Abstract: An electrical power converter including both DC and AC power terminals which are respectively connected to a DC current source and an AC voltage source, the latter comprising the grid of an AC power utility. A capacitor is coupled to the AC terminals along with a load. A control and gating circuit is coupled to the converter for supplying control signals for the thyristors of a converter bridge included in the polyphase converter. When disconnected from the utility grid, a relatively fast acting control loop is coupled to the control and gating circuit and is responsive to the current flowing in the capacitor and an AC reference voltage to accommodate the current source characteristics of the converter for maintaining an output voltage of substantially constant amplitude across the AC terminals.

Abstract: In an AC to DC power conversion circuit including a boost inductor connected in series between a full-wave rectifier and a DC load, a switch is connected to selectively shunt boost inductor current from the load. Switch conduction is controlled by a pulse width modulator generating switching pulses at a high fixed frequency. The pulse widths are automatically varied as a function of boost inductor current, load voltage, and an ideal sinusoidal waveform derived from the AC input voltage to force the boost inductor current to closely conform to the ideal sinusoidal waveform and thus minimize harmonic distortion, while achieving load voltage regulation and near unity power factor.

Abstract: A resonant, quasi-square wave, pulse width modulated (PWM) power source (33) for a connectorless power system is disclosed. The power source (33) supplies constant amplitude, fixed-frequency current to a supply loop (31) that is inductively coupled to a plurality of pickup loops (35), each of which supplies power to a remote unit, such as the seat regulator that supplies power to a group of commercial airplane seats (37). Precise, fixed-frequency control is provided by triggering a PWM with fixed-frequency sync pulses. Current control is provided by sensing the current applied to the supply loop, converting the magnitude of the sensed current to a voltage and using the voltage to control the width of the pulses produced by the PWM. The PWM pulses, in turn, control the width of the power pulses applied by the power source (33) to the supply loop (31). Stability and efficiency are improved by maintaining the output of the power source nearly resonant, i.e.

Abstract: A power supply apparatus has a switching device for converting a D.C. input to a high-frequency power; power conversion device for converting the high-frequency power into a plurality of D.C. output powers, a voltage maintaining device for sending a feedback signal to the switching device based on the value of one of the plurality of D.C. output powers thereby controlling the switching frequency and the duty ratio of the switching device in such a manner as to maintain the voltage of the one of the plurality of D.C. output powers constant, and a hysteresis device for varying the amount of the feedback in accordance with the load current on the D.C. output power used for the feedback so as to impart a predetermined hysteresis to the voltage change caused by a change in the load current.

Abstract: A parallel resonant converter as disclosed having in one embodiment a variable inductance and having a control circuit which operates the switches of the converter under zero voltage switching conditions and under continuous conduction mode by maintaining the quality factor of the resonant circuit generally equal to the DC conversion ratio.

Abstract: A circuit (20, 40, 50, 60) for generating an optimal compensating ramp voltage signal V.sub.cramp, with the minimum necessary slope, m.sub.c for a current mode DC/DC converter is shown. The adaptive compensating ramp generating circuit (20) is comprised of two voltage dividers (22, 24), the first voltage divider 22 divides an input voltage V.sub.IN and the second voltage divider 24 divides an output voltage V.sub.OUT. The first voltage divider (22) has two resistors (26, 28) in series: the first resistor (26) has a resistance of (1/B-1) * R ohms and the second resistor (28) has a resistance of R ohms. V.sub.OUT is divided by the second voltage divider (24) having two resistors (29, 30) in series: the first resistor (29) has a resistance of (1/A-1) * R ohms and the second resistor (30) has a resistance of R ohms. The constants A, B, and C are selected for the particular type of DC/DC converter employed. The divided voltages A * V.sub.OUT and B * V.sub.IN are input into a voltage controlled current source (32).

Abstract: A PWM inverter takes into account the harmonic distortion produced by a d.c. link which is not constant. The controller determines switching angles in real time eliminating the need for storing PWM patterns in ROM. This is made possible by a "quick" Fourier transform which allows the determination of switching events in a PWM inverter in real time. The inverter controller develops pattern sets for rejecting selected harmonics and accounts for known fixed harmonic distortion on the d.c. link by determining the set of switching angles for each controlled selected harmonic.

