Field assessment of induction motor efficiency through air-gaptorque

Induction motors are the most popular motors used in industry. This paper further suggests the use of the air-gap torque method (see J.S. Hsu et al., ibid., vol.10, no.3, p.471-7, 1995) to evaluate their efficiency and load changes. The fundamental difference between Method E and the air-gap torque method is discussed. Efficiency assessments conducted on induction motors under various conditions show the accuracy and potential of the air-gap torque method     

Motional time-harmonic simulation of slotted single-phase induction machines

Considering motional effects in the steady-state finite-element simulation of single-phase induction machines inevitably requires a transient approach. The resulting computation time seriously hampers the application of finite elements within technical designs. In this paper, time-harmonic finite-element simulation, as commonly applied to the three-phase induction machine model, is also enabled for single-phase motors by decomposing the air-gap field in two revolving fields in the opposite direction. The advantages and drawbacks of the novel approach are illustrated by a benchmark model. Issues such as ferromagnetic saturation, external circuit coupling, adaptive mesh refinement, and torque computation are addressed. The method is used to simulate a capacitor start/run motor.     

Analytical calculation of air-gap magnetic field distribution and instantaneous characteristics of brushless DC motors

This paper derives the relative air-gap-specific permeance distribution function by Schwarz-Christoffel transformation, considering the effect of slotting. Neglecting the iron saturation, and employing the analytical algorithm for partial differential equations, efficient and effective analytical calculations of no-load air-gap magnetic field distribution, armature field distribution, and phase electromotive force (EMF), are demonstrated, considering the stator slots. Subsequently, based on the main circuit topology of a brushless DC motor (BLDCM), the field-circuit coupling model is constructed for the motor, and then the phase current waveforms and load air-gap magnetic field distribution at any time are determined. Consequently, the instantaneous electromagnetic torque is computed, which underpins the quantitative analysis of torque ripple and the pulsation induced by commutation. Hence, the present work paves the way to precise prediction of the motor's performance and acoustic noise. It is a powerful tool for the design of surface permanent magnet brushless DC motors.     

Comparison of PM brushless motors, having either all teeth or alternate teeth wound

The electromagnetic performance of three-phase permanent magnet brushless motors in which the slot and pole numbers are similar and either all the teeth or only alternate teeth carry a concentrated coil is compared. Analytical and finite element techniques are employed to predict the air-gap flux density distribution, the cogging torque, electromotive force (EMF) and static torque waveforms, and the winding inductances, predicted results being validated by measurements. It is shown that there is a significant difference in both the phase and line EMF waveforms, the static torque-rotor position waveforms, and the self and mutual inductances.     

Online cage rotor fault detection using air-gap torque spectra

This paper discusses use of air-gap torque spectra as a means of identifying faults in cage rotors. Being dependent on both stator and rotor currents, the torque is very sensitive to faults in the rotor. Through a comparative study using a detailed machine model and the standard dq model, the paper shows that the characteristic frequencies generated by a particular fault are preserved even if the standard dq model is used for estimation of air-gap torque. This is validated through a practical hardware implementation for online spectrum estimation of air-gap torque using TMS320C31, where several faulted cage rotors were used for study.     

High-Frequency Loss Calculation in a Smooth Rotor Induction Motor Using FEM

In this paper, a new approach for the calculation of high-frequency losses in induction motors is presented. The input to the motors is assumed to be supplied from a sinusoidal voltage source. The method is based on the two-dimensional (2-D) field solutions of the magnetic circuit, obtained by using a nonlinear ldquoharmonicrdquo solution. Hence, the solution time is very short. From the ldquoharmonicrdquo solution, the air-gap field distribution as well as the fundamental frequency eddy current losses are determined. The high-frequency loss calculation is based on the assumption of a path for eddy currents within a lamination. A constant k is introduced that defines the width of the current flow path. The empirically found k value is verified by a theoretical calculation. The new method is applied to the calculation of losses of two smooth rotor induction motors. The prediction accuracy is found to be very good. The method is also applied to two open rotor slot motors to observe the change in the prediction accuracy. It is found that due to the small slot openings accurate predictions are still possible. The approach presented in this paper requires little time for loss calculation, and is very suitable for minimizing losses at the design stage.     

Determination of NEMA Design Induction Motor Parameters From Manufacturer Data

This paper proposes a simple method of determining the equivalent circuit parameters of National Electrical Manufacturers Association (NEMA) design A and B types of induction motors from standard manufacturer data such as rated output power, starting torque, breakdown torque, and efficiency and power factor at rated output power. A set of nonlinear equations for various quantities is first derived from the equivalent circuit with a single-cage rotor model, and then, equate to the corresponding actual values supplied by the manufacturer. These equations are then solved using a least-squares based algorithm to determine the motor parameters. The rotor parameters are considered as slip dependent to predict the starting torque of the motor and that requires refining the breakdown torque equation as well as the slip at which the breakdown torque occurs. The proposed method of determining the motor parameters is then tested on more than 300 large-size HV induction motors. The effectiveness of the proposed method is evaluated by calculating various external quantities of the motors through the estimated parameters and comparing them with the corresponding actual values supplied by the manufacturer.      

