Showing papers on "Harmonic published in 1988"

Multiple-harmonic conversion of 1064 nm radiation in rare gases

Abstract: The authors report the observation of very-high-order odd harmonics of Nd:YAG laser radiation in rare gases at an intensity of about 1013 W cm-2. Harmonic light as high as the 33rd harmonic in the XUV range (32.2 nm) is generated in argon. The key point is that the harmonic intensity falls slowly beyond the fifth harmonic as the order increases. Finally, a UV continuum, beginning at 350 nm and extending down towards the short wavelength region is apparent in xenon.

Programmed PWM techniques to eliminate harmonics - A critical evaluation

Abstract: Programmed pulsewidth modulators (PWMs) eliminating several lower-order harmonics generate high-quality output spectra, which in turn result in minimum current ripple and reduced torque pulsations, thereby satisfying several performance criteria and contributing to overall improved performance. Several programmed PWM switching patterns to eliminate harmonics in the output spectra of single-phase and three-phase inverters are possible. Each of these leads to a specific advantage in single- and three-phase inverters, depending on the application. First, the superiority of programmed PWM techniques over the conventional carrier-modulated PWMs is established. Next, a critical evaluation of the programmed PWM schemes on the basis of applications for single- and three-phase inverters is used to provide a framework and guidelines for the selection of the appropriate technique for each application area. Evaluation criteria include harmonic loss factor and total harmonic distortion factors defined at the input and output of the inverter terminals. Finally, a simple low-cost solution for obtaining the required PWM switching points is proposed. Selected results are verified experimentally. >

Induction motor analysis by time-stepping techniques

Abstract: A numerical approach is described that overcomes the deficiencies of other methods through the use of a two-dimensional, nonlinear, time-stepping finite-element method for excitation from a constant voltage source. Comparison of stator current for no-load and load conditions shows good agreement with test values on a large induction motor. It is shown that the method can provide a great deal of information on flux distribution and harmonic rotor bar currents. >

A new approach to harmonic compensation in power systems

Abstract: A novel approach to compensate for harmonics in power systems is proposed. The approach differs from conventional passive and active filters in its compensation principle. A practical system configuration to implement the approach is presented. It consists of a small-VA-rating PWM (pulsewidth-modulated) converter and a passive filter. The compensation principle is described, and compensation characteristics are examined theoretically and experimentally. The practicability and validity of the approach are experimentally demonstrated. >

A Mirror with an Intensity-Dependent Reflection Coefficient

Abstract: A device exhibiting a power-dependent reflection coefficient with fast time response and no frequency shift is proposed and analysed. It utilizes a combination of a nonlinear crystal for second harmonic generation and a dichroic mirror with high reflectivity for the second harmonic and partial transmission for the fundamental wavelenth. This device has been successfully used in a preliminary experiment to modelock a Nd:YAG laser.

PWM technique for power MOSFET inverter

Abstract: A sinusoidal PWM (pulsewidth-modulated) inverter suitable for use with power MOSFETs is described. The output waveforms in the proposed PWM inverter are investigated both theoretically and experimentally. A modulating signal for the three-phase PWM inverter is obtained by adding the harmonic components of integer multiples of three to the three-phase sine waves. By using the proposed modulating signal, the amplitude of the fundamental component is increased about 15% more than that of a conventional sine-wave inverter and the commutation number of the inverter is decreased to two-thirds of a conventional one. >

Harmonic current and reactive power compensation with an active filter

Abstract: A static VAr compensator (SVC) using an active filter has been developed that compensates reactive power, harmonic current, negative-phase current, and voltage fluctuations. The system configuration is described, and five types of control scheme for the filter, which are based on practical applications for various loads, and the performance characteristics for each type of control are analyzed. The active filter is shown by simulation to be more effective for suppressing arc-furnace flicker than the TCR (thyristor-controlled-reactor) SVC. >

Analytical analysis of the harmonic effects of a PWM AC drive

Abstract: A novel analysis is presented of the harmonic content of current, torque pulsations, and harmonic copper losses of a three-phase induction machine fed by a two-phase pulsewidth modulation (PWM) inverter. The purely analytical results are based on the assumption that the switching frequency is high compared with the fundamental frequency. It is shown that the results hold accurately for frequency ratios f/sub s//f/sub 1/>9. >

