Rotor Stack Research Articles

A novel cogging torque measurement method for multi slot/pole permanent magnet motor

AbstractThe adverse effect on speed ripple and vibration makes it necessary to evaluate the cogging torque. Due to the tolerance of stator and rotor stacks as well as nonuniform magnetization of permanent magnet, the amplitude of cogging torque is unequal in different rotor position. As a result, the verification of theoretical analysis becomes difficult. Therefore, the cogging torque average is used to weigh it. In view of the large least common multiple of multi slot/pole Permanent magnet motor, which means, there are multiple cogging torque cycles per rotor revolution, a cogging torque measurement method is proposed. Through simple devices such as arm, weight, digital gauge, resolver, and so on, the amplitude and period of cogging torque can be accurately measured without site restrictions. The correctness of the method is verified by simulation and experiment, and that can be used as a supplement to cogging torque evaluation for multi slot/pole PMM.

Permanent Magnet Synchronous Reluctance Machines With Axially Combined Rotor Structure

A permanent magnet synchronous reluctance machine (PMSynRM) is studied in this article. It has a stack of surface-mounted permanent magnet (SPM) rotor and a stack of synchronous reluctance (SynR) rotor, which are axially combined on the shaft. Unlike the traditional PM-assisted SynR machine (PMASynRM), the magnets are not located in the flux barriers of the SynR rotor, and therefore, the magnets and flux barriers have no more geometric confliction between them. The two rotor stacks can be circumferentially shifted to any relative angle. Clearly, the shifting angle and the axial length ratio between the two rotor stacks, together with the phase angle of the armature current vector, influence the motor performance, and their optimal design is detailed in this article. Analysis of the prototypes shows that the studied PMSynRM evidently exhibits higher torque density than the conventional PMASynRM.

Lateral and Torsional Vibration Monitoring of Multistack Rotor Induction Motors

In this article, both the lateral and torsional vibrations in multistack rotor induction motors (IM) are modeled in the same framework, and the corresponding precursors in the current waveform are analyzed in detail. Due to the very large rotor length-to-diameter ratio, rotor vibrations can become a major issue in these types of high-torque motors. In order to monitor the vibration components, the lumped mass-spring model for the rotors and couplings is developed to calculate the natural frequencies and torsional-mode shapes of the mechanical system. To model the electrical part, stator windings are split up into several identical windings that are connected in series and correspond to each rotor stack. Since the torsional vibration does not remarkably affect the winding symmetry, the dq model for each section is sufficient to describe the motor behavior during torsional oscillation. Similarly, for the lateral vibration, the mechanical system is modeled in all lateral directions and all modes. Due to the loss of symmetry, the abc model is used for each rotor and stator. In this modeling approach, all inductances (self and mutual of stator and rotor and the mutual inductances between the stator and rotor) are needed. To mimic the effect of lateral vibrations on the motor inductances, a new approach is developed using winding function theory. In this model, all inductances are formulated as a function of eccentricity severity and angle, and a new analytical IM model is developed to show the effects of lateral vibration. Finally, the experimental results are presented and compared with the analytical results.

High-Strength Rotor Design for Ultra-High Speed Switched Reluctance Machines

High speed machines are gaining popularity in more and more applications. Besides permanent magnet (PM) machines, switched reluctance machines (SRMs) are very competitive candidates for high-speed applications in high-temperature environments due to their single material rotor without PMs. However, the conventional rotor geometries of SRMs are not suitable for the ultra-high speed applications. In this article, a novel high-strength, high-torque-density, high-efficiency rotor design for the ultra-high speed SRMs is proposed. First, a three-dimensional finite-element analysis is conducted to calculate the von-Mises stress distribution of the proposed rotor and shaft. Then, in order to improve the bonding and robustness of the design, proper high-strength adhesives are used and modeled based on the cohesive zone model. A parametric study is conducted to find the optimal length of the rotor stack. A thermal stress case study is also conducted. Finally, several prototypes are built to validate the manufacturing feasibility of the proposed design.

