Abstract: A sensorless vector control system of an induction motor includes: a magnetic flux and speed controlling unit for receiving a predetermined command value and generating two phase voltages of DC component; a first reference frame converting unit for converting the two phase voltages of DC component into three phase voltages of AC component; an inverter for receiving the three phase voltages of AC component and driving an induction motor; a current detecting unit for receiving the three phase power of AC component flowing between the inverter and the induction motor, and detecting and outputting three phase currents of AC component; a second reference frame converting unit for receiving the three phase currents of AC component, and converting and outputting two phase currents of DC component; a magnetic flux and speed estimating unit for receiving the two phase voltage of DC component and the two phase currents of DC component, estimating a magnetic flux and speed required for a vector control; and a primary re

Abstract: An apparatus for producing a floating DC voltage source from an electric motor which may be a brushless DC motor. The apparatus includes a rectifier circuit for rectifying back electromotive force voltage from a winding of the motor and a clamping circuit for clamping the rectified voltage to produce a floating DC voltage which may be a TTL logic level voltage. A motor control circuit is disconnected from the winding to produce the back electromotive force voltage.

Abstract: The CPU 102 detects that the posture of the apparatus main body has been shifted from the normal posture into an abnormal posture on the basis of the acceleration information obtained as detection output of the acceleration sensor 41. Then, it restores the normal posture by means of a playback technique for controlling various drivers 3D through 7D, using route planning data stored in advance in the memory 101 for restoring the normal posture from a falling posture.

Abstract: A method and apparatus for controlling flux in an induction motor. During initial start of an induction motor drive the motor flux has to be charged up. The method provides time optimal flux forcing under limited inverter ampere capability. Command stator flux and current are generated and coordinated through a motor flux model. Charging of the motor flux and flux current are accelerated up to nominal operating values in coordinated manner.

Abstract: A robot apparatus such as an entertainment robot is configured to select a transmission destination at which a communication appliance is located, and transmits predetermined information to the selected transmission destination. The predetermined information may be a self-diagnostic result or information concerning its surrounding environment. If the transmission fails, the robot apparatus selects an alternative transmission destination whereat another communication appliance is located, and transmits the information to the alternative destination.

Abstract: A reversible DC motor drive includes a DC/DC converter having a DC input, a first output having a first pulsed DC voltage and first DC voltage value, a second output having a second pulsed DC voltage and second DC voltage value, and a third output having a third pulsed DC voltage and third DC voltage value. The first and second outputs are adapted for electrical interconnection with armature terminals of a DC motor, and the third output is adapted for electrical interconnection with a field terminal of the motor. A microprocessor routine calculates motor speed from the motor armature voltage, field current and armature current. An outer control loop for motor speed and two separately controllable inner control loops for armature and field current control the DC/DC converter responsive to the calculated motor speed and a speed reference in order to independently control the three DC voltage values.

Abstract: A motor apparatus, such as a DC brushless motor, is provided for the obtaining a position detection signal by detecting the intersection of the divided voltage of a bus voltage and three phases of the divided voltage of the three phases by a comparator which results in the central position of a flat portion. For the divided voltage for the three phases of the respective positive terminal of the comparator, the comparison voltage obtained by dividing the voltage between the divided voltage for the three phases of the following phase in the order and the divided voltage of the bus voltage is applied to the respective negative terminal. As the comparison voltage is phase shifted, the position can be detected by making a slant position ahead of the flat point of the intersection.

Abstract: The present invention provides a control algorithm which instantaneously detects magnetic pole positions in the inside of a motor including polarities of the motor. A controller 1 applies a minute voltage change vhd to a voltage command on a dc axis which constitutes an estimation magnetic pole axis of the motor 3, detects electric currents Idc, Iqc of a motor 3 and discriminates the polarities of the magnetic pole axes by making use of the difference between a cycle in which ripple components of the electric currents Idc, Iqc become positive and a cycle in which the ripple components become negative or the difference between positive-side and negative-side current change rates. Further, the minute voltage change vhd with respect to the above-mentioned voltage command is generated on both of the dc axis and the qc axis rectangular thereto, and the magnetic pole positions of the motor are directly estimated based on the current ripple components on the respective axes.

