A. Ometto

Uncertainty Issues in Direct and Indirect Efficiency Determination for Three-Phase Induction Motors: Remarks About the IEC 60034-2-1 Standard

Electric motors and the systems they drive are the single largest electrical end-use; it is estimated that they consume between 43% and 46% of all global electricity consumption. Recent studies show that efficiency values of induction motors can be successfully increased, with improvements up to 20%-30%. In 2009, the European Commission published a new regulation (640/2009) concerning requirements for the eco-compatible design of electric motors. In such a scenario, the experimental determination of induction motor efficiency is getting more and more important, because of the need of placing these motors in the right energy efficiency levels defined by international regulations. The correct motor classification strictly depends on the uncertainty associated with the efficiency determination. According to the IEC 60034-2-1 standard, the efficiency of three-phase induction motors can be determined by applying the direct or indirect efficiency technique. The direct procedure is suggested for low uncertainty efficiency measurements of single-phase induction motors and three-phase motors with rated power ≤1 kW. In this paper, we propose a comparative analysis between direct and indirect efficiency determination for three-phase induction motors, according to the IEC 60034-2-1 standard. The contribution of this paper consists in the comparison between the uncertainty calculations in indirect and direct efficiency determination for induction motors, to investigate the advisability of adopting the direct efficiency determination as reference method, because it is easier to implement and it has a good accuracy. This task is carried out starting from the accuracy requirements for measurement equipment, as demanded by the Guide to the Expression of Uncertainty in Measurement GUM. Some experimental results obtained with the indirect and direct efficiency determinations on a 3-kW three-phase two-pole induction motor are presented and discussed. A final suggestion related to a future version of IEC 60034-2-1 is also reported.

Supercapacitor to provide current step variation in FC PEM

The adoption of fuel cells for industrial applications, transportations and home power generation has widely increased during recent years. The performance evaluation of a fuel cell system requires both static and dynamic model, especially for the automotive applications which involve step variations of electric load. Besides, current step variations abruptly change the fuel cell voltage causing a non-linear behavior. In this case, a parallel supercapacitor becomes an important element to provide energy during transients. Fuel cell dynamic models presented in literature adopt chemical-physic-electrical parameters that are not usually provided by factories. In this paper, we concern the implementation of dynamic fuel cell model during step current transients from the electrical point of view. The model is used to evaluate the transient performance of the whole system, i.e. fuel cell and supercapacitor, by means of simulation software SIMULINK. Simulation results show gradual variations of fuel cell terminal voltage from initial steady state to final steady state values when a supercapacitor is parallel connected.

Minimum supercapacitor choice to provide current step variation in FC PEM

The adoption of fuel cells for industrial applications, transportations and home power generation has widely increased during recent years. The performance evaluation of a fuel cell system requires both static and dynamic model, especially for the automotive applications which involve step variations of electric load. Besides, current step variations abruptly change the fuel cell voltage causing a non-linear behavior (under-voltages). In this case, a parallel supercapacitor becomes an important element to provide energy during transients. Fuel cell dynamic models presented in literature adopt chemical-physic-electrical parameters that are not usually provided by factories. In this paper, we deal with the implementation of dynamic fuel cell model during step current transients from the electrical point of view [1]. The FC model is used to compute the minimum capacitance of the supercapacitor needed to obtain gradual variations of fuel cell terminal voltage from initial steady state to final steady state values.

The evaluation of the effects of the voltage amplitude modulations on induction motors

The voltage fluctuations of the electrical power supply can generate many undesirable effects on both industrial and domestic apparatuses. The aim of this paper is the evaluation of the effects of the low-frequency voltage amplitude modulation on induction motors. An experimental analysis has been applied to a three phase, 7.5 kW induction motor with the adoption of an ad hoc system for the EMC low-frequency immunity testing. The experimental results show that a few percent amplitude modulations can involve a critical behavior in terms of torque. As for the light flicker effect measurement, a statistical approach can be adopted for the evaluation of the torque fluctuation.

Analysis of PEM fuel cell – Supercapacitor – Battery pack system during standard cycle

Today the application of PEM (Polymer Electrolyte Membrane) fuel cell stack in the automotive field is a topic on central interest. Unfortunately, a PEM fuel cell stack cannot directly supply variable speed drives because of their non-linear behaviour in dynamic conditions. This paper is mostly concerned with the sizing of the supply system, PEM fuel cell-supercapacitor-battery pack, of an electric vehicle, based on the urban/suburban drive cycle NEDC (New European Driving Cycle). The average required power is used to design the fuel cell stack by imposing a preset operating point; the battery capacity is chosen in order to satisfy the constrain on the minimum value of the state of charge during the cycle; finally, the supercapacitor is determined by imposing the current sharing between battery and supercapacitor in transient condition.

