Modular Three-Phase Permanent-Magnet Brushless Machines for In-Wheel Applications

This paper describes the merits of a recently developed form of a three-phase permanent-magnet (PM) brushless ac machine in which the concentrated coils of each stator phase are wound either on adjacent teeth or on alternate teeth. Such a machine is often referred to as modular and offers a number of significant advantages over conventional PM brushless machines. For example, it results in a smaller number of slots for a given number of poles, which is a distinct manufacturing advantage, and yields a fractional number of slots per pole, which is conducive to low cogging torque. It also enables a significant increase in the achievable machine inductance to facilitate constant power operation over a wide speed range by flux weakening. However, the torque in modular machines is developed by the interaction of a high-order stator space harmonic MMF with the PMs, since the fundamental stator MMF has fewer poles than the PM rotor. Hence, significant eddy currents may be induced in the rotor by the fundamental and low-order space harmonic MMFs. The eddy-current loss can, however, be reduced by segmenting the magnets. Given that modular machines combine the high specific power and efficiency of conventional PM brushless machines with a high machine inductance, to enable a wide speed range, constant power operation, their potential for low manufacturing cost, and the fact that they have inherently low cogging torque, they are eminently suitable for in-wheel traction applications.      

Three-phase modular permanent magnet brushless Machine for torque boosting on a downsized ICE vehicle

The paper describes a relatively new topology of 3-phase permanent magnet (PM) brushless machine, which offers a number of significant advantages over conventional PM brushless machines for automotive applications, such as electrical torque boosting at low engine speeds for vehicles equipped with downsized internal combustion engine (ICEs). The relative merits of feasible slot/pole number combinations for the proposed 3-phase modular PM brushless ac machine are discussed, and an analytical method for establishing the open-circuit and armature reaction magnetic field distributions when such a machine is equipped with a surface-mounted magnet rotor is presented. The results allow the prediction of the torque, the phase emf, and the self- and mutual winding inductances in closed forms, and provide a basis for comparative studies, design optimization and machine dynamic modeling. However, a more robust machine, in terms of improved containment of the magnets, results when the magnets are buried inside the rotor, which, since it introduces a reluctance torque, also serves to reduce the back-emf, the iron loss and the inverter voltage rating. The performance of a modular PM brushless machine equipped with an interior magnet rotor is demonstrated by measurements on a 22-pole/24-slot prototype torque boosting machine.     

Optimization of a synchronous reluctance machine for a sensorless drive with a wide field-weakening range

For more than one century, electrical machines have been utilized for electrical drives. Nowadays, in most applications the electrical machine is fed by an inverter. Three types of machines are available for such purposes: the asynchronous induction machine, the permanent magnet excited synchronous machine and the synchronous reluctance machine. Reluctance machines represent an alternative to the other types when utilized in high-performance drives with a wide speed range. Due to the rotor saliency, these machines have an inherent suitability for a position-sensorless control. The parameters of a 5 kW machine with a maximum speed of 8000 rpm are evaluated by means of nonlinear finite element analyses. With regard to an application in a high-performance drive with a wide field-weakening range and a position-sensorless control scheme, the characteristics are calculated for the conventional reluctance machine as well as the reluctance machine with additional permanent magnets in the rotor. The comparison of the characteristics of the conventional reluctance machine and the permanent magnet assisted reluctance machine clearly shows the improved performance in terms of electromagnetic torque and power factor due to the interior permanent magnets. Thereby, the suitability for the application in position-sensorless drives due to the high effective saliency is preserved.     

Motion control with permanent-magnet AC machines

Motion control techniques have been developed to exploit the high efficiency and extremely fast dynamic response capabilities of permanent-magnet AC (PMAC) machines. Control techniques are reviewed separately for the two major classes of PMAC machines referred to as trapezoidal (i.e., brushless DC) and sinusoidal machines. While trapezoidal PMAC machine drives are distinguished by their controls simplicity and minimal sensor requirements, sinusoidal PMAC machine drives offer opportunities for extremely smooth torque production and extended high-speed operating ranges. Advanced PMAC machine control topics including sensor elimination techniques and robust servocontrol algorithms are reviewed, concluding with a discussion of PMAC machine drive application trends     

