Simulation Studies of the Transients of Squirrel-Cage Induction Motors

A new simulation approach is proposed in consideration of a saturation and a deep bar effect for the study of transients of three-phase squirrel-cage type induction motors. The mathematical model of an induction motor is expressed by the six differential equations of three-phase instantaneous voltage and current. The torque of an electric equation is related to the motion equations of motor and driven machine in the mathematical model. The values of reactance of stator and rotor are changed by the saturation of core caused by starting current. Also both the values of reactance and resistance of rotor bar are varied by the deep bar effect in the rotor core during starting. The calculation method of circuit constant that adds the influence of saturation and deep bar effect is proposed in this paper. The circuit constant of simulation model in consideration of saturation and the deep bar effect are decided by these computation methods in accordance with the conditions of rotation speed and current. If the large current flows, the leakage reactance of the stator and the rotor decreases by saturation. Moreover, the resistance of the rotor gradually decreases when the rotational speed rises from stop to synchronous speed, and the leakage reactance increases gradually. The calculated values were compared with the observed values of the examination machine of 1100 kW4P and an excellent agreement was obtained demonstrating the accuracy of the proposed simulation. Consequently, it is shown that the saturation and the deep bar effect are the essential factors to perform the accurate simulations of the induction motor. After checking the validity of the proposed approach, the simulation of the grounding faults was performed. In this study, all the simulation programs have been developed in the Matlab/Simulink environment.     

Advanced fault diagnosis of a DC motor

This paper presents a model of the DCc motor with an eccentric rotor. The winding function theory shows the effect of eccentricity fault on the motor inductances and the simulation is done using a nonsymmetric air-gap function. A modified equation is presented to show the existence of rotor slot harmonics in the DC motor current. To detect the eccentricity fault, a pattern recognition technique is utilized. The proposed algorithm works at steady state and uses armature current as input. The rotor speed is needed in order to provide the appropriate feature for the classifier. Therefore, rotor speed is estimated from the armature current using the commutation harmonics. The experimental results obtained from a 1/3-hp shunt DC motor verifies the proposed method. In order to cover different motor conditions, data are collected at different shaft speeds for both a healthy dc motor and a dc motor with an unbalanced load which exhibits static eccentricity.     

Modelling of synchronous machines for dynamic studies withdifferent mutual couplings between direct axis windings

Synchronous machine models commonly utilized for transient performance calculations normally disregard such effects as induction of currents on slot walls and rotor surfaces during canceling or blocking of field current and the difference of mutual coupling between direct axis windings. It is shown how such different couplings can easily be incorporated into the synchronous machine conventional modeling and how the characteristic reactance to be used in this representation can be determined through the reactive power rejection test with zero active power. The air gap flux saturation representation, when considered together with the inclusion of the effect of the difference of mutual couplings between direct axis windings, is also discussed     

Improvement in Modeling and Testing of Induction Motors

The paper presents a method for identifying the parameters of a new model of induction motor. This model has two rotor loops in order to take into account the deep-bar effect and a non-linear leakage reactance which represents magnetic saturation effect. These phenomena cannot be neglected when induction motor dynamic characteristics have to be evaluated. The test method is simple and non-iterative. Only two locked-rotor tests at rated current and different frequency are required to derive the parameters of the two rotor loops. A further locked-rotor test at a suitable current value is necessary to determine the non-linear leakage reactance of the model.     

Dynamic harmonic field analysis of a cage type induction motor whenmagnetic slot wedges are applied

Applying magnetic slot wedges to stator slot openings is an effective means to reduce the slot ripple harmonic components in the magnetic field. To investigate the effect of the wedges in detail, this paper proposes a new harmonic field analysis method that considers the rotor movement dynamically and the harmonic components of the secondary current as induced current by a two-dimensional finite element method (2D-FEM) using a time stepping technique. The differences in harmonic characteristics due to the use of plural wedges (a nonmagnetic wedge and magnetic slot wedges in different dimensions) for the stator slot opening of a three-phase cage induction motor are evaluated using the method. Suitable wedge dimensions to reduce the slot ripple harmonics are determined. From the comparison of calculated and experimental results of secondary current, the proposed method is shown to be appropriate and useful for the quantitative estimation of harmonic fields for induction motors     

Current monitoring for detecting inter-turn short circuits ininduction motors

In this paper the stator winding itself is used as the sensor for the detection of abnormalities in the stator winding. To achieve this task, detailed analysis of the air gap flux distribution and its dependence on the particular machine configuration is carried out. The analysis presented is based on the rotating wave approach which accounts for all the stator and rotor MMF harmonics, stator and rotor slot harmonics and harmonics due to saturation. It is shown that the most reliable indicators of the presence of the fault are the lower sideband of field rotational frequency with respect to the fundamental, together with some of the components that are related to slotting. Some of them increase as functions of the link current, in a range from 0 to over 10% and some components decrease in the range 0-12%     

Optimal Slot Opening Width for Magnetic Noise Reduction in Induction Motors

This paper presents a method to characterize the main magnetic force waves occurring in a sinusoidally fed induction machine. Three main force types are identified: slotting force waves, winding force waves, and saturation force waves. Slotting force waves are characterized in terms of number of nodes, velocity, propagation direction, and magnitude. On the ground of the expression of these forces magnitude, a method to cancel a given magnetic force wave by properly choosing the rotor slot or stator slot opening width is presented. This new method is validated with both simulations and experiments. Contrary to the common design rule that advices to decrease rotor and stator slot openings width in order to reduce magnetic noise, it is shown that a wider slot opening can lower the global noise level when properly chosen.      