Peaked-MMF smooth-torque reluctance motors

The authors investigate the steady-state torque characteristics of reluctance motors with nonsalient stator punchings, but with peaked rotating magnetomotive forces (MMFs). The torque calculation includes the effects of saturation and fringing and groove fluxes. The peaked rotating MMF is produced by properly coordinated current waveforms and winding. Peaked-MMF reluctance motors have tow major advantages: the torque is smooth and the flux per pole required to produce a given torque is lower than that of conventional reluctance motors. This property is most beneficial to two-pole reluctance motors, for a given frame whose bore diameters and slot areas can be increased significantly for higher ratings or better performance. Unlike switched reluctance motors, shaft encoders are not required for peaked-MMF motors     

Experimental determination of the forward and backward-field torquein shaded-pole motors

An experimental method for determining the forward- and backward-field torque and the amplitude of a pulsating torque at twice the line frequency in shaded-pole motors is developed on the basis of revolving-field theory. These parameters are determined from the induced voltage in a measured winding by segregating a forward and backward fundamental component using a spectrum analyzer. This torque-measuring method has been tested for a shaded-pole motor with one shading coil, and a shaded-pole motor with two shading coils per pole. The forward-field torque minus the backward-field torque, as measured by the measuring winding, agreed with that measured by a Prony-brake torque meter with sufficient accuracy. The amplitude of the pulsating torque also agreed approximately with that measured with an accelerometer. Each torque can be determined more precisely in this experimental technique than in existing calculation techniques     

Optimum geometry for torque ripple minimization of switchedreluctance motors

For switched reluctance motors, one of the major problems is torque ripple which causes increased undesirable acoustic noise and possibly speed ripple. This paper describes an approach to determine optimum magnetic circuit parameters to minimize low speed torque ripple for such motors. The prediction of torque ripple is based on a set of normalized permeance and force data obtained from numerical field solution for doubly-salient geometries. For that purpose a neural net is trained to extract the data needed to predict the torque produced by a given geometry and excitation at any position of teeth. Hence the static torque curve can be constructed and torque ripple can be found. The accuracy of the approach developed is illustrated by comparing measured and predicted torque for a switched reluctance motor. The optimum parameters for minimum torque ripple conditions are sought using the augmented Lagrangian method. The paper presents the optimization results, and then proceeds to determine the range of geometric parameters which keep the torque ripple within ±10 of the optimum value     

Control-based reduction of pulsating torque for PMAC machines

Control methods in torque pulsating reduction for surface-mounted permanent magnet motors are discussed in this paper. The pulsating torque is a consequence of the nonsinusoidal flux-density distribution caused by the interaction of the rotor's permanent magnets with the changing stator reluctance. The proposed control method is estimator based. To assure parameter convergence, Lyapunov's direct method is used in estimator design for the flux Fourier's coefficients. A novel nonlinear torque controller based on flux/torque estimate is introduced to reduce the influence of the flux harmonics. The influence of the cogging torque is considerably reduced at lower motor speed using the internal model principle and adaptive feedforward compensation technique. The overall control scheme and experimental results are also presented     

Estimation of induction motor double-cage model parameters from manufacturer data

This paper presents a numerical method for the estimation of induction motor double-cage model parameters from standard manufacturer data: full load mechanical power (rated power), full load reactive electrical power, breakdown torque, and starting current and torque. A model sensitivity analysis for the various electrical parameters shows that stator resistance is the least significant parameter. The nonlinear equations to be solved for the parameters determination are formulated as a minimization problem with restrictions. The method has been tested with 223 motors from different manufacturers, with an average value of the normalized residual error of 1.39/spl middot/10/sup -2/. The estimated parameters of these motors are graphically represented as a function of the rated power.     

Proposals for a practical calibration method for mechanical torque measurement on the wind turbine drive train under test on a test bench

The mechanical torque input into the wind turbine drive train is a very useful measurement for tests performed on a test bench. To ensure the accuracy and the reliability, an accurate calibration of the torque measurement must be carried out and repeated within a certain period of time. However, owing to the high torque level and large structure size, such a calibration is both expensive and time consuming. To overcome this challenge, a new calibration method is proposed here. The method is based on the electrical power measurement, where a high level of accuracy is much easier to achieve. With the help of a special test process, a relationship between the torque‐measuring signal and the electrical power can be established. The process comprises two tests with the drive train running in different operating modes. The calibration is possible by carrying out the same test process on several different torque levels. Detailed uncertainty analysis of the method is presented, whereby the uncertainty can be calculated by means of matrix operation and also numerically. As a demonstration, the implementation of the method on a test bench drive train that contains two 5‐MW motors in tandem with the motors operating in a back‐to‐back configuration is also presented. Finally, some variations on the method and possible ways of achieving better accuracy are discussed.     