Abstract: A power controller system for providing electrical power to a multiplicity of loads at outputs thereof under direction of a plurality of input controllers operating a corresponding plurality of output energization means with the controllers capable of having different output states in response to differnt system input signals, the controller having selected common components for forcing selected ones of said input controllers to terminate power to corresponding loads. Current sensing means upsetting current balances in bipolar transistors are used to sense beyond-limit currents.

Abstract: In the rectifier station (GR), the reference d-c current (J.sub.w) is automatically formed from the differences of a reference current-dependent control angle function and the current controller-dependent control angle (A) on the rectifier side, where the actual d-c current (J.sub.d) is termined by the power-dependent bucking voltage in the inverter state (WR), and that in the latter the difference of the actual d-c current (J.sub.d) and a reference d-c current obtained from the reference power (P.sub.W) through division by the actual d-c voltage (U.sub.d) influences a controller which sets the reference d-c current for the voltage controller (UR) in the inverter station (WR).

Abstract: A four-quadrant buck converter is described having a common leg of an inductor in series with an output capacitor, one power supply for providing a positive voltage output signal and negative voltage output signal to two solid-state switches joined at a common node, an output transformer whose primary is connected across the output capacitor and a pulse width modulated control circuit for operating the switches to produce a predetermined voltage across said output capacitor and for regulating the current out of the transformer. The control circuitry operates in response to a voltage signal from the output of the power supply, a voltage representative of the voltage at the output of the converter, a high frequency ramp voltage, an internal oscillator, and a voltage representative of the RMS current flowing on the secondary side of the output transformer.

Abstract: An electronic power supply converts alternating current to a regulated DC voltage with high efficiency. A switch connects the input AC to a DC output capacitor when the input voltage is greater than the output voltage and the output voltage is less than a set point value. The switch receives the input AC and rectifies the input voltage and applies the switch rectified voltage to the output. The voltage difference between the input voltage and the output voltage level at which the switch closes varies in accordance with the input and output voltages.

Abstract: A regulator is disclosed for regulating a DC power supply such as an automotive alternator or generator which has a wide speed range and is hence capable of producing an output voltage and power which varies between wide limits. The regulator comprises electronic circuitry which allows the voltage at the alternator to rise while limiting the current therethrough, typically by means of pulse-width modulation; a DC-DC converter connected to the load; and circuitry to control the output voltage of the DC-DC converter within close limits. A permanent magnet alternator provided with a regulator of the type described is also disclosed.

Abstract: An AC-DC converting apparatus includes a main circuit having a fully-wave rectifier for rectifying an alternating current from an AC power supply, a reactor connected to the rectifier, and a switching element for alternately causing the reactor to store and to release electrical energy. The converting apparatus further includes a current detection circuit for generating a current detection signal indicative of the current flowing into the apparatus, and a voltage waveform detecting circuit having a 4-diode bridge for rectifying the input current of the power supply, and a photocoupler for generating an input voltage waveform signal indicative of the rectified waveform of the input voltage. Positive reference waveform and negative reference waveform smaller in amplitude than the positive reference waveform are generated from the input voltage waveform signal.

Abstract: A PWM control apparatus for an interphase reactor multiplex inverter in which the load current in each phase output from the midpoint of each of the interphase reactors is quickly controlled by feedback control with a signal corresponding to the sum of the output currents of the inverters in the same phase, the voltage command value for each phase of each inverter is corrected with a signal corresponding to the difference between output currents of the inverters in the same phase (proportional to the circulating current), whereby the interphase voltage difference developed across the interphase reactor in each phase is quickly controlled and the circulating current in each phase is controlled to become extremely small.

Abstract: A power converter device that interchanges power between a first A. C. system and a second A. C. system, including an externally commutated power converter controlled such that the D. C. voltage of a D. C. line tracks a reference value, a voltage type self-commutated power converter which converts the output D. C. power of the externally commutated power converter to A. C. power under pulse width control and supplies the converted A. C. power to the second A. C. system, interchange power control means for controlling the phase difference angle of the voltage on the A. C. side of the voltage type self-commutated power converter and the voltage of the second A. C. system such that the interchange power tracks a reference value, and reactive power control means for controlling the control factor of the pulse width control such that the reactive power generated by the voltage type self-commutated power converter tracks another reference value.

