Effect of Slotting in Pm Electric Machines

ABSTRACT

The effect of sinttinq in permanent magnet electric machines differs from that in wound field ones. An analytically extended field model for slotted permanent magnet machines is presented in this paper. By using the model, the Carter's coefficient for permanent magnet machines can be easily deduced from the Carter's classical formula. The closed-form solution to the problem at hand is exact and much simpler than that of previously approximate methods.     

COMBINED FINITE ELEMENT AND ANALYTICAL METHODS FOR ROTOR DESIGN OF PERMANENT MAGNET SYNCHRONOUS MOTORS

ABSTRACT

While design procedures for conventional induction and synchronous machines are now mature and constrained by frame sizes and standards, there are opportunities to use modem CAD techniques to exploit the unique capabilities and characteristics of permanent magnet synchronous motors (PMSM). This paper presents a design methodology involving the use of an analytical model based on generalized machine theory, complemented by finite element analysis. The synergistic interaction of these two methods allows an engineer to design a PMSM to specific requirements with confidence. The effects of machine airgap and of the span angle and thickness of the permanent magnets are evaluated. A set of design specifications and a goodness function are established to select the preferred rotor geometry for a 30 hp prototype permanent magnet synchronous motor.     

The Fringing Effect in PM Electric Machines

ABSTRACT

To predict the fringing flux distribution along the axial length of a permanent magnet electric machine, a segmented permanent magnet model, in which magnets face a slotted and infinitely permeable boundary, is presented in this paper. By using the model and the method of Fourier expansion, the analytical solution to the fringing field problem can be easily determined. With the aid of the analytical results, the effective armature length for permanent magnet electric machines due to the axial extension of the magnet may be obtained.     

RUN-UP RESPONSE OF POLYPHASE PERMANENT MAGNET SYNCHRONOUS MOTORS

Abstract

An analysis to predict the transient run-up response of permanent magnet synchronous motors is drived based on the d and q axes model. It is based on the segmental reluctance approach with the addition of permanent magnets in the rotor. The computed simulation of the run-up characteristics are provided for variations in the values of stator resistance and permanent magnet excitation. A series of test results of a 3-phase, 2-pole, 1 hp permanent magnet motor are presented In order to verify the theoretical analysis. Effects of stator resistance on the air gap flux are also recorded experimentally. A reasonably close agreement between the measured and calculated values is obtained     

INVESTIGATION OF THE ROTOR OVERVOLTAGE IN SYNCHRONOUS MACHINES WITH RECTIFIER EXCITATION

Abstract

Electrical motors and generators used on mobile platforms--including aircraft and ship and land vehicles--are required to exhibit as high a power density as practically possible. With the advent of high-energy permanent magnets, it appears possible to build permanent magnet machines which can compete with electrically excited machines in terms of power density. This paper presents a discussion of the power density limitations as they apply to synchronous-type permanent magnet machines using rare earth cobalt permanent magnets. It also presents guidelines for future material development necessary to improve the power density of this class of machines.     

EFFECT OF PARAMETER VARIATIONS ON THE STABILITY LIMITS OF PERMANENT MAGNET SYNCHRONOUS MACHINES

ABSTRACT

This paper presents a comprehensive study of the effect of parameter variations on the stability behavior of permanent magnet (PM) machines. The state-variable equations are derived in terms of a set of non-dimensional parameters, so that their effects on the stability limits can be simply demonstrated. The stability is analyzed by means of state- space technique as it has the advantages of simplicity, accuracy and flexibility. The results of the analysis allow a choice of necessary and sufficient conditions for stable operation of the machine. The results also look interesting from a practical point of view, since a more realistic and efficient margin for stability limits can be defined. Simulation results are obtained using parameters of a 25-hp PM synchronous motor provided with a Neomax-35 magnets for excitation. Finally, useful conclusions have been provided, which are considered to be a good aid for designing these machines.     

Air Gap Field for Pm Electric Machines

ABSTRACT

The air gap field distribution in permanent magnet electric machines differs from that in wound field ones. In order to analyse the influence of magnet and air gap geometry on the shape of the air gap field in PM machines, an analytical method, which is much more economic and efficient than the numerical ones, is proposed in this paper. By using the method of Fourier expansion, a great set of curves for the air gap field characteristic coefficients in PM machines can be obtained. These design curves are necessary in simplified performance calculations for both AC and DC PM machines.     

TORQUE CALCULATION OF MACHINES WITH PERMANENT MAGNET MATERIALS

ABSTRACT

The prediction of torque in permanent magnet motors is essential for both their design and control. Most literature in the area uses the Lorenz equation for the force on a conductor in a magnetic field. This implies a model consisting of a slotless armature with the conductors distributed in the airgap. This is clearly not the case for the great majority of motors. In addition, this model is incapable of predicting such effects as cogging torque. A more fundamental approach is to use the energy transfer relationship, but this requires knowledge of the energy density of permanent magnet materials. There is surprisingly little agreement on the calculation of this energy.

