Line-start Permanent-magnet Machines Research Articles

Novel Steel-Bar Starting Cage Line-Start Permanent Magnet Machine With Spoke-Type Insulation Layers

A line-start permanent magnet (LSPM) machine can be regarded as the integration of an induction motor and a permanent magnet synchronous motor (PMSM), so it has both self-starting capability as the induction motor and high efficiency as PMSM. As the cage winding on the rotor of LSPM only works at the starting process, it competes with permanent magnets for the space of a rotor. To solve this paradox, a steel-bar starting cage-based LSPM machine is proposed and analyzed in this article. Steel has larger resistivity than aluminum and copper, which can increase the starting torque and reduce the impact current of cage winding at the starting stage. Due to high permeability of steel, magnetic flux can also flow through steel-bar cage winding at synchronous speed. A steel-bar starting cage and a traditional aluminum starting cage winding are compared based on a four-pole LSPM machine using finite-element analysis (FEA) in this article. It is verified that although the self-starting capability of steel-bar cage winding is reduced due to the leakage flux, the performance at synchronous speed such as torque density and torque ripple is improved.

Topology Optimization of Rotor Bars Geometry and Arrangement for a Line-Start Permanent Magnet Synchronous Machine

An optimally designed line-start permanent magnet synchronous machine may require a different squirrel-cage design than traditional induction machines. The objective of this paper is to apply topology optimization based on the normalized Gaussian network in order to find the optimal shape of the rotor bars for a line-start permanent magnet synchronous machine. This approach facilitates optimizing the shape of the bar, without the limitation of a predefined bar geometry and arrangement, as is usually considered for classic induction machine design. For this study, a previously designed four-pole line-start permanent magnet synchronous machine with a rated power of 1.5kW is used as an initial design. To verify the accuracy of performance evaluation by utilizing finite-element methods, a prototype of this machine was built and tested. Then, verified model of line-start permanent magnet machine is used for topology optimization of its rotor cage area. The outcome of the optimization is the Pareto front, from which three optimized designs are selected. Finally, these designs are analyzed and proved to have better steady-state performance than the initial machine. The results provide new insights for the design of squirrel-cage bars for line-start permanent magnet synchronous machines.

Optimal Design of Line-Start Permanent Magnet Synchronous Motor Based on Magnetic Equivalent Parameters

This paper used a lumped magnetic parametric approach-based analytical technique to design the 7.5 kW, three-phase line start permanent magnet (LSPM) motor. In order to enhance the efficiency and power factor (PF) of the prototype LSPM machine, an optimized slot shape of rotor permanent magnet (PM) was selected. The lumped magnetic circuit model is developed to present the magnetic characteristics, and analytical expressions are derived under the open circuit condition. The impact of the design variables on the analytical model air gap flux density distribution and magnetic flux leakage is studied for the LSPM. The design variables have a significant influence on the steady-state performance characteristics of the LSPM motor. Therefore, to verify the output results of the purposed model, the two-dimensional finite element analysis (FEA) was evaluated for the numerical solution of the analytical model.

Influence of rotor structures on line-start permanent magnet machines electromagnetic characteristics for pumping units application

Influence of rotor structures on line-start permanent magnet machines electromagnetic characteristics for pumping units application

Improvement of Starting Performance for Line-Start Permanent Magnet Motors by Winding Reconfiguration

Line-start permanent magnet (LSPM) machines have significant advantages in efficiency and power factor compared with regular induction motors; however, the poor starting torque and synchronization capability prevent the widespread of LSPM machines. In this article, winding reconfiguration methods are proposed to improve the starting performance and steady-state performance of LSPM machines. The turns-changing method is to increase asynchronous torque by reducing coil turns in starting, and the pole-changing method is to reduce and even eliminate the permanent magnet (PM) braking torque by changing the pole pair number of stator windings in the early starting period. Both of them can be adopted for an LSPM machine simultaneously to improve average torque in the starting stage. The starting performance is also compared between the turns-pole-changing LSPM machine and pole-increasing LSPM machine. Finite-element analysis is carried out to show the performance differences of LSPM machines using different starting strategies. A turns-changing LSPM machine is also manufactured and tested to validate the starting improvement.