Abstract: The present invention relates to a current limit circuit of an inverter refrigerator. The present invention comprises a current limit unit for outputting a current limit signal by comparing an excess current generation due to load change with a limit current level set in advance. Accordingly, the present invention can extend life span of a compressor by protecting a BLDC motor from the excess current and reducing permanent magnet demagnetization due to the excess current. In addition, the present invention can improve a mobility of a motor by maintaining uniform torque with stable current in the operation, and can operate a compressor stably without turning off by a momentary excess current in the compressor operation.

Abstract: An H-bridge motor driving circuit has, between a PWM comparator and a control circuit, first and second frequency dividers for frequency-dividing, by 2, an AND gate, OR gate, and first and second and second inverters. During a first period, the first and second MOS transistors are turned on, and the second and third MOS transistors are turned off, caused a current to flow through a motor. During a next second period, the third and fourth MOS transistors are turned on, and the first and second MOS transistors are turned off, causing a regenerative current to flow through the motor. During a next third period, the MOS transistors are turned on and off in the same manner as during the first period, causing a current to flow through the motor. During a final fourth period, the first and second MOS transistors are turned on, and the third and fourth MOS transistors are turned off, caused a regenerative current to flow through the motor.

Abstract: A system (10) for controlling the torque of an induction motor (12) utilizes a flux observer (14). The flux observer (14) receives stator current inputs (16, 18) and a rotor speed estimate (80) and then outputs a rotor flux estimate (20) that provides increased motor control stability at all speeds.

Abstract: The control device carries out a control of the induction machine based upon a flux estimation performed by means of a Luenberger state observer, the estimation error of which can be defined by means of a matrix having a plurality of eigenvalues, the magnitude of each of which is proportional to the magnitude of a respective pole of the induction machine, and the phase of each of which is rotated with respect to the phase of the corresponding pole of the induction machine by a given angle of rotation.

Abstract: The present invention overcomes the disadvantages of previously known motor controllers for centrifuge machines wherein a motor controller is provided for a centrifuge machine including a logic control module, one or more power cells, and one or more contactors. The logic control module is capable of interfacing with the main centrifuge controller and provides control over the power cells and contactors to provide a voltage ramp-up to accelerate the centrifuge basket. As such, the logic control module avoids the current draining problems associated with across the line starting of the centrifuge motor. The power cells receive a voltage from the main power supply, and output to the contactors variable power to control centrifuge motor speed. Further, the configuration of multiple contactors to reverse the power supplied to the centrifuge motor windings may eliminate the need for a second, reverse direction motor.

Abstract: A digital calculation apparatus 4 including a position-detection means 14 for detecting the magnetic pole position, based on the detected current difference values of the synchronous motor 1 in a two-phase short-circuit state, takes in current flowing in the synchronous motor by generating an interruption trigger signal in synchronization with the PWM signals for driving an inverter 3, and driving an A/D converter with the generated trigger signal.

Abstract: An air conditioning apparatus includes a DC power supply, at least one switching mechanism, and a drive circuit. The drive circuit includes an inverter including a plurality of switching elements. The inverter is directly connected to a positive terminal of the DC power supply and is directly connected to a negative terminal of the DC power supply. The drive circuit also includes a DC to DC converter. In one embodiment, a positive terminal of the DC to DC converter is connected to the positive terminal of the DC power supply via the at least one switching mechanism. In another embodiment, a negative terminal of the DC to DC converter is connected to the negative terminal of the DC power supply via the at least one switching mechanism. The drive circuit also includes a switching element circuit coupled to the inverter and to the DC to DC converter. The switching element circuit selectively activates and deactivates the plurality of switching elements.

Abstract: A driving system for driving an AC motor, which is capable of promptly switching to an individual inverter operation mode, wherein a control instruction for each of a plurality of power converters is generated by summing outputs of a plurality of current compensation units connected in parallel, and each of the plurality of current compensation units makes compensation to reduce a deviation of a detected output current value of an associated power converter from a current instruction value to be outputted from the power converter to zero.

Abstract: A control system for an electric vehicle, such as a golf car, that includes a plurality of power MOSFETs. The vehicle includes a shunt wound motor including an armature coil and a field coil. One of the MOSFETs is connected in parallel across the armature coil and another MOSFET is connected in series with the armature coil. The MOSFET across the armature coil includes a freewheel diode. When the motor is operating below its base speed, i.e., when the back EMF is below the power supply voltage, a current flow can be induced by momentarily turning on the series MOSFET, shorting the armature coil. Because of the residual inductance in the motor, a current flow in the counter clockwise direction is initiated at which point the series MOSFET is turned off. Consequently, a large voltage spike across the motor occurs which results in current flow back into the battery pack. By quickly turning the series MOSFET on and off the current flow can be maintained, and the regenerative braking can occur even at slow speeds.

