the track to create a series of travelling waves, each at a different frequency, in the segmented track.
Abstract: A system for supplying electric power to and automatically controlling the speed of driverless, electrically driven, wheeled vehicles on a guide way. The system includes a power distributing track divided into a plurality of power segments and a plurality of electrical power sources each cyclically operated at different frequencies coupled to different predetermined power segments of the track to create a series of travelling waves, each at a different frequency, in the segmented track. A vehicle including an AC induction driving motor coupled to one power segment of the track receives electrical energy at the particular frequency appearing at a particular moment at a particular power segment. The induction motor in the vehicle continues to receive that particular frequency and, hence, maintain a constant torque so long as the vehicle continues to travel along the track at the desired rate of speed. Should the speed of the vehicle exceed or drop below the desired rate of speed, the induction motor will receive a preceding or succeeding travelling wave of such frequency as to compensate for the increase or decrease in speed thereby causing the induction motor to maintain the desired vehicle speed along the track.
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01 Dec 1970TL;DR: In this article, an analysis of the sheet-secondary induction motor when the sheet is displaced sideways from a symmetrical position is presented, showing that the forces which act on the sheet are decentralising and that the magnitude of these forces and of the changes in propulsive force and winding impedance are dependent on five dimensionless parameters.
Abstract: An analysis is presented for the problem of the sheet-secondary induction motor when the sheet is displaced sideways from a symmetrical position. It is shown that the forces which act on the sheet are decentralising and that the magnitude of these forces and of the changes in propulsive force and winding impedance are dependent on five dimensionless parameters which are functions of the motor's design and its operating slip. Measurements made on a small linear motor, while indicating that the neglected skin effect and endwinding leakage fluxes can be significant, show reasonable agreement with the analysis. A means of reducing the decentralising force is suggested, and reference is made to a method of achieving lateral stabilisation of a freely suspended sheet.
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09 Oct 1970TL;DR: In this paper, a cycloconverter motor system consisting of 18 controlled rectifiers and 18 diodes is used to provide six unidirectionally conductive bridge rectifier switching means for supplying a three-phase AC induction motor from a threephase AC source.
Abstract: A cycloconverter motor system which includes 18 controlled rectifiers and 18 diodes connected to provide six unidirectionally conductive bridge rectifier switching means for supplying a three-phase AC induction motor from a three-phase AC source. The induction motor is provided with two identical threephase windings, each of which is comprised of three identical electrically isolated windings. Each of the six windings is connected with a respective one of the six unidirectionally conductive switching means. In this manner, each phase winding is comprised of two identical windings which are energized by their respective unidirectionally conductive switching means sequentially to provide oppositely directed magnetic fields and motor operation.
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20 May 1970TL;DR: In this paper, the authors describe an induction MOTOR that is driven by a direct electrical connection to the alternator, which supplies electric power to VEHICULAR COMPONENTS.
Abstract: THIS INVENTION RELATES TO AN INTERNAL COMBUSTION ENGINE SYSTEM IN WHICH THE COOLING FAN IS DRIVEN BY AN INDUCTION ELECTRIC MOTOR RATHER THAN MECHANICALLY BY THE ENGINE. THIS INDUCTION MOTOR IS A POLY PHASE INDUCTION MOTOR AND IS DRIVEN BY DIRECT ELECTRICAL CONNECTION TO THE ALTERNATOR WHICH NORMALLY SUPPLIES ELECTRIC POWER TO VEHICULAR COMPONENTS. THE INDUCTION MOTOR INCLUDES SQUIRREL CAGE ROTOR CONDUCTORS MOUNTED ON THE ROTOR AND DEEPLY BURIED IN THE ROTOR TO ENHANCE THE REACTIVITY OF THE MOTOR AT HIGH SLIPS.
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02 Mar 1970
TL;DR: In this article, a vehicle is equipped with a main drive system and an electric assist drive system which is actuated by the operator when the vehicle speed falls below some predetermined speed.
