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Three phase inverters with switching frequencies above 100 kHz have recently become feasible thanks to the significant progress made in wide band gap (WBG) semiconductor technology. The main drivers behind the intense research and development of these ultrahigh switching frequency pulsewidth modulation (PWM) inverters are the extraordinary gravimetric and volumetric power densities that can be achieved. On the other hand, not as much attention has been devoted to the loads that these inverters supply, quite frequently consisting in ac motors. The impact of sub-MHz PWM and of the associated tens of nanoseconds range switching transients on motor terminal overvoltage, common mode currents, and losses are still to be discussed exhaustively. The authors have thus decided to investigate what happens to induction motors (IM) supplied by inverters having switching frequencies up to 350 kHz. After an introduction on WBG semiconductor devices, the article presents some interesting experimental observations conducted on a 1.1 kW four-pole IM fed by a silicon carbide (SiC) inverter. Further insight is provided by tests on a laminated steel toroid, supplied by a single-phase full bridge SiC inverter. Discussions on the experimental results highlight the previously unreported impact of common mode losses, which become a predominant part of the PWM harmonic losses. [ABSTRACT FROM AUTHOR]

This article proposes an enhanced procedure for executing the standard variable load test that allows obtaining high correlation factors for indirectly measured stray-load loss during efficiency tests of induction motors. This helps to achieve the minimum values required by the international standards to consider the variable load test well executed, avoiding unwelcome test repetitions and leading to significant amount of time saved. With the machine at steady-state temperature, the enhanced procedure consists of alternatively applying load levels higher and lower than the rated load, so that the stator winding temperature zigzags around the rated value. Hence, multiple readings can be performed for each load point, having the machine in isothermal conditions. This allows averaging many measurements of the same load point to mitigate the impact of instrumentation and reading errors. The article includes load tests conducted on different induction motor sizes and pole counts, applying both the standard variable load test procedure and the proposed approach. The experimental results show that the proposed technique allows achieving higher correlation factors on the stray-load losses than the standard procedure. [ABSTRACT FROM AUTHOR]

This research work investigates the phenomenon of interbar currents in induction motors equipped with die-cast aluminum cages. Flowing in the lamination and distorting the bar current distributions, the interbar currents cause additional stray losses as well as an increase of the joule loss in the cage bars. The contact resistance between the bars and the rotor laminations is the key modeling element for a correct prediction of the interbar currents and the related extra losses. The study presents a new noninvasive method to obtain a reliable estimation of the contact resistance. The proposed hybrid approach is based on a mix of experimental data and Finite Element method (FEM)-based simulation results. A reliable three-dimensional FEM model of a four-pole 15-kW induction motor with closed rotor slots has been used to investigate the impact of variable contact resistance on the interbar currents, rotor joule losses, and the voltage drops along the bars, opening a new perspective for this complex phenomenon and its impact on the stray losses. Finally, the proposed methodologies are critically discussed and the obtained results compared with other relevant research works. [ABSTRACT FROM AUTHOR]

In the recent past, several successful experiences to modify standard efficiency induction motors in premium efficiency class (IE3) machines have been presented, considering simple and low-cost changes in the design and production processes. However, this approach—also known as no-tooling-cost process—seems to have reached its limit without offering appreciable margins above the IE3 efficiency levels. In order to avoid the electromagnetic redesign of entire machine series for an IE4 efficiency class, the authors investigated how to obtain further efficiency gains in the same volumes of IE3 induction motors. The study looked at unconventional modifications in the production process/design procedure that can still be considered to have minimal impact on manufacturers. In particular, stator winding layouts targeted at very low spatial harmonic content, rotor bar skew removal, and rotor slot insulation prior to aluminum cage die casting are investigated as potential low-cost solutions. [ABSTRACT FROM AUTHOR]

