Showing papers on "Three-phase published in 1975"

A three-phase differential relay for transformer protection

Abstract: Established harmonic restraint differential relay concepts have been enlarged upon to provide a new static multiple-winding transformer differential relay. The resulting design includes simplification of the frequency selective circuits and new techniques for rapidly determining the harmonic content of the applied currents. The relay is shown to have improved sensitivity, to be responsive to internal transformer winding faults in less than two cycles, and to the more severe type of fault in less than one cycle. Full security against inrush currents and overexcitation as well as to external faults is demonstrated. Poor performance of current transformers is shown not to adversely affect either the security or dependability of the relay. Heavy current test results are presented which confirm the relay principles.

Transistor bridge inverter motor drive having reduced harmonics

Abstract: A harmonic reduced pulse width modulation, variable speed transistor bridge inverter drive controls frequency and magnitude of stator voltage applied to an induction motor so as to maintain constant flux in the motor and permit maximum driving torque to be realized from the motor over a wide speed range. An oscillator generates a train of clock pulses whose frequency is proportional to an analog speed signal, and a three phase generator derives three phase reference waves having periods which include a predetermined number of clock pulses. A resettable volt/hertz integrator derives a train of ramp pulses which are synchronized to the clock pulses and vary in magnitude as a function of the integral of the output voltage from the bridge inverter with respect to time. The constant volt/hertz ramp signals are compared to first and second reference voltages to derive phase-displaced first and second control pulses. The fundamental output voltages of individual phases of the bridge are regulated in accordance with respective three phase reference waves, and pulse width modulating means switch the transistors in the phases of the bridge by two different width pulses whose leading edges are established by the clock pulses and whose trailing edges are respectively at the first and at the second control pulses and in such a manner that the bridge inverter applies twelve-step voltages to the motor stator windings in which the fifth and seventh harmonics are substantially eliminated and in which the voltage-to-frequency ratio is controlled to maintain constant flux in the motor.

Overvoltage phenomena associated with virtual current chopping in three phase circuits

Abstract: Switching of three phase load currents with low power factors, such as encountered in arc furnace installations can result in a series of reignitions and clearings. Depending on the switch performance and system parameters the switching transients may involve one, two, or three phases. The effect of virtual current chopping which can lead to line to line overvoltages is described. The phenomena are explained for simplified conditions, more complex situations are handled by digital computer studies.

Transistor inverter motor drive having voltage boost at low speeds

Abstract: A transistor inverter drive provides variable frequency and variable voltage power to an electric motor and pulse width modulates a predetermined number of harmonic-reduced constant volt/hertz pulses per cycle of motor frequency to selectively vary the motor stator voltage so that constant torque over a wide speed range can be accomplished and increases the number of constant volt/hertz PWM pulses per motor frequency cycle at low motor speeds to boost the stator voltage and thereby compensate for decrease in magnetizing current and in torque which would otherwise occur at low frequencies Particularly, the drive has first and second oscillators for respectively deriving trains of first and second clock pulses whose frequencies vary at different rates as a function of an analog speed signal; a frequency switch which selects the higher frequency train of clock pulses; a volt/hertz integrator which derives ramp pulses whose leading edges are established by the pulses from the frequency switch and which vary in magnitude as a function of the time integral of the inverter bridge output voltage; a first comparator for deriving a first control pulse when each ramp signal becomes equal to a first reference voltage; a three phase generator for deriving three phase reference waves synchronized to the first clock pulses and displaced 120° and each of whose period includes a predetermined number of first clock pulses; and means for controlling the fundamental output voltages of individual phases of the inverter bridge in accordance with respective reference waves and including pulse width modulation chopping means for controlling conduction of transistors of the bridge inverter by variable width PWM pulses whose leading edges are at the output pulses from the frequency switch and whose trailing edges are at the first control pulses, whereby the number of PWM pulses in each fundamental voltage cycle is controlled by the higher frequency pulse train and provides a boost in motor terminal voltage at low motor speeds

Phase sequence detector for three-phase ac power system

Abstract: The phase sequence of the phase currents supplied by a three-phase AC power supply to a phase-sensitive load (such as a three-phase AC motor) is monitored by developing three phase-displaced, rectangular shaped logic signals, respectively representing the three phase currents, and applying these logic signals to respective ones of the J, C and K inputs of a J-K flip-flop. When the load is correctly connected to the power supply, the phase relationship of the three logic signals will be appropriate to trigger the flip-flop to its set operating state. Any other sequence actuates the flip-flop to its reset state which, in turn, disconnects the load from the power supply.