On frequency estimation

Abstract: SUMMARY t This paper discusses a least-squares procedure and the use of the periodogram for isolating a discrete harmonic of a time series. It is shown that the usual asymptotics on estimation of frequency, amplitude and phase of such a harmonic have to be used with great caution from a moderate sample perspective. Computational issues are discussed and some illustrations are provided. Bolt & Brillinger (1979) make use of these asymptotic results. We consider a time series model of the form X, = a cos {wt + (f>) + e,, where e, is a stationary noise sequence, and one is interested in estimating the amplitude, frequency and phase of the harmonic component. The asymptotic theory of the least-squares estimates of these parameters has a long history. Whittle (1951,1953) obtained some of the earliest results. More recent results are by Hasan (1982), Hannan (1973) and Walker (1971), who formalize and extend Whittle's results. In these works it is shown that the asymptotic variance of the frequency estimate is of order n~3 and that the asymptotic variances of the other two components are of the more usual order n~\ These results extend when there are several harmonic components. The rate for the estimate of w seems almost unbelievably good, and our work was motivated by a desire to see how reliable the asymptotic theory is. In brief, we find that the product of the amplitude and the sample size, n, must be quite large in order for the asymptotic theory to be meaningful. If this product is not large, the frequency estimate is much more variable than indicated by the asymptotic theory and the amplitude estimate is severely biased. In applications in which the amplitude is small, giving rise to a small peak in the periodogram, these results suggest that naive application of the asymptotic theory to gauge resolution can be quite misleading. Section 2 of this paper is devoted to a review and examination of the asymptotic theory. We are also concerned with computational issues arising from the least-squares problem. This problem is nonlinear in the parameters, so that some sort of iterative search must be employed. Typically, search methods start from an initial guess and then proceed by a sequence of modified Newton-Raphson steps. For this nonlinear least-squares problem, it turns out that there are many local minima with a separation in frequency about n~l which makes the stationary point to which the iterative scheme converges extremely sensitive to the starting values, and this problem gets worse as the sample size increases. Furthermore, it follows from the results of § 2 that the estimate of the amplitude is very biased unless the frequency is resolved with order o(n~') so that failure to converge to the global minimum may give a very poor estimate of amplitude. The problem becomes

A new injection method for AC harmonic elimination by active power filter

Abstract: An injection method for an active filter which eliminates the harmonics present in AC lines by injecting PWM harmonic compensating current is proposed. In the proposed method, the active filter produces a pulsewidth modulation (PWM) current that cancels the existing harmonics up to any order completely. To generate such PWM current, both inverter and DC current source is needed. The current source can be replaced by a large inductor without any external power source. This can be achieved by providing the inverter with rectifying capability because the inverter has the same circuit structure as the rectifier. Therefore, the proposed model of PWM injection current includes not only the harmonic components to suppress the existing harmonics up to any order, but also the fundamental one, to raise the inductor current to any desired value. The characteristics of the injection method are investigated through a digital computer simulation. Feasibility is proved by the experimental results. >

Newton-type algorithms for the harmonic phasor analysis of nonlinear power circuits in periodical steady state with special reference to magnetic nonlinearities

Abstract: Computer algorithms of the Newton-Raphson type are derived for the harmonic analysis of systems containing nonlinear dynamic components in periodic steady state. The problem is formulated in the complex conjugate multiharmonic space that inherently represents the harmonic coupling between the different harmonic frequencies. The theory is applied to single-phase systems, including magnetic nonlinearities. >

A new family of resonant rectifier circuits for high frequency DC-DC converter applications

Abstract: A family of rectifiers suitable for operation at high frequencies is presented. These rectifiers use naturally occurring component parasitics to control diode switching thereby minimizing parasitic ringing and improving rectification efficiency by reducing the flow of harmonic currents. The input impedance of the resonant rectifier is linear, which makes possible an accurate adjustment of the rectifier to present the proper load impedance to an inverter. When a resonant rectifier is coupled to a resonant inverter in this manner, a fully resonant DC-to-DC converter is produced. With these circuits it is possible to achieve a very low input/output ripple and EMI since voltages and currents seen by the filters are confined to a very narrow frequency range compared to conventional squarewave converters. >

Voltage harmonics generated by voltage-fed inverters using PWM natural sampling

Abstract: The harmonics generated by natural sampled pulse width modulation are investigated experimentally and by theoretical models for the outputs obtained with triangular and sawtooth carrier signals. These equations are valid for synchronous and asynchronous operation. It is shown that triangular carriers generate less harmonics than sawtooth carriers, and that in synchronous operation with single-phase modulation, the carrier-to-output frequency ratio may be any integer value. >

A novel control of a current source active filter for AC power system harmonic compensation

Abstract: A novel control strategy for an active power filter that consists of a current-source PWM (pulse width modulation) converter and LC filters to eliminate the PWM carrier frequency components is proposed in order to obtain good harmonic compensation characteristics. The PWM converter is controlled with feedback loops of filter input currents and their derivatives to suppress the transient oscillations caused by the LC filters without inserting damping resistors. Optimum values of feedback gains are investigated. The cut-off frequency of the optimum design is about one seventh of the PWM carrier frequency, and the compensation delay time is two times as large as the carrier period. The performance of the active power filter with the proposed control strategy was confirmed by experiments in which the harmonic currents generated by a phase-controlled bridge converter were compensated. Input current after compensation was essentially sinusoidal, and the harmonic content was reduced to less than 4%. >

