Performance optimization of a photovoltaic induction motor pumping system

The performances of a photovoltaic pumping system based on an induction motor are degraded once insolation varies far from the value called nominal, where the system was sized. To surmount this handicap, an improvement of these performances by the optimization of the motor efficiency is described in this paper. The results obtained are compared with those of similar work pieces presented in the literature where the motor effeciency and air gap flux where optimized separatly. The simulation results show that the proposed system allows at the same time to combine the performances of the system with constant efficiency and the simplicity of implementation provided by the system with constant airgap flux.     

Improvement of induction motor drive systems supplied byphotovoltaic arrays with frequency control

The overall efficiency of an induction motor drive system, powered by a PV array, drops significantly when the insolation condition varies away from its nominal level. This problem can be overcame using a control method in which the frequency of the inverter's PWM control signal is adjusted according to the insolation and temperature conditions. The motor speed, and therefore, the power delivered to the load, are adjusted by controlling the inverter's frequency. This eliminates the mismatch between the maximum power that is available from the source and the power that is required by the load. Simulation results presented in this paper show that using the proposed control system allows the induction motor drive system to maintain its optimum efficiency and deliver consistently more power to the load when insolation and temperature vary from the nominal level. This method also offers an improvement in the system stability     

Study of a longitudinal flux permanent magnet linear generator for wave energy converters

A directly coupled linear permanent magnet generator of longitudinal flux‐type is investigated. The generator will be used for power take‐off in a wave energy converter. A combined field‐ and circuit model, solved by a time stepping finite element technique, is used to model and analyse the electromagnetic behaviour of the machine. A large number of simulations form the basis of a design study where the influence of armature current level, number of cables per slot, and pole width is investigated with respect to efficiency, generator size, and the load angle. A case study is performed for a chosen generator design. The electromagnetic behaviour is examined both for nominal load and for overloads. The generator has a nominal output power of 10 kW for a constant piston speed of 0.7 ms^?1. The electromagnetic efficiency at nominal load is 86.0%, the load angle 6.6°, and the power fluctuation 1.3%. At 300% overload the load angle barely exceeds 12° and the cable temperature is below 25°C provided that the stator back is thermally connected to the sea water. The numerical calculations have been verified for small speeds by experiments. Copyright © 2006 John Wiley & Sons, Ltd.     

A method for transient torque response improvement in optimum efficiency induction motor drives

Optimal efficiency control of induction motor drives implies operation at reduced flux levels with light loads. Two problems in light load operation are a large speed drop after sudden load torque increase and slow acceleration. In order to improve response in these transients, an algorithm for optimum dynamic distribution of the available maximum inverter current into the flux-producing and the torque-producing stator current components is developed in this paper. The proposed algorithm accounts for the main flux saturation effect in the machine and the dynamics of the flux variation. Its performance is illustrated by means of simulation and experimental results. Superiority of the developed algorithm over some of the existing methods is proved by comparing the speed drops, which result after sudden load torque increase during operation at light load, and by examining an acceleration transient under light load condition.     

Modeling of Saturated Induction Machines With Injected High-Frequency Signals

This paper analyzes the effects of injecting additional signals in induction machines for the purpose of speed control. A new saturation model able to correctly model the interaction between the added signal and saturation of the motor core due to the main torque-producing flux is presented. The introduction of a variable saturation factor is used to model the variation of the saturation level due to the additional signal. A third harmonic rotor circuit is also introduced to take account of the third harmonic component of the air-gap flux, due to saturation. An additional balanced voltage set is added to the normal supply to analyze the effects of such signals on a saturated induction motor. Simulation results of such a model both at no load and full load are presented together with experimental measurements.     

Optimal efficiency analysis of induction motors fed byvariable-voltage and variable-frequency source

The authors discuss the efficiency analysis and experimental data for an induction motor fed by a variable-voltage and variable-frequency (VVVF) source. Nonideal factors (core saturation, source harmonics, and skin effect) affecting the efficiency are included in the analysis to yield practical results from computer simulation. Based on the simulated results, an experimental system, composed of a DC link power converter with a VVVF feature, a dynamometer and a PC/AT, was developed to evaluate the efficiencies of the induction motor from low to rated speed and torque. Both analysis and experimental results indicate that the efficiency with VVVF is superior to that of constant flux operation. Experimental results showed that 10-15% improvement in the efficiency of a 2 hp induction motor at 0.4 per unit load using VVVF can be achieved as compared to the constant flux operation     