This paper uses an expression for energy density derived using a simple theoretical model, to develop an expression for torque for a permanent magnet machine with one slot per pole per phase. The expression for torque is then compared with the theoretical results from the conventional approach. The analysis is confined to the linear region of operation. The model is then generalised for any number of slots and phases using the Discrete Fourier Transform (DFT), and a general expression is developed for useful torque. This DFT model will be most appropriate for machines with a low number of slots per pole since the discrete nature of the machine is most apparent for these cases. Cogging and useful torques of a laboratory servo motor is predicted using the model and compared with measured values.     

CURVATURE EFFECTS IN RADIAL-FIELD PERMANENT MAGNET MACHINES

ABSTRACT

The paper describes two analytical models for predicting the magnet/airgap flux density distribution in radial-field machines having permanent magnets mounted adjacent to the airgap. The models differ in that inter-pole leakage is neglected in the first and accounted for in the second. However, amongst other things, both allow the determination of the spread of magnet working points due to flux focusing. Furthermore, it is shown that for machines in which the magnets are mounted on an inner hub an optimal magnet thickness exists for which the magnetic loading in the airgap is a maximum.     

ANALYSIS OF THE INTERFERENCE SYNCHRONOUS MACHINE

ABSTRACT

Interference synchronous machines operate at different speeds of the fundamental air gap field and of the rotor, because the fundamental permeance wave is produced by interference of open stator and rotor slots the numbers of which being slightly different. In the literature different names are used for this remarkable electrical machine (“Synchronous inductor motor”, “Vernier reluctance motor”). Interference synchronous machines are used as homopolar frequency generators with an axial field coil, as continuous running reluctance motors and as single- or double, - stack step motors with an axial permanent magnet.

In this paper the field of the interference synchronous machine is analysed by field wave theory. Speed,phase angle and length of the largest field waves reveal how synchronous and cogging torques are produced. Voltage and torque equations are derived by axis theory from the simplest model that is possible with five rotor teeth. The theory is applied on a fifty rotor teeth double-stack step motor. The inductances are determined by a difference method field calculation. The torque of the motor is then calculated and compared with measured values.     

DETERMINATION OF THE OPTIMUM ROTOR/STATOR DIAMETER RATIO OF PERMANENT MAGNET MACHINES

ABSTRACT

Interest in permanent magnet machines has gained momentum with the introduction of NdFeB alloys. However magnet cost is still relatively high and therefore design parameters have to be chosen carefully so as to minimize the volume of the magnet used for any particular application. This paper concerns the derivation of a mathematical expression that determines the optimum rotor/stator diameter ratio, known as the split ratio, in relation to other machine parameters. The analysis takes into account the demagnetizing effect of the load current on the magnet material and consequently the feasibility of the optimized design is examined. This analysis is then applied to several existing designs and the results indicate that the appropriate choice of the split ratio would lead to a significant reduction in the magnet volume and hence in the machine cost.     

Modeling and Design Optimization Of Permanent Magnet Motors

ABSTRACT

Variable-speed permanent (PM) magnet motors are being used in an ever-increasing range of industrial and commercial applications. The objective of this paper is to provide a basis for optimizing the design of such permanent magnet motors and, through this, to provide insight into a comparison of PM motors with other types of drive motor. First, design models are presented for the approximate analysis of torque capability, losses, thermal characteristics, magnet protection and power factor control. These models are then used in an optimization program using sequential unconstrained minimization techniques to produce designs for a wide range of motor torque ratings. In order to examine the potential for large PM motor drives, the design criterion chosen for emphasis in optimization is the minimum total lifetime cost, including the cost of losses. The high efficiency and good power-to-weight ratio of the optimized PM motors leads to a total present value cost which is expected to be significantly lower than achieved with induction motors.     

A PERMANENT MAGNET AC MACHINE STRUCTURE WITH TRUE FIELD WEAKENING CAPABILITY

ABSTRACT

New doubly salient ac machines in which the field excitation is developed by stationary permanent magnets has recently been undergoing development at the University of Wisconsin. Because of the unique geometry of the stator, these permanent magnets are readily accessible. Therefore, for experimental purposes, simple iron blocks have been used to “short” a large portion of the permanent magnet flux and thus reduce the back emf developed within the machine. However, for practical applications, field weakening schemes involving miniature linear actuators or high torque stepper motors are being explored, as well as machine designs with combined PM and electrical excitation. Preliminary experimental results show that a substantial reduction in the back emf can be readily obtained thus lending credibility to the concept of weakening air gap fields produced by stationary permanent magnets.     

