Commutation Angles Research Articles

Real-time implementation of harmonic elimination PWM technique for induction motor

This paper presents a new method to decrease the switching frequency of novel harmonic elimination Pulse Width Modulated (PWM) strategy for three-phase inverter. The torque ripple of the induction motor can be dramatically reduced by the new PWM technique. The three-phase inverter is associated with a passive LC filter. The commutation angles are predetermined off-line and stored in the microcontroller memory in order to speed up the induction motors on line control. Precalculated switching is modelled to cancel the greater part of low-order harmonics and to keep a single-pole dc voltage across the polarised capacitors. A passive LC filter is designed to cancel the high-order harmonics. This approach allows substantial reduction of the harmonic ratio in the ac main output voltage without increasing the number of switches per period. Consequently, the duties of the semiconductor power switches are alleviated. Simulation results are presented in order to illustrate such algorithm performance. To show the validity of our approach, DSP-based experimental results are presented.

Evaluation of Rotor Conducting Screens on the Single-Phase Switched Reluctance Machine

The single-phase switched reluctance machine has a simple and robust construction and is suitable for high-speed operation. Torque production is a function of the difference between the aligned and unaligned inductance, and increasing this difference theoretically will increase the torque. In this article, conducting screens are incorporated into the rotor of a prototype 2/2 single-phase switched reluctance motor. The effect of the conducting screens on current response, flux linkage, and output torque is evaluated using finite element analysis and validated experimentally. At fixed current conduction angles, conducting screens improve the rate of change of inductance at low speed, and enhance performance at high speed when the machine operates in the single voltage pulse mode. The conducting screens have a negative effect on the output torque when using optimal commutation angles. Reasons for this phenomenon are given.

Centroid PWM Technique for Inverter Harmonics Elimination

A novel harmonic elimination pulsewidth-modulation (PWM) technique for static dc/ac power inverters is proposed in this letter. The technique determines the commutation angles based on equilibrium points; which are obtained by calculating the centroid of the area under the curve of the sinusoidal function of voltage, within a time interval defined by the method of equal areas.

New aspects of inspecting commutation angles of high-voltage direct current systems

The conventional method of calculating high-voltage direct current (HVDC) converter commutation angles can result in very inaccurate results if the ripple of the DC is fairly high, or the HVDC link consists of capacitor commutated converters (CCC). For conventional line-commutated current source converters with a high DC ripple new analytical equations are presented for determining the commutation angles and the instantaneous values of DC DCside current at the firing instants. An iterative method for the calculation of commutation angles of CCC was also developed. The results are confirmed by digital simulation.

A review of iterative harmonic analysis for AC-DC power systems

Detailed models of a static power converter capable of representing the effects of AC and DC power system distortion, as well as harmonic modulation of the firing and commutation angles, are now becoming available. Using iterative techniques, such models are combined with linear harmonic penetration into AC network equivalents to provide accurate information about the power system harmonic content. This paper provides a critical review of the state-of-the-art in iterative harmonic analysis.

Steady-state model of direct connected generator–HVdc converter units in the harmonic domain

A new twelve-pulse unit connection model is formulated and solved in the harmonic domain. The model takes into account the effects of generator saliency and saturation, interaction of the converter with the DC system, and variation in the firing and end of commutation angles caused by AC voltage and DC current harmonics. The reactive power control by generator excitation in the absence of converter transformer on-load tap-changers is also modelled. The solution of a test system is validated by means of time domain simulations.

A Newton solution for the harmonic phasor analysis of AC/DC converters

The steady state equations that describe the power converter and DC system in the harmonic domain are solved by means of Newton's method, in a manner suitable for embedding in an iterative harmonic analysis of the complete power system. The solution includes the interaction of the power converter with the DC system, and the effect of variation in the firing and end of commutation angles caused by AC voltage and DC current harmonics. The convergence of Newton's method is investigated, and methods for accelerating the solution are implemented. Finally, the solution obtained is validated by means of time domain simulation.

Voltage, frequency, and phase-difference angle control of PWM inverters-fed two-phase induction motors

A phase-difference angle (PDA) controlled pulsewidth-modulated (PWM) inverter is proposed for a two-phase induction motor adjustable speed drive. Output waveforms are fixed over the whole operating range of the motor. The motor torque is controlled not by the modulation of the phase voltage, but by the PDA. Based on the selected harmonic elimination (SHE) PWM technique, the commutation angles of the output voltage are calculated. Several characteristics of the two-phase induction motor driven by the PDA inverter are analyzed. A hybrid PWM inverter is also proposed to compensate for the degradation of the efficiency at small PDA. Not only the PDA but also the voltage amplitude and frequency are used as the parameters for controlling the torque of the motor in the hybrid inverter. The speed characteristics of the two-phase induction motor driven by the hybrid PWM inverter are more flexible than when the motor is driven by the conventional PWM inverter, which requires adjustable communication angles. >

