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.     

A Novel Design of a Brushless DC Motor Integrated with an Embedded Planetary Gear Train

This paper presents a novel design by integrating a planetary gear train (PGT) within a brushless dc (BLDC) motor to be a compact structure assembly. It provides functional and structural integrations to overcome the inherent disadvantages of the traditional designs. The effects of the gear teeth on the magnetostatic field of the BLDC motor associated with the kinematics of the PGT are investigated. By designing the number of gear teeth integrated on the stator and magnet poles on the rotor, the cogging torque of the proposed motor configuration is effectively reduced. The magnetic field distribution and the output torque of the novel design with different standard gear modules are numerically calculated by two-dimensional finite-element analysis. Through the kinematic analysis that utilizes the fundamental circuits, feasible solutions for the number of gear teeth of the PGT are determined to meet the desired speed ratio.     

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     

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.     

Electrical Machines and Drives for Electric, Hybrid, and Fuel Cell Vehicles

This paper reviews the relative merits of induction, switched reluctance, and permanent-magnet (PM) brushless machines and drives for application in electric, hybrid, and fuel cell vehicles, with particular emphasis on PM brushless machines. The basic operational characteristics and design requirements, viz. a high torque/power density, high efficiency over a wide operating range, and a high maximum speed capability, as well as the latest developments, are described. Permanent-magnet brushless dc and ac machines and drives are compared in terms of their constant torque and constant power capabilities, and various PM machine topologies and their performance are reviewed. Finally, methods for enhancing the PM excitation torque and reluctance torque components and, thereby, improving the torque and power capability, are described     

Analysis of Negative-Saliency Permanent-Magnet Machines

In this paper, a negative-saliency permanent-magnet (PM) synchronous machine analysis is presented. This particular saliency feature is achieved by replacing a portion of the magnet material by a soft iron piece over the rotor pole. In this manner, the d-axis inductance is increased, whereas the q-axis inductance is almost not affected, leading to the condition that Ld is higher than Lq (negative saliency) corresponding to the inverse condition of typical PM machines. An expression for the optimum pole configuration is derived. It is shown that, with appropriate control of the stator current based on the machine's saliency, the unfavorable effects of magnet reduction on torque production can be compensated. It is also shown that the machine saliency affects the location of the operating points when it operates under vector control. Finally, the theoretical analysis is validated with experimental results obtained from a prototype axial-flux PM machine that exhibits negative saliency.     

Novel Switched Reluctance Machine Configuration With Higher Number of Rotor Poles Than Stator Poles: Concept to Implementation

There is a great demand for efficient, quiet, reliable, and cost-effective motor drives for propulsion systems in hybrid and plug-in hybrid electric vehicles. Owing to a rigid structure and the absence of magnetic source on the rotor, a switched reluctance machine (SRM) is inherently robust and cost effective. In spite of these advantages, several challenges in the control of this machine remain an issue, including high levels of torque ripple, acoustic noise, and a relatively low torque density. This paper presents a new family of SRMs which have higher number of rotor poles than stator poles. Using a newly defined pole design formula, several novel combinations of the stator–rotor poles have been proposed. From the simulation and experimental analysis of a prototype 6/10 configuration, it has been observed that this machine produces higher torque per unit volume and comparable torque ripple when compared to a conventional 6/4 SRM with similar number of phases and constraints in volume. The results presented in this paper make this family of machines a strong contender for survivable high-performance applications for automotive propulsion systems. The simulation and experimental results for the prototype 6/10 configuration have been presented and compared to a conventional 6/4 design for verification.      

A simple model for flux weakening in surface PM synchronous machines using back-to-back thyristors

Flux weakening in surface permanent magnet (PM) synchronous machines is revisited in this letter. The condition for achieving infinite constant power speed ratio (CPSR) is explained from the machine equivalent circuit and phasor diagram point of view. Back-to-back thyristors, or triac, switches feeding the three phases of a surface PM synchronous machine will be shown to be equivalent to a simple series reactance with respect to fundamental component behavior. Using such switches is equivalent to adding a series inductance to the machine. This additional inductance helps extend the CPSR of surface PM synchronous machines. This is significant because extending the CPSR of surface PM machines is usually a challenging task due to the presence of low-permeability surface magnets and the resulting low machine inductance.     