Computer-Aided Design Of A Permanent Magnet Motor

ABSTRACT

Although neodymium-iron-boron (Nd-Fe-B) magnets have high-energy product with suitable magnetic and physical properties for applications in electrical machines, the design of permanent magnet (PM) motors is a complicated problem due to their complex geometries and PM material characteristics. This paper initially compares various PM motor geometries and describes their PM material outlay, armature reaction and mechanical integrity. By the employment of appropriately located air slots to reduce the armature reaction in conjunction with flux enhancement arrangement without detracting from its mechanical integrity, a new rotor geometry for a high-field high-speed PM synchronous motor is proposed. Computer-aided electromagnetic calculation and graphical evaluation are employed for the design and optimization of the proposed PM motor. An idea of ‘computer job delegation’ is proposed and implemented within the computer system to provide full utilization of each computer's capabilities as well as parallel operation of the design and optimization process. The proposed rotor geometry and design philosophy have been implemented to design a 3.2 kW PM synchronous motor.     

Optimization of Flux Barriers of Line-start Synchronous Reluctance Motors for Transient- and Steady-state Operation

Abstract—In this work, an electromagnetic design procedure for a line-start synchronous reluctance motor is presented, and the main considerations are highlighted. To improve the steady-state performance of this motor, the geometry of the flux barriers as well as their number are optimized using an automatic optimization algorithm. For this purpose, two types of flux barriers, arc shaped and trapezoidal shaped, are formulated and studied by finite-element analysis. Analyzing the average cage torque during synchronization, effectiveness of the dq-axis rotor resistances on successful synchronization is discussed. To validate the results of this analysis, dynamic simulations are used, and the impact of the dq-axis rotor resistances on synchronization is investigated. In addition, the effect of the rotor bar position on the value of the dq-axis rotor resistances is discussed and analyzed by finite-element analysis. Finally, the impact of small pieces of permanent magnet in the flux barriers is studied. It is shown that against the permanent magnet-assisted motor, permanent magnets always enhance the motor performance; the steady-state characteristic of the line-start permanent magnet assisted motor could be deteriorated by permanent magnet.     

IMPACT OF SLOT HARMONICS ON LOSSES OF HIGH-SPEED PERMANENT MAGNET MACHINES WITH A MAGNET RETAINING RING

ABSTRACT

This paper presents an analytical method for determining the harmonic content in the flux pattern of permanent magnet synchronous machines due to the slotting of the stator. The analysis based on a rotor construction with magnets radially magnetized and a retaining ring to support them against the centrifugal forces. Expressions for the eddy current losses in the magnets and their retaining ring are derived in terms of the machine dimensions and the physical properties of magnets and ring. The accuracy of the method is examined by comparison with numerical flux calculations using finite element method. The impact of some physical quantities like machine speed and magnet conductivity on the slot harmonics losses is examined aiming to exhibit the role of the magnet retaining ring in contributing to these losses     

EDDY CURRENTS IN SOLID ROTOR PERMANENT MAGNET SYNCHRONOUS MOTORS FED BY VOLTAGE IINVERTER

ABSTRACT

The authors compare the performance of two permanent magnet synchronous motors fed by voltage inverter. The two machines differ in the means of holding the magnets. For one rotor, the magnets are held by a conductive shrink ring, for the other one by fibreglass tape. The influence of the conductivity of the shrink ring is analysed, not only in terms of the losses caused, but also in terms of operation modification     

STEADY STATE AND SENSITIVITY ANALYSIS OF HIGH-FIELD PERMANENT MAGNET MACHINES

ABSTRACT

In high-field electric machines employing powerful rare-earth magnets, the effect of saturation is quite pronounced and therefore, appropriate modifications to the classic two-reaction theory have to be considered to achieve more accurate and realistic prediction of the machine performance. This paper describes two approaches for modeling the steady-state operation of permanent magnet synchronous machines derived from a phasor diagram based on the two-reaction theory. The main feature of the proposed analysis is that both approaches consider the effect of core loss and do not require load angle values and hence, the need for shaft sensing equipment is avoided. The sensitivity of the models to the variation in machine parameters is investigated, and the validity of the analysis is confirmed experimentally.     

Characteristics of a permanent magnet type bearingless motor

Super high-speed and high-power electric machines are required for turbomolecular pumps and spindle drives. High rotational speed and high power drives can be achieved with bearingless motors. In this paper, a bearingless motor with the principles of permanent magnet type synchronous motors is proposed. High power factor and high efficiency can be expected in permanent magnet type bearingless motors. The proposed bearingless motor is a 4 pole permanent magnet synchronous motor, in which additional 2-pole windings are wound together with 4-pole motor windings in stator slots. With currents of 2-pole windings, radial magnetic forces are produced to support a rotor shaft. Principles of radial force production of surface-mounted permanent magnet bearingless motors are analyzed mathematically. It was found that radial forces are efficiently produced by employing thin permanent magnets on the surface of rotor iron core. A test machine was built in order to measure inductance functions as well as relationships between voltages and currents