Variable flux control of permanent magnet synchronous motor drivesfor constant torque operation

In this paper, a small signal model of permanent magnet synchronous machines is developed which includes both components of torque, i.e., magnet torque and reluctance torque. The effects of flux variations on the torque are analyzed by the use of the developed model. The off-line torque compensation method proposed for induction machines is then adapted to permanent magnet motor drives to achieve a constant torque, variable flux operation of the drives. A sensitivity analysis is performed to show that the off-line method is influenced considerably by machine parameter variations. Therefore the concept of forced compensation is introduced and an on-line torque compensation controller is proposed. Simulation results are presented to show the effectiveness of the proposed controller. An experimental vector controlled permanent magnet motor drive including the on-line torque compensation controller is implemented based on a TMS320C31 DSP to evaluate the method. The experimental results also confirm a desirable variable flux control of the motor drive under constant torque operation     

Novel permanent magnet motor drives for electric vehicles

Novel permanent magnet (PM) motor drives have been successfully developed to fulfil the special requirements for electric vehicles such as high power density, high efficiency, high starting torque, and high cruising speed. These PM motors are all brushless and consist of various types, namely rectangular-fed, sinusoidal-fed, surface-magnet, buried-magnet, and hybrid. The advent of novel motor configurations lies on the unique electromagnetic topology, including the concept of multipole magnetic circuit and full slot-pitch coil span arrangements, leading to a reduction in both magnetic yoke and copper, decoupling of each phase flux path, and hence an increase in both power density and efficiency. Moreover, with the use of fractional number of slots per pole per phase, the cogging torque can be eliminated. On the other hand, by employing the claw-type rotor structure and fixing an additional field winding as the inner stator, these PM hybrid motors can further provide excellent controllability and improve efficiency map. In the PM motors, by purposely making use of the transformer EMF to prevent the current regulator from saturation, a novel control approach is developed to allow for attaining high-speed constant-power operation which is particularly essential for electric vehicles during cruising. Their design philosophy, control strategy, theoretical analysis, computer simulation, experimental tests and application to electric vehicles are described     

A new flux-barrier-type reluctance motor with a slit rotor

A reluctance motor, with a rotor which has many slits, is proposed. The slits are used to generate magnetic flux in the d-axis direction. In practice, the rotor, which has a fewer number of slits, is desired to increase productivity. With a fewer number of slits, torque ripple becomes large. Therefore, torque ripple reduction methods are also proposed. In simulations and experiments, rated torque of the proposed motor is 1.7× as much as that of the conventional brushless DC motor with ferrite permanent magnets. The peak-to-peak (p-p) torque ripple is 1.5% p-p of the rated torque at the rated torque. The efficiency is 91% and the power factor is 61%, at the rated torque and 1200 min^-1. The characteristics of the field weakening and the constant power control are also shown. The breakpoint frequency of speed controls is about 200 Hz. Therefore, the proposed reluctance motor is feasible for general applications     

Sensorless torque control of SyncRel motor drives

This paper describes a direct self-control (DSC) scheme for synchronous reluctance motor drives. The presented DSC scheme develops a new torque control methodology that does not require any position transducer to synchronize the stator current vector with the rotor. Such a control strategy differs from the conventional DSC approach in order to fit some specific requirements of synchronous reluctance (SyncRel) machines. First, torque and rotor position are controlled instead of torque and stator flux as in a conventional DSC scheme. Second, the operating sector is selected according to the actual position of the current vector rather than the position of the stator flux. The proposed methodology allows simplifying implementation of the torque control on SyncRel drives and reducing the global cost for medium-performance electric drives. Simulations and experimental tests on a 1.5-kW motor drive are provided to evaluate the consistency and the performance of the proposed control technique     

Fractional-Slot Concentrated-Windings Synchronous Permanent Magnet Machines: Opportunities and Challenges

Fractional-slot concentrated-winding (FSCW) synchronous permanent magnet (PM) machines have been gaining interest over the last few years. This is mainly due to the several advantages that this type of windings provides. These include high-power density, high efficiency, short end turns, high slot fill factor particularly when coupled with segmented stator structures, low cogging torque, flux-weakening capability, and fault tolerance. This paper is going to provide a thorough analysis of FSCW synchronous PM machines in terms of opportunities and challenges. This paper will cover the theory and design of FSCW synchronous PM machines, achieving high-power density, flux-weakening capability, comparison of single- versus double-layer windings, fault-tolerance rotor losses, parasitic effects, comparison of interior versus surface PM machines, and various types of machines. This paper will also provide a summary of the commercial applications that involve FSCW synchronous PM machines.     

