Unity Power Factor Load Research Articles

A Hybrid PWM Strategy for Switching Loss Reduction in Three-Level Inverters

The advanced bus-clamping PWM (ABCPWM) sequences are well known to reduce line current THD in a three-level neutral point-clamped inverter. This paper deals with ABCPWM sequences employed optimally to achieve minimum switching loss throughout the linear operating region in a three-level inverter. The work extensively analyzes the performance of these sequences in terms of inverter switching loss and compares them with centered space vector modulation PWM (CSVPWM). Based on the analysis, a hybrid PWM scheme termed as <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">switching loss reduction</i> is proposed by optimally employing ABCPWM sequences in conjunction with variation in power factors. At a given average switching frequency, the proposed hybrid scheme reduces inverter switching loss compared to CSVPWM and the recently proposed discontinuous PWM-based optimization strategy at all power factors and modulation indices. With this scheme, switching loss reduces to 22% to 36% compared to CSVPWM and a maximum of 12% reduction than the existing optimization strategy. Furthermore, the scheme gives lower harmonic distortion compared to these two existing PWM schemes near high modulation indices and near unity power factor load. The theoretical, simulated, and real-time simulation results are presented with experimental results using TMS320F28335 DSP controller for validating the proposed work.

Efficiency characteristics for changes in amplitude and power factor of diode‐clamped linear amplifier unequally divided voltage power supply

AbstractA diode‐clamped linear amplifier (DCLA) is proposed to realize low electromagnetic interference (EMI) caused by switching operations. A new concept of the DCLA, namely the higher efficiency unequally divided voltage power supply DCLA (DCLA‐UeDV), is proposed. In previous studies, the optimal voltage ratio was derived, and a 1.7% points improvement in efficiency was realized for a rated unity power factor load. However, it has not been examined whether the conversion efficiency can be improved, except during the rated operation. In this study, operation of the DCLA is analyzed for changes in the amplitude of the output voltage and load power factor. Then, the advantageous operating region of the DCLA‐UeDV against DCLA‐eDV is clarified.

Inductance Measurement Methods for Surface-Mount Permanent Magnet Machines

Analytical performance estimation of a permanent magnet (PM) motor requires an accurate equivalent circuit model. In a lumped electrical model of a PM motor, resistance and inductances appear as passive elements, which are used to represent the phase winding resistance, inductance, core loss, etc. There is currently no available standard for parameter measurement of permanent magnet (PM) motors. In the literature, there are many studies on inductance measurement. However, they are applied to different types of motors. The purpose of this study is to evaluate those different inductance measurement methods, on the same motors, to identify whether they lead to the same result. Also, it was aimed to find out the difficulties involved in the measurement process. This study concentrates on determining the d-axis and q-axis inductances of two different surface-mount PM motors at standstill and under running conditions. The standstill measurement methods evaluated include the “current decay” method and the “DC inductance bridge” method as well as more common methods. The dependence of the inductances on the current magnitude, frequency, and excitation signal waveform is investigated. Measurements with PWM and sinusoidal AC voltage excitation are found to give similar results. The tests indicated that the “current decay” method is prone to measurement errors especially when the phase resistance is low. It is discovered that inductance measurements from standstill tests are independent of frequency for all practical purposes. Next, the same inductances are measured, while the test motors are running. The methods considered include; inductance measurement from no-load test, zero power factor load test, and unity power factor load test; while the machine is in generating mode. Furthermore, a new inductance measurement method is introduced where the measurement is made while the test motor is driven with a vector-controlled drive. Finally, inductance measurement results from different standstill tests and running tests are compared and evaluated.

Capability evaluation of four-leg DSTATCOM for compensating multifarious loads

A four-leg distribution static compensator (FL-DSTATCOM) has been investigated to achieve harmonic elimination, supply current balancing, compensation of reactive power and current in the neutral conductor of a three-phase, four-wire distribution system with various types of static, dynamic and combined loads. The capability of FL-DSTATCOM for compensating reactive power under dynamic condition is evaluated with induction machine having variable load torque. Also, the performance of FL-DSTATCOM is evaluated with combined load, which is a combination of dynamic and unbalanced static loads. The FL-DSTATCOM injects reactive and harmonics components of load current extracted using synchronous reference frame theory. With FL-DSTATCOM, the unbalanced, harmonic and reactive loads will appear to the main power supply as balanced, linear and unity power factor load. Hence, the main power supply would deliver only fundamental frequency positive sequence components of load current. The switching pulses are generated by employing hysteresis current controller. MATLAB/SIMULINK has been used for capability evaluation under multifarious loading conditions considering IEEE standard 519. The simulation results prove that the FL-DSTATCOM is capable of making the supply current as balanced and sinusoidal, maintaining the PF at point of common coupling near to unity and reducing the supply neutral current very close to zero.