Technical and economic considerations on induction motor oversizing

In industry, due to conservative system design, safety factors associated with uncertainty in the load requirements, discrete availability of commercial rated power, and/or load power variation, most three-phase squirrel-cage induction motors are oversized. Besides the extra capital investment, the oversizing of direct-on-line fixed-speed induction motors can lead to a significant efficiency and power factor reduction. However, the part-load efficiency of oversized motors can still be higher than the full-load efficiency of well-sized smaller motors because, in general, the nominal efficiency increases with the rated power. In this paper, an analysis of potential benefits and drawbacks of motor oversizing is carried out. On the basis of the catalogue technical data provided by one of the largest motor manufacturers for IE1-, IE2-, IE3-, and IE4-class four-pole induction motors, the main results of a simulation-based study on the oversizing energy efficiency and cost-effectiveness are presented. A method to estimate the motor efficiency and power factor for any load level using commercial catalogue data is proposed and applied. Some technical issues associated with motor oversizing are also briefly addressed. It is shown that, if the additional reactive energy consumption due to poorer power factor and the slight speed increase are ignored, for IE1-, IE2- and, to a much less extent, IE3-class motors, oversizing can be cost effective for many motor rated powers, resulting in a higher average efficiency and a lower motor lifecycle cost, as well as in an extended motor lifespan. For most IE3- and IE4-class motors, the oversizing is not cost effective because of the lower nominal efficiency gain when moving to a higher oversized rated power. Additionally, the oversizing impact on the motor energy consumption strongly depends on the load profile of the application. When an old motor fails, it will probably be an IE0- or IE1-class equivalent motor, and this situation provides a golden opportunity for replacing the old motor with a properly sized IE3- or IE4-class motor, which offers significantly higher efficiency for a wide range of loads.     

Minimum airgap flux linkage requirement for self-excitation instand-alone induction generators

A minimum airgap flux linkage is required for the self-excitation and stable operation of an isolated induction generator feeding an impedance load. With the aid of bifurcation theory, it is shown that the minimum airgap flux linkage requirement is the value at which the derivative of the magnetizing inductance with respect to the airgap flux linkage is zero. This minimum airgap flux linkage determines the minimum or maximum load impedance and minimum excitation capacitance requirements. This result is demonstrated using single-phase and three-phase induction generators     

A new sliding mode position controller with adaptive load torqueestimator for an induction motor

A new sliding mode control algorithm with an adaptive load torque estimator is presented to control the position of the induction motor in this paper. First, the rotor flux is estimated with the simplified rotor flux observer in the rotor reference frame and the feedback linearization theory is used to decouple the rotor position and the rotor flux amplitude. Then, a new sliding mode position controller with an adaptive load torque estimator is designed to control the position of the induction motor such that the chattering effects associated with the classical sliding mode position controller can be eliminated. Stability analysis is carried out using the Lyapunov stability theorem. Experimental results are presented to confirm the characteristics of the proposed approach. The good position tracking and load regulating responses can be obtained by the proposed position controller     

Novel Multiflux Level, Three-Phase, Squirrel-Cage Induction Motor for Efficiency and Power Factor Maximization

In the European Union, the average load factor of electric motors in both industrial and tertiary sectors is estimated to be less than 60%. However, in some industrial sectors, the average load factor for some motor power ranges can be as low as 25%. Most oversized three-phase induction motors operate with low efficiency and power factor, which is, by far, the most important cause for poor power factor in industrial installations. In the low-load operating periods, motor performance can be improved both in terms of efficiency and power factor if the magnetizing flux is properly regulated. In this paper, a multiflux level, three-phase, squirrel-cage induction motor is proposed, in which the efficiency and power factor can be both maximized as a function of load. This novel motor can be a surplus value in industry due to its flexibility, particularly, for variable load applications in which significant energy savings can be obtained, and can also be used as new or rewound general purpose spare motor (with several levels of voltage, magnetizing flux and/or power). The proposed motor has a stator winding with two sets of turns, sharing the same positions in the stator slots (which can be connected either in series or in parallel). Among all the possible stator winding connections, six modes were selected and analyzed (two of which are new). The basic principles for proper connection mode change are discussed. An electronic device and a contactor concept for automatic connection mode change are proposed. As far as the authors know, this concept is described and analyzed for the first time.     

