AN ANALYSIS OF SHADED POLE INDUCTION MOTORS WITH INCREASED STARTING TORQUE

ABSTRACT

A method of analysis of shaded pole induction motors with ununiform airgap is described. For the analysis, a dq model of a generalised machine theory with any number of stator windings arranged in an arbitrary way on the stator and ununiform airgap is chosen. The analysis involves resolution into a harmonic series, together with complex matrix inversion. Results of computational study of the effect of changes of airgap profile on the performance characteristics are presented. It is shown that by increasing the airgap under the part of a pole which la opposite to the shading coils, an increased starting torque can be obtained.     

A SINGLE PHASE LINEAR TRAVELLING WAVE INDUCTION MOTOR

ABSTRACT

A simplified analysis of a single phase linear travelling wave induction machine is described. The analysis shows that the design goodness factor is independent of variation in the width of the airgap, and predicts that the performance of the machine will not be degraded by an increase in airgap. A machine designed and constructed on the basis of the analysis, was used to investigate the possibility of varying the airgap to control the speed. The results of these investigations are fully discussed.     

INTERIOR PERMANENT MAGNET MOTOR HAVING SEVERAL IMPROVED FEATURES

ABSTRACT

The no-load airgap flux density distribution of most conventional permanent magnet motors is square in shape. The rotor configurations having interior magnets employed in these machines also lead to vastly different saliencies along the d- and q-axes giving a quadrature axis inductance larger than the direct axis inductance. A modified rotor configuration is presented that produces a near sinusoidal airgap flux density distribution which is achieved by shaping the rotor airgap profile while keeping the interior magnets rectangular in shape. As a result of this shaping the quadrature axis inductance is reduced significantly while there is almost no change in the direct axis inductance. It is also shown that the new rotor configuration leads to significantly reduced cogging torque and less saturation along the q-axis which assists in maintaining a linear torque versus current relationship in field oriented control employing the quadrature axis current. Airgap flux density levels, inductances and static torque characteristics obtained using the finite element simulation work are presented comparing them with those for a uniform airgap machine. There has been good agreement between the experimental and simulation results. The operational advantages gained by the improvements made are also briefly discussed.     

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.     

A Theoretical Study of Reduction Torque Pulsations in Low-Speed Self-Synchronous Motor Drives Fed through a Cycloconverter Link

ABSTRACT

A PWM strategy is developed to eliminate harmonics of the quasirectangular phase currents of two-phase feeding, self-synchronous motor that interact with the fundamental component of airgap flux to produce pulsating torques in the frequency range of mechanical response when the motor is driven through a midpoint, three-pulse cycloconverter under synchronous envelope control. After establishing the necessary PWM regime, an example is presented wherein harmonic content of phase currents and developed torque are calculated with and without modulation of phase voltages to verify the effectiveness of torque pulsation reduction and to assess the ohmic loss penalty incurred.     

低速大转矩多极永磁电机齿槽转矩削弱的有效方式

基于能量法和傅里叶分解的解析方法,推导了低速大转矩多极永磁电机齿槽转矩的解析表达式,基于所导出的表达式,研究了比较有效的抑制齿槽转矩的方法,应用有限元仿真分析方法知道,抑制该类电机齿槽转矩比较行之有效的的方法依次是,选择最优极弧系数、定子斜槽、极数和槽数的合理组合、采用分数槽绕组、设计不均匀气隙和磁性槽楔等。     

OPTIMAL WINDING DESIGN OF A PERMANENT MAGNET MOTOR FOR SELF-CONTROLLED INVERTER OPERATION

ABSTRACT

This paper describes a novel design of the three-phase synchronous motor which is tailored to match the operational characteristics of the self-controlled inverter. The design is based on a square waveform distribution of the rotor radial B-fields and square waveform distributions of the winding densities of the stator phases around the periphery of the airgap. The motoring torque has minimal time harmonics. The design achieves a maximum power density for the frame size of the motor. It is ideally suited for permanent magnet synchronous motors.     

SINGLE-PHASE INDUCTION MOTOR WITH ASYMMETRICAL STATOR

ABSTRACT

This paper describes a single-phase single-winding induction motor which is self-starting without the use of a shading winding. Starting torque is produced by a combination of several stator asymmetries: an airgap under the main pole which is-tapered down in the direction of rotation, poles without windings in the quadrature axis with solid iron flux bridges to the lagging sides of the main poles, and yoke sections which are larger on the leading than on the lagging sides of the main poles. A qualitative explanation of the action of these asymmetries is presented using equivalent circuits.     