Optimal modulation strategy for active front-end inverter feeding induction motors

Torque ripple in inverter fed induction motors is a critical problem if high switching frequency cannot be achieved because of the drive power rating or when switching losses must be kept low. In that case, the torque ripple minimisation should be one of the goals of the control strategies. The paper presents a PWM technique that can be used to minimise the steady state torque ripple of variable speed induction motor drives when high dynamic is not required, i.e. scalar control. The torque ripple is expressed as function of the DC bus voltage and of the notches introduced in the output voltage waveform of a VSI and it is minimised by finding out the optimum dc voltage value and modulation pattern. The control of the DC voltage does not increase the cost of the whole drive when a PWM AC/DC converter is already used to reduce the current harmonics and to improve the power factor. The proposed PWM strategy is compared with the space vector modulation and experimental results are reported and examined.

System PEM fuel cell-supercapacitor: Analysis in transitory conditions

Today the application of PEM (Polymer Electrolyte Membrane) fuel cell stack in the automotive field is a topic on central interest. Unfortunately, a PEM fuel cell stack cannot directly supply variable speed drives because of their non-linear behaviour in dynamic conditions. This paper is mostly concerned with the performance evaluation of the PEM fuel cell - supercapacitor system during repetitions of current step variations; in those cases, the supercapacitors cannot be completely recharged before supplying the load in the next dynamic, and the fuel cell stack can be overloaded. We propose the introduction of lithium battery packs and of a DC/DC converter. The obtained results show that with the proposed configuration. That will be useful in the design of the control strategy of variable speed drives supplied by the proposed system.

Survey about Classical and Innovative Definitions of the Power Quantities Under Nonsinusoidal Conditions

Abstract Today, electric power and energy measurements are widely required, practically in all the research, industrial and consumer applications. Power measurements are of importance primarily for the test, monitoring and maintenance of energy supply networks and electric equipment. The measurement of both electric power and energy is a still open research problem in the electrical engineering community. Phenomena like harmonic distortion, noise, transients, over-voltages and voltage dips have increased the difficulty in achieving accurate measurements, compared with the case of sinusoidal signals. Many of the non-active power component definitions that have been proposed cannot be implemented in the traditional electro-mechanical or solid-state meters, but require the adoption of more expensive and time-consuming digital techniques; recently, some new approaches for the definition of power quantities have been investigated. In this paper, a survey of the classical and innovative definitions is proposed, with the aim of summarize the different points of view outlined by the researchers. A case study of power factor correction in nonsinusoidal conditions is also presented, to give a numerical comparison about the power quantities measured according to the various approaches.

The modeling of a PEM fuel cell — Supercapacitor — Battery system in dynamic conditions

Today the application of PEM (Polymer Electrolyte Membrane) fuel cell stack in the automotive field is a topic on central interest. Unfortunately, a PEM fuel cell stacks cannot directly supply variable speed drives because of their non-linear behaviour in dynamic conditions. This paper is mostly concerned with the performance evaluation of the PEM fuel cell – supercapacitor system during repetitions of current step variations; in those cases, the supercapacitors cannot be completely recharged before supplying the load in the next dynamic, and the fuel cell stack can be overloaded. We propose the introduction of lithium battery packs and of a DC/DC converter. The obtained results shows that with the proposed configuration, the size of supercapacitors can be significantly reduced, reducing the stress on the battery pack without losses of performance. That will be useful in the design of the control strategy of variable speed drives supplied by the proposed system.

Effects of voltage amplitude modulations on mechanical vibrations in low voltage transformers

With the increasing use of sophisticated controls and equipment the continuity, reliability, and quality of electrical service has become extremely crucial to many power users. Electrical systems are subject to a wide variety of power quality problems which can interrupt production processes, affect sensitive equipment, and cause downtime, scrap, and capacity losses. Momentary voltage fluctuations can involve different kinds of undesired phenomena on electrical machines. This paper examines the impact of voltage fluctuations on the mechanical vibrations in low voltage transformers, with an experimental approach. The experimental analysis has been carried out testing a three phase, 10 kVA, low-voltage transformer. The measuring system is an ad hoc system for the EMC low-frequency immunity testing that generates sinusoidal and rectangular amplitude modulations. The experimental results show that a few percent amplitude modulations can involve a critical behavior in terms of vibrations, in addition to the magnetostriction, and can induce mechanical resonance phenomena