Timo P. Holopainen

Electromagnetic forces in cage induction motors with rotor eccentricity

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. The multi-slice, time stepping finite element analysis is used for solving the magnetic field and the forces are calculated from the air gap by a method based on the principle of virtual work. The forces are measured for a test motor equipped with active magnetic bearings. The active magnetic bearings are used to generate the eccentric rotor motions and also to measure the electromagnetic forces. The measured and calculated forces show relatively good agreement. The results show that superposition is valid when determining forces. This means that the calculated forces of different motions can be combined and the result is the force of the combined motion.

Effects of saturation on the forces in induction motors with whirling cage rotor

The effects of magnetic saturation on the radial magnetic forces in induction machines when the rotor is performing a cylindrical circular whirling motion are studied in this paper. Impulse method in the finite element analysis is used to calculate the forces and eccentricity harmonics from both the air-gap flux density and rotor currents. The forces are studied as a function of supply voltage in order to find the effects of saturation on them. The maximum radial force is found to be limited by saturation, which also couples the eccentricity harmonics together.

Spatial linearity of an unbalanced magnetic pull in induction motors during eccentric rotor motions

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.

Effects of equalizing currents on electromagnetic forces of whirling cage rotor

The electromagnetic force acts between the rotor and stator of an induction motor, when the rotor is performing cylindrical circular whirling motion with respect to the stator. The nonsymmetric flux distribution due to the eccentricity induces circulating currents in the rotor cage and parallel branches of the stator winding. These currents tend to equalize the flux distribution, and by doing this, reduce the electromagnetic force and change the direction of the force from the direction of the shortest air gap. Impulse method is utilized in the finite element analysis to calculate the frequency response of the force and the eccentricity harmonics of the flux density in the air gap and the circulating currents. The frequency responses were calculated for the test motors with different stator connections and with and without rotor cage. The frequency response function of the force is calculated for two induction motors with different geometry, one with open and another one with closed rotor slots. The results show that the equalizing currents in both parallel branches in the stator winding and rotor cage damp strongly the amplitude of the force. The damping effects depend on the whirling frequency, loading condition and the geometry of the machine.

Analysis of iron losses on the cutting edges of induction motor core laminations

A 37 kW induction motor is modeled numerically taking into account the deterioration of the magnetic material properties at the cut edges of the core laminations. The magnetization properties and specific loss curves of the damaged edge and the intact material have been estimated from measurements published earlier. The deteriorated edges affect the iron losses of the machine in two distinct ways. First of all, the reduced permeability at the edges causes higher local flux densities elsewhere, which increases the losses. On the other hand, the cutting also has a direct effect on the specific loss density at the edges. By numerical simulations we show that the main effect is the latter one, and that the effect of decreased permeability remains low. The iron losses increase up to 37.6 %. The torque and active power of the machine are not much affected by the cutting, although also the current and resistive losses slightly increase due to the decreased permeability at the edges.

Identification of magnetic properties for cutting edge of electrical steel sheets

Electrical steel sheets of motors and generators are usually shaped to the final form by punching. The punching and other cutting processes generate large plastic deformations and residual stresses. These are known to deteriorate the magnetic properties of the edge region. However, the characterization of this deterioration in the form of magnetic properties is missing. The main aim of this paper is to propose a method to identify the magnetic properties of the edge region based on experimental results. This approach is demonstrated by using previously presented test results for magnetic properties of rectangular strips. The width of these strips is varied, and thus, the share of the edge region to the whole can be used as a variable. Based on this variation, a simple model is developed and the model parameters fitted to the experimental results. The correspondence between the calculated and experimental results is good.

Space-Vector Models for Torsional Vibration of Cage Induction Motors

Electromagnetic response of induction motors to torsional vibration is studied using space-vector and finite-element (FE) models. Comparison of the results shows that the skin effect in the rotor bars plays an important role, and the basic space-vector model has to be updated to a double-cage or triple-cage model. The results of the higher order models agree well with those of the FE analysis as long as motors with semiopen rotor slots are studied. In case of closed rotor slots, the saturation of the iron bridges on top of the bars causes problems in modeling accuracy.

Space-vector models for torsional vibration of cage induction motors

Electromagnetic response of induction motors to torsional vibration is studied using space-vector and finite-element (FE) models. Comparison of the results shows that the skin effect in the rotor bars plays an important role, and the basic space-vector model has to be updated to a double-cage or triple-cage model. The results of the higher order models agree well with those of the FE analysis as long as motors with semiopen rotor slots are studied. In case of closed rotor slots, the saturation of the iron bridges on top of the bars causes problems in modeling accuracy.

Numerical Identification of Electromagnetic Force Parameters for Linearized Rotordynamic Model of Cage Induction Motors

The electromechanical interaction in cage induction motors induces additional forces between the rotor and stator. Recently, a linear parametric model was presented for these forces, and the corresponding model was combined with a mechanical rotor model. The electromagnetic system is usually non-linear due to the saturation of magnetic materials. Thus, the effective identification of the force parameters is crucial. Initially, the parameters were identified numerically from the response induced by the whirling rotor. Later on, a method based on the impulse response analysis was presented. The aim of this study was to present the theoretical background of this impulse method, and to present some useful additional features. The derived equations show the mathematical equivalency of this impulse method and the previous whirling method. The results show that the impulse method is numerically effective. In addition, the feasibility of thus obtained simple electromechanical rotor model was demonstrated.Copyright

Additional Losses of Electrical Machines Under Torsional Vibration

Electric drive trains have a torsional rigid-body vibration mode at a small, nonzero frequency. If an excitation occurs close to this frequency, the vibration amplitude may grow large and the electrical machine may suffer from significant additional losses. Standards set constraints on the oscillating torque in the shaft coupling and on the harmonics of line current. They indirectly limit the vibration amplitude and losses of the machines. Time-discretized finite-element analysis was used to study the losses of six induction and six synchronous machines under torsional vibration restricted by the constraints above. All the machines were supplied from sinusoidal voltage sources. In the worst cases of the induction motors, the vibration increased the electromagnetic total loss by about 20%. The constraints on synchronous machines are milder than those for induction machines. In this case, the maximum increase of the loss was 75%. The limit on the harmonic currents is essential from the loss point of view. Without this limit, the additional loss at the rigid-body resonance would lead to a temperature rise high enough to destroy the insulation system of the machine. The method of loss analysis was validated by measured results.