A Complete Time Domain Model of the Induction Motor for Efficiency Evaluation

The saturation of mutual and leakage inductances as well as the eddy currents effects are taken into account to represent an almost complete model of the induction motor. These effects are not trivial, when the stator voltage and/or frequency changes considerably. Therefore, the model is suitable for the motor performance studies under nonsinusoidal voltage waveform supplies. The effects of eddy currents are represented by a simple double cage model equivalent to the deep bar cage. A practical method for finding the double cage parameters is outlined and the results obtained were used in the simulation. The accuracy of the model is emphasized by comparing the simulation results with test results of the stator inrush current during starting under no-load conditions. The model is used for calculating the induction motor losses in the time domain, when the motor is fed from nonsinusoidal voltage waveform supplies. Samples of the obtained results from different voltage waveforms are given together with that of a pure sinusoidal voltage supply for comparison.     

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.     

Smooth starting of slip ring induction motors

The starting of three-phase wound rotor induction motors by stepped resistances added in the rotor circuit suffers from several problems. This paper suggests a novel method of motor starting by inserting a parallel combination of resistance, inductance and capacitance in the rotor circuit. The proposed method guarantees a smooth and high starting torque over most of the starting period besides a controlled starting current. A mathematical model is developed based on a frequency domain quasi-steady state equivalent circuit. An optimization technique is then applied to determine the values of the rotor added elements such that minimum starting time is achieved subject to current and torque constraints. Experimental verification has been carried out and the computed results are in good agreement with the measured values     

A combined finite element-state space modeling environment forinduction motors in the ABC frame of reference: the blocked-rotor andsinusoidally energized load conditions

The combined finite element-state space (CFE-SS) modeling environment was used to predict the performance of a 1.2 hp, three-phase case-study squirrel cage induction motor under blocked rotor and typical load operating conditions. The nature of this CFE-SS environment allows one to rigorously account for the impact of space harmonics generated by the magnetic circuit, winding, and cage geometric, as well as layout peculiarities and magnetic saturation, on the current and torque profiles, and ohmic losses in the stator armature and cage. This includes the ability to predict the profiles of connector and bar currents. The results of the CFE-SS simulations compare favorably with blocked rotor and load experimental test data. Potential capabilities of this CFE-SS modeling environment, and its use in impacting motor design decisions, are discussed in the light of reported findings     

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.     

On-site efficiency evaluation of three-phase induction motor based on particle swarm optimization

On-site efficiency determination of induction motor is essential in industrial plants for saving the energy consumption. This paper presents a new application of particle swarm optimization (PSO) approach for field efficiency evaluation of induction motor based on a modified induction motor equivalent circuit. The stray-load loss is considered in the equivalent circuit by adding an equivalent resistor in series with the rotor circuit and its value is derived from the assumed stray-load loss recommended in IEEE Std. 112. The PSO approach uses the information about the stator current, stator voltage, input power, stator resistance and speed of the motor and determines the equivalent circuit parameters. Once these parameters are known, the efficiency of motor can be evaluated. The simulation results on a 3.75 kW motor are presented and compared with the results of torque gauge method (TGM), equivalent circuit method (ECM), slip method (SM), current method (CM) and segregated loss method (SLM). The results reveal that the proposed method can evaluate the efficiencies of motor with less than 3% error under normal load conditions. Consequently, the method can be used in motor energy management system for improving the overall energy savings in industry.     

Pattern recognition-a technique for induction machines rotor brokenbar detection

A pattern recognition technique based on Bayes minimum error classifier is developed to detect broken rotor bar faults in induction motors at the steady state. The proposed algorithm uses only stator currents as input without the need for any other variables. Initially, rotor speed is estimated from the stator currents, then appropriate features are extracted. The produced feature vector is normalized and fed to the trained classifier to see if the motor is healthy or has broken bar faults. Only the number of poles and rotor slots are needed as pre-knowledge information. A theoretical approach together with experimental results derived from a 3 hp AC induction motor show the strength of the proposed method. In order to cover many different motor load conditions, data are obtained from 10% to 130% of the rated load for both a healthy induction motor and an induction motor with a rotor having 4 broken bars     

Real-Time Speed and Flux Adaptive Control of Induction Motors Using Unknown Time-Varying Rotor Resistance and Load Torque