Segregated phase comparison system

Abstract: This disclosure relates to phase comparison relaying systems for a three phase power transmission line in which the current is measured at each end of each phase conductor. The phasor difference of the currents in a selected two of the phase conductors is determined at each end of the selected conductors and is phase compared to determine the location of phase-to-phase faults whether or not a ground fault is involved while the phasor sum of the current in all three of the phase conductors is determined at each end of the phase conductors and is phase compared to determine the location of phase-to-ground faults.

Missing phase detection circuit for use with three-phase power sources

Abstract: A circuit for detection of a missing phase voltage condition, and certain other malfunction conditions, in the output of a three-phase A.C. alternator or power supply. Three pulse forming circuits are provided with their input ends each connected to receive a different one of the phase voltages. Each pulse forming circuit provides an output pulse signal for a duration corresponding to the excursion of its corresponding phase signal in a predetermined one of the voltage polarities of the power supply. The outputs of the pulse forming circuits are connected to individual inputs of an OR gate circuit. The OR circuit is adapted to clamp the trigger input of a retriggerable monostable multivibrator to ground during presence of a pulse output from any of the pulse forming circuits. The duration of a triggered output state of the multivibrator is chosen to be greater than the period of alternation of the phase voltages. A construction may be provided in which the pulse forming circuits and the input channels of the OR gate network are formed of opto-isolator units to achieve electrical isolation of the input and the output stages.

A controllable secondary multikilowatt dc current source with constant maximum power factor in its three phase supply line

Abstract: A type of industrial size dc converters for submegawatt applications is described and illustrated with the characteristics of a small scale 10 kW model. High conversion efficiency near 97 percent is maintained at full power and with an internal frequency of operation near 10 kHz. The system is characterized by the behavior of a true current source with inherent short circuit capability of its output terminals. All power is processed via a firmly controlled high Q series resonant circuits. Feasibility of power densities in excess of 1 kW/kg component weight is demonstrated. This high power density reflects itself in low quantities of material needed for construction of these systems. Operation of the dc converter as part of a rectifier-filter-converter system, powered from a three phase supply line is described. Test data demonstrate a high power factor p.f. ≃ 0.95 for all conditions of converter loading. Data obtained from a thyristor bridge-filter system are presented and subjected to comparative analysis.

Inverter arrangement with two controlled three-phase inverters

Abstract: An inverter arrangement comprising two controlled three-phase inverters is disclosed. The inputs of these inverters are preferably connected to a common DC voltage source, while the output voltages delivered by the inverters are of rectangular waveshape and are added together by means of transformers to form resultant output voltages. In accord with the invention, in order to obtain resultant output voltages whose harmonics are of orders which do not change when voltage control is applied, all the output inverter voltages are caused to comprise in each period a positive and a negative voltage pulse, each of which pulses has breaks or no-voltage intervals of controllable width positioned at predefined places within the pulse.

Frequency converter for three phase machines - gives twelve pulse output by use of two inverter stages

Abstract: Undesirable losses and torque modulations can be minimised by using a twelve pulse output. In a frequency converter with an intermediate d.c. stage this is achieved by adding a second output inverter connected via its own smoothing inductor to the output of the supply commutated converter. The inverter sections are arranged to have 30 deg. displacement between output voltages. The output is typically commoned by a choke coil in each phase with a centre tap from each coil feeding the load machine. The system is applicable to any sort of rotating field load.