Abstract: An available input of to a motor controller is used to measure the motor power supply voltage. Typically, a scaled down version of the voltage is sensed. The microprocessor is used to calculate the motor control current, in that case, is then calculated from the scaled down version. A scale factor that depends upon the sensitivity of the motor current to the motor supply voltage is calculated by the microprocessor. Driving parameters in the controller are then adjusted to account for the scale factor. In a preferred embodiment, the driving parameters are PWM parameters, and the microprocessor multiplies the measured power supply voltage to obtain a scale factor that can then be multiplied by the nominal PWM values to obtain the new PWM values.

Abstract: A power electrical system is disclosed for a microturbine power generator. The invention permits the microturbine to be started using an external DC power source. The DC voltage is converted to a variable DC voltage by means of a bi-directional buck-boost circuit, DC bus and a DC-to-AC converter. The DC-to-AC converter produces at its output a fixed voltage pattern whose frequency is gradually increased in concert with the DC voltage, to accelerate the microturbine from standstill to startup speed. Once the microturbine is started, the excitation is discontinued, and the DC bus and DC-to-AC are used to produce output AC power at a voltage level and frequency to match an electrical load.

Abstract: A DC motor control method and apparatus for reducing time required for deceleration without degrading positioning accuracy. In a device which drives a mechanism by using a DC motor as a power source, in deceleration of the DC motor, when the mechanism arrives at a predetermined position, a velocity command value to the motor, generated in accordance with a cubic function having a mild curve profile, is changed to a constant value, thus the velocity is discontinuously reduced.

Abstract: Disclosed is a starting device and method for eliminating a peak current introduced when an inductive circuit such as a motor or a directcurrent (DC) motor is actuated, The starting device includes a signal generator electrically connected to the inductive circuit and generating a first signal corresponding to a signal output by the inductive circuit, a comparative circuit electrically connected to the signal generator for converting the first signal to a second signal to be compared with a reference signal to generate a control signal, and a controlling device electrically connected with the inductive circuit and the comparative circuit for receiving the control signal and gradually increasing a current input into the inductive circuit corresponding to the control signal, thereby eliminating the peak current introduced when the inductive circuit is actuated.

Abstract: A robot system having a plurality of component units. The robot system includes a first storage device, and a second storage device. The robot system further includes a plurality of control device, with a first component control device provided with the first component and a second component control device provided with the second component wherein the first component control device controls the first component device and the second component control device controls the second component.

Abstract: A motor drive control system suitable for control of a drive of a motor, such as a brushless motor or a linear motor, having a plurality of excitation phases, through the use of rectangular waves. A drive means is provided for producing an excitation signal to be supplied to each of the excitation phases of the motor and a control means for determining a direction of the excitation signal for each of the excitation phases and for conducting ON/OFF switching. The control means controls the rate of change of the excitation signal to be switched at the switching operation.

Abstract: A motor control system 16 for use within an electric vehicle 10 having an induction motor 12. Control system 16 utilizes a torque control module 18, a vector control module 20 and a space vector PWM module 22 to efficiently and accurately control the torque provided by motor 12.

Abstract: A motor current detection amplifier detects a motor current's areas in two phase periods with reference to a motor voltage. A controlling microcomputer computed the ratio of the motor current areas of the two phase periods and provides the same as phase difference information which is in turn used to control a motor drive voltage to apply a sine wave of a predetermined period to the motor's coil to allow 180°-driving including a sine wave conduction resistant to noise and achieving a reduced cost hike.

Abstract: A driving device for a motor-driven compressor including: (a) a motor-driven compressor for compressing a refrigerant; (b) a DC power supply serving as a power source of the compressor; (c) a capacitor connected in parallel with the DC power supply; (d) a switch provided between the DC power supply and the capacitor, and closed when the compressor is operated and opened when the compressor is stopped; (e) a driver for converting electric power supplied from the DC power supply via the switch and capacitor into driving power for the compressor, and for outputting the driving power thereto; (f) a controller for instructing the driver to operate or stop the compressor; and (g) a discharge control unit provided in the driver and controlling the driver so that the capacitor is discharged using the compressor as a medium, after the instruction to stop the compressor has been given.