Line-Start Vernier Permanent Magnet Machines

Vernier permanent magnet (VPM) machine has become a good candidate to drive the pump and fan loads, since these loads always operate at low speed and require high output torque. In these applications, the machine operates near a fixed speed and requires no speed adjustment capability. Hence, the inverter can be replaced by a line-start unit, and the machine can be connected directly to the grid, which reduces the weight, volume, and cost of the whole system. However, the novel magnetic gearing effect in the VPM machine causes two issues. First, the abundant magnetic field harmonics interact with the line-start unit and produce strong braking torque even at steady-state operation; second, large harmonic leakage inductance reduces the starting capability. Therefore, the traditional method to build a line-start machine is not suitable for the VPM machine. In this article, a line-start VPM (LSVPM) machine topology is proposed. By analyzing two issues above, the structure and pole-pair of both stator armature and rotor wound windings are carefully selected, and finally pole-changing strategy is chosen to start the machine. The performances of the proposed machine are verified by finite element analysis (FEA), which shows it has 49% higher torque density than regular line-start permanent magnet machine, and has the capability to start with the rated pump load. A prototype also has been built and tested to demonstrate the analysis and simulation.

Analytical Modeling for Optimal Rotor Shape to Design Highly Efficient Line-Start Permanent Magnet Synchronous Motor

In this study, an analytical technique for the rotor geometry optimization based on lumped magnetic parametric approach is used to design a two-pole, three-phase, 7.5-kW line-start permanent magnet (LSPM) synchronous motor. The permanent magnet shape substantially affects the air-gap flux density distribution, back electromotive force (EMF) as well as the copper loss, which have a great impact on the performance characteristics of the permanent magnet synchronous motors. Principal advantages involve in adjusting the rotor shape are to achieve the effective air-gap flux density and optimize the fundamental component of the back EMF with low harmonic content for minimum ripple torque. Therefore, to enhance the efficiency (η) and power factor, an optimized slot shape considering various design parameters is selected for the permanent magnet of the rotor in the prototype LSPM machine. A linear saturated lumped magnetic parametric model is developed to exhibit magnetic characteristics, and analytical equations are acquired under the open-circuit condition without considering the slotting effect for design simplicity. The influence of design variables on the air-gap flux density distribution and the flux leakage is investigated precisely using an analytical circuit model. A parametric study of the prototype model demonstrates that the steady-state performance of the LSPM motor are significantly influenced by the design variables. The inductance saliency ratio and electromagnetic torque components are carefully analyzed in terms of their effects on the load characteristics of the LSPM motor in order to determine the optimal shape of the PM slots and the magnetic flux barriers. The validity of the proposed method has been checked by evaluating the numerical solution of the analytical model using a two-dimensional finite element method.

New Trends in Energy Efficient Electrical Machines

Today high efficiency electrical machines are a new and mandatory trend in the motor production in Europe and United States of America. Lately there are developed also higher efficiency classes as IE4 (Super Premium Efficiency) and IE5 (Ultra-Premium Efficiency). It is well known that IE4 electrical motors are available on the market and IE5 machines are taken into account by several big producers. Efficiency upgrade of the motors has very good consequences on the environmental problems. The efficiency classes (IE1-IE4) are described in the international standard IEC 60034-30. Line-start permanent magnet machines and Squirrel Cage induction motors can be upgraded, in order to have a bigger efficiency, according to IE4 specifications. In the case of variable speed machines variable reluctance synchronous motors are also included in the efficiency class IE4. In order to improve a motor to comply with the IE5 efficiency NdFeB or Ferrite magnets should be used.

