Effects of the Slot Harmonics on the Unbalanced Magnetic Pull in an Induction Motor With an Eccentric Rotor (original) (raw)
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Electromagnetic forces in cage induction motors with rotor eccentricity
IEEE International Electric Machines and Drives Conference, 2003. IEMDC'03., 2003
1 -The paper deals with the electromagnetic forces in induction machines when the rotor is performing eccentric motion with respect to the stator. The studied eccentric motions of the rigid cage rotor are cylindrical circular whirling motion, symmetric conical whirling motion and the combination of these two basic modes of eccentric motions.
Spatial linearity of an unbalanced magnetic pull in induction motors during eccentric rotor motions
COMPEL - The international journal for computation and mathematics in electrical and electronic engineering, 2003
There is an unbalanced magnetic pull between the rotor and stator of the cage induction motor when the rotor is not concentric with the stator. These forces depend on the position and motion of the centre point of the rotor. In this paper, the linearity of the forces in proportion to the rotor eccentricity is studied numerically using time‐stepping finite element analysis. The results show that usually the forces are linear in proportion to the rotor eccentricity. However, the closed rotor slots may break the spatial linearity at some operation conditions of the motor.
Rotordynamic Simulation Method of Induction Motors including the Effects of Unbalanced Magnetic Pull
IEEE Access
This paper presents an optimal method for rotordynamic simulation of induction motors, including the effects of unbalanced magnetic pull (UMP). The developed simulation method containing the UMP model is simple but still accurate for the actual design process of induction motors. The UMP model is simplified by using the magnetizing current without calculation of the rotor current. The effects of the slot opening and saturation are initially incorporated into the model by using the Carter factor. To improve the accuracy of the model, the magnetizing current is calculated by the finite element analysis (FEA), and the proposed correction factor is also built into the model. Moreover, mixed eccentricity is modeled and applied to the time step rotordynamic simulation for considering the actual rotor eccentricity condition. Based on the developed UMP and eccentricity models, rotordynamic simulation methods within the induction motor design process are proposed and tested in a standard four-pole induction motor. The simulation results show that inclusion of the UMP force reduces the critical speeds and generates electromagnetic excitation. The study further shows that the effects of UMP vary with a change in static eccentricity, dynamic eccentricity, slip, and bearing stiffness. Finally, based on the results, a utilization plan of the developed methods is proposed. INDEX TERMS Induction motor, mixed eccentricity, rotordynamics, unbalanced magnetic pull. NOMENCLATURE Bẟ Air-gap magnetic flux density C Damping matrix c Correction factor edy Amplitude of dynamic eccentricity emix Amplitude of mixed eccentricity est Amplitude of static eccentricity F Magnetomotive force Fg Gravity force matrix Fub Unbalance force matrix Fump UMP force matrix Fx Net UMP force in horizontal direction Fy Net UMP force in vertical direction G Gyroscopic matrix Im Magnetizing current K Stiffness matrix kC,s Carter factor for stator kC,r Carter factor for rotor kC,tot Total Carter factor Kump UMP stiffness matrix lst Stator stack length M Mass matrix N Number of turns in a winding n Degree of polynomial p Pole pair number q Displacement vector ̇ Velocity vector ̈ Acceleration vector r Air-gap radius wfν Winding factor t Time variable
Calculation of unbalanced magnetic pull in induction machines through empirical method
IET Electric Power Applications
Unbalanced Magnetic Pull (UMP) is an important factor to study to improve the reliability of induction machines. UMP occurred in a machine when the existence of rotor eccentricity which causes an uneven distribution of magnetic flux in the airgap. In this study, a UMP damping coefficient is introduced to explain the UMP damping effect from the counteracting flux produced by a parallel connected cage rotor. An empirical method is proposed to calculate the UMP using the damping coefficient and an analytical model. Using the proposed method, a 4-pole and 8-pole squirrel cage induction machine with static eccentricity are investigated, which uses inputs from both Finite Element Analysis and experimental work. Then, the UMP calculation for a dynamic eccentricity with the extracted parameters is done to verify the empirical method. A slip control method is described which uses a UMP/Torque ratio to find the operating slip with the lowest UMP. Comparisons between results with and without slip control are made on both induction machines. It shows that great reduction of UMP can be achieved when the machine is lightly loaded.
