Abstract: In a current-type converter system in which a DC current is fed to a load through a pulsating current suppressing DC reactor from a current-type converter constituted by self-extinction elements connected in a bridge circuit, a converter protection apparatus is arranged such that when a fact that a terminal voltage of the DC reactor becomes an overvoltage is detected, the self-extinction elements are turned on so as to form a closed circuit including the DC reactor, the load, and the converter so that the overvoltage can be suppressed and continuous operation of the converter can be carried out.

Abstract: A regulated chopped DC power supply comprising an alternator with a stator and a rotor, the alternator having a three-phase winding for providing a high voltage output (1, 2, 3) and a star-connected three-phase winding for providing a low voltage output. Also included is a control circuit (14) for controlling a rectifier (12) which is connected to the high voltage output (1, 2, 3) of the alternator to produce a regulated chopped dc output (52) for powering electrical appliances. The control circuit (14) periodically interrupts the rectifier (12) at a frequency to interrupt substantially simultaneously all of the phases of the high voltage output.

Abstract: The invention relates to a supply circuit for an X-ray emitter usable in radiology. The circuit has a transformer, whereof the primary is connected to the three-phase power supply and whereof the secondary is connected to the X-ray emitter. The primary circuit is put into operation by the closing of a controlled switch. This switch has a set of closing thyristors. The switch also has a correctly charged capacitor connectable to the set of thyristors. For opening the circuit, the capacitor is connected in such a way that it supplies a reverse current to the thyristors of the set of thyristors. Thus the capacitor is recharged with the reverse polarity. During a following closing - opening sequence of the switch, the terminals of the capacitor are connected to the set of thyristors by switching their connection. In this way, the capacitor is always correctly connected for fulfilling its function during the blocking of the thyristors.

Abstract: An apparatus of the invention includes an AC power source for providing AC electric power, a thyristor converter, being formed of self-extinctive switching elements, for converting the AC electric power to a DC excitation current, a coil for storing magnetic energy corresponding to the excitation current, and a bypass circuit coupled in parallel to the coil. The above appartus uses a method having the steps of: (a) circulating the excitation current between the thyristor converter and the coil when the excitation current reaches a reference value, and (b) commutating the circulated excitation current from the thyristor converter to the bypass circuit after a given elapsed period of time from when the excitation current reaches the reference value, so that the coil generates a high voltage or induces a large current corresponding to the magnitude of the stored magnetic energy.

Abstract: A method and apparatus for converting the output of a polyphase AC machine into a DC current by capacitor assisted line commutation within the rectifying circuitry.

Abstract: The present invention provides an optically controlled power converting apparatus using light trigger/light quench electrostatic induction thyristors, as switching elements, which can execute the switching operations at a high speed being when they are supplied with light trigger pulses and light quench pulses. By supplying the light trigger pulses and light quench pulses at the timings corresponding to the pulse width modulation, the light trigger/light quench electrostatic induction thyristors can perform predetermined power converting operations.

Abstract: In order to produce direct voltages with greater flexibility, a rectifier (1) according to the invention comprises an alternating voltage measuring sensor (25), a comparator (22), controllable switches (160 to 183) and a switch control (20), through which the switching states of the switches are alterable in dependence on the output signal of the comparator, in order so to connect at least one of the direct voltage outputs (11, 12, 13) for certain time spans during each alternating voltage phase one after the other with the alternating voltage inputs of the rectifier that a pulsating direct voltage drops at it relative to a fixed or variable direct voltage potential. The switching instants are determined through constant measurement of the instantaneous alternating voltage values and their comparison with predeterminable direct voltage levels or alternating voltages, which serves as reference values.

Abstract: A current-source power converting apparatus having a three phase AC-DC converter composed of self-extinction devices for converting an AC power furnished from an three phase AC power source into a DC power and a DC-AC inverter connected with the AC-DC converter through a DC reactor for re-converting the DC power into a three phase AC power to supply the re-converted power for a load. When the failure of the AC power source is detect according to one embodiment, the AC power source is detached from the AC-DC converter and a battery is connected between arbitrary two phases at the input end of the AC-DC converter. After that, the DC power of the battery is supplied for the DC-AC inverter intermittently by switching the corresponding self-extinction devices of the AC-DC converter and controlled by varying the duty ratio of the switching operation.