Induction motors variable speed drives diagnosis through rotor resistance monitoring

Induction motor driven by vector control method makes high performance control of torque and speed possible. The decoupling of flux and electromagnetic torque obtained by field orientation depends on the precision and the accuracy of the estimated states. Rotor asymmetries lead to perturbations of air gap flux patterns in induction machines. These perturbations in flux components affect the electromagnetic torque, as well as stator currents and voltages. This paper first investigates the control of the induction motor using an extended Kalman filter (EKF) for a direct field-oriented control. It then studies the broken rotor bars (BRBs) fault by the monitoring the rotor resistance. The hypothesis on which the detection is based is that the apparent rotor resistance of the motor will increase when a rotor bar breaks. The rotor resistance is estimated and compared with its nominal value to detect BRBs fault. The EKF estimates the rotor flux, speed and rotor resistance on line by using only measurements of the stator voltages and currents. Simulation results show the effectiveness of the proposed method in the cases of load torque perturbation and speed reversion.     

Performance of a domestic pellet boiler as a function of operational loads: Part-2

Emissions and efficiency of a pellet boiler (40 kW) at nominal load were compared with emissions and efficiency at reduced load, while fired with six biomass pellets. The pellets include reed canary grass (Phalaris arundinacea), pectin waste from citrus shells (Citrus reticulata), sunflower husk (Helianthus annuus), peat, wheat straw (Triticum aestivum) and wood pellets. The measurements of emissions comprised of carbon monoxide (CO), nitrogen oxides (NO_x), sulphur oxides (SO_x) and flue dust mass concentrations (using DINplus and isokinetic sampling techniques). Emissions varied as a function of operational loads, for each type of pellets.The CO emissions were insignificant with reed canary grass (RCG), citrus pectin waste (CPW) and straw pellets at nominal load, however, at reduced load same pellets emitted 1.9, 4.0 and 7.4 times higher CO than wood pellets, respectively. Peat pellets emitted maximum CO at nominal load (4221.1 mgNm⁻³, 12.6 times higher than wood pellets) however; at reduced load CO emission was insignificant. The highest NO_x emissions were reported with CPW, which were 3.4 and 4.6 times higher than wood pellets at nominal load and reduced load, respectively. Dust emissions were highest with sunflower husk and lowest with RCG pellets, at both operational modes. The best performance was reported with wood pellets, followed by RCG and pectin pellets, however, wood pellets combustion emitted 1.7 and 2.0 times higher dust_DINplus than RCG at nominal and reduced loads, respectively. Not only fuel specific combustion optimization but also operational load specific optimization is essential for efficient use of agro-pellets in this type of boilers.     

Exergy-energy analysis of full repowering of a steam power plant

A 320 MW old steam power plant has been chosen for repowering in this paper. Considering the technical conditions and working life of the power plant, the full repowering method has been selected from different repowering methods. The power plant repowering has been analyzed for three different feed water flow rates: a flow rate equal to the flow rate at the condenser exit in the original plant when it works at nominal load, a flow rate at maximum load, and a flow rate when all the extractions are blocked. For each flow rates, two types of gas turbines have been examined: V94.2 and V94.3A. The effect of a duct burner has then been investigated in each of the above six cases. Steam is produced by a double-pressure heat recovery steam generator (HRSG) with reheat which obtains its required heat from the exhaust gases coming from the gas turbines. The results obtained from modeling and analyzing the energy-exergy of the original steam power plant and the repowered power plant indicate that the maximum efficiency of the repowered power plant is 52.04%. This maximum efficiency occurs when utilizing two V94.3A gas turbines without duct burner in the steam flow rate of the nominal load.     

Impact of burner cycle time in liquid fuel pressure burners on the thermal efficiency of heating appliances

Heating appliances used in buildings are designed according to building's heating load as a function of outdoor design air temperature. Usually this temperature is quite lower than the average external air temperature during the heating season. For most of the heating season, heating appliances do not operate at their nominal heating capacity. This requires boilers to operate at a lower thermal efficiency. Furthermore, on–off cycling of the pressure oil burner increases the amount of emissions of harmful substances into the flue gases. In many systems, a built-in burner is harmonized to the heating power of a boiler. The heating appliance's thermal efficiency appears to depend on burner cycle time and boiler heating load. Thermal efficiency of the heating appliance drops during shorter burner cycle times. The shortest burner cycle time appears at approx. 50% of the boiler heating load. Interestingly, in cases where the boiler heating load is not equivalent to rated (design) boiler capacity, two methods of calculating boiler thermal efficiency produce different results.     

