Showing papers on "Three-phase published in 1994"

Indirect sensing for rotor flux position of permanent magnet AC motors operating over a wide speed range

Abstract: This paper describes an indirect sensing, or sensorless, method for rotor flux position for brushless permanent magnet (BPM) motors operating over a wide speed range, while keeping maximum torque per ampere and/or maximum efficiency capabilities. The method described is particularly applicable to trapezoidal back emf type of BPM motors. The typical trapezoidal waveform of the motor internal voltages (or back emf) contains a fundamental and higher order frequency harmonics. In particular, the third harmonic component is extracted from the stator phase voltages while the fundamental and other polyphase components are eliminated via a simple summation of the three phase voltages. The resulting third harmonic signal keeps a constant phase relationship with the rotor flux for any motor speed and load condition, and is practically free of noise that can be introduced by the inverter switching, making this a robust sensing method. In contrast with indirect sensing methods based on detection of the back-emf signal that require heavy filtering, the third harmonic signal needs only a small amount of filtering to eliminate the switching frequency and its side bands. As a result, the method described here is not sensitive to filtering delays, allowing the motor to achieve a good performance over a wide speed range. Motor starting is also superior with this method since the third harmonic signal can be detected and processed at lower speeds than for the conventional method of back-emf sensing.

Small-signal modeling and control of three-phase PWM converters

Abstract: The paper presents a systematic approach to small-signal modeling, and control design of three-phase PWM converters. The well established design procedures used in DC-DC converter control design, are adapted for the three-phase converter control based on the similarity in the small-signal dynamics of the three-phase converters and their DC-DC counterparts. The approach is especially beneficial in three-phase rectifier control, which is reduced to a single-input single-output system after closing the current loops. The modeling approach is verified on a 10 kW three-phase boost rectifier switching at 15.6 kHz with DSP control. A wide-bandwidth output voltage control loop is then designed based on the verified small-signal transfer functions. >

A novel three-phase utility interface minimizing line current harmonics of high-power telecommunications rectifier modules

Abstract: Based on the combination of a three-phase diode bridge and DC/DC boost converter a new three-phase three-switch three-level PWM rectifier system is developed. It can be characterized by sinusoidal mains current consumption, controlled output voltage and low blocking voltage stress on the power transistors. The application could be, e.g., for feeding the DC link of a telecommunications power supply module. The stationary operational behavior, the control of the mains currents and of the output voltage are analyzed. Finally, the stresses on the system components are determined by digital simulation and compared to the stresses in a conventional PWM rectifier system. >

A complete DC and AC analysis of three-phase controlled-current PWM rectifier using circuit D-Q transformation

Abstract: A recently proposed circuit P-Q transformation is used to analyze a three-phase controlled-current PWM rectifier. The DC operating point and AC transfer functions are completely determined. Most features of the power converter are clearly interpreted. They are: (1) the output voltage can be controlled from zero to maximum; (2) the system is equivalently an ideal current source in the steady state; (3) the system can be described as linear circuits; and (4) the input power factor can be arbitrarily controlled within a certain control range. >

Power system applications for phasor measurement units

Abstract: State-of-the-art technology now permits measurement and analysis of power system performance on a scale not previously possible. Synchronized sampling, derived from the GPS (Global Positioning System), and high accuracy sigma-delta analog-to-digital converters form the basis for a system that can measure the state of the power system at a given instant over any area. Samples are acquired 12 times per cycle and are processed by a recursive discrete Fourier transform (DFT) algorithm. This produces the magnitude and angle of the input signal for each sample. The resulting phasors can be combined to produce a positive sequence phasor for a set of three phase inputs. The positive sequence quantities can be used to calculate voltage and current phase and magnitude, real and reactive power, frequency, and impedance. Since the positive sequence parameters are acquired at the same instant (within 1 microsecond), the state of the system at the measured nodes is known at the sample time. Applications to fault recording, disturbance recording, transmission and generation modelling verification, and power system stabiliser testing, are discussed. >

Digital-signal-processor-based control of three-phase space vector modulated converters