Abstract: A method and apparatus for deriving characteristics of a utility signal for synchronizing the output of a motor drive with the utility for a transfer or capture operation. With the drive in idle, the voltage and phase angle of an input to the drive are detected by instrumentation of the drive. The voltage and phase angle of a signal from a utility are detected by instrumentation of the drive at the output of the drive. A phase angle difference and scaling factor are determined to correlate the input signal to the motor drive with the signal at the output of the motor drive. The phase angle difference and scaling factor are stored and subsequently used to derive the signal at the motor due to the utility at the time of the transfer or capture procedure.

Abstract: In a robot device, its functions and performance can be improved with ease. According to the present invention, control device detachably mounted on a prescribed component unit for driving and controlling each of the component units in a prescribed state is provided and thereby, the control device can be easily exchanged for a new one. Thus the robot device in which its functions and performance can be improved can be realized. Further, according to the present invention, a robot device comprises storing device detachably mounted on the prescribed component unit for storing desired behavior type information and thereby, the storing device can be readily exchanged for storing device storing behavior type information different from the behavior type information stored in the former storing device. Thus, the robot device in which its functions and performance can be improved with ease can be realized.

Abstract: An electric power steering device constructed to detect a steering force by a driver and a vehicle speed respectively by a torque sensor and a vehicle speed sensor, set a steering force assisting current for a motor by a steering force assisting current setter using the detected steering force and the detected vehicle speed, set an upper limit value of a motor current by a motor current upper limit value setter based on a power function of a deviation between the set steering force assisting current and a predetermined motor current reference value, and perform overheat protection by setting an upper limit of a motor current applied by a motor driving circuit based on an integrated value of the motor current squared, whereby a problem of an insufficient steering assisting force when stationary steering is repeated many times while parking in a garage in a narrow parking area is solved.

Abstract: A control apparatus for estimating the flux, the current, and the speed of an AC motor using the current and the voltage, and controlling the vector of an AC motor using the estimated speed, a flux command, and a torque current command. The speed is estimated by adding a product of the deviation between the actual value and the estimated value of a magnetization current, the level of a torque current correspondence value, the sign correspondence value of a primary frequency command value, and a gain to an outer product of an estimated current deviation and an estimated flux. Thus, a stable speed estimating operation can be performed to successfully operate the motor in a low speed area in which a voltage frequency applied to the motor is extremely low.

Abstract: The present invention concerns a method for symmetrizing asymmetric faults occurring within a pull chain comprising a synchronous or asynchronous motor controlled by a two-level or multi-level inverter consisting of static switches using semiconductors such as IGBT or bipolar transistors. The invention comprises detecting whether the fault connects or may connect a motor phase with the top point or the bottom point of the inverter power supply, and short-circuiting only the half of the inverter affected by the fault on the basis of the detection.

Abstract: A speed controller circuit for a motor including a ramp function generator having a first input connected to a source of a speed demand signal and a second input connected to a ramp step signal. The ramp function generator develops a frequency ramp that has a slope is initiated by the speed demand signal which is adjusted in response to the ramp step signal. The ramp function generator also provides a frequency signal wherein the ramp function increases or decreases the frequency according to the speed demand. A V/F circuit to the ramp function generator and a space vector modulation circuit are connected to the V/F circuit. The output of the space vector modulation circuit is applied through power switching devices to the motor for speed control. A PI controller circuit is connected to a motor current feedback signal and to a maximum current signal, which provides the current limit of the control system.

Abstract: A system for providing DC current to a rotating superconducting winding is provided. The system receives current feedback from the superconducting winding and determines an error signal based on the current feedback and a reference signal. The system determines a control signal corresponding to the error signal and provides a positive and negative superconducting winding excitation voltage based on the control signal.

Abstract: Disclosed is an improved three-phase direct-current (DC) brushless motor with Hall elements, in which two Hall elements instead of three Hall elements are placed upon the driving circuit of the DC brushless motor to sense the variation of the magnetic flux during the operation so as to reduce the manufacturing cost and also improve the operation precision.

