Design Guidelines for Fractional Slot Multi-Phase Modular Permanent Magnet Machines

This paper presents the design considerations for a fractional slot multi-phase modular permanent magnet (PM) machine with single-layer concentrated windings. The winding factors for various slot/pole number combinations are calculated to identify the optimal slot/pole number combinations for different phase numbers. In addition, the electromagnetic performance influenced by flux gaps (FGs), such as air-gap MMF, back-EMF, cogging torque, on-load torque and torque ripple, etc., are comprehensively investigated by using the 2-D finite element (FE) method. Several general rules with respect to the influence of FGs on multi-phase modular PM machines performance are established. The prototypes of modular PM machines are built and the finite element results are validated with experiments

Demagnetization withstand capability enhancement of surface mounted PM machines using stator modularity

The flux gaps in alternate stator teeth of the modular PM machines can have a significant impact on the total magnet flux density, and hence, the potential magnet reversible/irreversible demagnetization under flux weakening operations or short-circuit conditions. Such a problem has not been studied in literature and will be investigated in this paper. The influence of flux gaps on the d-axis inductance and the potential peak short-circuit current is analysed for different slot/pole number combinations. It is found that the flux gaps will affect both the d-axis inductance and open-circuit flux linkage, and hence reduce short-circuit current of machines with pole number (2p) smaller than slot number (Ns) while they will increase the short-circuit current of machines with 2p > Ns. However, the opposite phenomena can be observed for demagnetization withstand capability. For machines having 2p Ns, this capability can be improved. Other parameters, such as magnet thickness, temperature, etc., have also been accounted for in the demagnetization analysis. Tests have been carried out to validate the predictions of inductances and short-circuit current as well as performance such as phase back-EMF, cogging torque and static torque for machines with one defective magnet, which represents the case of partially demagnetized magnets

Comparison of flux-weakening control strategies of novel hybrid-excited doubly salient synchronous machines

For hybrid-excited doubly salient synchronous machine, both the field excitation current and the d-axis current can be utilized to adjust the flux-linkage, which provides more flexible control parameters for flux-weakening operation. In this paper, three flux-weakening control methods, i.e. utilizing field excitation current alone (Method-I), utilizing armature current alone (Method-II), and optimal method (Method-III), are proposed and compared. All three methods can achieve similar torque performance in the constant-torque region. In the flux-weakening region, Method-I exhibits low torque and limited operating speed range. The operating speed range can be further extended by Method-II and Method-III. In addition, Method-III can provide a higher efficiency in flux-weakening region than Method-II since the copper loss of field winding can be decreased in proportion to the reduction of field excitation current. Those flux-weakening control methods are verified by experimental results

Uncontrolled generator fault protection of novel hybrid-excited doubly salient synchronous machines with field excitation current control

In the safe-critical applications such as electric/hybrid electric vehicles, reliability of the machine drive system is vital. Among the various machine fault types, the uncontrolled generator fault (UCGF) at high speed is one of the most serious faults that could damage the machine drive system. In this paper, the fault protection capability of a novel hybrid-excited doubly salient synchronous machine is examined to illustrate its great capability to prevent the dangerous overvoltage issue from the UCGF. PM flux of the novel hybrid-excited doubly salient synchronous machine is inherently short-circuited when the field excitation current is not fed into the excitation windings. Therefore, when the UCGF is detected in the high speed region, the DC-link overvoltage issue can be effectively reduced by forcing the field excitation current to zero. Since only the field excitation current is utilized to protect the machine and power inverter from the UCGF, it is easy to implement and the concept of proposed control strategy can also be applied to other hybrid-excited machines. Finally, the proposed control strategy is verified by both simulation and experimental results on the hybrid-excited doubly salient synchronous machine

Molecular motion and mobility in an organic single crystal: Raman study and model

We report Raman spectra of the organic semiconductor 5,6,11,12- tetraphenyltetracene (rubrene) in the temperature range 30-300 K. The linewidths of certain low-frequency peaks increase significantly, especially in the range 150-200 K. These peaks correspond to the vibrations of the phenyl side groups of the rubrene molecules, and their couplings to intermolecular vibrational modes. We propose a model in which the strong increase in mobility observed with increasing temperature between 30 and 150 K results from disorder as the phenyl groups exchange sides of the backbone plane and break the symmetry. This model explains previous experimental observations of structural and calorimetric changes near 150 K