A novel harmonic power filter

Abstract: A novel harmonic power filter system consisting of passive LC filters and voltage source pulsewidth-modulated (PWM) converters is described. This filter system has the merits of both conventional passive LC filters and conventional active power filters. The operating principle and a control scheme for the system are described. Compensation characteristics are discussed from both a theoretical and experimental point of view. The practicability and validity of compensation for large-capacity thyristor converters is experimentally demonstrated. >

A model for the effect of stress on the low‐frequency harmonic content of the magnetic induction in ferromagnetic materials

Abstract: A simple model used previously by the authors to explain stress variation of magnetic hysteresis is now employed to explain the effect of stress on the amplitudes of the first‐ and third‐order harmonics of the magnetic induction signal resulting from application of an ac magnetic field of low frequency to a steel specimen An improved expression for the effective field contribution Hσ due to stress has been derived from thermodynamic considerations

Design and performance of a high frequency link induction motor drive operating at unity power factor

Abstract: The design and performance of a complete three-phase converter system and field-oriented induction motor drive based upon a 20 kHz AC link is described. By using the same converter for the AC input side and the output load side, it is shown that power can be transferred in either direction. It is also shown that, with the use of a current regulator, both power flow on the link and the link voltage amplitude can be regulated. In addition, by suitable feedback control, the power factor at the input to the converter can be adjusted to unity. Both computer and experimental results show unity power factor operation, low harmonic current in both the input and output of the system, and bidirectional power flow capability. >

Digital spectra of nonuniformly sampled signals. II. Digital look-up tunable sinusoidal oscillators

Abstract: For Part I see ibid., vol.37, no.2, June 1988. The author presents theories and applications of a digital spectrum analysis technique for a class of nonuniformly sampled signals. The structure of the harmonic components present in a digitally synthesized sine wave is analyzed using a table look-up method. The digital table look-up method offers several desirable features, such as high-frequency stability and precision control of both the frequency and the phase of the generated sine wave. However, undesirable spurious harmonic components are generated when one tries to tune to different frequencies by manipulating the memory-addressing mechanism rather than loading a new waveform sample into the waveform memory, which is generally time-consuming and sometimes infeasible. The frequencies, amplitudes, and phrases of all the spurious harmonic components are derived in closed form. >

Theory of Bernstein wave coupling with loop antennas

Abstract: Coupling to ion Bernstein waves near the first harmonic of the ion cyclotron resonance with coil antennas is investigated by using a plane layered model of a tokamak plasma. The boundary conditions in vacuum are solved analytically for arbitrary orientation of the antenna and Faraday screen conductors, in terms of the surface impedance matrix of the plasma for plane waves. The latter is evaluated by solving the wave equations in the plasma by taking into account finite Larmor radius and finite electron inertia effects, cyclotron and harmonic damping by the ions, and Landau and collisional damping by the electrons. – Applications to the Alcator C tokamak give reasonable agreement between the calculated and measured radiation resistance when the first ion cyclotron harmonic is just behind the antenna; outside this range, the calculated resistance is lower than the experimental one. In general, the coupling efficiency is found to be very sensitive to the edge plasma density, good coupling requiring a low density plasma layer in the vicinity of the Faraday screen. Coupling also improves with increasing ion temperature in the scrape-off layer and is appreciably better for antennas with antisymmetric than with symmetric current distribution in the toroidal direction.

Frequency converting apparatus

Abstract: A frequency converting apparatus including a sample circuit and a bandpass filter wherein an input analog signal is sampled at predetermined periods only for predetermined time intervals to produce or increase the harmonic components of the original analog signal, a desired frequency component is extracted through the bandpass filter from the harmonic components to thereby convert the original signal having a particular frequency band to a signal having a higher-frequency band.

High impedance fault analyzer in electric power distribution

Abstract: A system (Fig. 3) and method for detecting an abnormality in a network (10) for distributing or transmitting electric power at a predetermined fundamental frequency. The system produces a signal (40) representing the fundamental frequency and another signal (42) representing a harmonic current occuring in the network. The phasor relation between the fundamental voltage and harmonic current representing signal are compared (44). The system produces a signal (82) indicating the occurence of a high impedance fault in response to a predetermined change in the compared phasor relationship. Embodiments (Figs. 5 and 6) are disclosed utilizing expression of signals within the network in polar coordinates, as well as embodiments (Fig. 11) utilizing signals expressed in rectangular coordinates. Such embodiments include both circuitry for detecting zero crossing phenomena, as well as circuitry for producing representation of signal phasor products.