Abstract: An induction motor system operable in a wet environment comprises an induction motor and an apparatus for controlling the speed of the motor. The motor is adapted to be coupled to an AC source for supplying an AC signal. The motor includes a rotor having rotor laminations. The reflected rotor resistance over reflected rotor reactance ratio of the motor is adjusted to be greater than one. The controlling apparatus includes a switching device, user controls, controller means and isolation means. The switching device is connected in series with the motor and is operative in either a high impedance state wherein significant current flow through the motor is prevented or a low impedance state wherein current flow through the motor is substantially undisturbed. The user controls provide motor speed input signals.

Abstract: If a speed instruction value is less than a predetermined value, the output current of a power-conversion unit is controlled to be larger than a current value in an ordinary no-load operation, or a frequency instruction value is calculated based on a speed instruction value in place of an estimated speed.

Abstract: In the synchronous motor controlling apparatus and method of the invention, a current is detected and subjected to coordinate transformation onto &ggr;-&dgr; axis assumed for the rotor. A correction term is obtained on the basis of the derived current and an arbitrarily given correction current instruction. A &ggr;-&dgr; axis voltage instruction is computed by adding a value obtained by multiplying the correction term by a proper gain to a voltage equation in a steady state of the synchronous motor. Consequently, a control method of small computational load, which does not require gain adjustment according to the inertia of a motor load, can be attained.

Abstract: A method and apparatus for switching an AC motor between two power supplies such as supply mains and a variable frequency AC motor drive. In a transfer procedure, the motor drive is coupled to the motor to power the motor. When the supply main is connected to the motor, a resulting current spike in the output of the motor drive is detected to disconnect the motor drive from the motor very quickly while limiting current. In a capture procedure, both power supplies are coupled to the motor with the motor drive in an idle mode. When the supply mains are disconnected from the motor, the voltage change in the output of the motor drive is detected to quickly activate the motor drive to power the motor.

Abstract: A robot system wherein the CPU 102 detects that the posture of the system main body has been shifted from the normal posture into an abnormal posture on the basis of the acceleration information obtained as detection output of the acceleration sensor 41. Then, the body restores the normal posture by a playback technique for controlling various drivers 3D through 7D, using route planning data stored in advance in the memory 101 or restoring the normal posture from a falling posture.

Abstract: A method and a device are described for determining an angular velocity of the rotor of a polyphase machine operated by field orientation without a transmitter, wherein control signals, a stator current model space vector and a conjugated complex reference space vector are calculated by a signal processor containing, among other things, a complete machine model and a modulator, as a function of a flux setpoint, a torque setpoint, a d.c. voltage value, measured power converter output voltage values and system parameters, with a real stator current space vector which is measured and the calculated stator current model space vector are being multiplied by the calculated conjugated complex reference space vector, and the imaginary components of the results being compared with one another, and the system deviation determined therefrom being used to adjust the angular velocity of the rotor as a system parameter in such a way that the system deviation thus determined becomes zero.

Abstract: A method of regulating a rotary machine powered by a voltage inverter receiving, as input a control voltage delivered by a regulator servo-controlling the electromagnetic torque on a reference value, the regulator receiving as inputs the reference value and sat least one sampled signal representative of the torque of the machine, and proceeding at each sampling instant, to predict the torque value at a following sampling instant and to modify the control voltage accordingly.

Abstract: An electric drive apparatus and method for controlling AC motors wherein the motor output is a function of the AC input power to the electric drive. Means are provided for detecting the voltage being supplied by the source of AC power to a drive circuit, and for controlling the output to the multi-phase AC load based on the detected power level. A controller monitors the motor voltage, current, and speed or frequency while also receiving a command for torque or torque-producing current. The method provides for the operation of the drive circuit in the event of either a partial or complete loss of AC input power. During such an occurrence, the controller will cause the circuit to generate sufficient negative torque to cause a power flow from the motor to the drive circuit to substantially equal the inherent losses in the drive and motor to avoid loss of energy from the DC filter of the drive circuit. Motor torque and speed can be quickly restored when the input power is re-established.

Abstract: Provided to the motor controller is means which can alter a conversion ratio with which a current value detected by a current detector is converted into a unit (such as voltage) easily usable in motor current control. According to the maximum current value of the motor and the demagnetization limit current of the motor, an optimum conversion ratio is selected and switched over. By making it possible to select and preset a conversion ratio, a plurality of motors can be controlled by one motor controller. Therefore, restrictions imposed on the combination of the motor controller and the motor are lightened.