Abstract: The paper presents the implementation of a DSP-based controller for three-phase, space-vector modulated converters. The implementation is illustrated for the control of a 2 kW, ZVS matrix power converter-based three-phase PWM rectifier. The controller features very high data processing speed (converter switching frequency of 100 kHz), and provides high-quality, low-distortion power converter input currents and output voltages. The controller can be implemented using only a few standard integrated circuits, providing high reliability and low cost. >

Space vector modulated, zero-voltage transition three-phase to DC bidirectional converter

Abstract: Operation and implementation of the novel, space vector modulated, zero-voltage transition, three phase voltage source inverter/boost rectifier is presented. The converter is intended for high performance, medium power applications requiring bidirectional power flow. The proposed modified space-vector modulation allows all switches to be operated with soft-switching and constant frequency. The modulation algorithm also eliminates any low frequency distortion caused by the zero-voltage transitions and can be applied to any soft switching PWM three-phase converters. A simple, digital signal processor based implementation of the modulator and current regulators, operating with 30 /spl mu/s sampling time, is described. Measured efficiency of the 30 kHz, IGBT prototype is around 95%, and distortion of the three-phase currents is extremely small. >

Fault mode single-phase operation of a variable frequency induction motor drive and improvement of pulsating torque characteristics

Abstract: Improved reliability and fault tolerant operation of power converter systems are extremely important for industrial AC drives. The paper considers variable frequency variable voltage operation of a three-phase induction motor in single-phase mode for two common faults of a three-phase inverter, i.e., open base drive and device short-circuit. The motor performance has been extensively analyzed in single-phase mode and remedial strategies have been developed to neutralize large second and other lower order harmonic pulsating torques. In a single-phase open loop volts/Hz control made of a faulty three-phase inverter, it has been demonstrated that odd harmonic voltages at appropriate phase angles can be injected to neutralize the low frequency pulsating torques so as to permit smooth drive operation. It has been shown that the pulsating torque can be further reduced by load dependent flux programming rather than operating with constant rated flux. >

Asymmetrical three-phase load-flow study based on symmetrical component theory

Abstract: For asymmetrical three-phase load-flow study, two methods based on symmetrical component theory, the bus admittance method and the decoupling compensation method, are proposed. By using the decoupling feature of symmetrical-element models in a positive-negative-zero sequence reference frame, such as those of generators and transformers, the bus admittance method can obviously increase the speed of three-phase load flow computation without losing its accuracy. By further using the weak coupling feature of the asymmetrical transmission line model in a positive-negative-zero sequence reference frame, the decoupling compensation method breaks the three-phase coupled model into three sequence-decoupled models with the injection of certain compensation currents into both terminal buses of the line, and thus opens up a way to the utilisation of parallel processing. The validity of these methods is verified by numerical examples.

An ideal PWM-VSI inverter using only one current sensor in the DC-link

Abstract: This paper describes how to improve the performance of open loop, low cost PWM-VSI controlled AC machines by the use of one current sensor. The performance is especially improved at low speed where nonlinearities like blanking-time and voltage drop are dominant. An often used modulation technique is described and basic compensation techniques are presented. Only one current sensor in the DC link is used for reconstruction of all three phase currents and the currents are used for compensation. Different limitations are discussed. The compensation techniques are tested in a 16-bit microcontroller based inverter. Tests show that the phase currents can be reconstructed by measurement of the DC link current both at low and high speed. They also show that a highly improved torque-speed characteristic can be obtained by using the compensation techniques and an almost ideal inverter is obtained. >

Simple high performance three-phase boost rectifiers

Abstract: Important advantages of using a DC rail diode in the three-phase PWM boost rectifier are recognized, including reduced diode reverse recovery and high reliability, as well as guaranteed six-step operation. Several simple soft-switched three-phase boost rectifiers were developed based on using the DC rail diode, further improving the converter performance by minimizing the switching losses. All these advantages can be achieved with three simple independent analog controllers. Consequently, these new rectifiers can run at much higher switching frequencies, and achieve higher efficiency and higher power density with lower cost and higher reliability than the conventional one. >

Control of front-end three-phase boost rectifier

Abstract: Modeling and control of a three-phase PWM boost rectifier is presented. A current controller in rotating coordinates assures unity input power factor and fast output voltage regulation. Sensitivity of stability margins to different loads is analyzed. Two typical loads, a DC-DC power converter and an inverter supplying an AC motor, are considered. Linear and nonlinear current controllers in the direct axis are compared. With the nonlinear controller, which employs load current feedforward, the rectifier sensitivity to load variations is greatly reduced and a single optimal output voltage compensator can be designed for all load conditions. Small-signal output impedance is significantly improved, and faster transient response is obtained. The results are illustrated with simulations of a practical 10 kW rectifier. >