Abstract: A regenerative d-c power supply comprises a bridge rectifier, a series-connected smoothing choke, a shunt-connected filter capacitor and a switching circuit consisting of a first diode disposed in a positive line of the d-c power supply between the smoothing choke and one terminal of the capacitor, a second diode disposed in a negative line of the power supply between another terminal of the capacitor and the rectifier and a pair of converter valves in the form of transistors connecting the cathodes of the diodes to one another and the anodes of the diodes to one another. Each converter valve is operated in the forward energy transmission direction by a respective first pulse sequence having pulses staggered in time with respect to the pulses of the other first pulse sequence. The converter valves are operated in the reverse energy transmission direction by respective second pulse sequences each having pulses staggered in time with respect to the pulses of the other second pulse sequence.

Abstract: A microprocessor determines the zero cross-over times and direction of the phase voltages of the AC power line; a multiplexer has three parallel inputs which are respectively connected to the three phase conductors of the line and an output delivering a voltage formed by successive portions of the phase voltages; a flip-flop sensitive to the change of sign is connected to the output of the multiplexer; and EXCLUSIVE OR gate has an input connected to the output of the flip-flop and an other input to an output of the processor; and a NAND gate has an input connected to the output of the EXCLUSIVE OR gate and another input which receives a signal formed of a succession of square waves forming enabling windows.

Abstract: A way of placing chokes with air core, such as thyristor commutating chokes, in a three-phase alternating current rectifier. With a view to eliminating the drawbacks of air-core chokes especially in three-phase rectifying applications, the three chokes of three phases, wound in the same winding direction, are placed on one and the same plane at the corner points of an imagined triangle which brings the chokes as close together as possible.

Abstract: A process and device are provided for determining the mean value of the current delivered by a thyristor rectifier bridge fed by a multiphase AC network. Said device comprises successively an operational amplifier whose inverting input receives a signal representative of the current supplied by the bridge and is connected to its output by a capacitor, a blocking sampler and an analog-digital converter. The switch and the blocking sampler are controlled by a microprocessor so as to obtain an integration cycle and end of cycle sampling during a period zone of value (T/6), (T) being the period of the network.

Abstract: A dv/dt protection circuit device for an AC-DC converter apparatus including control board switching arrangement having a current cut-off function which conducts an interrupts current from a power source to a load and refluxes the load current while the current is uninterrupted by a full-wave bridge rectifier circuit having a plurality of arms. Each arm of the bridge rectifier circuit connects a reactor for suppressing a rate of change of a power source line voltage of the bridge rectifier circuit at the switching time of the switching arrangement and a condenser is provided for absorbing an overvoltage generated at an inductance on the side of the power source, the condenser being connected on the side of the power source connected to the bridge rectifier circuit.

Abstract: A ramp generator includes a master counter repeatedly ramping through for ramp-intersect operation of a digital pulse generator, while firing at the intersection, or when above the reference will cause a complementary digital change to define with the master counter count a ramp count on a subsequent ramp and adjacent zone below, thereby to adjust rapidly to a fast change of reference.

Abstract: Disclosed is a control circuit for a power converter apparatus using self-extinction devices as switching devices, in which turn on and turn off operations of the self-extinction devices are controlled by gate pulses which have been subject to pulse width modulation with a pulse train having a pulse width which is sinusoidally varied, and, at the same time, the self-extinction devices in same phase are made conductive in a predetermined period corresponding to an output voltage reference value to make an output of the power converter apparatus be in a shorted state in the sense of DC current to thereby control the output of the power converter apparatus.

Abstract: An SCR firing circuit allows the firing point to be changed. The firing circuit includes a ROM which has an output for each gate of each SCR. The ROM is programmed to fire the SCRs in a desired sequence. The ROM is driven by a counter, which counts pulses from an oscillator, the frequency of which is variable. The difference between the zero cross of the waveform and the ROM output is compared and averaged. The comparator output is summed with a variable DC demand voltage. The sum is applied to an integrator, which provides an integrated output to the oscillator to change the frequency of the oscillator if the firing point is changed.

Abstract: A turn-off circuit for GTOs has a three-phase, full-wave rectifier. The output voltage of the three-phase, full-wave rectifier is applied as a turn-off power source to a charge/discharge capacitor.

