Showing papers on "Three-phase published in 1974"

Method and control apparatus for static VAR generator and compensator

Abstract: A static VAR generator circuit having a control system adapted to utilize the values of three phase load currents to generate time delayed firing angles for thyristor controlled inductors which are utilized with parallel capacitors to provide unity power factor and balanced electrical currents to the AC source. The time delayed firing angles are calculated from integrating furnace load currents over prescribed intervals during real time to thus maintain balanced load current at specified phase angles, usually zero, in the three phase system.

Calculation of Losses in Steel Enclosures of Three Phase Bus or Cables

Abstract: This paper derives a method of calculating losses in steel enclosures that contain three equilaterally spaced balanced three phase conductors. Results are presented in a series of graphs which can be used for design purposes. Experimental data on a 30 inch (76cm) steel sheath confirmed the theoretical calculations.

A Comparative Study of Symmetrical Three-Phase Circuits for Phase-Controlled AC Motor Drives

Abstract: The speed of an induction motor in certain types of drives can be adjusted by phase control of the applied voltage. Three pairs of inverse-parallel thyristors are required for three-phase bipolar symmetry, but several different arrangements of the thyristors and motor windings are possible. A direct series connection of thyristors and windings in each phase can be made; the three phases can then be connected in delta, wye, or wye with neutral return. Alternatively, a delta connection of thyristor modules can be inserted in the opened neutral junction of wye-connected windings. Representation of the motor as a counter EMF in series with leakage reactance allows a simplified analysis of the current waveforms in terms of the thyristor conduction angle. With a given fundamental current, the current harmonics in the motor windings and in the supply lines for each of the circuits can then be compared. The results are equally applicable to thyristor-controlled inductive loads such as may be used for reactive power adjustment.

Electric heating element control circuit

Abstract: A circuit for connecting to an alternating current electric network having at least one phase a high-power electric heating device, such as a sauna heater, provided with a plurality of electric heating elements. Each phase of the network is evenly divided into a number of separate part leads such that the total number of part leads are arranged in two equal series. A first switch is arranged to simultaneously open and close all of the part leads of both series. A second multipolar switch controlled by a thermostat is adapted to open at least one of the separate part leads of one of the series. Each heating element is connected in circuit with one part lead from each series of part leads. The network may be three phase with the heating elements being delta-connected.

D.C. voltage transformation in high tension systems

Abstract: A transmission substation for transforming a high D.C. voltage over an intermediate three phase A.C. network into a lower D.C. voltage. The substation includes an arrangement of inverters and rectifiers for effecting the transformation with reduced losses. Also encompassed is a similar reversed arrangement for stepping up from a lower D.C. voltage to a higher D.C. voltage.

Apparatus for generating three phase sinusoidal voltages

Abstract: Apparatus for inverting input DC source voltage to generate three-phase, sine-wave, line-to-line output voltages. A first inverter produces three-phase, flat-top, full-wave voltages in relation to a point of reference potential on three-phase output conductors and a second inverter operating in synchronism with the first inverter generates a single-phase, flat-top voltage at three times the frequency of the output of the first inverter. A transformer having a plurality of voltage taps is coupled between the point of reference potential and the second inverter. First and second banks of voltage level selector controlled rectifiers are coupled to the voltage taps and are selectively gated to produce alternate pairs of stepped ascending and descending voltages which are selectively coupled to the three phase output conductors by voltage distributors to complement the three-phase, flat-top, full-wave voltages to complete the line-to-neutral, three-phase waveforms. These waveforms combine to form threephase, line-to-line voltages. The voltage level selector controlled rectifiers which are commutated naturally during generation of the stepped voltages are coupled directly to the voltage distributors and the voltage level selector controlled rectifiers which require auxiliary commutation are coupled to the voltage distributors through commutating switches. Current flow through the commutating switches is reduced by capacitors coupled across the three-phase output conductors to provide for a leading load power factor. The capacitor values may be selected so as to reduce the current through the commutating switches to zero for all expected load conditions to thereby allow the elimination of the commutating switches and the level selector controlled rectifiers coupled thereto.