Real-Time Speed and Flux Adaptive Control of Induction Motors Using Unknown Time-Varying Rotor Resistance and Load Torque

In this paper, an algorithm for direct speed and flux adaptive control of induction motors using unknown time-varying rotor resistance and load torque is described and validated with experimental results. This method is based on the variable structure theories and is potentially useful for adjusting online the induction motor controller unknown parameters (load torque and rotor resistance). The presented nonlinear compensator provides voltage inputs on the basis of rotor speed and stator current measurements, and generates estimates for both the unknown parameters and the nonmeasurable state variables (rotor flux and derivatives of the stator current and voltage) that converge to the corresponding true values. Experiments show that the proposed method achieved very good tracking performance within a wide range of the operation of the induction motor (with online variation of the rotor resistance: up to (87%). This high tracking performance of the rotor resistance variation demonstrates that the proposed adaptive control is beneficial for motor efficiency. The proposed algorithm also presented high decoupling performance and very interesting robustness properties with respect to the variation of the stator resistance (up to 100%), measurement noise, modeling errors, discretization effects, and parameter uncertainties (e.g., inaccuracies on motor inductance values). The other interesting feature of the proposed method is that it is simple and easily implementable in real time. Comparative results have shown that the proposed adaptive control decouples speed and flux tracking while standard field-oriented control does not.      

A new composite adaptive speed controller for induction motor basedon feedback linearization

A new adaptive control technique is proposed to control the speed of the induction motor in this paper. First, the rotor flux is estimated with the simplified rotor flux observer on the rotor reference frame and the feedback linearization theory is used to decouple the rotor speed and the flux amplitude. Then, a new composite adaptive control algorithm based on an integral cost function is designed to control the speed of the induction motor. The overall speed control system is verified to be stable and robust to the parameter variations and external disturbances. Experimental results are provided to demonstrate the effectiveness of the presented approach. The good speed tracking and load regulating responses can be obtained by the proposed controller     

Optimal Efficiency Control of an Induction Motor Drive

This paper describes a practical method for achieving optimal efficiency over the complete operating range of a variable speed drive. The proposed system adaptively adjusts the flux level in the motor based upon a direct measurement of the power input to the drive. An internal field orientation torque control loop and a speed regulator are employed to maintain the load speed requirements. Experimental results describing the efficiency optimization and the dynamic behavior of the drive at reduced flux are presented. The influence of the tuning of the field oriented controller on the efficiency of the drive is experimentally investigated.     

An efficient high performance voltage decoupled induction motordrive with excitation control

The state of the art in indirect slip frequency-controlled induction motor drive systems is fast response, high performance, voltage decoupling control. However, decoupling control needs to operate at a constant rotor flux, which makes energy conversion inefficient. A variable-flux decoupling model of a voltage-fed induction motor which provides optimal efficiency and quick response is proposed. An optimization scheme determines the flux level for maximum efficiency at any operating condition, and a coordination controller assures quick torque response without torque pulsations. Application to a 100 hp and a 7.5 hp motor shows that a substantial saving in controllable losses during low-load operation is possible while maintaining high performance     

The transient and qualitative performance of a self-excitedsingle-phase induction generator

The modeling and transient performance of a single-phase induction generator with series or parallel connected load is the theme of this paper. The system of equations are expressed in terms of flux linkages and includes the effect of magnetizing flux linkage saturation. Generator self-excitation and voltage collapse phenomena are simulated. The balance of the paper deals with the qualitative behavior of the generator using concepts of harmonic balance and system bifurcation     

Passivity-based sliding mode position control for induction motor drives

In this paper, a passivity-based sliding-mode controller is proposed to control the motion of an induction motor. At first, the induction motor is proved to be a state strictly passive system. Then, a sliding-mode position controller with an adaptive load torque estimator is designed to control the position of the induction motor such that the chattering effects associated with a classical sliding-mode position controller can be eliminated. The stability analysis of the overall position control system is carried out by the passivity theory. The proposed approach is robust with regard to variations of motor mechanical parameters and load torque disturbances. Finally, experimental results are included to demonstrate that good position tracking can be obtained without the rotor flux observer.