SPEED AT BREAKDOWN TORQUE OF A SINGLE-PHASE INDUCTION MOTOR

ABSTRACT

The purpose of this paper is to present an “exact” analytical expression to calculate speed at breakdown torque and the breakdown torque of single-phase induction motors. From the theoretical expression for torque developed by a single-phase motor, a sixth order polynomial in speed at breakdown torque is obtained, and solved, using a well known method. A real root for per unit speed, i.e. the ratio of speed to the synchronous speed, that lies between 0 and 1 is selected and the actual breakdown torque is determined by substituting it in the torque equation. The breakdown torque, also termed as maximum torque, so obtained has been found to yield better correlation between the predicted and the tested results than the only existing empirical relation. Steps to calculate speed at maximum torque are also suggested. For an arbitrarily selected single-phase induction motor, a curve of speed at breakdown torque as a function of r₂/X is also included for the purposes of illustration and understanding.     

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.     

TURBINE-GENERATOR SHAFT TORQUES AND FATIGUE LIFE EXPENDITURE PREDICTED BY FREQUENCY DOMAIN AND TIME DOMAIN METHODS FOLLOWING SEVERE DISTURBANCES ON THE SYSTEM SUPPLY

ABSTRACT

This paper examines precision of predicting time responses for torque in turbine-generator-exciter shafts together with fatigue life expenditure of shafts by frequency domain analysis not analysed in the literature heretofore following incidence and clearance of worst-case disturbances on the electrical supply system. Effect of both governor and avr action following a range of disturbances is also illustrated. The effect of steam and electrical viscous damping on maximum and minimum shaft torques predicted by frequency domain analysis for a range of disturbances as a function of the fault clearing time is also depicted

The analysis is based on Fourier analysis of generator airgap torque following incidence and clearance of a severe supply network disturbance or following mal-synchronisation to obtain torque excitation which acts on the generator rotor corresponding to each modal vibration. Amplitude and phase of each modal vibration is thereby determined. Using appropriate damping, time responses for shaft torque at each shaft cell is constructed by summing components which correspond to each modal vibration. These time responses are compared with those obtained by solution of more than 50 differential equations which simulate the shaft train, turbine, generator, governor and avr

It is shown that time responses for transient turbine-generator-exciter shaft torques together with maximum shaft torques and fatigue life expenditure of the shaft can be predicted faithfully by frequency domain analysis taking due account of magnitude and phase of each modal vibration, and damping, following (i) worst-case Line-Line-Line, Line-Line, and Line-Ground disturbances from full-load to no-load with clearance, and (ii) mal-synchronisation. Simulation of damping of rotor swing, together with governor action, is important in making precise assessments of transient turbine shaft torque (and fatigue life expenditure of the shaft) at shaft locations which are close to die generator

660 MW, 1000 MW and 1300 MW two-pole Machines are analysed.     

ROLE OF TORQUE ANGLE IN INVERTER-FED INDUCTION MOTOR DRIVES

ABSTRACT

It is shown from the induction motor differential equations that changes in load torque, speed reference and torque angle are accompanied by a transient change in the speed of the air-gap mmf. The time constant associated with the torque angle change is different for a voltage source and a current source inverter-fed induction motor. This aspect throws light on the instability of current source inverter-fed induction motor on open loop operation. Evidently, the solution to overcome this instability is to use the torque angle for feedback control. It is proved that torque angle feedback alone is not sufficient for stabilization of the current source inverter-fed induction motor.     

TRANSIENT PERFORMANCE OF A GROUP OF INDUCTION MOTORS WITH TERMINAL CAPACITORS FOLLOWING SUPPLY DISCONNECTION

ABSTRACT

In this paper a model representing a group of three-phase induction motors, with terminal power-factor correcting capacitors, following disconnection of the group with its terminal capacitors as one unit from the supply, is derived. From this model the transient behavior of the motors of the group and the interaction between them, following supply disconnection is investigated, and the effect of the terminal capacitors on the transient behavior of each motor of the group, regarding terminal voltage, stator currents, airgap powers, is shown.     

Digital Simulation Of A Vector Current Controlled Axially-Laminated Anisotropic (ALA) Rotor Synchronous Motor Servo-Drive

ABSTRACT

It has been recently demonstrated that axially laminated anisotropic (ALA) rotor reluctance motors have high torque density, power factor and efficiency due to unusually high L_d/L_q ratios. For variable-speed vector-controlled drives no starting cage on the rotor is required. Through digital simulations this paper explores the steady-state torque and power factor capabilities and the transient performance of a vector ac current-controlled ALA-rotor motor drive. Fast torque and speed responses are demonstrated for a two-pole motor up to a speed of 2500 rpm. Good performance down to 0.1 rpm is obtained through an adaptive proportional integral (PI) speed controller.     