Harmonic resonance at a medium sized industrial plant

Abstract: Harmonic resonance is a concern for industrial plants because approximately 50% of the normal electrical load consists of computer equipment, switch mode power supplies and AC induction motor adjustable speed controllers. Due to the nonlinear characteristics of these loads, significant harmonic voltages and currents are produced on the electrical distribution system. Measurements were made with a harmonic analyzer at several locations in an electrical distribution system to quantify harmonic currents and voltages. The system impedance diagram, prepared from the system one line diagram, was used to model the electrical system and calculate power system natural frequencies. Calculations indicated harmonic resonance occurs near the seventh harmonic frequency depending on configuration of the electrical power system. The plant equivalent circuit was prepared for several harmonic frequencies to show the amplification of currents and voltages due to the effects of harmonic resonance. Calculations to determine network harmonics are presented using a simplified system representation in combination with traditional circuit reduction methods. The results of these calculations are shown to be valid by actual system measurement. >

PSPICE simulation of three-phase inverters by means of switching functions

Abstract: Static power converters can be analyzed by means of widely available circuit simulation software packages such as PSPICE. However, they are usually modeled as a set of real switches, which results in long execution times and possible convergence problems in the case of complex circuits. This paper proposes macromodels to simulate three-phase power converters on such packages. The proposed macromodels are based on converter switching functions rather than actual circuit configuration, and they are suited for steady state and large signal transient analysis at system level. In this approach, voltage source inverters (VSI), current source inverters (CSI), and controlled rectifiers (CR) are simulated as multiport networks avoiding the physical nonlinear micromodels of the power switches. Computer memory and the run-times required for the simulation are thereby minimized. Complete examples of VSI, CSI and CR, with different PWM techniques, are given with specific reference to the PSPICE software to illustrate the effectiveness of the proposed models. >

A novel three-phase single-switch discontinuous-mode AC-DC buck-boost converter with high-quality input current waveforms and isolated output

Abstract: In this paper, a new three-phase single-switch AC-DC flyback converter system is presented. The system operates in the discontinuous mode. The simple structure of its power and control circuit, low mains current distortion, and resistive fundamental behavior, as well as the high-frequency isolation of the controlled output voltage, have to be pointed out. Besides the analysis of the stationary operating behavior, the dependencies of the peak values, average values, and rms values of the device currents, and of the maximum blocking voltages across the power electronic devices on the circuit parameters, are given as analytic approximations. The theoretical analysis is verified by digital simulation. >

An improved voltage vector control method for Neutral-Point-Clamped inverters

Abstract: A new PWM method for Neutral-Point-Clamped (NPC) inverters is introduced. The method is based on closed-loop control of the line-to-line voltage vectors. It uses independent hysteresis comparator controllers to regulate the direct and quadrature axis components of the three phase output voltages. The controllers select the appropriate inverter output voltage vectors through an EPROM table. The closed-loop control allows a high performance over the whole range of operation, even when low speed devices such as the GTO are used. A neutral-point potential control is described, which is capable of stabilizing the variations within fixed limits during steady and transient states. The principle of the method is discussed and the vector selection technique is presented. The effectiveness in the output-voltage-waveform generation and the balance of the DC-link capacitor voltages are verified by simulation and experiment. >

Cross current control for parallel operating three phase inverter

Abstract: A synthesis method of the cross current control system for parallel operating three phase PWM inverters is proposed. The mathematical descriptions of the steady state, transient response and higher harmonic components of cross current are derived to obtain a guide of design. Simulations are provided to confirm such a theory. >

Power converter with emergency operating mode for three phase induction motors

Abstract: A DC-to-AC power converter that is used to drive a three-phase motor while providing for fail soft and emergency operation. The power converter employs a conventional three half-bridge arrangement of power poles and adds a fourth pole to provide for an emergency operation in the event of failure of one of the three normal poles. The fourth (emergency operation) pole is coupled between a pulse width modulated controller and the neutral wire of the motor and is powered by first and second DC power busses. The controller is configured to disable a failed one of the three normal poles, and to control the fourth pole subsequent to disabling of the failed pole. Fuses or other disconnect devices are also added between the three normal poles and the respective windings of the motor. The present power converter allows continued motor operation at a reduced power level in the event of a failure of one of the power poles. The present invention has application to electric vehicle power systems and may be used in a critical process where temporary operation must be maintained.