Force density limits in low-speed permanent-magnet machines due to saturation

This work discusses the practical limits imposed by magnetic saturation for the force density in low-speed permanent-magnet electric machines. The force density dependence on current density and slot depth is investigated with the aid of finite-element modeling. For saturation reasons, shallow slots are more favorable for achieving high force densities. However, for thermal reasons, deeper slots become favorable. Therefore, an optimum slot depth that maximizes the force density for each current density level exists. The maximum allowable slot depth range for four low-speed applications has been identified for a given maximum motor diameter.     

The Interactions of Turbine Hot Streak and Leakage Flow from the Inter-Platform Gap

ABSTRACT

The paper presents the effects of the leakage flow from the inter-platform gap on the migration of hot streak in a first stage turbine by three-dimensional unsteady Reynolds-Averaged Navier-Stokes simulations. Five circumferential positions of the hot streak are considered. The comparisons between the results with/without slot leakage show significant differences. The leakage changes the hot streak in the vane passage significantly and it protects the vane suction surface trailing from the hot gas. The leakage also changes the secondary flow and results in forming a new couple of vortices in the vane passage. In general, rotor passage, the hot gas usually gathers in the rotor hub and the pressure surface. In the present study, the leakage coolant from upstream slot is entrained to the unsteady rotor secondary flows and transported toward the rotor hub and pressure surface effectively. The cooling effect is related to the relative circumferential positions between the hot streaks and slot. When the hot streak is positioned at the slot suction side, the time-averaged temperature reduction on the blade leading edge can be more than twice of that for the hot streak at the slot pressure side.     

Neural network-based modeling and parameter identification of switched reluctance motors

Phase windings of switched reluctance machines are modeled by a nonlinear inductance and a resistance that can be estimated from standstill test data. During online operation, the model structures and parameters of SRMs may differ from the standstill ones because of saturation and losses, especially at high current. To model this effect, a damper winding is added into the model structure. This paper proposes an application of artificial neural network to identify the nonlinear model of SRMs from operating data. A two-layer recurrent neural network has been adopted here to estimate the damper currents from phase voltage, phase current, rotor position, and rotor speed. Then, the damper parameters can be identified using maximum likelihood estimation techniques. Finally, the new model and parameters are validated from operating data.     

An induction machine model for predicting inverter-machineinteraction

The conventional qd induction motor model typically used in drive simulations is very inaccurate in predicting machine performance, except perhaps for the fundamental component of the current and the average torque near rated operating conditions. Predictions of current and torque ripple are often in error by a factor of two to five. This work sets forth an induction machine model specifically designed for use with inverter models to study machine-inverter interaction. Key features include stator and rotor leakage saturation as a function of current and magnetizing flux, distributed effects in the rotor circuits, and a highly computationally efficient implementation. The model is considerably more accurate than the traditional qd model, particularly in its ability to predict switching frequency phenomena. The predictions of the proposed model are compared with those of the standard qd model and to experimental measurements on a 37 W induction motor drive     

A unified approach to main flux saturation modelling in D-Q axismodels of induction machines

Main flux saturation is most frequently modelled by selecting either stator and rotor d-q axis currents or stator and rotor d-q axis flux linkages as state-space variables. This paper attempts to unify main flux saturation modelling in d-q axis models of induction machines by presenting a general method of saturation modelling. Selection of state-space variables in the saturated machine model is arbitrary and appropriate models in terms of different state-space variables result by application of the method. A couple of models, obtainable with different selection of state-space variables, are presented. The cross-saturation effect is explicitly present in all the models, except for the one with stator and rotor flux linkage d-q axis components as state-space variables. The models are verified by simulation and experimental investigation of induction generator self-excitation     

Finite-Element Simulation of Turbine-Generator Terminal Faults and Application to Machine Parameter Prediction

A finite-element, time-stepping technique is described for simulation of balanced and unbalanced terminal faults on a turbine-generator. Magnetic saturation, induced currents in the rotor body, wedges and field winding, and the relative motion between the rotor and stator windings, are modelled. Calculated values of induced field winding and stator phase currents, following a 0.5 p.u. sudden shortcircuit on a 660 MW machine, are shown to be in close agreement with test results. The numerical model has been used to simulate terminal short-circuits at different pre-fault voltage levels so that the effect of stator saturation can be observed.     

