Assessment of interbar currents in double-cage induction motorswith broken bars

Research has shown that it is possible for currents to flow in a broken rotor bar of a squirrel cage induction motor by means of interbar currents. Such research dealt only with single-cage motors. Further work was necessary when considering double-cage motors. This paper proves theoretically and experimentally that interbar currents also exist in double-cage motors. The influence of interbar currents on the current distribution around the broken bar is also shown     

Copper die-cast rotor efficiency improvement and economicconsideration

Significant motor efficiency improvement can be achieved by substituting aluminum with copper die-cast rotor in a squirrel cage induction motor. This paper summarizes the experiments conducted by Westinghouse Motor Company Canada Ltd. in our copper die-cast rotor induction motors project     

An analysis of bearingless AC motors

Several types of AC bearingless motors are proposed. These bearingless motors have conventional four-pole stator windings and additional two-pole windings, whose currents produce radial forces acting on the rotor. General expressions of the machine inductances and radial forces are derived for the cylindrical rotor and salient-pole motors. No-load characteristics of laboratory squirrel-cage induction and reluctance-type synchronous bearingless motors are provided. The test motors were successfully driven by the control circuits     

Determination of interbar current effects in the detection ofbroken rotor bars in squirrel cage induction motors

In present monitoring techniques for the detection of broken rotor bars in squirrel cage induction motors, the assumption is made that a broken bar conducts no current. This causes a magnetic imbalance, which can then be detected to indicate a broken bar. In certain motors, however, large currents are still able to flow in the broken bar by means of interbar currents. This paper deals with the effects these currents have on broken bar monitoring. The work shows theoretically and experimentally that interbar currents reduce any magnetic imbalance brought about by the broken bar     

Online rotor mixed fault diagnosis way based on spectrum analysis of instantaneous power in squirrel cage induction motors

Broken rotor bars and eccentricity are common faults in squirrel cage induction motors. These two faults usually occur simultaneously. This paper will deal with this kind of mixed fault. It is well known that the characteristic frequency of broken bars in the stator line current of a squirrel cage induction motor is very near to that of the fundamental component. In the spectrum of the stator line current, the characteristic components related to broken rotor bars are always submerged by the fundamental one, and it is difficult to detect the broken bar fault at an early stage. In our work, instantaneous power of the motor is used as the quantity to be monitored. Theoretical analysis indicates that the spectrum of ac level of the spectrum of the instantaneous power is clear from any component at the fundamental supply frequency, and the fault characteristics can be highlighted, which is effective toward the separation of mixed faults and the quantification of the fault extent. Experimental results have demonstrated the effectiveness of the proposed technique.     

A new graphical approach to three phase two speed induction motorwinding design

It has already been shown how to obtain 2-speed winding squirrel-cage induction motors using pole amplitude modulation (PAM) with 6 terminals to the winding. Phase modulation has been used as well to obtain 2-speed winding squirrel cage induction motors with simpler C-terminal change-over switch only. This paper presents a new powerful method (based on the graphical representation of phase distribution) which enables the designer to make his choice easily between PAM and PM. Particular example has been chosen for the presentation together with some experimental results obtained on a trial model     

H-G diagram based rotor parameters identification for inductionmotors thermal monitoring

In this paper, an effective online method for induction motor parameter identification, especially rotor parameters, based on the H-G diagram is presented for motor thermal monitoring purposes. The H-G diagram is established from the analysis of the induction motor measurement of active and reactive power consumption for each operating point. Computer simulations and experimental tests, carried out for a 4 kW four-pole squirrel cage induction motor, provide an encouraging validation of the proposed thermal monitoring technique. The process should be refined for a possible industrial application     

Impact of wind farms on a power system. An eigenvalue analysis approach

This paper analyzes the frequency dynamic behavior in a power system with a high wind power penetration. To this end, wind farms equipped with squirrel cage and doubly fed induction generators are compared. Aspects of the modeling of the different kinds of wind generation and power systems are cited. Then, it is shown, through an eigenvalue analysis, that wind farms equipped by doubly fed induction machines, adequately controlled, can contribute to improve the frequency dynamics. Simulations are presented which verify the theoretical results.     