Abstract: A pulse arc welding power supply incorporating one or more series resonant converters (SRC) for converting D.C. power to A.C. power which is then rectified to supply welding current. Each series resonant converter includes a series resonant tank circuit consisting of a capacitor, an inductor and the primary winding of a transformer whose secondary winding supplies current to the welding arc. The undesirable secondary output capacitor of the prior art is eliminated, as all the capacitance in the series resonant circuit is located on the primary side of the transformer. Current sensors sense the ringing current in the resonant circuit and also the load current and produce respective feedback signals which are used to control the switching frequency of an SCR bridge in the tank circuit, thereby maintaining the welding current at a desired value.

Abstract: A permanent magnet rotor type electrical alternator-SCR power supply having a unique series resonant LC circuit connected to the alternator and resonant close to the lowest expected frequency of the alternator.

Abstract: The present invention discloses a speed control apparatus for an AC motor which vector-controls, for example, a drive current of the AC motor such as an induction motor, synchronous motor and AC commutator motor with a feedback current control loop providing an inverter utilizing a semiconductor element for power amplification such as power transistor and compensates for non-linear current characteristic of the drive current generated by dead time resulting from ignition delay of a semiconductor element with a current compensation control element provided within the current control loop, thereby controlling the rotation speed and command response speed of an AC motor.

Abstract: In a resonant regulator power supply, a resonant circuit is coupled to a source of alternating input voltage that is controllable in operating frequency. A power transformer primary winding is coupled across the capacitive element of the resonant circuit for generating an output voltage across a secondary winding. A control circuit varies the operating frequency in a negative feedback loop to regulate the output voltage on the positive slope of the frequency characteristic curve. A frequency limiting circuit prevents the operating frequency from passing through the resonance frequency of the resonant circuit.

Abstract: A solar energy electric generating system for space vehicles is provided which directly generates alternating current from ambient light without the use of inefficient power conversion. Light incident to photocell arrays is mechanically or electrically gated to produce fluctuating DC. A number of transforming devices are provided to convert the fluctuating direct current to alternating current. By varying the frequency and duty cycle of the pulsing output voltage, frequency and phase relative to some reference may be controlled. Additionally, direct current may also be produced efficiently from the same photocell arrays. A closed-loop system is provided for regulation. The rotational torque produced by a rotating shutter version may also be used, under inertial navigation control, to provide a rotational torque for vehicle positioning.

Abstract: A power converter apparatus for a DC power transmission system or a frequency conversion system includes a control apparatus that functions such that during the operation of converters, when the output voltage of a constant reactive control circuit reaches the output voltage of a margin angle limited control circuit, a shunt reactor is interrupted or a shunt capacitor is closed, and when the output voltage of the constant reactive power control circuit reaches a control delay angle corresponding to the curve which has a difference .DELTA.Q compared with the curve P-Q at a minimum margin angle .gamma. min, the shunt reactor is closed or the shunt capacitor is interrupted.

Abstract: A device which generates a symmetrical output voltage system at the output of a transformer with a neutral point which can carry current on the secondary side or a Y-point former, even if the load is asymmetrical, each primary terminal being fed from a pair of bridge arms of a pulsed inverter. The device further comprises a decoupling network in which the conductor voltages measured on the secondary side are converted into substitute actual values according to the structure of the transformer circuit, which actual values represent symmetrical output voltages or a likewise symmetrical system with a free Y-point. These substitute actual values are controlled to form a system of symmetrical reference values.

Abstract: The invention concerns a converter (2) provided with GTO thyristors the frequency and characteristic shape of its output voltage (U.sub.A) being constrained by a load (3). The frequency of the converter (2) is matched to the constrained output voltage (U.sub.A) with the aid of a regulating circuit (4). Non-simultaneously switched GTO thyristors are switched on and off in a predetermined timed relation to each other (FIG. 3).

Abstract: A high-voltage generator for electrostatic sprayer devices includes a frequency-clocked power amplifier for feeding the primary of a transformer whose secondary is connected to a high-voltage cascade or multiplier circuit. The power amplifier is connected to a controllable low-voltage d.c. voltage source and to a controllable frequency generator, whereby the control of the d.c. voltage source and of the frequency generator is provided by a microcomputer such that the transformer is optimally matched for all voltages appearing at the high-voltage output of the cascade.