Electromechanical interaction in rotordynamics of cage induction motors
Journal of Sound and Vibration, 2005
Eccentric rotor motion induces an unbalanced magnetic pull between the rotor and stator of cage induction motors. Recently, a linear parametric model of this eccentricity force due to the arbitrary rotor motion was presented. The purpose of this study is to combine this electromagnetic force model with a simple mechanical rotor model, and further, to demonstrate the rotordynamic response induced
The main proposal in this work is the theoretical- experimental analysis of the three-phase induction motor operation under different rotor slot inclination. The linear mathematical model(2) for the motor takes in consideration space harmonics of magnetomotive force (MMF) distribution. The motor feeding is done through a PWM inverter with sinusoidal modulation (PWSM), which means that the time harmonics must be also considered. This study allow us to draw some conclusions on how the slot rotor inclination can change the induction motor behavior.
Analytical analysis of rotor slot harmonics in the line current of squirrel cage induction motors
Journal of electrical engineering, 2006
This paper describes a new approach for analysing the effect of the space distribution of rotor bars in squirrel cage induction machines on the generation of rotor slot harmonics (RSH). An analytical expression of the stator current has been developed. The proposed expression is based on the linkage inductance expression derived using the winding function approach (WFA) and its decomposition into Fourier series. This approach describes the necessary relationship required for the presence of rotor slot harmonics and explains how the stator current is influenced by voltage unbalance. Simulations results have shown excellent match with theoretically predicted harmonic components.
Calculation of Radial Forces in Cage Induction Motors at Start—The Effect of Rotor Differential
IEEE Transactions on Magnetics, 2000
In this paper, the radial forces in an induction motor are calculated using finite element analysis. These radial forces (or unbalanced magnetic pull-UMP) are generated when the rotor is eccentric. The work illustrates the importance of higher winding harmonics and rotor differential leakage in the starting UMP. Examples of a 6 pole machine with 26 and 40 bar rotors show that increasing the bar number and air-gap length will reduce the UMP. Further studies are carried out using parameter variation and a 10-pole machine is also addressed, where experimental results exist, in order to validate the calculation.
2007
This paper presents the development of mathematical models and simulations of magnetic field, and mechanical vibration in a three-phase squirrel-cage induction motor. Its aim is to compare the vibration magnitude when the motor possesses different geometrical rotor-slot shapes. The cross sectional areas of two typical semi-closed slot shapes (rectangular and round shapes) are kept equally constant according to the IEEE standard. Under an assumption of sinusoidal motor excitation, the simulation works employ the finite element method (FEM) and the Newton-Raphson method to solve time varying nonlinear equations. The numerical solutions obtained indicate the electromagnetic force distribution over the motor cross sectional area. Such forces cause mechanical vibration in the motor. To evaluate this vibration, the displacement of stator inner perimeters was observed carefully. As a result, the round rotor slot gives 4.8% less vibration than the rectangular rotor slot does.
Air-gap force distribution and vibration pattern of Induction motors under dynamic eccentricity
Electrical Engineering, 2008
A method for determining the signatures of dynamic eccentricity in the airgap force distribution and vibration pattern of induction machine is presented. The radial electromagnetic force distribution along the airgap, which is the main source of vibration, is calculated and developed into a double Fourier series in space and time. Finite element simulations of faulty and healthy machines are performed. They show that the electromagnetic force distribution is a sensible parameter to the changes in the machine condition. The computations show the existence of low frequency and low order force distributions, which can be used as identifiable signatures of the motor condition by measuring the corresponding low order vibration components. These findings are supported by vibration measurements and modal testing. The low frequency components offer an alternative way to the monitoring of slot passing frequencies, bringing new components that allow to discriminate between dynamic eccentricity and rotor mechanical unbalance. The method also revealed a non linear relationship between loading, stress waves and vibration during dynamic eccentricity.