Measurement of short circuit load losses in large three phase power transformers using an alternating current comparator bridge

Abstract: The application of a high-voltage alternating-current-comparator capacitance bridge to the measurement of the short circuit load losses of large three phase power transformers at rated current is described. Input of the transformer current to the bridge is provided through a 1000/1 ratio two-stage current transformer with a single turn primary winding Insulated for 50 kilovolts. Voltage input is provided through a loss-free gas dielectric capacitor which serves as the reference. Measurements on typical transformers ranging in size from 450 MVA to 750 MVA; with currents from 600 to 2100 amperes and short-clrcuit power factors from one percent to four percent, are discussed. Comparisons to appropriately corrected wattmeter measurements (aided by low-loss capacitors connected in parallel with the transformer) indicate an overall agreement of better than one percent.

Active symmetrical component network for protective relays

Abstract: A circuit responsive to selected symmetrical components of current in a three phase alternating current system. The component values of the circuit can be selected so that the circuit output is proportional only to the negative sequence components of current, only to the positive sequence components, or to both the positive and negative sequence components of current. The positive sequence response is 180° out of phase with the negative sequence response and the amount of positive sequence response can be adjusted without affecting the magnitude or phase of the negative sequence response.

Three phase a.c. motor drive system

Abstract: A system for controlling the speed, torque and power of three phase a.c. motor. A binary logic circuit generates a variable frequency three phase signal which controls three driver pairs in an inverter to selectively apply either the positive voltage or the negative voltage of a direct current power source to each of the three motor phases. The three phase binary driver control signal is pulse frequency modulated below 60 cycles per second to operate the motor in a constant torque mode without varying the rectifier output voltage. The logic control signal also includes a delay to protect the drivers by assuring that one driver of a pair is "off" prior to turning the other driver "on". In addition the system comprises high response voltage and current limiting circuits to protect the system and to reduce the overcapacity requirement of the components. An accelerator/decelerator circuit achieves a nearly linear characteristic through the use of switching transistors to permit substantial cost savings.

Low power solid state three-phase overcurrent/undercurrent protection circuit

Abstract: A three-phase current sensing fault detection network has been developed employing an all solid state design. Systems equipped with such a network will be deprived of three-phase power in the event that either excessive or insufficient current is monitored in any or all of the three-phase power lines. The system design can be expanded to include a system shutdown capability for any number of sensing inputs from various critical portions of the circuit being monitored.

Three-phase brushless DC motor

Abstract: A three-phase brushless DC motor has three power transistors with their collector-emitter paths connected to the respective stator windings, and two Hall effect devices each having a pair of control current electrodes and a pair of Hall voltage electrodes. One of the Hall voltage electrodes of the Hall effect devices is connected to the base electrodes of the first and second power transistors and the other Hall voltage electrodes are connected together by a combining network which delivers a resultant voltage to the base electrode of the third power transistor. A voltage dividing and balancing network is connected between the base electrodes of the first and second transistors and between the control current electrodes of the Hall effect devices to drive the stator windings with three-phase commutating currents of equal amplitude.

Method of stepwise speed control and three-phase squirrel-cage motor

Abstract: A method of stepwise speed control, and three-phase squirrel-cage motor, that comprises main windings connected either in a delta or in a star circuit, the main windings being connected to at least one set of additional windings that are also connected either in delta or in star. The method permits at least five different speeds of rotation without the use of a transformer.

Three phase controlling system

Abstract: In a three phase load controlling system two bilateral switching circuits control the current in the first and the third phases; the first phase is later than the third. The second phase is uncontrolled. The gating signals applied to the bilateral switching arrangement connected in the first phase are advanced by a constant degree with respect to the voltage of the first phase. The gating signals applied to the bilateral switching arrangement connected in the third phase are synchronized with the voltage of the third phase.

Three phase full wave DC motor decoder

Abstract: A three phase decoder for DC motors is disclosed which employs an extremely simple six transistor circuit to derive six properly phased output signals for fullwave operation of DC motors. Six decoding transistors are coupled at their base-emitter junctions across a resistor network arranged in a delta configuration. Each point of the delta configuration is coupled to one of three position sensors which sense the rotational position of the motor. A second embodiment of the invention is disclosed in which photo-optical isolators are used in place of the decoding transistors.