Abstract: An inverter control circuit for a power supply source comprising an inverter and a commercial power source (reserve power source) and adapted to connect the inverter normally to a load and to change the inverter with the commercial power source uninterruptedly by means of a thyristor switch to supply power uninterruptedly to the load in case the inverter malfunctions. The inverter control circuit comprises a digital PLL circuit for synchronism control, a bias signal, and a parallel connection of a synchronism control circuit and a synchronism detecting circuit, and is capable of uniterruptedly changing the inverter to the commercial power source by being switched to biasing signals by means of the switch at the timing of the synchronism detecting of the synchronism detecting circuit in case the synchronism control circuit malfunctions.

Abstract: A power supply circuit of switching regulator type includes a converter transformer defined by a first winding for receiving input power from a DC source and second winding for producing output power, a switching transistor for alternately making and breaking electric connection between the DC source and the first winding by alternate turn on and off operations of the switching transistor, and a rectifying circuit connected to the second winding for rectifying the output power. A power supply circuit further includes a first error detector for detecting a drift in the rectified output DC current and for producing a first control signal, a second error detector for detecting a drift in the rectified output DC voltage and for producing a second control signal, and a controller for controlling the on and off operations of the switching transistor based on either one of the first control signal and the second control signal so as to stabilize the rectified output power.

Abstract: The invention relates to a blocking oscillator switched mode power supply which is supplied with a primary voltage U.sub.p obtained by rectification of the AC line voltage. Because the line voltage varies, the secondary voltage which supplies electrical equipment is stabilized by means of a switched mode power supply. The maximum power from the supply generally depends on the primary voltage U.sub.p. The present invention has the objective of eliminating this disadvantage of the known switched mode power supplies. To this end, according to the invention, a correction circuit actuated via a voltage divided is provided, the voltage divider being connected to the primary voltage U.sub.p. The correction circuit is designed so that it keeps the breakpoint of the secondary current I.sub.s constant.

Abstract: A power supply 10 providing a DC output voltage comprises a voltage input circuit 11, a charge buildup and transfer circuit 14, a switch including SCR3 for selectively interconnecting the input circuit 11 and the charge buildup and transfer circuit 14, a trigger circuit including a unijunction transistor gating circuit 16 for triggering the SCR3, and a sensing circuit 22 including a current reference 26 for sensing the current output to the load 38 and comparing the output to current reference 26 while providing a comparison signal to the trigger circuit 16 so as to modify the triggering of SCR3 and thus the interconnection of the voltage input circuit 11 and the current buildup and transfer circuit 14.

Abstract: An arrangement for converting AC input power to DC power at a reference voltage including an output filter capacitor which is connected to the input AC line when the output voltage is less than or equal to the desired reference voltage and when the input AC voltage is less than a predetermined threshold. In this manner, charging pulses are generated twice during each half cycle of rectified AC power.

Abstract: A single DC power switching converter feeds two DC load circuits from two transformer secondary windings. One winding is tightly coupled to the primary and its output voltage is controlled using pulse-width modulation. The other secondary winding is loosely coupled to the primary so that its leakage inductance resonates with a secondary capacitor such that its output is controlled by converter frequency adjustment. Thus, both output voltages are controlled using a single power switching stage.

Abstract: The static converter of the invention comprises a central processor (15) measuring the output voltage (TM3), the output current (TM4), the rectifier current (S) and the reverse bias time (8) of the thyristors of the switch (5). From this information it drives, through interfaces (17, 18, 20) the starting thyristors (6) comprising a series resonating circuit intitiating the oscillations of the parallel resonant load circuit (2,3).

Abstract: A load control circuit having an input terminal adapted to receive any one of a number of different input voltage signals including signals of at least first and second type. These signals may be, respectively, DC and AC input signals. Control circuits are provided for operation of the load from the input voltage signal. This control circuit includes at least first part responsive only to said input voltage signal being of first type for coupling said signal of first type to the load, and a second part responsive only to the input voltage signal being of second type for coupling the signal of second type to the load.

Abstract: The power supply converts alternating current voltage to a direct current voltage which is in turn converted to a pulse width modulated voltage. A circuit arrangement is provided responsive to the pulse width modulated voltage to produce N output direct current voltages each having a different predetermined value, where N is an integer greater than one. A feedback circuit is provided between the circuit arrangement and the circuit providing the pulse width modulated voltage to provide a substantially constant value for the N output voltages when a varying load is presented to the power supply. A current transformer arrangement is coupled in series in the feedback circuit to provide wide bandwidth and low droop without oscillation for the power supply when the power supply is subjected to a step load (a step from no-load up to full-load).