Calculation of harmonic frequencies and harmonic force fields by the hartree‐fock‐slater method

Abstract: Results are presented from Hartree-Fock-Slater (HFS) calculations on harmonic frequencies and force constants of H2O, H2S, NH3, PH3, CH4, SiH4, and C2H4. Both frequencies and force constants were calculated by a numerical (finite difference) differentiation of analytical energy gradients. It is shown by a comparison with experimental data and results from ab initio Hartree-Fock (HF) calculations that the HFS-method provides harmonic frequencies and force constants in at least as good agreement with experiment as the HF-scheme.

Circular harmonic phase filters for efficient rotation-invariant pattern recognition

Abstract: A generalized approach for pattern recognition using spatial filters with reduced tolerance requirements was described in some recent publications. This approach leads to various possible implementations such as the composite matched filter, the circular harmonic matched filter, or the composite circular harmonic matched filter. The present work describes new examples leading to very high selectivity filters retaining rotation invariance and reduced requirements on device resolution. Computer simulations and laboratory experiments show the advantages of this approach.

A pitch‐synchronous analysis of hoarseness in running speech

Abstract: A method of pitch‐synchronous acoustic analysis of hoarseness requiring a voice sample of only four fundamental periods is presented. This method calculates a noise‐to‐signal (N/S) ratio, which indicates the depth of valleys between harmonic peaks in the power spectrum. The spectrum is calculated pitch synchronously from a Fourier transform of the signal, windowed through a continuously variable Hanning window spanning exactly four fundamental periods. A two‐stage procedure is used to determine the exact duration of the four fundamental periods. An initial estimate is obtained using autocorrelation in the time domain. A more precise estimate is obtained in the frequency domain by minimizing the errors between the preliminary calculated power spectrum and the predicted spectrum spread of a windowed harmonic signal. Analysis of synthesized voices showed that the N/S ratio is sensitive to additive noise, jitter, and shimmer, and is insensitive to slow (8 Hz) modulation in fundamental frequency and amplitude. An analysis of pre‐ and postoperative voices of six patients with benign laryngeal disease showed that the N/S ratio for vowel /u/ in running speech consistently improved after surgery for all subjects, in agreement with their successful therapeutic results.

Reduction of undesired harmonic components

Abstract: A method and means for reducing the energy of undesired r.f. harmonics generated in the highest power amplifier of a transmitter by providing at least one alternate path for the undesired harmonics incorporating a relatively low powered power amplifier for at least one undesired harmonic component and causing the harmonic in the alternate path to have an amplitude that is substantially equal to the corresponding harmonic output from the main path and also causing the harmonic in the alternate path to be substantially out of phase with the harmonic in the main path and then combining the harmonics of the paths. This procedure is only feasible if the output of the highest power amplifier is processed by the main path circuit connected between the highest power power amplifier and the antenna so as to attenuate the undesired components sufficiently so that the harmonics power levels are a very small percentage of desired output wave.

Flat-top windows for PWM waveform processing via DFT

Abstract: New flat-top windows by which per forming the measurement of the parameters that characterise periodic signals are presented. It is shown how, by applying these windows in the frequency domain beyond the FFT, the negative effects of both the spectral leakage and the harmonic interference are reduced without employing interpolation algorithms. The high accuracy of the results is confirmed by an error analysis. These windows are very useful in the determination of the harmonic content of the inverter output PWM waveforms supplying electrical motors, in order to qualify the entire drive.

Plates with regular stiffening in acoustic media: Vibration and radiation

Abstract: Flat plates and cylindrical shells with regular and identical stiffening constitute spatially periodic structures. Specially convenient methods of vibration analysis are available for these, some of which are suitable for the inclusion of the effects of fluid loading from adjacent acoustic media. This paper outlines the nature of free wave motion in periodic structures stiffened either in one direction or in two orthogonal directions. The structural response to distributed harmonic pressure fields is analyzed by using displacement functions consisting of a series of space harmonics. The sound radiated or transmitted by the vibrating structure is also found. The method of space harmonics is next combined with the method of phased array receptance functions to yield equations for the propagation constants of fluid‐loaded periodic plates, stiffened in just one direction. Some calculated propagation constants are presented. Recent developments in the application of the hierarchical finite‐element method to periodic structures are described. Computed results are presented for the propagation constants of an orthogonally stiffened plate, without fluid loading. Consideration is given to those frequency ranges and wave motions that could radiate into an adjacent medium.

Active control of low‐frequency harmonic sound radiated by a finite panel

Abstract: Quadratic optimization theory is applied to determine the minimum sound power output of a simply supported panel in a baffle when the radiation is controlled with additional ‘‘secondary’’ monopole sources. The theory presented produces analytical results for the optimal complex strengths of the secondary sources and the resulting value of minimum power output. The results are valid in the low‐frequency limit when the panel dimensions are much smaller than the wavelength of the radiated sound. The results demonstrate the importance of matching the secondary source distribution to the type of panel mode and can be explained using Maidanik’s modal radiation classification scheme.