New ventilation cooling for modular PM machines utilizing flux gaps and rotor ducts

This paper investigates a new ventilation cooling technology for modular permanent magnet (PM) machines. Air is pumped into the stator flux gaps and rotor ducts. A significant temperature reduction for the modular machines can be observed by introducing ventilated air. The flux gaps within the modular machines provide extra coolant path, notably modify the flow profile. Moreover, the flux gaps also provide additional contact area between machines and coolant, improving heat transfer rate. Comparative study between different machine configurations, including different inlet and outlet areas, different topologies of shaft, and different stator topologies have been investigated. Their influences on machine cooling have been investigated using computational fluid dynamics (CFD) modelling. Furthermore, since the coolant (air) will pass through the stator-rotor annular gap, i.e., airgap, as well as rotor ducts, the rotor speed will influence the cooling efficiency of the ventilation cooling system. This influence comes from shock loss, friction loss, and combining flow phenomenon. A series of experiments have been conducted to validate the CFD simulations

Investigation of ferrofluid cooling in modular permanent magnet machines

Compared with conventional non-modular machines, the flux gaps in alternate stator teeth of the modular machines can increase the winding factor and provide flux focusing effect, and hence can increase the torque/power density. In addition, the flux gaps can also be used as cooling channels to improve the thermal performance. This paper investigates an advanced cooling approach using ferrofluid as coolant to fill in the cavity around end-windings and flux gaps. The multiphysics modelling in this paper has shown that the influence of the flux gaps on machine thermal performances comes from two aspects: (1) the gravitational effect, the flux gaps allow more self-circulating coolant to transfer heat to the housing. This helps to reduce machine temperature by around 5°C for a flux gap width of 2mm; (2) the magnetic body force, i.e., the thermomagnetic effect. This is very effective in non-modular machine cooling. But its efficiency slightly deteriorates in the modular machines. This is because the self-circulation of ferrofluid in the flux gaps due to the gravitational effect has been negatively affected by the thermomagnetic effect

Influence of Carbon Concentration on the Superconductivity in MgCxNi3

The influence of carbon concentration on the superconductivity (SC) in MgC$_{x}$Ni$_3$ has been investigated by measuring the low temperature specific heat combined with first principles electronic structure calculation. It is found that the specific heat coefficient $\gamma_n=C_{en}/T$ of the superconducting sample ($x\approx1$) in normal state is twice that of the non-superconducting one ($x\approx 0.85$). The comparison of measured $\gamma_n$ and the calculated electronic density of states (DOS) shows that the effective mass renormalization changes remarkably as the carbon concentration changes. The large mass renormalization for the superconducting sample and the low $T_{c}$(7K) indicate that more than one kind of boson mediated electron-electron interactions exist in MgC$_{x}$Ni$_3$.Comment: 4 pages, 4 figure

Study of J/psi decays to Lambda Lambdabar and Sigma0 Sigma0bar

The branching ratios and Angular distributions for J/psi decays to Lambda Lambdabar and Sigma0 Sigma0bar are measured using BESII 58 million J/psi.Comment: 11 pages, 5 figure

Search for the Rare Decays J/Psi --> Ds- e+ nu_e, J/Psi --> D- e+ nu_e, and J/Psi --> D0bar e+ e-

We report on a search for the decays J/Psi --> Ds- e+ nu_e + c.c., J/Psi --> D- e+ nu_e + c.c., and J/Psi --> D0bar e+ e- + c.c. in a sample of 5.8 * 10^7 J/Psi events collected with the BESII detector at the BEPC. No excess of signal above background is observed, and 90% confidence level upper limits on the branching fractions are set: B(J/Psi --> Ds- e+ nu_e + c.c.)<4.8*10^-5, B(J/Psi --> D- e+ nu_e + c.c.) D0bar e+ e- + c.c.)<1.1*10^-5Comment: 10 pages, 4 figure