Leakage flux and force calculation on power transformer windings under short-circuit: 2D and 3D models based on the theory of images and the finite element method compared to measurements

Abstract: The paper compares different techniques based on the theory of images and the finite element method enabling to calculate the leakage field and the static electromagnetic forces on the windings of power transformers. The models developed have been applied to the one phase part of a three phase shell type power transformer and the results are verified experimentally. Both cases, that of approximate two dimensional analysis and the three dimensional configuration, have been studied. A particular three dimensional finite element formulation involving only a scalar potential was found to be suitable for this class of problems. >

Losses due to rotational flux in three phase induction motors

Abstract: This paper discusses rotational losses and how they are produced in the core materials of induction motors. These losses are largely caused by flux that rotates in the plane of the machine laminations. This suggests that steel specification for applications to rotating machines should be given in terms of rotational loss data as a material characteristic, in much the same fashion as Epstein test results are provided for alternating losses. If a standardized test for rotational losses were to be used, steel producers could rationally investigate the effects of composition and processing variables. This is necessary in order to produce low loss steels for motor applications. Reduction of rotational losses in motor cores could significantly lower AC machine operating costs and contribute to the growing interest and design of high efficiency induction motors. The paper describes a test procedure for determining rotational losses in a sample. It then compares the results with standardized tests from an Epstein test procedure. It is seen that there are significant differences in loss results obtained for the rotational test versus the alternating current test. The authors have investigated a time harmonic finite element formulation utilizing Magnet 2D, a commercially available package. The paper includes a brief analysis of a typical problem using this tool. >

A three-phase regulated PWM rectifier with on-line feedforward input unbalance correction

Abstract: The analysis and design of a direct six-switch three-phase PWM rectifier, capable of correcting input unbalance, is presented. Based on the input source positive and negative sequence components, an unbalanced transfer matrix in terms of input phase voltages is derived. An online method is used to implement the transfer matrix function and generate the switch gating signals. As compared to other unbalance correction methods, the proposed approach is very simple to implement. It uses only a few discrete analog and digital components. The algorithm of the proposed approach is described in this paper, and results are verified from a 1 kVA breadboard set-up. >

New single-switch three-phase high power factor rectifiers using multi-resonant zero current switching

Abstract: New single-switch three-phase high power factor rectifiers are introduced which have continuous input and output currents. By the use of a multi-resonant scheme, the transistor operates with zero current switching and the diodes operate with zero voltage switching. These multi-resonant rectifiers with a single transistor are capable of drawing a higher quality input current waveform at nearly unity power factor and lower stress than the quasi-resonant rectifiers. Buck-type converters are used for the power stage, and hence the output voltage is lower than the input voltage. Moreover, these rectifiers have a wide load range and low stress on semiconductor devices. Simulation and experimental results are presented. >

Active filter control

Abstract: A set of command signals is generated for application to the respective lines of a four-wire three phase power distribution system to compensate for harmonic distortion and neutral unbalance. A transform operation is performed upon a first set of signals, which collectively represent the uncompensated load current in each of the main transmission lines of the power distribution system, to generate a second set of signals which collectively represent the active and reactive components of the current in the system. Signal components at the fundamental frequency of the three phase system are removed from the active and reactive components to provide only harmonic active and reactive components. A reverse transform operation is performed upon the harmonic components to provide a set of command signals.

A three-phase high power factor PWM ZVS power supply with a single power stage

Abstract: This paper introduces a new switching mode power supply with the following properties: zero-voltage switching; pulse-width modulation at constant frequency; three-phase input with high power factor; and low input current distortion, using a single power stage formed by a full-bridge DC to DC power converter. Circuit operation, theoretical analysis, simulation, and design procedures and examples are presented. A laboratory prototype rated at 1.5 kW and operating at 100 kHz has been fabricated and tested successfully. The corresponding experimental results are included in the paper. >

Which is better at a high power reactive power compensation system, high PWM frequency or multiple connection?