Diagnostics of bar and end-ring connector breakage faults inpolyphase induction motors through a novel dual track of time-seriesdata mining and time-stepping coupled FE-state space modeling

This paper develops the fundamental foundations of a technique for detection of faults in induction motors that is not based on the traditional Fourier transform frequency domain approach. The technique can extensively and economically characterize and predict faults from the induction machine adjustable speed drive design data. This is done through the development of dual-track proof-of-principle studies of fault simulation and identification. These studies are performed using our proven time stepping coupled finite element-state space method to generate fault case data. Then, the fault cases are classified by their inherent characteristics, so-called "signatures" or "fingerprints." These fault signatures are extracted or mined here from the fault case data using our novel time series data mining technique. The dual-track of generating fault data and mining fault signatures was tested here on three, six, and nine broken bar and broken end-ring connectors in a 208-volt, 60-Hz, 4-pole, 1.2-hp, squirrel cage 3-phase induction motor     

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     

Three-phase self-excited induction generators: an overview

Induction generators are increasingly being used in nonconventional energy systems such as wind, micro/mini hydro, etc. The advantages of using an induction generator instead of a synchronous generator are well known. Some of them are reduced unit cost and size, ruggedness, brushless (in squirrel cage construction), absence of separate dc source, ease of maintenance, self-protection against severe overloads and short circuits, etc. In isolated systems, squirrel cage induction generators with capacitor excitation, known as self-excited induction generators (SEIGs), are very popular. This paper presents an exhaustive survey of the literature over the past 25 years discussing the process of self-excitation and voltage buildup, modeling, steady-state, and transient analysis, reactive power control methods, and parallel operation of SEIG.     

On the theory of 3-phase squirrel-cage induction motors includingspace harmonics and mutual slotting

General equations for squirrel-cage induction motors are derived based on the real geometry of the motor. The squirrel cage is described by its meshes; no equivalent windings are used. By means of complex time-dependent transformations free angles are introduced which simplify the set of equations when the specific geometrical properties of both types in which the asynchronous machines fundamentally can be divided are taken into account. The equations are general in the sense that all space harmonics are taken into account, due to the MMF and double slotting. This provides a better calculation of the synchronous, pulsating, and asynchronous torques. The final equations enable the formulation of some specific properties of both types in connection to their electromechanical behavior. Further, they are valid for star and delta connections and for any arbitrary source voltage     

A survey of shaft voltage reduction strategies for induction generators in wind energy applications

Common mode voltage generated by PWM inverters and the parasitic couplings of the machine structure in high frequencies create a model for the system which leads to an induced voltage on the shaft. Shaft voltage became a dominant side effect of power electronic converters since they are widely used in wind turbine applications to prepare desirable frequency and suitable control on active and reactive power. This voltage is known as the main cause of many unwanted problems such as leakage current, ball-bearings damages and reduction of generator's life time. In this paper, pulse width modulation strategies have been presented for two-level and three-level back-to-back AC–DC–AC converters in order to reduce or eliminate common mode voltage of these converters for reduction or elimination of shaft voltage in squirrel cage and doubly fed induction generators. Applying these techniques lead to complete elimination of shaft voltage for squirrel cage Induction generator with back to back AC/DC/AC converter and a 66 percent reduction of the shaft voltage generation in the Doubly Fed Induction Generator. Simulation results and mathematical analysis have been presented to investigate proposed techniques.     

What stator current processing-based technique to use for induction motor rotor faults diagnosis?

In recent years, marked improvement has been achieved in the design and manufacture of stator winding. However, motors driven by solid-state inverters undergo severe voltage stresses due to rapid switch-on and switch-off of semiconductor switches. Also, induction motors are required to operate in highly corrosive and dusty environments. Requirements such as these have spurred the development of vastly improved insulation material and treatment processes. But cage rotor design has undergone little change. As a result, rotor failures now account for a larger percentage of total induction motor failures. Broken cage bars and bearing deterioration are now the main cause of rotor failures. Moreover, with advances in digital technology over the last years, adequate data processing capability is now available on cost-effective hardware platforms, to monitor motors for a variety of abnormalities on a real time basis in addition to the normal motor protection functions. Such multifunction monitors are now starting to displace the multiplicity of electromechanical devices commonly applied for many years. For such reasons, this paper is devoted to a comparison of signal processing-based techniques for the detection of broken bars and bearing deterioration in induction motors. Features of these techniques which are relevant to fault detection are presented. These features are then analyzed and compared to deduce the most appropriate technique for induction motor rotor fault detection.     