Abstract: An inverter control system for selecting an angle set defining an inverter output waveform in response to various operating conditions of the inverter. The inverter control system selects an angle set in response to signals representing the power factor of the load and the normalized DC bus voltage to minimize the harmonic content of the inverter output waveform and to compensate for voltage drops occurring between the DC power supply and the load. The inverter control system is also responsive to the inverter filter output current to detect fault conditions and in response thereto, to select at least one angle set defining a waveform to reduce the voltage at the load to zero while allowing the filter output current to increase to a given percentage of the rated current.

Abstract: A power converter circuit, which is voltage-regulated and current-limited by a peak current control circuit, significantly reduces the current tailout effect by decreasing the frequency of operation of the converter during low voltage output current-limited operation by increasing the nonconduction interval of the power switch while maintaining a fixed conduction interval.The peak current control circuit periodically initiates current conduction in the power switch in response to a clock pulse, and terminates conduction when a peak current level is attained. The peak current control includes circuitry to compare decaying current in the filter circuit during nonconducting intervals with a preset current tailout threshold level. The periodic triggering of the power switch into conduction is inhibited until current has decayed to this current tailout threshold level.

Abstract: In the frequency control mode, a switch is ON. A power difference is fed back to an adder through a limiter and is compared with an output from a power setter. A difference signal from the adder is supplied to an adder to obtain a difference with detected power. By the feedback loop, the preset power is set to be substantially equal to the detected power. While the difference is greater than a predetermined value, an output from a level detector is at logic level "0". Even if a mode switch instruction is received in this state, an AND gate does not produce an output signal and the mode changing is not performed.

Abstract: An inverter apparatus includes a plurality of gate-turn-off thyristors being provided at each end of thyristor-branches, and bias sources or capacitors connected in parallel with the gate-turn-off thyristors for applying reverse voltage to the thyristor-branches when the gate-turn-off thyristor is turned off.

Abstract: A method and apparatus are provided for an improved power inverter circuit, including a separate starting inverter having an A.C. output applied across a load prior to energizing the main inverter, and driven by a series of repeating gating signals. On achieving a preselected load condition, the starting inverter is removed from the circuit and the main power inverter is energized.

Abstract: A method and apparatus for controlling the frequency of a power inverter for converting D.C. current into A.C. current having a frequency controlled by gating signals, which apparatus and method includes a load voltage detector for deriving a voltage wave; a load current detector for deriving a current wave; a phase shifter to shift the phase of one of the waves by 90.degree.; a multiplier to multiply the voltage and current waves; one with the other to produce a product; a reference signal for comparison with the product to create a control signal; and, a gating signal controller responding to the control signal.

Abstract: A control circuit for a flyback stepcharger comprising a source producing a urrent proportional to a power supply output in order to charge a capacitor. When the capacitor reaches a predetermined level the charging stops and switching transistors to the primary of the transformer of the stepcharger are turned on. When the primary current reaches another predetermined level the switching transistors are turned off. The source charging the capacitor is restarted a set time after the switching transistors are turned on. The stepcharging is terminated when the DC voltage of the power supply output reaches its set value as measured when the switching transistors are on.

Abstract: A fault protective system for a load commutated inverter motor drive including an AC to DC source side converter coupled from an AC source to a DC to AC load side converter via a DC link circuit and wherein the load side converter supplies AC power of varying magnitude and frequency to the synchronous motor load. A pair of microcomputers interlinked by a common digital memory respectively control the inverters and interface with hardware type sensor means for detecting the occurrence of a plurality of different types of faults. Additionally, a number of stored instructional sets or programs are respectively included in both microcomputers to detect by software means another plurality of faults. The detection of a fault on either side of the link circuit is communicated to the respective computer which subsequently communicates with the other microcomputer to effect a coordinated protective action in response to thyristor cell failures, circuit failures, and abnormal operating conditions.

Abstract: In a switching regulator including a pulse-width modulator for controlling the pulse width of a base input applied to a pair of transistors which are alternately turned on and off by the base input, the pulse-width modulator is provided, by an error amplifier, with an input corresponding to the magnitude of a detected output voltage, and variations in the input applied to the pulse-width modulator for increasing the pulse-width of the aforementioned base input are moderated by means of a time-constant circuit and a circuit for causing the time-constant circuit to be charged and discharged.