Abstract: A vehicle is equipped with a main drive system and an electric assist drive system which is actuated by the operator when the vehicle speed falls below some predetermined speed. A prime mover for the vehicle, in addition to driving two wheels directly through a primary transmission, drives a three-phase electrical alternator. The alternator energizes an induction motor with a torque-regulating control system for independently driving two other wheels of the vehicle to provide greater traction at low speeds. The rotor circuit of the induction motor includes a rectifier bridge which feeds a line-commutated inverter for coupling the slip power back to the stator input. The control system determines the firing angle for conduction of the switches in the inverter to control the output torque of the induction motor as a function of speed. The system includes a shaping network which defines the desired speed-torque characteristic for accelerating the vehicle; and it generates a signal representative of a demand torque for a given motor speed. Another circuit sensing rotor current generates a signal representative of the load torque. A comparison circuit receives the two signals representative respectively of the actual torque and the desired torque for that speed; and it generates an error signal for controlling the firing angle of the switches in the inverter circuit such that the motor and vehicle are accelerated along the torque-speed envelope of the shaping network once the assisting system is actuated. Thus, the control system regulates the output torque of the assisting induction motor as a predetermined function of vehicle speed.
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TL;DR: A review of the Squirrel-Cage induction motor's plugging characteristics can be found in this paper, where the authors also discuss the potential pitfalls of plugging applications with low-inertia squirrel-cage induction motors.
Abstract: Plug stopping a low-inertia squirrel-cage induction motor can provide a very quick stop, but the motor draws high peak currents from the power system and delivers high transient torques to the driven machinery. This engineering phenomena is well documented, yet the potential pitfalls of plugging applications are often not appreciated. The induction motor is so widely used in industry that a review of its plugging characteristics may be useful.
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20 Mar 1970TL;DR: In this paper, a rotor resistor assembly for a wound rotor induction motor includes resistors formed by metal bars which are wound in circular coils extending circumferentially around the rotor shaft.
Abstract: A rotor resistor assembly for a wound rotor induction motor includes resistors formed by metal bars which are wound in circular coils extending circumferentially around the rotor shaft. The resistors are mounted on the rotor shaft by a support member which includes pivotal linkages movably connecting the resistors to the support member. The resistors respectively expand and contract radially when heated and cooled and a counterweight included in the pivotal linkages maintains radial tension on the coiled bar resistors when the assembly is rotated at high speeds. A fan impeller arrangement extending from the support member circulates cooling air through the resistors.
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29 Jul 1970
TL;DR: In this paper, a split phase induction motor including a main winding and a start winding is used to de-energize the start winding after the motor has started, a solid state electronic switch is connected in series with the start wound and an oscillator is connected to actuate the switch.
Abstract: This disclosure deals with a split phase induction motor including a main winding and a start winding. To de-energize the start winding after the motor has started, a solid state electronic switch is connected in series with the start winding and an oscillator is connected to actuate the switch. An electrical element having an impedance which varies with the temperature thereof is connected to control operation of the oscillator. When the motor is initially energized, the temperature of the element is low, the oscillator operates, and the switch is closed. As the motor speeds up, the element is heated, such heating changing its electrical impedance. When the temperature of the element reaches a certain value, the oscillator is turned off, the switch is opened and the start winding is de-energized.
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03 Jun 1970TL;DR: In this article, a high resistance rotor, high slip induction motor is described where the high resistance is achieved by omitting alternate ones of the conductor bars in the squirrel cage and leaving the remaining apertures open.
Abstract: A high resistance rotor, high slip induction motor is disclosed wherein the high resistance is achieved by omitting alternate ones of the conductor bars in the squirrel cage and leaving the remaining apertures open. Normal high conductivity metal such as aluminum or copper is used for the conductor bars and by omitting alternate bars there is a saving of metal and an increase of resistance plus an increased cooling capacity in the periphery of the rotor to achieve a design class C or D motor. The foregoing abstract is merely a resume of one general application, is not a complete discussion of all principles of operation or applications, and is not to be construed as a limitation on the scope of the claimed subject matter.
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09 Feb 1970
TL;DR: In this paper, a three-phase, two-speed pole-changing induction motor using the pole changing method of pole-amplitude modulation, which has a single-layer stator winding, diamond or concentric coils being used as preferred, is presented.
Abstract: A three-phase, two-speed pole-changing induction motor, using the pole-changing method of pole-amplitude modulation, which has a single-layer stator winding, diamond or concentric coils being used as preferred. The stator coil layout is derived from the stator coil layout of a corresponding double-layer winding for the same alternative pole-numbers. The present invention provides a two-speed, three-phase alternating current electric motor having a single layer stator winding with alternative connections providing alternative polenumbers by a modified method of pole-amplitude modulation.