Abstract: Power delivered to a load by a three-phase supply is controlled by six thyristors which are controlled by a control signal generator comprising a programmed microcomputer (2), a zero-crossover detector (3), a clock pulse generator (6) and a counter (7) so that the thyristor conduction angles are determined by a digital signal applied to an input (17) of the microcomputer. In order to obtain a fast response to changes in the supply frequency and to take into account any unbalance between the supply phases, the microcomputer repeatedly measures the actual length of each nominally 60.degree. portion of a complete period of the supply by starting the counter at a predetermined point within each portion, the counter subsequently being stopped by the output of the detector when the next zero-crossover occurs and then being read by the microcomputer.

Abstract: An AC-to-DC power conversion by connecting GTO elements to the positive arms of a three-phase full wave bridge circuit and by repeating the mode of supplying power to a load from an AC power supply by controlling the GTO elements, and the mode of making the current flowing to the load flow back at the time of interruption of the power supply mode, when the power supply mode and the flow-back mode are switched over, overvoltage at the time of the switching is suppressed by simultaneously causing the power supply and flow-back currents to flow.

Abstract: A microprocessor-based solid state AC-DC converter system makes use of the microprocessor facility to implement sequential thyristor firing at selected delay angles. A first timer at the AC line fundamental frequency supplies to the microprocessor a numerical master ramp count, from which individual ramp counts are calculated which relate to the individual thyristors to be fired in sequence. From such an individual ramp count by comparison with a delay angle reference count the microprocessor calculates the number of units of time needed until zero count on the individual ramp. A second timer is preset at an initial count value equal to such calculated number of units of time and the second timer is run to count-down. The thyristor next to be fired is triggered when zero count occurs.

Abstract: A phase control device for a power converter with a plurality of controlled rectifier elements, including a phase detector for detecting a phase of an AC voltage of the power converter to produce a phase signal. A processing circuit for receiving an electrical quantity appearing at an AC side or a DC side of the power converter as a feedback signal and for comparing the feedback signal with a reference signal to produce a phase control signal so that the feedback signal becomes equal to the reference signal, and a firing pulse determination circuit for comparing the phase signal with the phase control signal to produce a plurality of firing pulses, which are adapted to be connected to selected of the controlled rectifier elements to fire the controlled rectifier elements. The processing circuit includes a monitoring circuit which judges whether or not the phase control device has failed based on the phase signal, the phase control signal and a specific firing pulse from the firing pulse determination circuit.

Abstract: A phase detecting apparatus comprises a first PLL and a second PLL having a response time shorter than the response time of the first PLL. An AC voltage signal produced by the first PLL which is in phase with the detected AC voltage except until the expiration of the response time of the first PLL after an abrupt change in the phase of the detected AC voltage is normally applied to the second PLL. Until expiration of the response time of the first PLL after rising of the detected AC voltage to a certain level, the detected AC voltage is applied, in place of the AC voltage signal produced by the first PLL, to the second PLL. The second PLL provides a digital signal indicative of the phase of the detected AC voltage.

Abstract: A power converter utilizes first and second thyristor bridges energized by two AC power sources which are insulated from each other. The power converter includes first and second auxiliary thyristor circuits each containing plural thyristors, and a commutation circuit. The cathode of each thyristor of first auxiliary thyristor circuit is connected to one of the AC power sources, and the anode of respective thyristors are connected each other at a common node. The anode of each thyristor of second auxiliary thyristor circuit is connected to the other of the AC power sources, and the cathode of respective thyristors are connected each other at another common node. The commutation circuit is connected between the common nodes of the first and second auxiliary thyristor circuits.

Abstract: A multiple output DC power supply having a single closed loop control system for two or more power amplifiers which provide matched DC outputs. The control system is responsive to the sum of the DC signals output from the power supply to derive an average error signal. A switching logic circuit compares signals representing the unregulated AC power driving the power amplifiers to a reference signal in order to generate a set of control signals for each of the amplifiers, the average error signal biasing the reference signal so as to vary the time of occurrence of the control signals and regulate the DC outputs.