Determination of radial-forces in relation to noise and vibrationproblems of squirrel-cage induction motors

The radial electromagnetic forces in induction motors play an important role in the production of audible noise and vibrations. The magnetic flux pulsations at the iron surfaces produce these radial forces, which act on the stator and rotor structures. An analysis for the calculation of the various field harmonics and radial forces in squirrel cage induction motors is presented in this paper. To verify the validity of the analysis, a squirrel cage induction motor is analyzed. Theoretical and experimental results are presented with a view to determine the actual role played by the air-gap harmonic fields on the radial forces. Also, the effects of loading on the radial forces and the ensuing vibrations are closely examined     

Asynchronous Performance Prediction of AC Permanent Magnet Motor

many permanent magnet synchronous motors are run up from standstill by a sudden connection with an ac supply. The cage windings in rotor slots produce induction motor torque to run up the rotor, but two torque dips are produced in the torque-slip curve of the permanent magnet motor; one is caused by the rotating permanent magnet, and the other by the magnetic and the electric asymmetry between the direct and quadrature axes. Therefore, the prediction of the asynchronous performance is very important, as motors cannot be run up, unless the minimum values of the torque dips exceed the load torques at the slips. In this paper, ``harmonic permeance coefficient' is newly introduced, which is used to combine the finite element field solution with the calculation of air gap inductance, and equations for the calculation of asynchronous performance of permanent magnet motors are also expressed. The calculated value by these equations agreed well with the experimental results.     

Core loss in buried magnet permanent magnet synchronous motors

The steady-state core-loss characteristics of buried-magnet synchronous motors operating from a sinusoidal constant frequency voltage supply are investigated. Measured and calculated core loss, with constant shaft load, is shown to increase with decreasing terminal voltage due to an increase in armature reaction-induced stator flux-density time harmonics. Finite-element modeling is used to show that the additional loss due to the time-harmonic fields can increase core loss by a factor of six over the loss associated with only the fundamental component field at low motor flux levels. A simple air-gap model of motor flux components shows that this increased loss is due to localized rotor saturation. Thus, stator-core harmonic fields should be expected for all buried-magnet rotor synchronous motors (with or without a cage) operating at low flux levels. This factor becomes increasingly important when the motors are operated in the high-speed low-flux mode in conjunction with a variable-speed drive     

Characteristic Analysis of Multilayer-Buried Magnet Synchronous Motor Using Fixed Permeability Method

The direct (d)- and quadrature (q)-axis inductance characteristics of the interior permanent-magnet synchronous motor (IPMSM) are presented, with an emphasis on the cross-coupling effects. Nonlinear variation of phase inductance is investigated, according to the rotor position on entire stator current excitations. We also propose a new analytical method, called the fixed permeability method (FPM). We applied this method (FPM) to steel-cored permanent-magnet linear synchronous motors for large powered linear motional applications. Experimental data were found to corroborate the findings obtained by the FPM method. We also used FPM to analyze magnetic saturation characteristics of the prototype IPMSM, and compared the air-gap flux density and$d$- and$q$-axis inductance with the conventional finite element analysis results.     

A novel calculation method for iron loss resistance suitable in modeling permanent-magnet synchronous motors

This paper proposes a calculation method for iron loss resistance, suitable for modeling permanent-magnet synchronous motors. The proposed method is based on the linear feature between semi-input power and square of speed electromotive force. The iron loss resistance is calculated from the slope of this linear function in the offline manner. The advantage of the proposed method is that the iron loss resistance can be calculated directly without any measurements related to mechanical loss. In addition, the proposed method can be executed at any load conditions. The validity of the proposed method is experimentally confirmed by the comparison between the actual torque and the calculated torque containing the iron loss resistance.     

A new method for quality monitoring of induction motors

Catalog data of three-phase induction motors specify values of some operational parameters in addition to the nameplate information. These parameters include the efficiency, power factor, starting current, maximum torque, rated torque, and starting torque. At the end of the manufacturing process, extensive testing is required to ensure that a specific motor meets the specifications. In this paper, a method is presented to ensure that the motor meets the operational parameter specifications, including the tolerances that might be imposed using the results only of the no-load test and the blocked-rotor test     

Comparison of the nature of torque production in reluctance andinduction motors

The nature of torque production is different in reluctance and inductance motors. One significant difference occurs in a reluctance motor that has nonsalient stator punching and a salient motor. When the flux per pole is small in such a motor, the torque can still be high, as long as the rate of energy change with respect to the rotor angular displacement at the rotor pole fronts and pole ends is high. A theoretical foundation to improve the torque capability of reluctance motors is provided. Effects of saturation and stray-load loss are also studied. Experimental results show agreement with theoretical conclusions