In this paper, an algorithm for direct speed and flux adaptive control of induction motors using unknown time-varying rotor resistance and load torque is described and validated with experimental results. This method is based on the variable structure theories and is potentially useful for adjusting online the induction motor controller unknown parameters (load torque and rotor resistance). The presented nonlinear compensator provides voltage inputs on the basis of rotor speed and stator current measurements, and generates estimates for both the unknown parameters and the nonmeasurable state variables (rotor flux and derivatives of the stator current and voltage) that converge to the corresponding true values. Experiments show that the proposed method achieved very good tracking performance within a wide range of the operation of the induction motor (with online variation of the rotor resistance: up to (87%). This high tracking performance of the rotor resistance variation demonstrates that the proposed adaptive control is beneficial for motor efficiency. The proposed algorithm also presented high decoupling performance and very interesting robustness properties with respect to the variation of the stator resistance (up to 100%), measurement noise, modeling errors, discretization effects, and parameter uncertainties (e.g., inaccuracies on motor inductance values). The other interesting feature of the proposed method is that it is simple and easily implementable in real time. Comparative results have shown that the proposed adaptive control decouples speed and flux tracking while standard field-oriented control does not.      

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     

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 nonconventional method for fast starting of three phasewound-rotor induction motors

This paper presents a nonconventional method for the fast starting of three-phase wound rotor induction motors. This is achieved through shunting the stator and rotor windings across the supply for a predetermined short period. By this connection, the motor produces an extremely high starting torque. The feasibility of this basic idea has been confirmed through investigating the starting transients corresponding to this mode of operation. For this purpose, a rigorous state-space mathematical model has been developed and simulated. The validity of the proposed method and the findings from the mathematical model have been confirmed experimentally     

Detection of rotor slot and other eccentricity related harmonics ina three phase induction motor with different rotor cages

Detection of rotor slot and other eccentricity related harmonics in the line current of a three phase induction motor is important both from the viewpoint of sensorless speed estimation as well as eccentricity related fault detection. However, it is now clear that not all three phase induction motors are capable of generating such harmonics in the line current. Recent research has shown that the presence of these harmonics is primarily dependent on the number of rotor slots and the number of fundamental pole pairs of the machine. While the number of fundamental pole pairs of a three phase induction motor usually is within one to four (higher pole pairs are generally avoided due to increased magnetizing current), the number of rotor slots can vary widely. The present paper investigates this phenomenon further and obtains a hitherto nebulous theoretical basis for the experimentally verified results. Detailed coupled magnetic circuit simulation results are presented for a four pole, three phase induction motor with 44, 43, and 42 rotor slots under healthy, static, dynamic and mixed eccentricity conditions. The simulation is flexible enough to accommodate other pole numbers also. These simulations are helpful in quantifying the predicted harmonics under different combinations of load, pole pair numbers, rotor slots and eccentricity conditions, thus making the problem easier for drive designers or diagnostic tools' developers. Data from three different induction machines, namely, a 4 pole, 44 bar, 3 hp, a 4 pole, 28 bar, 3 hp and a 2 pole, 39 bar, 100 hp motor have been used to verify the results experimentally. The simulation and the experimental results clearly validate the theoretical findings put forward in this paper     

A new energy recovery double winding cage-rotor induction machine

This paper proposes a new double winding induction machine and its speed control methods. The machine consists of two stator windings and one cage rotor. One stator acts as a motor and the other as a generator. By controlling the voltage supplied to the secondary or the generator winding, the rotor speed can be adjusted. The machine has a similar speed control characteristic to that of a slip-ring induction motor equipped with the rotor energy recovery scheme. The construction, principle, equivalent circuit, and speed control schemes of the new machine are presented. The performance characteristics of the machine are analyzed using the equivalent circuit and verified by experimental results.     

Detection of broken bars in induction motors using an extendedKalman filter for rotor resistance sensorless estimation

This paper deals with broken bars detection in induction motors. The hypothesis on which detection is based is that the apparent rotor resistance of an induction motor will increase when a rotor bar breaks. To detect broken bars, measurements of stator voltages and currents are processed by an extended Kalman filter for the speed and rotor resistance simultaneous estimation. In particular, rotor resistance is estimated and compared with its nominal value to detect broken bars. In the proposed extended Kalman filter approach, the state covariance matrix is adequacy weighted leading to a better states estimation dynamic. Its main advantage is the correct rotor resistance estimation even for an unloaded induction motor. As part of this estimation process, it is necessary to compensate for the thermal variation in the rotor resistance. Computer simulations, carried out for a 4 kW four-pole squirrel cage induction motor, provide an encouraging validation of the proposed sensorless broken bars detection technique     