Abstract: A control circuit for a power converter which converts between polyphase periodic power and unidirectional power is disclosed. The control circuit includes first means for sensing a parameter of the periodic power and a ramp generator coupled to the first means for generating a ramp signal having a frequency equal to the frequency of the periodic power. A second means senses a parameter representative of a condition at the output of the power converter and a comparator combines the outputs from the ramp generator and the second means to generate a pulse train having a frequency equal to the frequency of the periodic power but shifted in time relative thereto, with the shift being dependent upon the output parameter. A phase-locked loop, of PLL, multiplies the frequency of the pulse train and additionally develops a signal which is coupled to the ramp generator to maintain the amplitude of the output therefrom at a constant level irrespective of changes in frequency of the periodic power.

Abstract: A method of adaptively controlling the time of rendering the individual controlled devices of a controllable power conversion bridge includes the sensing of the instant in time at which the current from the bridge to a load becomes discontinuous. From the time of this sensing, the time period to a predetermined time such as a normally calculated time of rendering the controlled devices conductive is derived. This time period is then multiplied by a multiplier and the resultant product is combined with the predetermined time to provide an actual time of rendering the controlled devices conductive.

Abstract: The power delivered from a three-phase A-C power source and through a network of SCR's (connected, for example, to form a full wave rectifier bridge or to provide an A-C switch) to a load circuit is adjusted by controlling the conduction angles of the SCR's. The control is achieved by developing, from only one of the three alternating phase voltages provided by the A-C power supply, a reference square wave signal of the same frequency and whose amplitude excursions are precisely synchronized or keyed to the zero voltage crossings of the selected phase voltage, even though that phase voltage may include undesired harmonics, noise, transients or other distortion components.

Abstract: A high response motor controller is disclosed which operates directly from a multi-phase AC power line without a DC link. The controller directly converts power from a fixed frequency multi-phase input to a D.C. or variable frequency output current. The output frequency is not limited by the AC power line frequency and the switching frequency is independent of the AC power line frequency. Controlled switching devices are connected between the multi-phase power input line and the power output line. Analog input signals are compared with signals representative of actual motor currents to produce current error signals. These current error signals control the conduction state of the controlled switching output transistors by a two state modulation process.

Abstract: A method and apparatus for use in conjunction with a power conversion system employing a plurality of switching elements for controlling the electrical power supplied to a load from a source includes the sensing of the instantaneous voltage furnished to the load and providing a signal representing the time derivative of the sensed voltage. The derivative signal thus developed is compared with predetermined reference values to develop output signals which are logically combined to provide a final signal representative of the instant in time at which one of the switching elements is commutated (turned off).

Abstract: A superconduction power storage apparatus includes a superconduction coil for storage and release of electrical energy, and a separately excited thyristor conversion unit having d.c. terminals connected to the said superconduction coil and a.c. terminals connected to an a.c. power source. The conversion unit is arranged so as to convert the electrical energy from a d.c. form to an a.c. form and vice versa between the superconduction coil and the a.c. power source. An on-load voltage regulator is connected between the a.c. side terminals of the thyristor conversion unit and the a.c. power source, and is arranged for adjusting the voltage appearing at the a.c. terminals of the thyristor conversion unit.

Abstract: A microprocessor-based static converter control circuit includes a free-running counter which is synchronized with the powerline phase and frequency using a zero-crossing detector circuit. Firing angle commands are received and converted to a corresponding firing count. The microprocessor outputs a firing code for the thyristor firing pulse circuit when the count of the free running counter reaches the firing count.

Abstract: An electrical power system has a variable speed multi-phase alternator. A multi-phase full wave rectifier with controlled rectifier elements converts the electrical power to DC. The DC voltage is regulated by a control circuit which determines the angle of conduction of the rectifier elements. If the alternator frequency is excessive, the controlled rectifier elements are not reliably commutated. The control circuit compares the alternator frequency with a reference. When the frequency is above a selected value, rectifier operation is modified either for half wave rectification or for full wave rectification of less than all of the phases from the alternator. Additional switching time is provided, improving reliability.

Abstract: A coil for a 12-pulse transformer-rectifier apparatus assembled on a three-leg magnetic core comprising a half primary winding sandwiched between two half secondary windings of substantially the same height as the half primary winding. A pair of coils associated with one leg of a three-leg magnetic core in a 12-pulse transformer-rectifier, are oriented in opposition to one another, and have all their terminal conductors extending parallel to one another with transversal connectors in closely-related parallel planes.