Induction motor loss segregation and efficiency measurement requires loading dynamometers and other equipment, such as a variable voltage sinusoidal power supply. These are expensive and not often available even when a loading device is accessible. Variable frequency drives are now widely used for operating induction machines and are more widely available and less expensive. However, their use in place of a fixed frequency sinusoidal power supply to directly measure machine efficiency is intriguing, but inherently challenging due to the pulsewidth modulation (PWM) output voltage. This paper presents some investigations for using variable frequency drives to perform IEEE 112B and IEC 60034-2-1 tests. The use of the drive, the measurement criteria, and procedures are reported and discussed. The results presented demonstrate the feasibility of the proposed concept for measuring machine efficiency with a PWM power source. [ABSTRACT FROM AUTHOR]

This Special Section on "Optimization of Electrical Machine Designs" summarizes the state-of-the-art on the latest researches carried out on the topic, both in the academic and industrial environments. Basically, all the electrical machine topologies and optimization techniques have been discussed in this Section, suggesting to the readers interesting solutions of several optimization problems related with modern applications. The second part of the special section (the first one has been published in the December 2017 issue of the IEEE Transactions on Industrial Electronics) includes 20 papers from a total of 44 accepted contributions. The studies included in the Part II are categorized in a table and are outlined to some extent in order to help the readers to quickly compare the main peculiarities of the mathematical approaches and the practical implementations. The Part II paper categorization of this Special Section presents a second set of four papers focused on the permanent magnet synchronous machines [ABSTRACT FROM PUBLISHER]

The determination of induction machine torque and current speed characteristics relies on methods based on either direct testing or calculation, using a machine equivalent circuit (EC). Both methods may lead to significant errors. As a contribution to a still open discussion, this paper presents the challenges encountered using these methods and an approach is presented in this paper to overcome the inherent challenges. The proposed approach considers saturation, and compensates for skin effect and machine temperature to improve accuracy. Five induction motors of different pole pairs were analyzed and tested to provide the understanding of the underlying issues of predicting torque and current characteristics of induction machines. The findings indicate that, for direct testing, multiple data points closer to nominal voltage are required. In the case of EC modeling, proper correction of the model for temperature and saturation leads to improved prediction of the torque characteristics of general purpose induction machines. [ABSTRACT FROM AUTHOR]

Electrical engineers rely on analytical models to design industrial induction motors for different customer-driven applications. These models are fundamental and do not normally consider additional core losses due to stamping or welding or the impact of being inverter fed. To overcome the resulting gap between analytical results and measured data, these equivalent circuit models are traditionally adapted by correction factors. This paper presents a methodology allowing the parameterization of such an analytical model based on inverter-fed load and no-load measurements carried out for several frequencies. The results show a fair agreement over a wide range of operating points, which is acceptable for daily regular industrial applications. [ABSTRACT FROM AUTHOR]

A simplified analysis of influence of pole pairs on the steady-state behavior of induction motors is presented. The analysis makes simplifying assumptions to express relationship between key steady-state performance parameters of 2-pole reference motor compared to its multiple pole counterparts. The analysis is performed using considerations of fixed geometrical dimensions on one hand and fixed output power on the other hand. Available manufacturer catalog data of medium voltage (MV) induction machines from one manufacturer are used to evaluate the soundness of the analysis. Also test and catalog data analyses of commercially manufactured low-voltage (LV) machines from different manufacturers are evaluated to determine extent to which they conform to the theoretical considerations of the analyses. The preliminary findings indicate more agreement with the MV motors than the LV motors. [ABSTRACT FROM AUTHOR]

An introduction to various articles published within the issue is presented including the design of a sports car's direct-driven interior permanent-magnet synchronous motors (IPMSMs), a claw-pole double-rotor synchronous machine and the electromagnetic sizing procedure for an electrical machine.