Analysis of the Double Output Induction Generator Using Direct Three-phase Model Part II - Harmonic Analysis

This paper investigates the harmonic content of the rotor current, rotor voltage and the torque of the variable speed constant frequency double output induction generator (VSCF-DOIG). The waveforms of these variables were generated using a direct three-phase model and then analyzed numerically for their harmonic content using a Fourier series subroutine. In addition, this paper investigates the stator distortion currents that are being induced in the stator windings by the rotor current harmonics at frequencies which are not an integral multiple of the stator fundamental frequency. As a check on the analysis the calculated stator current waveform is compared with the experimental oscillogram.     

Speed control of a three-phase induction motor based on robust optimal preview control theory

A synthesized method for speed control of a three-phase induction motor (IM) based on optimal preview control system theory is implemented in this article. An IM model comprises three-input variables and three-output variables that coincide with the synchronous reference frame that is implemented using the vector method. The input variables of this model are the stator angular frequency and the two components of the stator space voltage vector, whereas the output variables are the rotor angular speed and the two components of the stator space flux linkage. The objective of the synthesized control system is to achieve motor speed control, field orientation control, and constant flux control. A novel error system is derived and introduced into the control law to increase the robustness of the system. The preview feed-forward controller, which includes the desired and disturbance signals, is used to improve the transient response of the system. A space vector pulse-width modulation (PWM) control technique for voltage source-fed IM is prepared for microprocessor-based control. Spectral analysis of the output voltage is evaluated to predict the effect of the proposed space vector modulation technique on the dynamic performance of the IM. The optimal preview controlled system is implemented, and its applicability and robustness are demonstrated by computer simulation and experimental results.     

Analysis of torques in large steam turbine driven inductiongenerator shafts following disturbances on the system supply

The paper first summarises the advantages of steam turbine-driven induction generators over conventional generators such as low cost, less maintenance, rugged and brushless rotors (squirrel cage type, no need for synchronisation, etc.), together with problems concerning excitation (VAr compensation at loads etc). A mathematical model of the induction generator simulated in direct-phase quantities where saturation of the magnetising reactances is simulated and saturation of stator and rotor leakage reactances is ignored is developed and employed for detailed simulation of the machine. Discrete-mass models of the machine shaft where both steam and electrical viscous damping is simulated are employed in comparing transient shaft torsional response evaluated by time domain simulation and frequency domain analysis following incidence and clearance of severe system faults. The paper then investigates torsional response following incidence and clearance of severe supply system disturbances, when the rotor is stationary and when running at close to synchronous speed unexcited, and following malsynchronisation when excited by a controlled VAr source, together with torsional response following bolted stator-terminal short-circuits at full-load and no-load following switching in of the induction generator onto the system supply. It examines precision of predicting torque in turbine-generator shafts by frequency domain analysis not analyzed for induction generators in the literature heretofore following incidence and clearance of worst-case disturbances on the supply. Effect of steam and electrical damping on maximum shaft torques predicted by frequency domain analysis is also illustrated. The results illustrate there is no tendency for shaft torques to become more onerous as the fault clearing time is increased as is the case for shaft torques in large synchronous machines. Three large two-pole machines of rating of up to a few hundred MWs are analysed     

Reduced order models for induction motors with two rotor circuits

Induction motor behavior is commonly simulated by a fifth order differential equation model which includes two stator state variables, two rotor state variables, and shaft speed. Normally two more variables must be added to account for the effects of a second rotor circuit representing deep bars, a starting cage, or rotor distributed parameters. This paper presents a technique for including the effects of a second rotor circuit without adding more state variables to the fifth order model. Transient, as well as steady state effects are included. The model may be useful in cases where switching introduces transient torques and losses that are invisible to the fifth order model, and it is not desirable to expand the model to include two more state variables. A laboratory test is included     

Thermal analysis of an electrical machine taking into account theiron losses and the deep-bar effect

The temperature distribution over the cross-section of an asynchronous squirrel cage machine is calculated by taking into account the copper losses and the iron losses at the steady-state condition. Under transient conditions, the deep-bar effect in the rotor is taken into account. Due to the deep bar effect, the current densities and thermal sources in the bar change and as a consequence the resistances, the leakage reactances, the currents and the thermal sources in the stator copper are also altered. Resistances depend upon temperatures     

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