Control characteristics of the SRM drives. I. Operation in thelinear region

The control characteristics of switched reluctance motor (SRM) drives are analyzed for operation of the motor in the linear region of its magnetic characteristics. After reviewing the motor operation, the authors consider the current-fed and voltage SRM drives. For both types of drives, the control variables and the related ranges are identified, the relationships between such variables and the average motor torque are calculated, and the torque capability is found. The basic schemes for the speed control of the SRM drives are also formulated     

Control and dynamics of constant-power-loss-based operation ofpermanent-magnet synchronous motor drive system

The operational envelope of electrical machines is limited by the maximum permissible power loss of the machine at any given speed. The control and dynamics of the permanent-magnet synchronous motor (PMSM) drive operating with a maximum power loss versus speed profile is proposed in this paper. The proposed operational strategy is modeled and analyzed. Its comparison to the conventional strategy of limiting current and power to rated values demonstrates the superiority of the proposed scheme. The implementation of the proposed strategy is developed. It is achieved with an outer power loss feedback control loop. This has the advantage of retrofitting the present PMSM drives with the least amount of software/hardware effort. The PMSM drives in this case then can use the existing controllers to implement any torque control criteria, such as constant torque angle, unity power factor, constant air-gap flux linkages, maximum torque per unit current, or maximum-efficiency operation. Experimental verification of the new operational strategy is provided. The concepts presented in this paper can be applied to all other types of motor drives     

Sensorlose INFORM-geregelte Permanentmagnet-Außenläufermotoren für Industrie, Automotive und Traktion

In this paper, the optimization of external rotor permanent magnet motors with respect to sensorless position detection using the INFORM method is presented. The external rotor type offers special advantages in drives with high torque demand, especially at low speed and standstill. Typical applications are elevators, starter-generators and traction drives. Furthermore, the paper shows the calculation of reluctance properties due to saturation and geometry using numerical field calculation. This is an important fact for high-performance position estimation using the INFORM method. The accuracy of position estimation reaches a few electrical degrees; therefore it is fully sufficient for practical application in the mentioned fields. At the Institute of Electrical Drives and Machines at Vienna University of Technology, such PM external rotor drives have been designed, built and tested. The sensorless operation has been verified successfully.     

Control of switched reluctance drives for electric vehicleapplications

Dynamic controllers of switched reluctance drives adjust at least three variables, i.e., current amplitude, turn-on, and turn-off angles. In electric vehicle (EV) applications high efficiency of the drive over a wide speed range, wide torque bandwidth, and low torque ripple under varying DC-bus voltage conditions are important design goals. Hence, controllers of switched reluctance drives for EVs usually have a complex structure. In this paper, the demands on control accuracy of switched reluctance machine traction drives and the traction controller sampling frequency, which are necessary to take advantage of the switched reluctance machine dynamic capabilities, are discussed. To integrate the traction drive, the control commands need to be actualized with a sampling frequency of at least 100 Hz to meet the high-dynamic requirements of modern vehicle control systems, e.g., active cruise control, antislip control, and active damping of mechanical drivetrain oscillations. It is found that the switching angles have to be adjusted within one-tenth of a mechanical degree. This study shows that switched reluctance drives can fulfill all requirements needed for electric propulsion using standard microcontrollers or digital signal processors     

Direct torque control of induction machines with constant switching frequency and reduced torque ripple

Direct torque control (DTC) of induction machines is known to have a simple control structure with comparable performance to that of the field-oriented control technique. Two major problems that are usually associated with DTC drives are: switching frequency that varies with operating conditions and high torque ripple. To solve these problems, and at the same time retain the simple control structure of DTC, a constant switching frequency torque controller is proposed to replace the conventional hysteresis-based controller. In this paper, the modeling, averaging, and linearization of the torque loop containing the proposed controller followed by simulation and experimental results are presented. The proposed controller is shown to be capable of reducing the torque ripple and maintaining a constant switching frequency.     