Current sensing protector system for A. C. motors

Abstract: The specification discloses a system for preventing current overload in a three phase dual speed A. C. motor in both high and low speed modes. The system includes current transformers for sensing the current level in each of the three phases applied to the motor. Circuitry is provided to detect the occurrence of a current level above each of a plurality of levels depending upon the speed of the motor. Circuitry is responsive to the detection of an overload current level in order to operate a switching transistor to terminate conduction of a triac in order to terminate the current flow through the phases. A light emitting device is connected to the switching transistor to emit light upon the occurrence of the overload condition. A light sensor senses operation of the light emitting device and initiates a timing cycle by a timing circuit during which the triac may not be rendered conductive.

Arrangement for synchronizing static thyristor converters for supplying three phase motors

Abstract: An arrangement for synchronizing two static converters supplying three phase motors so that one converter may be taken off the line and the other put on the line without disturbance in which the firing pulses for the thyristors in the converters and generated in the converter control unit are used to make phase and frequency comparisons in order to initiate switching.

Sources of harmonic currents in slip-ring induction motors

Abstract: Several features which could cause an unsaturated 3-phase induction motor to draw nonsinusoidal currents from a supply of negligible harmonic content are examined. An analysis is developed which provides a simple method for the prediction of the harmonic-current magnitudes and frequencies. A comparison of predicted and measured current harmonics is presented to substantiate the theory.

Arc-welding power source

Abstract: A multi-phase SCR-rectifier controlled arc welding power source having improved operating and internal protection characteristics. The transformer has an improved core and winding design having a built-in internal reactance and a core arrangement which is scrapless and very compact. The output has an adjustable, drooping, volt-ampere curve such that the welding current may be readily preset or adjusted to any desired value. The power source includes an unusually high inductance for a three phase source so as to give a very stable arc with a control circuit having provisions to properly control the conduction angle of the SCR's so as to overcome the disadvantages of such a large choke on starting or short circuiting so as to give optimum welding characteristics. Further the ratio of short circuit current to welding current is controlled and decreases with an increase in the welding current. Protection against damage of the power source is provided in the event of extended short circuit or overload. Compensation for line voltage variation is also provided.

Method and apparatus for fault detection in a three-phase electric network

Abstract: The voltage-to-ground or neutral of one phase conductor of a three-phase line is compared with the mesh or interphase voltage of the other two phases on an amplitude basis and the comparison result is used to trigger a threshold switch if the former is much smaller than the latter. The threshold switch may be connected to the selector of an excitation system of a network protection assembly or a nonselective excitation system may utilize the outputs of the threshold switches for each of the three comparisons, in order to produce selective triggering of a network protection assembly.

Design of a thyristor frequency converter aided by computer simulation

Abstract: This paper describes a method of designing a thyristor frequency converter for a source of induction heating, which converts three phase input power of power frequency to single phase high frequency power without using the intermediate DC link How to determine the circuit constants and the rating of necessary devices such as inductors, capacitors, and thyristors is described with circuit analysis This design is carried out by the combination of calculating some approximated mathematical expressions and using computer simulation It is shown that this method is successful in quick determination of parameters

Selsyn digital transducer

Abstract: A selsyn adapted for operation as a digital transducer. The three phase winding of the stator is connected as a primary. The rotor, having a single phase winding, is connected as a secondary. A three phase bridge circuit is electrically coupled to the three phase winding. Optical couplers are coupled to the three phase bridge circuit to deliver six conduction pulses per cycle. A detector of the rotational displacement intelligence of the rotor is coupled to the single phase winding to deliver an output pulse as a function of the alignment of the single phase winding with one of the three phase windings, the alignment occurring every 60* of rotor revolution. A logic circuit is coupled to the optical couplers and to the rotational displacement detector for identifying the coincidence of the six conduction pulses with one of the output pulses respectively, each coincidence delivering a digital bit which is a function of the rotor displacement.