Loss of Life in Induction Machines Operating With Unbalanced Supplies

This paper estimates motor life when a motor is supplied with a combination of over- or undervoltages with unbalanced voltages. The motor life is predicted by estimating the stator winding insulation life of squirrel cage motors using Arrhenius' equations. Electrical and thermal models are used to calculate motor losses and temperatures, respectively. The thermal model parameters are obtained from simple motor testing techniques rather than from complex methods requiring motor design data     

A comprehensive method for the calculation of inductance coefficients of cage induction machines

In this paper, a new comprehensive method for the calculation of inductance coefficients of squirrel cage induction machine based on combined winding function approach (WFA) and magnetic equivalent circuit (MEC) is presented. By taking into account machine geometry, rotor skewing, stator and rotor slots effects and type of windings connection, this method is able to model most of the important features of an induction machine. The effects of each machine parameter on the inductance coefficients are verified. Also, effects of several rotor asymmetries on these inductances are shown. Simulation results are verified by more elaborate nonlinear finite element model and finally with experimental results.     

3-D electro-magnetic analyses of a cage induction motor with rotorskew

The paper studied the performance characteristics of a three-phase cage induction motor using a 3D finite element technique. Especially, the effects of rotor skew and the rotor end-rings and the distribution of the electromagnetic field toward the axial direction, which have not been able to be analyzed accurately by 2D analysis, were investigated. Since the 3D analysis enabled the analysis of the continuity of the air gap flux density at the core ends and to take into consideration the rotor end-ring impedance, the motor parameters were calculated with high accuracy, compared with 2D analysis. Firstly, the authors made the relation of the rotor end-ring current with the rotor bar current clear, by analyzing the harmonics of the rotor bar current and the rotor end-ring current in the cases of motors without rotor skew and with rotor skew. Secondly, the torque and the rotor axial force were calculated from the distribution of the electromagnetic field obtained by the 3D analysis in the case of rotor skew. They also calculated the axial forces on the both end-rings in the case of rotor skew. The results obtained in the paper show the importance of 3D analysis for the design of induction motors     

Unbalanced transients-based maximum likelihood identification of induction machine parameters

The paper describes an effective formulation of a maximum-likelihood identification algorithm for linear estimation of the equivalent-circuit parameters of cage-type (single cage and double cage) or deep-bar induction motors with measurement and process noises. A complete generalized model for symmetrical and asymmetrical test analysis of induction machines is developed for this purpose. The paper outlines the theory and reasoning behind the proposed statistical-based treatment of online data derived from generalized least-squares estimator and a Kalman filter. The method is successfully applied to online double-line independent finite-element (FE) short-circuit-simulated records of a deep-bar-type induction motor.     

Instantaneous Shaft Radial Force Control with Sinusoidal Excitations for Switched Reluctance Motors

Unbalanced radial forces acting on a rotor shaft exist in motor applications where the external load is not balanced or when the rotor is not centered causing a nonuniform air gap. These forces are undesirable as they cause motor vibrations. In view of its special structure, the shaft radial force and the torque of a three-phase 12/8 pole switched reluctance motor (SRM) can be separately controlled by proper pole current selection in the energized phase. Therefore, radial forces can be produced intentionally to cancel the existing radial force produced by rotor eccentricity and the unbalanced load inertia. The motor vibrations are thereby reduced. In this paper, a sinusoidal current excitation scheme is proposed for the torque and radial force control of a 12/8 pole SRM. When controlled with the selected sinusoidal currents, the SRM can simultaneously produce the desired shaft radial force in any rotational plane direction and the required rotational torque. As all pole currents are individually controlled, a more sophisticated phase commutation strategy is also proposed that provides smoother torques and radial force ripples.     

Dynamic simulation of dynamic eccentricity in inductionmachines-finding function approach

The paper describes a method for the dynamic simulation of dynamic rotor eccentricity in squirrel cage rotor induction machines. The method is based on a winding function approach, which allows for all harmonics of magnetomotive force to be taken into account. It is demonstrated how this complex dynamic regime can be modeled using the mutual inductance curves of symmetrical machine using proper scaling techniques. Experimental results demonstrate the effectiveness of the proposed technique and validate the theoretical analysis