Starting from 2011, in order to reduce electricity consumption and carbon dioxide emissions, the European Minimum Energy Performance Standard will impose minimum efficiency values for electric motors introduced into the European market. The new European standard requirements will impose nontrivial efforts to manufacturers, particularly for small- and medium-sized producers. In this paper, the axial lengthening of the magnetic core is considered as an effective solution to improve the induction motor efficiency class. In fact, this design solution does not require changing stator and rotor laminations, and it can be considered as a “no tooling cost” (NTC) solution to improve the rated efficiency. Starting from standard-efficiency totally enclosed fan-cooled machines, one size has been selected, and a “lengthened” motor prototype has been designed and analyzed using a very fast and reliable design procedure based on a coupled analytical–finite-element approach. The experimental results confirm the accuracy of the proposed design method and validate the proposed NTC solution to get higher efficiency induction motors. [ABSTRACT FROM AUTHOR]

The improvement of induction motor efficiency over the years has been incremental in nature. One of the well-known methods used to increase the efficiency of induction motors, cost-effectively, is by increasing the stack length. There are many motor manufacturers with proprietary designs that are optimized for different parameters, e.g., cost, volume, weight, copper, iron, etc. Therefore, the effectiveness of incrementally improving the efficiency for motors of various designs and from different manufacturers presents an interesting research. This paper seeks to determine the incremental improvement of design efficiency of motors that are already subjected to commercial production. Motors from five manufacturers were tested, torn down, and analyzed, and their design efficiency improvement is investigated. [ABSTRACT FROM PUBLISHER]

In this paper, the effects due to the presence of closed or open rotor slots in high-speed induction motors are presented. In particular, the problem linked to electromagnetic effects on the motor current is analyzed in detail. Two 32 000-r/min induction motor prototypes, the former with open rotor slots and the latter with closed ones, have been built and tested in order to verify the theoretical analysis. A modified steady-state equivalent circuit to take into account the electromagnetic effects of rotor closed slots is proposed, and experimental tests have been performed to validate the proposed model. The presented study is focused on high-speed induction motors for turbomolecular vacuum pumps. [ABSTRACT FROM AUTHOR]

This paper proposes methods and algorithms for the estimation of induction motor equivalent circuit parameters, starting from the geometrical and electrical data generally available after an electromagnetic design. This part includes an approach for the skin effect determination in arbitrarily shaped rotor slots, based on the elementary layer method. In addition, a complete analytical development for the fundamental airgap flux density, including magnetic saturation phenomena and no-load characteristic determination, is reported. Finally, a comparison between the computed and measured values is provided and discussed for several induction motors. [ABSTRACT FROM AUTHOR]

This paper deals with methods and algorithms for the estimation of induction-motor equivalent circuit parameters starting from the geometrical and electrical data generally available after an electromagnetic design. The main aim of this paper is to help the readers pass from the theoretical analysis to the numerical code suitable to be included in its own design software. For this reason, the step-by-step algorithms are included in this paper. Some of the methods are based on well-known approaches while the other ones are original. For the latter ones, a complete discussion of the theoretical analysis is reported and discussed. The first part deals with the stator- and rotor-resistance computations and the determination of the leakage inductances based on an original flux classification. Finally, the skewing slot effects on the machine parameters are provided. The comparison between computed and measured values confirms the accuracy of the proposed computational algorithms. [ABSTRACT FROM AUTHOR]

In this paper, the effects of the sinusoidal supply voltage amplitude on the stray-load losses in small/medium-size induction motors are analyzed and discussed using a simplified per-phase equivalent circuit, which has recently been proposed by the authors. In this circuit, the stray-load losses due to the airgap spatial harmonics effects on the rotor squirrel cage are modeled using a suitable additional resistance connected in series with the stator impedance. Then, this paper's main aim is to further prove the validity and the accuracy of the proposed model and to increase the familiarity with the new equivalent circuit parameter. By an experimental campaign conducted on a four-pole 7.5-kW totally enclosed fan-cooled induction motor, several data have been collected and used to determine the stray-load losses for different values of the supply voltage, in accordance to the IEEE Std. 112-B. The same data have been used to determine the additional resistance as a function of the supply voltage. The obtained results show that, also in conditions different from the rated ones, the additional resistance is reasonably able to take into account the considered stray-load losses in the machine power balance. [ABSTRACT FROM AUTHOR]