Evaluation of the Efficiency of Line-Start Permanent-Magnet Machines as a Function of the Operating Temperature

The standard squirrel-cage induction machine has nearly reached its maximum efficiency. In order to further increase the energy efficiency of electrical machines, the use of permanent magnets in combination with the robust design and the line start capability of the induction machine is extensively investigated. Many experimental designs have been suggested in literature, but recently, these line-start permanent-magnet machines (LSPMMs) have become off-the-shelf products available in a power range up to 7.5 kW. The permanent magnet flux density is a function of the operating temperature. Consequently, the temperature will affect almost every electrical quantity of the machine, including current, torque, and efficiency. In this paper, the efficiency of an off-the-shelf 4-kW three-phase LSPMM is evaluated as a function of the temperature by both finite-element modeling and by practical measurements. In order to obtain stator, rotor, and permanent magnet temperatures, lumped thermal modeling is used.

Influence of Supply Voltage Distortion on the Energy Efficiency of Line-Start Permanent-Magnet Motors

Similar to standard three phase direct on line induction motors are three phase line-start permanent-magnet machines (LSPMMs) susceptible to grid voltage distortion. Although straight forward adaptation of harmonic loss mechanisms for standard squirrel-cage induction motors to LSPMMs seems appropriate, this study will indicate that harmonic loss calculations of induction machines are unsuitable to give accurate or even reasonable estimations of the influence of supply voltage distortion on the energy efficiency of LSPMMs. This research will specifically address the interaction of rotor-induced harmonic currents. Finite-element modeling is used in order to address the effect of supply voltage distortion on the segregated losses of an LSPMM. Finite-element modeling (FEM) allocates the majority of the harmonic loss to the rotor and shows shifting the relative phase angle between identical magnitudes of superimposed supply distortion of fifth and seventh order can result in a 30% reduction of rotor loss and 0.5 percentage points in overall energy efficiency. The results obtained by FEM are validated by practical measurements.

Development of a Novel Magnetic Circuit Model for Design of Premium Efficiency Three-Phase Line Start Permanent Magnet Machines With Improved Starting Performance

This paper presents challenges in an ongoing project related to the design of a premium efficiency three-phase line start permanent magnet synchronous motor (LSPMSM) and proposes an exclusive solution through development of a novel magnetic circuit model. Since the LSPMSM is rapidly replacing the conventional induction motors in industrial applications such as pumps, fans and conveyors which require the machine to operate under both frequent start-stop and steady-state conditions for a wide range of time, a trade-off has to be reached between their starting performance and efficiency post-synchronization. Until now this has only been possible with time consuming FEA based design methodology. Hence, understanding the need and scanty availability of literature on simplistic and yet precise magnetic circuit based design of three-phase LSPMSM, this manuscript proposes exclusively a novel magnetic circuit model to design LSPMSM with improved starting performance. Firstly, the aforementioned design issues have been illustrated using different machine configurations developed. Further, a detailed procedure to deal with the design issues with the help of the developed magnetic circuit model is discussed and validated by developing a machine where the trade-off has been reached.

Line-Start Permanent-Magnet Machines Using a Canned Rotor

Various line-start permanent-magnet (PM) machines have been around for many years: these offer the advantages of line-starting as an induction machine and running synchronously as a PM machine, both from a fixed-frequency supply. The major disadvantage is the uncertainty of successful synchronization, particularly with loads combining high inertia with high load torque. Virtually all conventional line-start PM machines use a cage-type induction winding embedded into the rotor along with the magnets. This paper describes an unusual variant of this type: comprising a very simple rotor construction of surface-mounted rotor magnets contained within an annular conducting rotor can which acts as the induction winding but can also provide environmental containment if the application requires it. The purpose of this paper is to describe the canned motor concept in detail so that its potential for other motor/drive applications can be assessed. This paper also develops and describes an analytical dynamic electromechanical model using ldquolayer theory.rdquo Experimental results demonstrating successful operation of a 2.5-kW prototype machine are presented together with the simulated results using the dynamic model. The influence of certain key design features on the synchronization process is also included.

Some Tricks for Modeling Rotating Electrical Machines Using Finite Elements

Despite worldwide efforts and many recent advances, electrical machine modeling using finite elements can be computationally very expensive, especially if three-dimensional models are required. This contribution is concerned with speeding up the process. Some examples of line start permanent magnet machines and induction motors are provided.