Abstract: A circuit for controlling the power delivered to a load from an alternating current source includes a bridge rectifier coupled to the alternating current source for developing full and half-wave rectified signals in response thereto. The full and half-wave rectified signals are algebraically added to form a reference signal which is applied to one input of a comparator, the other input comprising an adjustable direct current control signal. The output of the comparator comprises a pulse width modulated signal which is used to control the state of a switching transistor connected in series with the load between the rectifier node developing the full-wave rectified signal and a point of reference potential.

Abstract: An improvement of a thyristor converter drive wherein a thyristor converter drives a load, such as an electric machine, with which a series resonance circuit (1) has been connected in parallel. The components (2,3) of the series resonance circuit (1) are so selected that the charging phase and discharging phase of the circuit have equal length.

Abstract: A gate pulse phase shifter is designed so as to generate pulses at intervals of 60.degree. regardless of distortions in the AC power supply voltage, and so that, for linearity in control, the phase reference signal and the DC output voltage of AC-DC converters connected to the device are proportionally related.

Abstract: A rectifier system in which the harmonic content of the A.C. current is reduced in a particularly efficient manner. Two or more 3-phase thyristor bridges are connected in parallel to the A.C. supply, and to the DC output by way of interphase-transformers. For the positive thyristors, and for the negative thyristors separately, the conduction transitions within each bridge are staggered from one bridge to the next so that the total conduction on each phase follows a stepped characteristic approaching a sine wave and thus having a smaller harmonic content than a comparable arrangement having uniform firing angles from bridge to bridge.

Abstract: A circuit arrangement for the compensation of voltage variations of the electrical AC polyphase power line for an AC-DC converter (3).The invention comprises a first circuit (5) for determining the voltage variations at the input of each trigger rectifier (Th.sub.1, Th.sub.2, Th.sub.3) of a converter (3) and a compensating circuit (6) for compensating the firing angle of each trigger rectifier. The compensating circuit (6) is connected to an output (7) of the first circuit (5) for determining the variations in the input voltage of each rectifier. Outputs of the compensating circuit (6) are connected to control inputs of the pulse generator (4) for controlling the firing angle of each rectifier so as to adjust the period of the firing pulses of each trigger rectifier (Th.sub.1, Th.sub.2, Th.sub.3). The invention is particularly applicable to electrical DC power supplys for data processing systems.

Abstract: A three-phase regenerative rectifier provides power flow from ac to dc using a three-phase rectifier bridge. The reversal of the input ac voltage at the input frequency commutates current in the rectifier devices. A power transistor or field-effect transistor is coupled in inverse parallel with each rectifier device to provide a path for current from the dc output of the rectifier back to the ac supply during regeneration. The rectifier devices can be phase controlled thyristors or diodes. Control means sense the three-phase voltage and switch the transistors in phase with the ac supply. Commutation of the transistors is provided by their self-contained turn-off capabilities.

Abstract: A valve actuator comprises a variable speed motor, a speed control circuit for controlling the motor speed including speed selector means by means of which the desired motor speed can be set to different values by adjustment means. A torque limit control circuit is arranged to stop said motor when a pre-set torque limit has been exceeded. The torque limit control includes a torque sensor arranged to provide a measure of torque from one or more torque-related electrical parameters of the motor for example a torque related current flowing in the motor, and adjustment means for setting torque limit control circuit to different torque limits.

Abstract: Magnetizing current supplies are disclosed for supplying current for magnetizing parts in a magnetic particle inspection system, including SCR's connected in circuit with a three-phase voltage step-down transformer to accurately control the duration and magnitude of magnetizing current. In one type of supply, six SCR's are connected between magnetizing apparatus and terminals of three center tapped secondary windings of a three-phase voltage step-down transformer, the SCR's being operable to rectify current and also being connected to a phase control circuit which controls firing times of all SCR's to control the magnitude of the magnetizing current.

Abstract: An inductive storage circuit for generating high voltage pulses includes a quenching circuit and a discharge circuit each connected in parallel with a storage inductor. One branch of the quenching circuit includes a quenching capacitor and one branch of the discharge circuit includes a resistor and a diode in series. These two branches have a common junction, to which is connected a quenching thyristor that forms the second branch of each of the quenching and discharge circuits. Thus, the quenching thyristor is in series with each of the quenching capacitor and the discharge resistor.