Abstract: A magnetic bearing apparatus for the journalling of a body (3), in particular of a rotor (3), comprises a stator (2) with a control winding (22) having at least three loops (22a, 22b, 22c) for the production of a magnetic control field by means of which the position of the body (3) relative to the stator (2) can be regulated, with each loop (22a, 22b, 22c) belonging to a different electrical phase, and a control device (4) which in a first operating mode supplies each loop (22a, 22b, 22c) with in each case a phase current (Ia, Ib, Ic) or in each case a phase voltage (Ua, Ub, Uc) as a control parameter. Means are provided in order to regulate the control parameter for each loop (22a, 22b, 22c) independently of the control parameter for the other loops (22a, 22b, 22c) as well as a monitoring unit (5) which can switch the bearing apparatus into a second operating mode in which a reduced number of phases, which is at least two, produces the magnetic control field.

Abstract: A motor control apparatus and a method according to the invention are directed to detecting a motor velocity. Leading and trailing edges of two pulse signals are distinctively detected from one another, and those pulse signals have cycles which are proportional to the motor velocity, and a phase difference between the pulse signals is about one quarter of a single cycle from one another. Then, a period of time between the pulse edges in the same direction of the same pulse signal is measured, and the period of time between the pulse edges is used to sequentially convert it into the motor velocity and thereby detect the motor velocity.

Abstract: The invention concerns a method for varying and regulating the speed of a motor powered from a voltage source via a triac, comprising, at least once every alternating voltage interval, the following steps: computing a surface value (SI) corresponding to the current absorbed (I(M)) in a time window (FT) defined between a current switching-on time (ti) and a following current switching-off time (tf); computing an interval of the next triac-blocking time (t′); and controlling the starting of the triac when the blocking time (t′) has elapsed. The invention is useful in particular for household electric and house automation appliances operating at variable speed.

Abstract: A control algorithm or method for use in controlling a voltage-fed induction machine. The control algorithm includes the following steps. The DC link voltage supplied to an inverter driving the induction machine is monitored. When the DC link voltage is high enough, the algorithm controls the amount of current supplied to the induction machine to provide current controlled operation of the induction machine. When the DC link voltage is not high enough to control the current under transient conditions, the induction machine is controlled by imposing the maximum possible phase voltage and optimal slip angle on the machine to provide maximal torque per ampere operation of the induction machine. The maximal torque per ampere operation is performed when either of the following conditions is met: a) the torque level required by the induction machine is such that efficiency optimization cannot be performed, or b) current regulators approach saturation.

Abstract: Methods and systems consistent with this invention detect a failed short thyristor in a solid-state controller for delivering power to a load during reduced-voltage operation. Methods and systems consistent with this invention detect a voltage across the thyristor, and indicate a failed short thyristor when the absolute value of the voltage across the thyristor remains below a threshold value during a predetermined period of time. Methods and systems consistent with this invention detect a failed open thyristor in a solid-state controller during full-voltage operation. Methods and systems consistent with this invention detect a voltage across the thyristor, and indicate a failed open thyristor when the absolute value of the voltage across the thyristor exceeds a threshold value.

Abstract: This invention relates to a brushless motor driving system for driving an outdoor fan of an air conditioner. The driving system comprises a Hall probe for detecting a position of a rotor of a brushless motor, an inverter circuit consisting of a plurality of switching elements connected in bridge-connection, the inverter circuit supplying ON-OFF electric current to stator coils of the brushless motor, and a control means for switching the switching elements of the inverter circuit based on the output signal of the Hall probe. Before starting the motor, the control means controlls the switching elements to supply DC PWM-controlled current to the stator coils of the motor in synchronization with the output signal of the Hall probe, so that the rotor is braked, stopped and set to home position.

Abstract: A noise attenuation circuit is disclosed for use in a motor drive system. The motor drive system includes a pulse width modulated inverter providing three phase power output on three phase conductors for driving a motor. The noise attenuation circuit includes a common mode transformer including a common mode choke having a core, with first, second and third choke windings on the core, each connected in series with one of the three phase conductors, and a fourth winding on the core. A three phase iron core transformer creates a neutral point representing common mode voltage. The iron core transformer has three primary windings connected in a wye configuration. A capacitor circuit is connected between the iron core transformer and the three phase conductors to prevent saturation of the iron core transformer during pulsed DC operating conditions. The fourth winding is connected between the neutral point and the DC bus and develops a common mode voltage that is equal and opposite to that existing in the system.