Method and apparatus for compensating reactive power

Abstract: An arrangement for compensating reactive power in a three phase network using two converters with the same no load d-c voltage in which the two converters are coupled in parallel on the d-c side and shorted by a common choke with the one converter operated with a delayed firing time and the other with an advanced firing time and a d-c current in the choke regulated to a constant value by influencing the firing angles of the two converters in opposite directions.

Speed controllable drive for an asynchronous motor

Abstract: A speed controllable three phase drive including an asynchronous motor having a slip ring rotor which can be connected to a three phase network and a d.c. motor mechanically coupled to the asynchronous motor with the d.c. motor fed through an uncontrolled rectifier connected to the slip ring of the asynchronous motor has a controlled static converter in series with the uncontrolled rectifier and d.c. motor. In an alternate embodiment the d.c. motor is replaced by a static converter motor fed by a controlled inverter connected to the uncontrolled rectifier. The controlled static converter is coupled to the three phase network to result in a three phase drive with a greater speed range and a lower reactive power requirement as compared to subsynchronous static converter cascades.

Method of reducing current unbalance in a three-phase power transmission line operating with one faulty phase

Abstract: A method for reducing current unbalance in the electric system of a three-phase A.C. power transmission line operating in an incomplete phase regime using the wires of the line and transformers electrically coupled thereto. A disconnected wire of the line operating in the incomplete phase regime is grounded at one end thereof and a suitable alternating voltage source is connected at the other end thereof to increase the current flowing through the transformer neutrals, thus reducing current unbalance in the electric system. An embodiment of the invention provides for the use of a lightning protection wire rope instead of the wire in case the latter is broken.

Hand held variable speed drill motor and control system therefor

Abstract: A high speed hand held polyphase motor and a control system therefor is disclosed. The motor is of the induction type, designed for use in oral surgery and the like. The control system includes a bridge type inverter having a separate pair of solid state switching devices such as transistors connected in a bridge configuration across a pair of direct current power supply conductors for each phase winding of the motor. A switch control circuit is connected between the solid state switches and a variable frequency clock signal generator for turning the switches of each pair on and off in a given phase sequence to provide three phase current for the motor windings. The speed of the motor is proportional to the clock signal frequency. A voltage regulating circuit is connected between the power supply conductors and a source of direct current voltage to supply a direct current voltage across the supply conductors which varies in accordance with the clock signal frequency over a pre-established range. The control system further includes a torque demand adjustment circuit coupled to the voltage regulating circuit to permit the voltage across the supply conductors to be independently adjusted over a preset range of clock signal frequency.

Drive for automatic door opener - has single or multiphase motor whose speed is varied by phase angle shift

Abstract: The system is used for swivelling hinge doors or for sliding doors. The door is rapidly opened and its last stage in the opening movement is relatively slow. The speed is controlled by using a single or multiphase alternating current motor whose rotating speed can be adjusted by supplying direct current components and alternating current components. The direct current proportion can be varied by a phase angle shift actuating system. This arrangement simplifies the actuating system and a small number of electrical components eliminate the use of a braking cylinder. The motor has a pole change arrangement for changing the rotating direction. Separate phase angle shift control systems are used for determining the basic speed on opening and closing and for establishing the damping speed at the end of the opening and closing movement. The phase angle shift control systems are connected to different phases of the three phase current supply.

System and method of measurement of insulation qualities of three-phase power equipment

Abstract: Three-phase power equipment while still connected to the three-phase line has the insulation qualities of its various phases tested. All ground connections of said equipment except one are disconnected and that one is reconnected through ground current sensing means. Coupling units which may be capacitors are respectively connected between each of the three phases of the power line to which the power equipment is connected and ground. Each coupler unit is a reference phase current sensing means. Adjustable phase and magnitude adjustor means are used to couple the respective phase current sensing means to a summing means so that they are adding. The ground current sensing means is similarly coupled to the same summing means but opposed to phase current effects. A null detector coupled to the summing unit detects when balance occurs. Each of the phases is checked in sequence using fixed predetermined settings of the phase and magnitude adjusting means for the other two phases and separately and sequentially varying the settings of capacitance and dissipation factor for the phase under test until nulling occurs. Deviation in the settings of the adjustable phase and magnitude adjustor means from previous settings are considered in determining whether changes suggest deterioration of insulation and in which phase such deterioration may be occurring.