Abstract: A digital phase-shifting circuit employed for a thyristor converter, which operates on a polyphase power supply. In the digital phase-shifting circuit which is simply constructed as a whole, a counter portion consists of an N-bit binary counter and a divide-by-six ring counter, and a comparator portion consists of a common comparator which compares data bits and simply constructed two-bit comparators that are provided for each of the phases.

Abstract: A rectifier controller for controlling bridge connected silicon control rectifiers (SCRs) (1 to 6) of a three-phase rectifier which is used to supply a direct current (DC) voltage to an inverter (314) used for injecting pulse coded audio frequency signals on a power line. The controller supplies gate control signals to the SCRs (1 to 6) of the rectifier bridge in accordance with the envelope of the three-phase supply voltages and provides for protection of the inverter (314) when shoot-through conditions occur in the inverter (314) and provides for output DC voltage adjustment by controlling the firing time of the rectifier bridge SCRs.

Abstract: An improved method of electric fish trawling which utilizes the fright-reaction exhibited by fish exposed to an electric field is disclosed. The method utilizes a trawling net equipped with electrodes which emit DC pulses having a repetition rate of six cycles per second or less, a pulse duration of approximately one millisecond, and producing a field having a voltage gradient of one-hundred eighty volts per meter. A pulse generating circuit, which uses full-wave rectified single phase power or three phase, half-wave rectified power for a condenser discharge circuit is also provided.

Abstract: The effect of commutation notches 35 appearing in the nominally sinusoidal alternating current (AC) line-to-line voltage, for example, v.sub.ab used to synchronize the firing of thyristors in three phase, phase-locked power converters 12 and 14 is essentially eliminated by reconstructing the corrupted line-to-line voltage by summing the integral of the corrupted line-to-line voltage .PSI..sub.ab with a signal proportional to the product of the delta load current, for example, i.sub.ab and the commutation inductance L.sub.c. The composite waveform .PSI..sub.ab +i.sub.ab .times.L.sub.c) resulting from the summation has well defined zero crossings 41 from which synchronizing pulses are generated for synchronizing the phase-locked thyristor firing circuit 18 implemented by either software or hardware techniques. Additionally, the reconstructed waveform is used to control commutation, particularly in the control of a load side converter i.e. inverter 14 which is utilized in an AC motor drive system.

Abstract: A conventional on-off voltage regulator responsive to the output voltage of the rectifier of a vehicular battery charging system, operates three circuits each containing a pulse transformer and its driving transistor and other components for controlling the three SCRs that provide the negative side of the rectifier bridge, so that they will either pass entire halfwaves or block completely. The other three rectifier elements of the bridge are ordinary diodes. With this arrangement the SCRs may have the same rating as the other diodes of the bridge, and the thermal load is evenly distributed, while an alternator having a permanent magnet field can be used.

Abstract: An ac to dc power conversion apparatus employing commutated thyristors has a series circuit comprising a capacitor 8 and an impedance element 9 connected at the input side of a thyristor conversion unit 4. The capacitor reduces notch voltages generated during the commutation of the thyristors, and the impedance element prevents the capacitor from resonating with other inductive reactances in the circuit. A switching device 10 is connected in parallel with the impedance element, and is closed during the commutation of the thyristors to maximize the notch voltage reduction.

Abstract: An electric power source device for electrolytical production or coloring of alumilite, wherein pulses are generated at a predetermined phase of A.C. source, the pulses are frequency-divided by a non-contact circuit and, depending on the results of the frequency division, analogue switches respectively comprising a non-contact circuit are driven to actuate rectifier units for rectifying the A.C. source. Outputs of alternately reversed polarities with distinctly sharp rising and falling are provided.

Abstract: In power conversion apparatus such as a naturally commutated cycloconverter for use in converting the output of a variable speed generator to a constant frequency, there is provided an end stop control circuit for ensuring commutation of power switching devices that includes timing wave generators for both rectification and inversion end stops. The timing wave generators are phase locked with the generator outputs but are controllable by end stop bias signal derived from a variable system parameter that may be independent of the source, such as load current to provide prompt protection against converter malfunctions due to excess load currents.Further provided is an excess current detector employing, for each phase, a pair of high slew rate, high current unipolar output op amps connected in a voltage follower configuration with a common pull-up resistor and a common load capacitor to develop a peak current signal representing the greater of the positive and negative peaks of the alternating phase current.