In this paper, an easy method for the iron loss prediction in pulsewidth-modulation (PWM) inverter-fed induction motors is presented. The method was initially proposed and validated for the prediction of iron losses in a nonoriented soft magnetic material with PWM supply. Starting from the iron losses measured with a sinusoidal supply and the PWM waveform characteristics, a fast and reliable prediction of the iron losses in a motor can be obtained too. The method requires the separation of the iron losses in the hysteresis and the eddy current components with a sinusoidal supply, plus the average rectified and rms values of the applied PWM voltage. The proposed method has been proved on an induction motor prototype that is able to provide a good accuracy in the iron loss measurement. The comparison between the measured and predicted iron losses with PWM supply has shown an excellent agreement with an error lower than 5%, confirming the method validity. [ABSTRACT FROM AUTHOR]

This paper intends to develop a more accurate approach for determining the no-load iron losses in pulse width modulation (PWM) inverter fed induction motors. The proposed method is validated by means of a prototype motor with a plastic rotor cage. The iron losses have been computed by the timestepping finite element method, both with sinusoidal and PWM supply. The iron losses have then been estimated by adding up the contribution generated by orthogonal components of the flux density, as if the iron losses generated by these components were independent phenomena. The rotational hysteresis losses, as well as excess ones, have been calculated applying a correction factor based on experimental data. These factors are a function of the peak flux density and ellipticity of the B vector loci. Experimental validations are provided for several frequency and magnetic saturation values. In addition, this paper demonstrates the necessity to consider the harmonics initial phase in order to increase the accuracy in the iron loss prediction. [ABSTRACT FROM AUTHOR]

In this paper, the determination of the end space induction motor heat transfer coefficients is presented, and the methodologies used are examined closely. Two "ad hoc" prototypes have been built and a test bench completed. This paper reports the setup of the test procedures and results obtained in detail. As the end windings are the hottest points of the motor, particular care has been devoted to the determination of the heat transfer coefficient concerning the end-winding structure. The results obtained are of fundamental importance for the determination of the thermal resistances between end windings and end caps. These can then be used in thermal networks usually adopted in thermal model analysis. [ABSTRACT FROM AUTHOR]

In this paper, the stray load losses due to the air gap spatial harmonics effects on the rotor squirrel cage are considered and a simplified induction motor equivalent circuit able to include them in the machine power balance is reported. In the proposed model, an additional resistance appears in series with the stator impedance. This additional parameter has been defined on the basis of power balance considerations applied to the induction motor classical theory. The complete theoretical analysis together with the guidelines to measure the equivalent circuit parameters are reported in detail. Finally, the proposed approach has been experimentally verified using five total enclosed fan cooled induction motors. [ABSTRACT FROM AUTHOR]

In this paper, the authors discuss the problem concerning the determination of some thermal parameters which are very complex to compute. These parameters play an important role in thermal networks usually adopted for electrical machine thermal analysis. In particular, in this paper, the following thermal parameters are analyzed: equivalent thermal resistance between external frame and ambient, equivalent thermal conductivity between winding and lamination, forced convection heat transfer coefficient between end winding and end-caps, radiation heat transfer coefficient between external frame and ambient, interface gap between lamination and external frame, air cooling speed, and the bearings effective thermal resistance. While the information given in this paper is mainly related to induction motors, it is also of general use for the designers of other types of machine. The reported results have been validated using induction motors with power ranges from a few kilowatts up to 1 MW. [ABSTRACT FROM AUTHOR]