Abstract: In general, higher PWM frequency gives better performance at small or medium capacity inverters. This paper deals with the comparisons between two systems, SB and MB at a multi-10 MVA GTO power converter for reactive power compensation from the viewpoints of desired switching frequency and converter connections. SB is a single bridge system consisting of a three-phase bridge power converter unit and a transformer and its PWM frequency varies from 450 Hz to 1800 Hz. The line frequency is 50 Hz. 1 MB or 3 MB is a multiple bridge system consisting of plural single- or three-phase power converter units and transformers. Two kinds of systems using different transformer windings are considered. Type A uses conventional but complicated zigzag windings and type B simple star and delta windings. The switching frequencies are chosen the lowest, 50 Hz for 1 MB and 150 Hz for 3 MB. The evaluated items are harmonic distortion, power loss, GTO utilization factor and control response. A simulation study shows that MB is superior to SB in any case, even in the case of severe line faults where very quick response is requested. A new control strategy is adopted for obtaining quick response. Type B system has almost the same characteristics as type A system and can easily conquer the DC magnetization of the power converter transformers that may be encountered at a line voltage disturbance. >

PWM converters for three phase AC power control and AC to DC conversion

Abstract: PWM converters provide buck, boost, buck-boost, flyback, and Cuk configurons for three phase AC-AC power control. In an AC-DC embodiment, only a single power conversion circuit is used in a configuration including a pair of series connected gate turn off devices which are oppositely poled and oppositely timed, are connected directly to an AC input source, and provide an output to a high frequency isolating transformer. The transformer provides an output to a rectifier, which thus provides the DC output.

Dc/ac inverter apparatus for three-phase and single-phase motors

Abstract: Electrical power apparatus is disclosed for selectively powering either a three-phase ac motor or a single-phase ac motor. A dc/ac inverter is configured to provide appropriately phased input signals to the three input terminals of whichever motor is being driven, with no substantial hardware reconfiguration being required. In addition, a special clamp circuit prevents an excessively high voltage from being applied to the inverter by a variable-voltage power source such as a photovoltaic array, whereby the inverter need not be sized to accommodate voltage levels substantially greater than its design voltage.

Alternating current generator with direct connected exciter winding

Abstract: A three phase four pole alternating current generator includes an AC exciter including a series winding for each phase of the main generator windings and a shunt winding for each of the main phase windings. The series exciter windings includes two coils. In a high Y connection of the winding, the two coils are connected in series with each other and with the main phase winding. In the low voltage Y connection, series coils are separated and each is located in a paralleled leg of the same main phase winding. The shunt winding is connected directly across a corresponding main phase winding. An electronic voltage regulator includes phase voltage sensing which is coupled to a reference control circuit to produce a control signal if the voltage varies from a reference voltage. The current to the shunt winding of the exciter is controlled to maintain the desired output voltage. The system establishes and maintains a selected voltage in a balance condition with minimum circuit connection and improved overall performance and is applicable for all sizes of generators of a particular design.

Vector control of induction motor with split phase stator windings

Abstract: A vector controlled scheme is described for an induction motor with split phase windings. Such a motor is obtained by splitting the phase windings of a conventional three phase motor with an angular separation of 30 electrical degrees between the axes of the two halves. In the proposed scheme, the motor is run as a three phase machine by connecting the split phase windings in series. However the winding taps are utilised for making voltage measurements. Based on a space phasor model of the split phase machine, a method of acquiring the position of the rotor flux is developed. The scheme is based only on the voltage and current measurements, without the need for position encoders. It is also independent of machine resistance values. Results from computer simulation as well as from an experimental prototype drive are presented and discussed. >

Step-up/down three-phase AC to DC convertor with sinusoidal input current and unity power factor

Abstract: A novel three-phase AC to DC power convertor is proposed to achieve sinusoidal current waveform and unity power factor at the input, and to simplify the control of all switches. In addition, the new convertor has both step-up and step-down capability and no current sensor is required for the current control. The steady-state analysis such as DC model, harmonic analysis of the input current and some design considerations are presented. Finally, some experimental results are given for demonstration.