Hybrid torque modeling of spherical actuators with cylindrical-shaped magnet poles

A ball-joint-like three-degree-of-freedom (3-DOF) spherical actuator which features a ball-shaped rotor with multiple permanent magnet (PM) poles and a spherical-shell-like stator with air-core coils is proposed to achieve omni-directional smooth motion in only one joint. Unlike previous study in which dihedral-shaped PMs are employed as the rotor poles, this paper utilizes cylindrical-shaped PMs to facilitate the fabrication and reduce the system cost significantly. Torque output of the spherical actuator is formulated with a hybrid method, i.e., using both analytical and experimental methodologies. Specifically, the analytical torque model of spherical actuator with dihedral-shaped PM poles is derived. Then a research prototype with cylindrical-shaped PM poles is developed, and a torque measurement testbed is built up to conduct experiment on the prototype. As the torque variation trend of actuators using two different types of PM poles with respect to the rotor orientation is similar, parameters in the analytical model are adjusted to fit with the experimental measurements. The resulting torque model can be employed for real-time motion control of the actuator. The cylindrical-shaped PM poles also reduce the inertial moment of the rotor by 60%, which is favorable for achieving better dynamic performance of the spherical actuator.     

Analytical and experimental investigation on the magnetic field and torque of a permanent magnet spherical actuator

This paper presents the torque model of a ball-joint-like three-degree-of-freedom (3-DOF) permanent magnet (PM) spherical actuator. This actuator features a ball-shaped rotor with multiple PM poles and a spherical stator with circumferential air-core coils. An analytical expression of the magnetic field of the rotor is obtained based on Laplace's equation. Based on this expression and properties of air-core stator coils, Lorentz force law is employed for the study of the relationship between the rotor torque and coil input currents. By using linear superposition, the expression of the actuator torque in terms of current input to the stator coils can be obtained in a matrix form. The linear expression of the actuator torque will facilitate real-time motion control of the actuator as a servo system. Experimental works are carried out to measure the actual magnetic field distribution of the PM rotor in three-dimensional (3-D) space as well as to measure the actual 3-D motor torque generated by the actuator coils. The measurement results were coincident with analytical study on the rotor magnetic field distribution and actuator torque expressions. The linearity and superposition of the actuator torque were also verified through the experiments.     

Identification and compensation of torque ripple in high-precisionpermanent magnet motor drives

Permanent magnet synchronous machines generate parasitic torque pulsations owing to distortion of the stator flux linkage distribution, variable magnetic reluctance at the stator slots, and secondary phenomena. The consequences are speed oscillations which, although small in magnitude, deteriorate the performance of the drive in demanding applications. The parasitic effects are analyzed and modeled using the complex state-variable approach. A fast current control system is employed to produce high-frequency electromagnetic torque components for compensation. A self-commissioning scheme is described which identifies the machine parameters, particularly the torque ripple functions which depend on the angular position of the rotor. Variations of permanent magnet flux density with temperature are compensated by on-line adaptation. The algorithms for adaptation and control are implemented in a standard microcontroller system without additional hardware. The effectiveness of the adaptive torque ripple compensation is demonstrated by experiments     

High-bandwidth current control for torque-ripple compensation in PMsynchronous machines

Active compensation of torque harmonics in high-performance synchronous permanent magnet (PM) motor drives requires high-bandwidth current control. It is demonstrated that proportional integral (PI) current control exhibits performance limits, even when feedforward compensation of the rotor induced voltage and the stator inductance drop is used. High bandwidth requirements are satisfied using a digital deadbeat current controller. Sampling time delays are eliminated to the extent possible by means of a current predictor. The current controller and the predictor refer to a model of the parasitic effects of the PM synchronous machine that is acquired and adapted to parameter changes in real time. Stator current distortions due to deviations from the sinusoidal flux linkage distribution are thus eliminated. The control system facilitates compensation of high-frequency torque ripple of the machine     

Design and Analysis of a Permanent Magnet Spherical Actuator

This paper has proposed a 3-DOF spherical actuator consisting of a ball-shaped rotor with a full circle of permanent- magnet (PM) poles and a spherical-shell-like stator with two layers of circumferential air-core coils. One key feature of this design is the parametrization of PM and coil poles. Based on the torque model of the PM spherical actuator, the relationship between poles' parameters and torque output can be demonstrated. As a result, the actuator design aiming at achieving maximum torque output can be carried out from the relationships. Another advantage of this spherical actuator is its singularity-free workspace, which is verified with the actuator torque model and condition numbers.     