A performance analysis of three-phase and dual three-phase (DTP) induction pulsewidth modulation (PWM) inverter-fed motor drives is conducted in this paper. The focus is on the efficiency performance of high-frequency DTP machines compared to their three-phase counterparts in low/medium power applications. For this purpose, a DTP machine, having two sets of stator three-phase windings spatially shifted by 30 electrical degrees (asymmetrical six-phase winding configuration), was tested for both six-phase and three-phase winding configurations under the same magnetic conditions. Simulation and experimental results are presented to evaluate the efficiency performance of three-phase and dual-three induction motor drives employing PWM voltage source inverters. [ABSTRACT FROM AUTHOR]

This paper deals with the endwinding cooling problems of totally enclosed fan-cooled (TEFC) induction motors. In order to obtain information about the phenomena involved in the motor end space, three ‘ad hoc’ prototypes have been build. The complete test-bench setup, together with the followed test procedures, is reported in detail. The measurement results have shown that all the motor-part overtemperatures (winding, endwindings, stator lamination, and external motor frame) decreasing, with the inner air speed increasing. The measured motor overtemperatures and losses allow the thermal-resistance identification of a simplified thermal model suitable to describe the thermal behaviors of the prototypes. By the endwinding-to-motor frame thermal resistance, the related heat-exchange coefficients have been evaluated as a function of the rotor speed. The proposed procedure allows separating the forced-convection contribution by the other thermal-exchange phenomena that occur in the end-space regions. The obtained heat-transfer coefficients are in agreement with the results reported in the past literature. [ABSTRACT FROM AUTHOR]

In order to predict conducted electromagnetic interference in inverter-motor drive systems, high-frequency (HF) motor models are requested and the involved parameters have to be available. In previous studies, the authors have presented an accurate HF model for induction motors and they have defined the procedures to identify the model parameters. In this paper, these results are extended to several types and sizes of industrial ac motors such as induction, synchronous reluctance (without interior permanent magnets), and brushless motors. The model parameter-identification procedure has been improved, and it is based on a least-squares data fitting applied to the measured magnitude and phase-frequency-response curves of the phase- to-ground and the phase-to-neutral impedances. The aim of this paper is to provide quick indications to select the suitable values of the HF model parameters, with reference to the size and type of the ac motor, to evaluate the HF voltage and current components in inverted-fed ac motor systems. [ABSTRACT FROM AUTHOR]

Vector control accuracy of induction motor drives is affected by variations of motor parameters that are treated in the control algorithm as constant values, and by the phenomena that are not modeled at all and are therefore unaccounted for in the controller. Detuning sources of the first type include variations of rotor and stator resistance, mutual inductance, and leakage inductances, while the second category includes stator iron (core) losses. All these sources of detuned operation have been studied in a considerable depth in the past. It appears that the only potential source of detuned operation, which has never been studied before, is the stray load loss (SLL), which belongs to the category of unmodeled phenomena. This paper develops an analytical mode! that characterizes detuning due to SLLs in indirect rotor-flux-oriented (RFO) current-fed induction motor drives in steady-state operation by means of the orientation angle error, actual to reference rotor- flux ratio, and actual to reference-torque ratio. A quantitative assessment of the impact of SLL on accuracy of rotor-flux-oriented control is performed, with the necessary motor parameters obtained from IEEE 112-B standard measurements and subsequent equivalent circuit parameter fitting. Detuning is also examined in transient operation. It is shown that, although SLLs are comparable to the iron losses in the studied machine (of approximately the same value in the rated operating point), their impact on accuracy of vector control is much smaller when compared to the iron loss induced detuning. [ABSTRACT FROM AUTHOR]

This paper deals with the role of the radiation thermal resistances in industrial motors and describes a test set useful for the resistance evaluation. The test set is based on a vacuum chamber and on a numerical acquisition system, and the resistances are obtained by monitoring motor temperatures during a heating test. An analytical model that describes how the radiation resistance depends on the motor temperature is also provided. Experimental results obtained on a totally enclosed fan-cooled 150-W induction motor are shown and discussed. [ABSTRACT FROM AUTHOR]