Traction Synchronous Homopolar Motor: Simplified Computation Technique and Experimental Validation

Synchronous homopolar motors (SHMs) have been attracting the attention of researchers for many decades. Various mathematical models of SHM have been proposed to deal with its complicated magnetic circuit. Among them, there are time-consuming 3D finite element models (FEM), equivalent circuit models neglecting some significant features of the machine design, and 2D FEM models with virtual excitation winding distorting its magnetic field picture. This paper proposes a novel 2D FEM of SHM and shows that since there are no sources of excitation in the cross-sections of the rotor and stator stacks, no virtual elements are required. This model uses the general solution of the Gauss’s law for magnetism containing excitation flux. The model is based on a set of magnetostatic boundary value problems for various rotor positions. The set of boundary problems is completed with the excitation equivalent circuit. The losses in the armature and field windings and the stator and rotor magnetic cores are computed in postprocessing. All these computations are carried out for a single combination of stator and rotor stack. A symmetrization algorithm is proposed to extend the obtained results to the whole SHM. A comparison of the theoretical and experimental data for a nine-phase three-section 320 kW SHM is carried out. These SHMs were used in a mining truck with a carrying capacity of 90 tons.

Experimental and Theoretical Research on Cogging Torque of PM Synchronous Motors Considering Manufacturing Tolerances

This paper investigates the cogging torque caused by manufacturing tolerances through analytical, finite element analysis (FEA), and experimental methods. Based on experiments, it is found that the stator and rotor tolerances of PM motor generate low-frequency components of cogging torque, such as the multiples of rotor pole and stator slot numbers. First, analytical calculation is used to find out the frequency spectrum components of the cogging torque. Then, FEA is used to confirm the analytical results and to determine which tolerances play dominant role on causing these components. Furthermore, the sensitivities of the key tolerances are illustrated. These can provide the designer a direction to design a motor with low sensitivity to tolerances. Because the tolerances of stator and rotor stacks are from stamping die, these tolerances are almost constant. A simple manufacturing method, i.e., rotation lamination, is proposed to manufacture the improved stator and rotor stacks. After using the proposed method, the effects of stator and rotor tolerances are individually validated through experiments.

FE-BE computation of electromagnetic noise of a permanent-magnetic excited synchronous ma-chine considering dynamic rotor eccentricity

Electromagnetic noise in Electrical Machines (EMs) occurs due to vibrations caused by magnetic forces acting onto rotor and stator surface. This is the dominant source for the considered permanent-magnetic excited synchronous machine in this paper. The radiated electromagnetic noise is sequentially calculated by a Finite Element (FE) and Boundary Element (BE) computation. An electromagnetic FE model is created to determine magnetic forces. Structure-borne sound and rotor dynamics are calculated using a structural dynamic FE model for the EM housing and the rotor. In order to predict resonance frequencies and amplitudes as reliable as possible, it is important to know the direction-dependent stiffness of the laminated rotor stacks and mechanical joints as well as their structural damping. Thereby, the properties of the laminated stack can be determined experimentally by a shear and dilatation test. Mechanical joint properties can be modelled by Thin-Layer Elements (TLEs) and the overall damping by the model of constant hysteretic damping. The radiated sound power is determined by a direct BE computation. The influence of dynamic rotor eccentricity on radiated sound power is examined for a run-up of the EM. All FE models are verified by data from experimental modal analysis.

Effect of Lamination Stack Ends and Radial Cooling Channels on No-Load Voltage and Inductances of Permanent-Magnet Synchronous Machines

The effect of lamination stack ends and radial cooling channels on the effective stator stack length, no-load voltage, and synchronous inductances of a permanent-magnet synchronous machine are studied using three-dimensional finite element analysis. The stack ends and the radial cooling channels decrease the equivalent electrical length of the machine, thus leading to the fact that a two-dimensional (2-D) approach overestimates the no-load voltage. The effect of stack ends and radial cooling channels on the electrical length of the machine can be compensated by making the rotor stack and permanent magnets longer than the corresponding stator stack. It is shown that in case of no radial cooling channels, synchronous inductances are typically overestimated with the 2-D approach. However, in the case of stator structure having several radial cooling channels, inductances are underestimated using the 2-D approach.

Cage induction motors with inter-rings

A general form of analysis is described that is capable of modelling cage induction machines with inter-rings (conducting rings joining all the bars of the cage, positioned along the length of the rotor stack). The model is verified using a commercially-manufactured motor. It is shown that, as far as torque is concerned, inter-rings have an effect which is opposite to that introduced by skewing the rotor.