A transient induction motor model including saturation and deep bareffect

Transient cage induction motor models for use in inverter-fed drives and controllers are reviewed. A simple transient model is presented that includes rotor deep bar effect and magnetic saturation of the magnetising and rotor leakage flux paths. The improved model requires motor details in the form of simple impedance versus frequency characteristics which can be obtained from a variety of external sources. These can range typically from detailed steady-state finite-element solutions to simple experimental measurements. The model is verified experimentally using a 75 kW, 4 pole vector controlled AC motor drive     

Main flux saturation modelling in double-cage and deep-barinduction machines

The available models of saturated double-cage and deep-bar induction machines are the current state-space model and the flux state-space model, where state-space variables are selected either as stator current and currents of both rotor cages, or stator flux linkage and flux linkages of both rotor cages. This paper presents a number of models of saturated double-cage (deep-bar) induction machines where alternative sets of state-spate variables are selected. The method of main flux saturation modelling relies on recently introduced concept of `generalised flux space vector', which has originally been developed for modelling of saturated single-cage induction machines. The procedure and the novel models are verified by experimental study and simulation of self-excitation process in a double-cage induction generator     

High-Frequency Loss Calculation in a Smooth Rotor Induction Motor Using FEM

In this paper, a new approach for the calculation of high-frequency losses in induction motors is presented. The input to the motors is assumed to be supplied from a sinusoidal voltage source. The method is based on the two-dimensional (2-D) field solutions of the magnetic circuit, obtained by using a nonlinear ldquoharmonicrdquo solution. Hence, the solution time is very short. From the ldquoharmonicrdquo solution, the air-gap field distribution as well as the fundamental frequency eddy current losses are determined. The high-frequency loss calculation is based on the assumption of a path for eddy currents within a lamination. A constant k is introduced that defines the width of the current flow path. The empirically found k value is verified by a theoretical calculation. The new method is applied to the calculation of losses of two smooth rotor induction motors. The prediction accuracy is found to be very good. The method is also applied to two open rotor slot motors to observe the change in the prediction accuracy. It is found that due to the small slot openings accurate predictions are still possible. The approach presented in this paper requires little time for loss calculation, and is very suitable for minimizing losses at the design stage.     

Experimental characterization procedure for use with an advanced induction machine model

An advanced induction motor model that includes stator leakage saturation, rotor leakage saturation, magnetizing saturation, and distributed system effects in the rotor circuits has been set forth. This model is considerably more accurate than traditional models, particularly in terms of predicting switching-frequency dynamics. The model proposed is very general in terms of the range of magnetic properties that can be incorporated. This paper provides suggestions for specific forms for the leakage and magnetizing characteristics and derives the resulting small-signal impedance and large-signal steady-state equivalent circuit. Based on these results, a test procedure for experimentally characterizing the machine is developed. The application of the procedure to a 50-hp test machine is included as an example.     

The prediction of the hydrodynamic performance of marine current turbines

The development of a blade element momentum (BEM) model for the hydrodynamic design of marine current turbines is presented. The model includes routines for interpolation of 2D section data and extrapolation for stall delay. The numerical model is compared with experimental data obtained from tests of an 800 mm diameter model rotor carried out in a cavitation tunnel. The theoretical predictions are in good agreement with the experiments. Using this validated model, a typical 3D rotor is used to demonstrate parametric variations of the design parameters. The effect of tip immersion on possible cavitation is assessed for this rotor. The model is then used to solve the dynamic effects of a tidal profile. The effect of an increase in blade roughness is presented, indicating a relatively small reduction in power. This work demonstrates that the numerical model developed can provide a useful tool for the investigation of the hydrodynamic design and operation of marine current turbines.     

Theory and analysis of three-phase series-connected parametricmotors

This paper presents the steady-state performance of a three phase wound-rotor parametric motor. This type of motor can be practically realized by connection of stator and rotor phases of a conventional wound-rotor induction machine. The analysis is based on the d-q axes model, from which a phasor diagram is presented. The analysis is extended to include the magnetic saturation effect. Comparison between theoretical and experimental results showed a satisfactory agreement proving the validity of the mathematical model as well as magnetic saturation effect representation. Also the motor stability is investigated     

An improved model for saturated salient pole synchronous motors

An improved model for the transient analysis of saturated salient pole synchronous motors is presented. With the aid of saturation factors obtained by test or with finite elements, Park's equations for a synchronous machine are modified to independently account for the saturation of the magnetizing flux linkages in the region of the stator teeth and rotor pole face as well as saturation of the total flux linking the stator core. The model is used to calculate the starting performance for a direct online start as well as the transient performance during a load change. The model shows improvement over more traditional models, indicating that representation of both main flux and core saturation are important for synchronous machine analysis