Abstract: A high power generator for an ion discharge device used for the thermal treatment of metals in a rarified gaseous atmosphere. A variable width pulse generator is used to vary the input power to an inductance means. The frequency of the input pulses is held constant, while the pulse width is varied. An arc protection circuit is provided to sense and prevent an incipient arc across the secondary of the transformer. The arc protection circuit includes a voltage detector and a current detector with logic means to sense an under voltage-over current condition that presages the beginning of an arc. A second embodiment is disclosed which uses two circuits in parallel to drive the ion generator. One circuit is charged while the other is discharged. A third embodiment is disclosed which was a thyristor network for selectively loading the primary circuits of a pair of inductance circuits in response to an inductance-capacitance time constant.

Abstract: A boost power supply having a full-wave rectifier driving an inductor, diode, and the parallel combination of a filter capacitor and load. A shunting switch connected between the inductor and diode is periodically closed to draw current through the inductor, thereby charging the inductor. The switch is then opened so that the inductor, while discharging, generates a relatively high voltage which is applied to the load and filter capacitor through the diode. The shunting switch is opened and closed in a manner which causes the input current to follow a sinusoidal waveform in phase with the input voltage with an amplitude that maintains the output voltage of the power supply constant. Specifically, a harmonically pure sinusoidal waveform is multiplied by a voltage inversely proportional to the output voltage.

Abstract: A load resistance control circuit compares the actual value of a non-zero temperature coefficient load resistance to a desired load resistance magnitude, and generates a control signal varying as a function of the deviation of the actual load resistance from the desired resistance. A fixed portion of each of the load voltage and load current are sampled and compared to references, set such that the sampled portions will each be equal to an associated reference value, at the same time, if the load resistance is of the desired magnitude. An analog comparator can be used to add or remove charge to or from an integrating capacitor responsive to the actual load resistance being different from the desired load parameter. In a digital embodiment, a counter is incremented or decremented responsive to the actual load resistance differing from a desired value. The capacitor voltage or digital count then is used to affect a load resistance change, as, e.g.

Abstract: There is provided an improvement in a power inverter for converting D.C. voltage into an alternating voltage by alternating gating D.C. current across a load. This type of inverter employs a gating circuit for providing gating signals in two sets of switches at a frequency which controls the power factor of the voltage and current applied to the load, means for detecting the power factor, means for comparing the detected power factor with a reference signal indicative of a desired operating power factor and means for adjusting the frequency in accordance with the comparison whereby the power factor tends to be at the desired power factor level. The improvement in this type of inverter is the provision of means for changing the reference signal during the operation of the inverter in accordance with a control signal so that the desired power factor can be changed during start up of the inverter.

Abstract: An induction heating cooking apparatus comprises a self-controlled inverter which comprises a series connection of an induction coil and a switching element, a resonance capacitor connected in parallel with the switching element, and a flywheel diode. An inverter driver circuit triggers the inverter with a starting pulse obtained from a starting signal generator. A voltage signal associated with a current flowing through the switching element is obtained by means of a current transformer. A voltage source of the inverter is implemented as a ripple current voltage source. A comparator of a gate signal generator compares a reference level changing in accordance with the ripple current voltage and the voltage detected by the current transformer, thereby rendering the switching element non-conductive when the voltage level reaches the reference level.

Abstract: A selective commutation technique used with an inverter eliminates one of the two commutation pulses at those points at which there is a particular relationship of inverter operating characteristics. The power factor angle .theta., the voltage phase angle .delta., and the voltage pattern notch angle .beta. are sensed to determine this particular relationship. Because at the switching point there is no current flowing through the gated thyristor, the gated thyristor can then be transitioned to the nonconducting state merely by the removal of its gate signal. A single makeup pulse is then rung through the system to initialize the commutation capacitors for the next switch point.

Abstract: Disclosed is a class of power converter which extracts electric energy from a source of A.C. or D.C. power and conveys this energy through series resonant circuits to two or more loads in an independently controllable form.

Abstract: A first PWM inverter for converting the ac voltage of an ac power source to a dc voltage is connected through a dc circuit with a second PWM inverter for driving an ac motor. A smoothing capacitor is provided in the dc circuit. The firing angles of the first inverter are so controlled that the voltage in the dc circuit may be kept at a predetermined value and the second inverter is controlled in accordance with the speed instruction signal for the ac motor, whereby the ac motor can operate a regenerative operation without inducing higher harmonic components.