Steady State Analysis of Thyristor Controlled Three Phase Induction Motors Using State Space Techniques

Abstract: The steady-state analysis of a three phase induction motor with a pair of back to back connected thyristors in each line is presented. A mathematical model suitable for the analysis is developed. The model retains the stator currents unaltered, whether the neutral is connected or not. This facilitates the application of current constraints introduced by thyristors and of boundary matching conditions needed for the steady-state analysis. State space modelling is employed and the initial condition vector pertaining to steady-state is obtained directly. The need for digital simulation of the characteristics of thyristors and integration from arbitrary initial conditions are avoided. The method is well suited for digital solution, easy to programme and fast in execution.

Three-phase arc furnace - with device for determining arc voltages has comparator integrators and differentiating elements

Abstract: Comparators measure the difference between the phase voltage with respect to the neutral point in the melt, and they measure the sum of active voltage drops on the corresponding connection, and the sum of reactive voltage drops proportional to the derivatives with respect to time of currents in the other two connections. A magnetic volta-meter followed by an integrator is allocated to each connection. A current transformer followed by a differentiating element is provided in each phase. A wire parallel to the connections couples the melt neutral point with the measurement point for the phase voltages and is run as far as possible from the furnace; measurement wires directly connected to the furnace transformer terminals are pref. twisted together.

Three-phase arc furnace - has device for arc voltages measurement, using comparator measuring phase voltages

Abstract: The phase voltages are measured with respect to the melt which is the neutral point, and the difference is formed in separate comparators between the phase voltage and the sum fo teh active voltage drop in the corresponding conductor, and reactive voltage drops proportional to derivatives with respect to time of currents in the other two connections. Corrections are applied to the comparators derived in electronic circuits from the shift of zero crossings of arc voltage and current produced by change of connection positions, resulting in a change or reactive voltage drops.

Two and three-phase oscillators with all-pass network having amplitude responsive time constant

Abstract: Two-phase and three-phase electronic oscillators comprising an integrator coupled in a loop with an all-pass network. The input resistor of the integrator is an incandescent lamp filament having a positive temperature coefficient of resistance, which adjusts the time constant of the integrator as a function of the amplitude of the oscillator voltages, thereby enforcing the condition for oscillation. In the two-phase version, the input and output voltages of the integrator are precisely in quadrature, and have precisely the same amplitude. In the threephase version, an amplifier having a gain equal to square root 3 is interposed in the feedback path between the output and input of the oscillator to provide three voltages which are 120* out of phase and of precisely the same amplitude.

A New and Simple Three-Phase Thyristor Inverter for Variable Frequency Operation

Abstract: A new three-phase inverter circuit is described and analyzed for the condition of an inductive load. The circuit does not require an auxiliary supply or any auxiliary thyristors to assist commutation. All commutation capacitors are placed across the dc side but are used in the same way as capacitors that are normally used for ac side commutation. The output voltage of the inverter is a quasi sine wave controlled by a simple firing circuit. The author was a visiting Research Fellow with the Department of Electrical Engineering and Electronics, the University of Manchester Institute of Science and Technology, Manchester, England.

DC arc welder unit - with constant power regulation using variable firing angle control stages

Abstract: The main current supply to the welding electrode and work piece is through a halfwave-phase control- rectifier circuit using thyristors and in the case of a three phase mains connection the rectifier configuration is six phase with balancing choke. An auxiliary phase controlled (ie VFA) rectifier unit with thyrectors is used to feed a fixed load resistor and provides constant power control by using the difference voltage between the p.d. a cross that load and an adjustable reference p.d. as the control input to the trigger pulse generators which fire the thyristors. Improved quality weld due to accurate smooth regulation.

Calculating the efficiency of three-phase induction motors

Abstract: A method for computing the efficiency of a three-phase induction motor using the input phassor currents for blocked motor, no load, and running with load, is presented.