背绕式有槽永磁同步电机的电磁场解析模型

以1台5 kW背绕式高速永磁同步电机为研究对象,建立其电磁场解析模型。将电磁场求解域划分为气隙子域、永磁体子域、槽口子域和槽子域,求解相应的拉普拉斯方程或泊松方程,解析模型计及电枢反应场、永磁场和定子开槽的影响。计算了该电机的气隙磁密、绕组磁链、绕组反电动势、齿槽转矩和电磁转矩,并将结果与二维有限元法计算结果和试验数据比较,比较结果说明了解析模型的准确性。最后以槽口开度为变量,研究其对气隙磁密分布和齿槽转矩的影响。     

TORQUE VECTOR CONTROL (TVC) OF AXIALLY-LAMINATED ANISOTROPIC (ALA) ROTOR RELUCTANCE SYNCHRONOUS MOTORS

Abstract

Axially-laminated anisotropic (ALA) rotor reluctance synchronous motors (RSMs) have been proved recently capable of higher torque density, higher power factor and efficiency compared to their induction motor counterpart with the same stator. Vector current control systems have been proposed for such drives, based on maximum available torque constant current angle or minimum current for a given torque constant current angle or combined current-voltage control. In an effort to do away with the current controller limitations, especially in the high speed region, the paper proposes a torque vector control (TVC) system which is a direct stator flux and torque control through a table of optimum switchings in the voltage-source PWM inverter. A sliding mode speed controller is added. Extensive digital simulations are presented. They demonstrate the TVC fast response, wide speed range and load rejection performance.     

TORQUE VECTOR CONTROL (TVC) OF AXIALLY-LAMINATED ANISOTROPIC (ALA) ROTOR RELUCTANCE SYNCHRONOUS MOTORS

Abstract

Axially-laminated anisotropic (ALA) rotor reluctance synchronous motors (RSMs) have been proved recently capable of higher torque density, higher power factor and efficiency compared to their induction motor counterpart with the same stator. Vector current control systems have been proposed for such drives, based on maximum available torque constant current angle or minimum current for a given torque constant current angle or combined current-voltage control. In an effort to do away with the current controller limitations, especially in the high speed region, the paper proposes a torque vector control (TVC) system which is a direct stator flux and torque control through a table of optimum switchings in the voltage-source PWM inverter. A sliding mode speed controller is added. Extensive digital simulations are presented. They demonstrate the TVC fast response, wide speed range and load rejection performance.     

LAMINATED SALIENT-POLE MACHINES: NUMERICAL CALCULATION OF THE ROTOR IMPEDANCES,STARTING CHARACTERISTICS,AND DAMPER-BAR CURRENT DISTRIBUTION

ABSTRACT

This paper describes a numerical method for calculating the self and mutual reactances of all the electrical circuits in a machine on the basis of field computation and the given machine constructional dimensions. Subsequently, the asynchronous starting characteristics as well as the damper-bar currents are also computed.

Each of the damper bars is treated individually so that it can assume any arbitrary spatial position, e.g., along the pole surface or in the interpole region. Furthermore, the damper cage can either be complete (with damper ring) or be of the pole-grid type (without pole connection). In the analysis provision is also made for a pole-shoe collar under the lower face of the pole shoe.

All feasible types of airgap patterns can be precisely accounted for. The saturation influences are neglected, however, it is also possible to consider these approximately by modifying the virtual airgap and the damper-slot opening.     

APPROXIMATE ANALYSIS OF TEMPERATURE RISE IN AN INDUCTION MOTOR DURING DYNAMIC BRAKING

ABSTRACT

The problem of two-dimensional transient heat flow in the stator of induction motor is solved by use of a finite element formulation and arch-shaped elements in the r-θ plane of the cylindrical co-ordinate system. The shape functions and exact solution matrices are derived algebraically for utmost economy in computation. The temperature distribution has been determined considering convection from the stator frame and cylindrical airgap surface. Finally the temperatures obtained by this two dimensional approximation have been compared for different stator currents considering the time required for each stator current during the transient.     

Control Algorithm for the Inverter Fed Induction Motor Drive with DC Current Feedback Loop Based on Principles of the Vector Control

ABSTRACT

This paper brings out a control algorithm for VSI fed induction motor drives based on the converter DC link current feedback. It is shown that the speed and flux can be controlled over the wide speed and load range quite satisfactorily for simpler drives. The base commands of both the inverter voltage and frequency are proportional to the reference speed, but each of them is further modified by the signals derived from the DC current sensor. The algorithm is based on the equations well known from the vector control theory, and is aimed to obtain the constant rotor flux and proportionality between the electrical torque, the slip frequency and the active component of the stator current. In this way, the problems of slip compensation, Ri compensation and correction of U/f characteristics are solved in the same time. Analytical considerations and computer simulations of the proposed control structure are in close agreement with the experimental results measured on a prototype drive.