Elektronische Korrekturstromspeisung bei permanentmagnetisch erregten Antrieben

Many servo applications require electrical drives showing a good torque performance without disturbing torque pulsations especially in the low speed range. Usually the improvement of the torque performance is obtained by constructional design. This paper presents a method for optimizing the electrical power supply of brushless permanent magnet excited synchron motors. In automatically running calibration processes the current wave forms for suppressing the torque pulsations are calculated on a computer controlled motor test stand. Using an efficient processor, the current curve generator can be easily integrated into the software of the motor electronic.     

Interior Permanent Magnet Motors With Reduced Torque Pulsation

In this paper, three-phase interior permanent magnet brushless DC motors are analyzed. The effect of magnetization direction, number of stator slots, winding distribution, skew angle, current waveform, and advance angle on torque pulsation is examined. Finite element method is used to calculate the torque, reluctance torque, back iron flux density, tooth flux density, detent torque, and back electromotive force of the motors. Switching instants are calculated such that the reluctance torque can be utilized and maximum torque with reduced pulsation is achieved. Experimental results to support the simulation findings are included in this paper.     

Theoretical modelling and experimental identification of nonlinear torsional behaviour in harmonic drives

The demand for accurate and reliable positioning in industrial applications, especially in robotics and high-precusion machines, has led to the increased use of harmonic drives. The unique performance features of harmonic drives, such as high reduction ratio and high torque capacity in a compact geometry, justify their widespread application. However, nonlinear torsional compliance and friction are the most fundamental problems in these components, manifesting themselves as a combination of stiffening spring together with hysteresis at reversal points. Accurate modelling of the static and dynamic behaviour is expected to improve the performance of the system.This paper offers a model for torsional compliance of harmonic drives. A statistical measure of variation is defined, by which the reliability of the estimated parameters for different operating conditions, as well as the accuracy and integrity of the proposed model, are quantified. The model performance is assessed by simulation to verify the experimental results.Two test setups have been developed and built, which are employed to evaluate experimentally the behaviour of the system. Each setup comprises a different type of harmonic drive, namely the high load torque and the low load torque harmonic drive. The results show an accurate match between the simulation torque obtained from the identified model and the measured torque from the experiment, which indicates the reliability of the proposed model.     

Determination of converter control algorithms for brushlessdoubly-fed induction motor drives using Floquet and Lyapunov techniques

This paper presents a method of analyzing the stability of brushless doubly-fed machines (BDFM) in the synchronous mode of operation. Unlike the stability analysis of conventional induction and synchronous machines for which the linearized state equations are time-invariant, the linearized state equations of the BDFM are time-varying and, consequently, their eigenvalue analysis cannot be performed directly. However, since the system matrix of the linearized equations is a periodic function of time, the generalized theory of Floquet can be applied to transform the time-varying system of equations into an equivalent set of equations with a constant system matrix. Eigenvalue analysis can then be employed to analyze the stability characteristics of given equilibrium points. Investigation into the stability of a proof-of-concept BDFM in the synchronous mode of operation using the transformed linearized model shows good correlation between theoretical and experimental results. In addition, the possibility of designing a BDFM drive with inherent open-loop stability over the entire speed range is demonstrated. Finally, under stable steady state synchronous operation, power converter control methods are discussed and it is shown that scalar control methodologies similar to those for induction motor drives can be applied to the BDFM     

Bifurcation Analysis of Five-Phase Induction Motor Drives With Third Harmonic Injection

The interest in variable-speed multiphase induction- motor drives has substantially increased in recent years and novel proposals show good prospects for industrial implementation in high-power applications. The additional degrees of freedom existing in multiphase machines have allowed for new applications with high torque density by current harmonic injection in concentrated winding machines. This paper addresses, for the first time, the bifurcation analysis of a five-phase induction-motor drive when a third harmonic is injected for torque-enhancement purposes. The main focus of this paper is to present a mathematically based study of the nonlinear dynamics of the proposed drive with torque enhancement. The overall bifurcation analysis for both concentrated and distributed winding machines confirms that the harmonic injection provides not only torque enhancement but also more robust controllers. This further advantage offers improved performance of multiphase drives compared to their three-phase counterparts.