Compacting of concrete using vibrating blocks - involves using electric motor-driven imbalance vibrators using three phase current

Abstract: Concrete is compacted by using a three-phase current having a frequency which varies either continuously or in small steps, to drive the electric motors of imbalance vibrators. The amplitude of the current is variable in dependence upon the frequency. The vibrators comprise at least one imbalance weight mounted non-rotatably on the electric motor shaft and at least one other imbalance weight which rotates in an adjacent region on the shaft defined by a stop fixed to the shaft and ends stops. The rotating weight moves freely between the fixed stop and its opposite end position, or is released against an elastic retaining force, and is also freely movable in its rotational area, so that centrifugal force of the vibrator may be controlled by changing the direction of rotation or countercurrent braking of the motor by reversing the current phases.

Detector for direct current - gives instantaneous reading of current in rectifying networks without voltage reaction

Abstract: The apparatus is used in a bridge type rectifier network fed by three-phase a.c. The load is supplied by d.c. from switching rectifiers so that a variable current is produced. To measure the current, a transformer is used, producing a three phase a.c. proportional to the a.c. input. The three-phase a.c. from the rectifier is rectified, using a set of non-switching rectifiers. A resistor is connected across the measuring rectifier bridge output. A seco nd resistor is connected to one of the terminals and to an amplifier, with its positive terminal connected via a resistor to earth. A resistor and capacitor are connected in parallel across the amplifier.

Parallel-coupled static inverters - supply three phase load under control of single unit via transformer windings

Abstract: The static inverter circuit consists of two or more, parallel-coupled static inverters (1, 2) supplied from the same d.c. network (N) and controlled by only control unit (St). The 3-phase outputs of the two inverters are coupled to opposite ends of the 3-phase transformer's three windings. Tappings along these windings lead off to the single 3-phase load (L). If three inverters are to be run in parallel, then the tappings of the windings between the first two inverters are coupled to the ends of a second 3-phase transformer's windings. The second transformer's opposite ends are connected to the third static inverter's outputs.

Asynchronous motor operation circuit - uses three phase invertor and compares nominal and actual currents

Abstract: The circuit has a measurement element for load current, a reference element, a switching stage for reference and control currents comparison and for derivation of switching signals. It has further a control unit for switchable rectifiers of the inverter. Switching signals trip in a first operational zone the invertor between driving voltage and zero voltage, and in a second operational zone between zero voltage and counter voltage. Depending on the position of the reference element, second operational zone is blocked when driving is required, and first operational zone is blocked when braking is required.

Three-phase heating circuit for continuous-flow water heaters - with one triac has one pair of contacts to control star-coupled heating elements

Abstract: The three-phase heating circuit, for continuous flow water heaters, has three star coupled heating elements and only one triac. The triac acts as a power switch and is located between one heating element and the phase supply line for that element. The triac is coupled by its gate to a control circuit actuated by a pair of mechanical contacts. The contacts belong to a pressure difference switch which responds to water flow. The control circuit also operates an auxiliary relay in one of the other phase supply lines. The circuit uses a reduced number of components.

Electronic three-phase four-wire system watt-hour meter

Abstract: An electronic three-phase four-wire system watt-hour meter equipped with a single operational circuit which multiplies the voltage and the current of each phase of a three-phase alternating current power source of which electric energy is to be measured. Electronic switches are provided which work on voltage and electronic switches are provided which work on current in each phase. Said electronic switches are cyclically opened and closed by three-phase pulse signals provided for control. The operational circuit produces, in the form of a current, an output corresponding to the sum of the electric power of each phase of said three-phase alternating current source.