Effect of supply voltage asymmetry on IRP p – q -based switching compensator control

Results of a study on how the supply voltage asymmetry affects the reference signal for switching compensator control, in a situation when this signal is generated using the instantaneous reactive power (IRP) p-q theory are presented. According to the IRP p-q approach, the compensator should compensate IRP and the alternating component of the instantaneous active power of the load. However, that in the presence of the supply voltage asymmetry, even an ideal, unity power factor load has an instantaneous active power with a non-zero alternating component is demonstrated. According to IRP p-q theory-based approach, it should be compensated and this requires that a distorted current be injected into the distribution system. It means that in the presence of the supply voltage asymmetry, the algorithms based on the IRP p-q theory generate a non-sinusoidal reference signal for the compensator control, even when voltages and currents in the system are sinusoidal.

Performance Analysis of Three-Phase Self-Excited Induction Generator Using Genetic Algorithm

This paper presents a new technique for the analysis of a three-phase self-excited induction generator feeding a three-phase balanced unity power factor load. A genetic algorithm has been proposed to find the generated frequency and magnetizing reactance simultaneously. This leads to a complete analysis of the self-excited induction generator. Computed results have been compared with experimental results. The comparison confirms the validity and accuracy of the proposed method.

Fault tolerant, variable frequency, unity power factor converters for safety critical PM drives

The aerospace industry is currently considering a constant voltage, variable frequency supply for aircraft power systems. This variable frequency system is based on a generator directly driven from the aero-engine, in which the frequency is dependent on engine speed. A number of safety critical loads are placed on this system, and it is essential that failure of any one load does not affect the operation of another. The paper develops the concept of fault tolerant input converters for an electric fuel pump. The converters always act as a unity power-factor load, and can tolerate a range of failures, whilst maintaining both the operation of the pump and minimal impact on the electrical supply. Two converter types are proposed and compared for this application. The operation of these two unity power factor converters from a variable frequency supply is demonstrated. The effect of faults in the selected converters on converter operation and the supply itself is then discussed.

Implementation of a controlled rectifier using AC-AC matrix converter theory

It is well known that a PWM-controlled rectifier can offer advantages of reduced low-order harmonics and unity input power factor when compared to a conventional thyristor converter. However, theoretically optimum PWM strategies are often difficult to implement physically or are not easily extended to regenerative operation. The authors propose an alternative PWM strategy based on AC-AC matrix converter theory, which generates only high-order switching harmonics, presents a unity power factor load to the supply, implicitly extends to regeneration (and operation with a center tapped DC output), and is feasible to physically implement for real-time output voltage control. Both the theory and physical simulation results are presented. >

Synchronous machines with contrarotating armatures: steady-state behaviour

A synchronous machine can be constructed with two armatures and one field. If the armatures contrarotate, the armature-reaction flux in the quadrature axis, ideally, is eliminated. The machine therefore operates with a high saliency ratio. The voltage regulation for a unity power-factor load is then only affected by winding resistance and leakage reactance. This unique feature allows the machine to have a peak power capability several times that of conventional machines of the same size. Two contrarotating machines have been designed and tested. One of the arrangements has steady-state behaviour approaching the expected ideal behaviour. The test machine built has three rotatable machine elements which allows the examination of a number of operating modes.

Hall effect wattmeters

A transmission-type wattmeter can be formed utilizing the Hall effect to give wide-range multiplication over a frequency range extending from the low audio-frequency region to the SHF band. At the low frequencies, the Hall device can take the form of a thin wafer of indium antimonide mounted in series with the center leg of a cup core magnetic circuit. An audio-frequency wattmeter employing such a structure has been analyzed, constructed and tested. The wattmeter has a range of zero to 400 milliwatts when used in a 600-ohm circuit, and displays a frequency error of less than plus and minus 3.5 per cent over a frequency range of 100 to 6000 cps for a unity power factor load. In the SHF range, a cavity and electric probe are arranged to provide excitation for the indium antimonide wafer in the manner first employed by Barlow. The cavity, about one wavelength long, is separated from the waveguide in which power flow is to be measured by a thin brass wall, and is coupled to the waveguide by a rectangular slot in the top wall of the waveguide. The semiconductor wafer is placed in the center of the cavity, at a position having a maximum in magnetic field and a minimum in electric field for a given cavity excitation value. The electric probe, which extends from the cavity to the waveguide, provides excitation current for the semiconductor element. Three tuning adjustments are required, two for the cavity and one for the probe. A wattmeter embodying this technique has been found to provide a sensitivity of one dc microvolt per milliwatt of transmitted power at a frequency of 9.40 Gc.