In this paper, the thermal behavior of two induction motors (2.2 and 4 kW, four poles) and two synchronous reluctance motors [(SynRMs) transverse-laminated] are investigated and compared. Both motor types use the same stator but have different rotors. Using a lumped-parameter simulation program, a thermal analysis has been also carried out, and the obtained results have been compared with the experimental ones. A direct comparison of the thermal behavior of the two motor types has thus been made for constant load and constant average copper temperature conditions. Inasmuch as the SynRM has negligible rotor losses compared with the induction motor, it is capable of a larger rated torque, from 10% to more than 20%, depending on the relative size of end connections and motor length. [ABSTRACT FROM AUTHOR]

With the ever-increasing pressure on electric motor manufacturers to develop smaller and more efficient electric motors, there is a need for more thermal analysis in parallel with the traditional electromagnetic design. Attention to the thermal design can be rewarded by major improvements in the overall performance. Technical papers published to date highlight a number of thermal design issues that are difficult to analyze. This paper reviews some of these issues and gives advice on how to deal with them when developing algorithms for inclusion in design software. [ABSTRACT FROM AUTHOR]

In this paper, the analysis of some possibilities for increasing the induction motor efficiency using production techno logical process modifications is reported. This approach is known as the "no tooling cost" (NTC) strategy because it does not require a complete redesign of new laminations with a consistent cost in terms of investments. The paper shows the results obtained by a full experimental approach, using "ad hoc" prototypes. The NTC design modification and the technological processes analyzed in this paper have been done on totally enclosed fan-cooled standard induction motors. Obviously, the original motors have been com- pared from the energetic point of view with these prototypes. The energetic performance has been measured in accordance with the WEE Std. 112-96 Method B. In particular, the following modifications, for obtaining an increase in efficiency, have been taken into consideration: rotor with copper bar included in the slot before the aluminum die cast, increase of the core axial length, and annealing of the stator core. [ABSTRACT FROM AUTHOR]

With the increasing pressures on electric motor man ufacturers to develop smaller and more efficient electric motors, there is a trend to carry out more thermal analysis in parallel with the traditional electromagnetic design. It has been found that attention to thermal design can be rewarded by major improvements in the overall performance. Thus, there is a requirement for accurate and reliable thermal analysis models that can be easily incorporated into motor design software. In this paper, emphasis is given to thermal sensitivity analysis of totally enclosed fanooled induction motors. In particular, thermal parameters are modified and their effects on the temperature rise shown. The results are useful for identifying the most important thermal parameters and enable robust designs to be developed that are insensitive to manufacturing tolerances. [ABSTRACT FROM AUTHOR]

Motor efficiency has to be measured or calculated in accordance with international standards. The most important standards are the IEEE 112-B, IEC 34-2, and JEC 3. In this paper, a comparison of the measurement procedures defined by these international standards is reported, together with some comments on the prescribed methodologies. The comparison is based on experimental results obtained by tests on four general-purpose three-phase induction motors. The stray-load loss measurement represents a critical key for the correct evaluation of the motor efficiency. For this reason, a critical analysis of this type of losses has been performed. In particular, in order to understand which are the most critical quantities that influence their evaluation, the stray-load loss sensitivity to the measurement errors is analyzed. In the final part of the paper the temperature influence, on the conventional iron losses, is experimentally analyzed. The performed tests show that the temperature difference between the no-load test and the motor real operative conditions is not negligible. [ABSTRACT FROM AUTHOR]

Presents a study which verified a simplified thermal model for variable-speed self-cooled induction motors. Review of a thermal model proposed by P. Mellor and D. Turner; Hypotheses assumed regarding the simplified thermal model; Comparison between the proposed model and the Mellor and Turner model; Estimation of the cooling air speed and fan characterization.

An introduction is presented in which the editor discusses various reports within the issue on topics including current technologies for high-speed electrical machine applications, a novel motor topology with a lateral stator used for drilling, and use of amorphous laminations on induction motor.