Performance of a double-star synchronous generator with bridge rectified output

In this paper, the performance of a double-star synchronous generator with bridge rectified output is studied from the fundamental machine equations. The generator has two stator windings shifted by /spl pi//6 electrical radians, which with their bridges can be connected either in series or in parallel. Therefore the double-star synchronous generator with rectifier load can have two major operation modes. Using the fundamental machine equations, the expressions of displacement angles and commutation angles are derived under these operation modes. The performance of the double-star synchronous generator with bridge rectified output can be predicted from given load current, field current, generator speed, bridge delay angle and its parameters. Theoretical results are compared with experimental results obtained from a laboratory machine. >

Effect of DC ripple and commutation on the line harmonics of current-controlled AC/DC converters

Line harmonics are usually predetermined under the simplified assumption that the DC current is sufficiently smoothed. But in practical operation of controlled converters, e.g., feeding medium-sized DC drives, DC ripple often cannot be neglected. For the predetermination of line harmonics, this means that the operating point is specified by the EMF and the mean value of the DC current, i.e., the associated control and commutation angles are unknown. The consideration of the basic effects of commutation and DC ripple leads to line current harmonics that are separated into positive and negative sequence systems. Selected results of systematic evaluations show the amplitude and phase as functions of DC reactance as well as of the EMF and mean DC current in the range of usual low- and medium-voltage supply networks. In comparison with the conventional precalculations assuming sufficient DC smoothing, significant deviations occur in the case of higher DC ripple.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Microprocessor Control of DC/AC Static Converters

Microprocessor control of power electronic systems offers the possibility of improvements in reliability, maintenance and servicing, and increased control flexibility. This paper describes several pulsewidth modulation (PWM) waveforms where a predetermined number of unwanted harmonics can be canceled. The advantages and disadvantages of the different methods of delaying the signals in small microprocessor controlled PWM inverter drive systems are considered. Several examples of single-phase and three-phase PWM inverters, with the laws which govern the commutation angles, output waveforms, and their respective frequency spectra are presented.

Distribution System Harmonics: Controls for Large Power Converters

Because of the anticipated applicaton of large ac/dc power converters on utility distribution systems, the Electric Power Research Institute (EPRI) funded Project RP1024-1 to study distribution system surge and harmonic characteristics. The harmonics produced by large converters depends on converter type (line-or self-commutated), converter pulse number, thyristor firing angle controls, commutation angles, voltage regulation techniques, voltage unbalance, and?very importantly?network impedance frequency response. Of particular significance is the fact that a great deal of effort is still required to determine tolerable levels of harmonic voltages and currents on distribution systems. Harmonic controls for power converters-important because of their effect on converter cost and efficiency-are now specified on the basis of engineering data which need to be enhanced by further testing and analysis. In the face of observable and ever-increasing background levels of distribution system harmonics, more precise anticipation of future concerns is required. Test methods and instrumentation need to be standardized to ensure conformance to yet-to- be-determined permissible harmonic limits. This paper discusses line-and self-commutated power converters and techniques for controlling the harmonics associated with them and presents the principal results of EPRI Project RP1024-1.

Optimal Pulsewidth Modulation for Feeding AC Motors

A pulsewidth modulation (PWM) technique is proposed, intended particularly for the reduction of the harmful effects of the harmonics in the inverter-fed drive systems. The approach is based on the minimization of the rms value of the current harmonics in the motor by a proper choice of the commutation angles. Switching patterns are calculated for a different number of commutations per period, for both single-phase and three-phase half-bridge inverters. In the case of the single-phase, an analytical procedure is developed which simplifies the computations. Comparisons are made of the results thus obtained with those given by other PWM techniques in use.

Characteristics of a diode-bridge-loaded synchronous generator without damper windings

The paper extends the theoretical study of diode-bridge-loaded synchronous generators without damper windings to loads with commutation angles equal to or greater than 60° (modes 2 and 3). The equations necessary to establish the performance in these operating modes, and a simple method of establishing the boundaries between the modes, are given. Constant flux linkages are assumed. Experimental results from tests on a 5 kVA laboratory generator have been used to check the computed theoretical results obtained by using the published equations. A larger, turbogenerator exciter has been used to illustrate the theoretical difference between operation in mode 1 and mode 2.

Theory of the Three Phase Salient Pole Type Generator with Bridge Rectified Output - Part II

Using two approximations, the original set of basic equations of Part I applying to a Star connected stator winding is converted into explicit expressions for the average field current If, the commutation angles β and μ, the instantaneous values of all rotor currents and the DC output voltage. The particular characteristics of a Delta connected stator winding are discussed and its equations derived. All expressions are shown both in absolute as well as in per-unit values and can also be used as the initial approximation for the iterative process required to solve the basic set of exact equations of Part I. A numerical example demonstrates the capabilities of the equations.