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29 Jul 1970
TL;DR: In this article, a symmetrical conductor switching device is used for providing the switching function with the switching device being switched to a low impedance state, effectively connecting the start winding in circuit, when a charging device is charged to a predetermined level.
Abstract: An electronic switch circuit for controlling the start operation of a single phase induction motor of the timed start type in which the start winding is disconnected a controllable time interval after the start cycle is initiated. A symmetrical conductor switching device is used for providing the switching function with the switching device being switched to a low impedance state, effectively connecting the start winding in circuit, when a charging device is charged to a predetermined level. A positive temperature coefficient thermistor is used for varying the voltage supplied to charge a charging capacitor and to vary the time constant of the charging capacitor. A phase shift capacitor connected in series with the charging capacitor causes the charging current of the charging capacitor to lead that of the applied line voltage and permit the switching device to be rendered conductive in a desired time relationship with the voltage supplied to the motor.
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01 Jul 1970TL;DR: In this article, a simple equation is given which describes the effect on these torques, and the precise amount of skew can be important, and one has to weigh the advantages against such disadvantages as an increase in loss and leakage reactance.
Abstract: The vast majority of induction motors have skewed slots, and one of the principal reasons for employing skew is to reduce the cogging tendency. It is shown that skewing can effectively achieve this purpose, and a simple equation is given which describes the effect on these torques. However, the precise amount of skew can be important, and one has to weigh the advantages against such disadvantages as an increase in loss and leakage reactance.
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TL;DR: In this article, the extinction angle of a phase-controlled induction motor is defined as a function of the firing angle and the motor parameters, and the α-β-0 transformation has been used to obtain a complete solution of the voltage equations for a constant speed of the motor.
Abstract: Phase-controlled thyristors connected back-to-back can be used to control the speed of induction motors. The motor windings carry a discontinuous pattern of current. The extinction angle of the thyristors is a function of the firing angle and the motor parameters. The α--β--0 transformation has been used to obtain a complete solution of the voltage equations for a constant speed of the motor. An approximate solution as well as the experimental results are presented.
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14 Aug 1970
TL;DR: In this article, a system employing gyroscopes driven by AC motors at synchronous speeds is described, where the motors are controlled so that their rotors always occupy the same synchronous position with respect to the rotating magnetic fields in the motors.
Abstract: In a system employing gyroscopes driven by AC motors at synchronous speeds the motors are controlled so that their rotors always occupy the same synchronous position with respect to the rotating magnetic fields in the motors. This control, which makes the drift of the gyroscope due to rotor unbalance constant, is achieved by generating a rotor marker pulse each time the rotor passes through a predetermined angular position and causing the rotor to be positioned so that this pulse occurs at a preselected point in the cycle of energization of the AC motor.
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05 Jan 1970
TL;DR: In this article, a speed control system for an induction motor includes an inverter coupled to the motor stator winding and a second induction machine with its rotor coupled to a motor shaft.
Abstract: A speed control system for an induction motor includes an inverter coupled to the motor stator winding and a second induction machine with its rotor coupled to the motor shaft. The stator of the second machine is energized with a constantfrequency voltage. The frequency of the rotor voltage of the second machine is determined by the stator voltage frequency and the shaft speed. The signal from a reference frequency generator and the rotor frequency are applied to a phase comparator and a synchronizing device, the output signal thereof being applied to a limiter. The rotor frequency is also applied to a frequency detector, the output signal of which is applied to a filter, a differential stage, a device for regulating the inverter voltage or current and a frequency generator that controls the inverter frequency. The filter passes the AC component to the limiter. The output of the differential stage also is applied to the limiter. The limiter output is applied to the frequency generator and to the regulating device to control the motor speed and to maintain the motor flux constant.
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24 Feb 1970
TL;DR: An induction motor rotor in which conducting bars and end rings are preformed ladder-like for insertion in the slots and the bars have the same circumferential widths as the lands of the laminated core between them and substantially the same radial thickness as the radial depth of the slots in which they rest as discussed by the authors.
Abstract: An induction motor rotor in which conducting bars and end rings are preformed ladderlike for insertion in the slots and the bars have the same circumferential widths as the lands of the laminated core between them and substantially the same radial thickness as the radial depth of the slots in which they rest. The bars are stamped without waste of material from heavy strip stock to provide interdigitating fingers whose length is at least 50 percent of the slot length and provide identical half-ladder units which are then shaped to the rotor slot pattern and inserted in the rotor slots where they are induction soldered for circuit continuity and conductivity.