Design equations for the tooth distribution of stepping motors

Two design equations for stepping motors are developed in terms of number of phases, number of poles per phase, number of stator teeth per pole, number of steps per revolution, step angle, tooth pitch of the rotor, and tooth pitch of the stator. Two tooth distribution configurations are considered. In one type of motor, the tooth pitch of the rotor is not equal to the tooth pitch of the stator. In the second type, the two pitch angles are equal, but there is a pole-to-pole offset in the stator tooth distribution to generate the necessary driving torque. A typical use of the design equations is illustrated using numerical examples. The design equations can be used in the evaluation of existing motors and in the design of new motors     

Sensorless control of induction Machines at zero and low frequency using zero sequence currents

This paper considers both flux and rotor position estimations for sensorless control of delta-connected cage induction machines (IMs) at low and zero frequency operation. The variation of leakage inductance due to either saturation or rotor slotting is tracked by measuring the derivative of the zero sequence current in response to the application of appropriate voltage test vectors. The method requires only a single extra sensor. It requires access to machine phase windings and is appropriate for integrated-type induction motor drives. Both a closed-slot and an open-slot machine is used to demonstrate rotor flux and rotor position tracking, respectively. Experimental results are presented showing sensorless torque control and sensorless speed and position control at low and zero frequencies.     

High-speed torque control of brushless permanent magnet motors

A method of converter control that improves the high-speed torque of brushless permanent-magnet (PM) motors is presented. The method consists of modulating the converter conduction intervals and their phase relative to the rotor position in order to deliver current to the stator windings at high speeds where the motor back EMF approaches the convertor rating. A microprocessor-based controller is used in the implementation. With this control, operation of the PM motor drive at its maximum ratings can be extended to higher speeds     

An Optimal Control Technique for Multiphase PM Machines Under Open-Circuit Faults

In this paper, an optimal control technique for n-phase permanent-magnet (PM) machines under various open circuit faults is presented. Under the fault conditions, the currents in the healthy phases are controlled to compensate phase loss and to produce the required output torque. The proposed control technique ensures continuous operation of the machines while producing minimum torque ripples and minimum stator ohmic loss. The control technique is based on the instantaneous power balance theory. To set the summation of the phase currents equal to zero, a constraint is incorporated in the derivation of the control technique. A five-phase PM machine is considered to demonstrate the proposed open circuit fault-tolerant control strategy. Simulation and experimental results are provided for validation.     

Steady-State Performance and Stability Analysis of Mixed Pole Machines With Electromechanical Torque and Rotor Electric Power to a Shaft-Mounted Electrical Load

This paper presents the steady-state model, performance, and stability analysis of a mixed pole machine with a new operational mode which provides a rotor torque and an n -phase rotor electrical output power to a shaft-mounted rotating electrical load. The machine operated under this mode can be used in applications that require contactless power, such as in robotics, or applications that require independent control of both rotor torque and rotor electric power, such as for contactless rotational antennas and turret systems. The performance assessment includes electromagnetic torque, electrical efficiency, mechanical efficiency, and total efficiency based on both simulation and experimentation. The effect of electrical loading and stator voltage on both rotor torque and rotor electric power is also considered. The machine steady-state stability is introduced by plotting the machine operating characteristics that determine all stable operating regions of the machine under the proposed mode of operation.     

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.     

Winding distribution effects on induction motor rotor fault diagnosis

The sidebands around stator currents harmonics as a potential tool for supporting the diagnosis of rotor faults in induction motors are analyzed in this paper. The presence of broken bars introduces high frequency components in the machine currents spectrum in addition to the characteristic sidebands around the fundamental component. These additional components are due to the interaction between, rotor asymmetry and either the voltage harmonics, or winding distribution, or rotor slots. In particular, the components at frequencies near to fifth and seventh harmonics, produced by the interaction between the rotor faults and the harmonics of the spatial distribution of stator windings, are analyzed in this work. A multiple coupled circuit model of the induction motor is used to evaluate the sensitivity of these components for different stator winding configurations, load level, supply voltage conditions, and different number of broken bars. Simulation results showed that a particular analyzed component near to fifth harmonic depends mainly on fifth harmonic of winding distribution, which remains almost constant for most common distributions. Therefore, it is expected that this component should be found in most motors with broken bars. Finally, experimental laboratory results and two industrial cases that validate the analysis are presented.