Skewed Slots Research Articles

Numerical models for rotor cage induction machines using finite element method

In this paper, the authors propose and compare 2D and 3D finite element models to study cage induction machines. Both possibilities of a skewed and unskewed rotor slots are considered. For electromagnetic equations, they use the vector potential formulation. The magnetic and electric circuit equations are solved simultaneously. The movement is taken into account by an air gap element in 2D and slip surface in 3D. In the 2D model, the skew of the rotor bars is considered by solving a succession of 2D problems.

Magnetic field analysis in condenser motor considering skewed slot effects

This paper describes a useful method to analyze the nonlinear magnetic fields in a condenser motor considering skewed slot effects by 2-D FEM coupled problems. In this method, the exciting circuit equations, the rotor circuit equations including the currents in the end ring and the magnetic field equations are solved simultaneously. Therefore the input currents, the rotor bar currents and the currents in the end ring are obtained directly. Moreover this method is developed to consider the skewed slot effects by modifying the rotor circuits. The validity of this method is shown by comparison between the computed results and measured ones.

An Analysis on Performance Improvement of Electrostatic Motor by Skewing Electrodes.

The authors have developed high-power electrostatic motor called Dual Excitation Multiphase Electrostatic Drive (DEMED). This motor possesses 230W/kg of power/weight ratio and this is much higher than those of conventional electromagnetic motors. However, the motor has some problems with the quality of its output, such as low positioning resolution or much viblations, since its force generation is rippled. In case of electromagnetic motors, torque or force ripple is removed by skewing slots of their armatures. Similaly, the force ripple of DEMED is expected to be removed by skewing their electrodes, instead of slots. In this paper, the effect of skewing electrodes of DEMED is analyzed, and the analytical results show that appropriate skewing will suppress the ripple considerably and will raise effective force of DEMED.

Pulsating torque minimization techniques for permanent magnet AC motor drives-a review

Permanent magnet AC (PMAC) motor drives are finding expanded use in high-performance applications where torque smoothness is essential. This paper reviews a wide range of motor- and controller-based design techniques that have been described in the literature for minimizing the generation of cogging and ripple torques in both sinusoidal and trapezoidal PMAC motor drives. Sinusoidal PMAC drives generally show the greatest potential for pulsating torque minimization using well-known motor design techniques such as skewing and fractional slot pitch windings. In contrast, trapezoidal PMAC drives pose more difficult trade-offs in both the motor and controller design which may require compromises in drive simplicity: and cost to improve torque smoothness. Controller-based techniques for minimizing pulsating torque typically involve the use of active cancellation algorithms which depend on either accurate tuning or adaptive control schemes for effectiveness. In the end, successful suppression of pulsating torque ultimately relies on an orchestrated systems approach to all aspects of the PMAC machine and controller design which often requires a carefully selected combination of minimization techniques.

Magnetic field analysis in shaded pole motor taking skewed slot effects into account

In this paper the authors present a method to analyze the nonlinear magnetic fields with accuracy using two-dimensional finite element method in a shaded pole motor. The rotor circuit equations are coupled with the magnetic field equations, to estimate the rotor currents directly. Likewise the shading coil currents are calculated directly by coupling problems. This method is developed by modifying the rotor circuit equations to analyze the magnetic fields in the motor considering skewed slot effects. When the computed results by proposed method are compared with the experimental values, good agreements are obtained. >

Effect of Casing Treatment on Aerodynamic Performance of a Contrarotating Axial Compressor Stage

The paper reports the results of an experimental investigation on the effect of casing treatment on aerodynamic performance of a low-speed contrarotating axial compressor stage having a hub-tip ratio of 0.667. Three types of casing treatments, namely axial slots, axial skewed slots and circumferential grooves, have been examined. Flow structure downstream of the rotors is examined for treated casing and compared with solid casing. The results show a significant improvement in stall margin and efficiency during off-design operation of a contra stage.

Experimental verification of stray losses in cage induction motors under no-load, full-load and reverse rotation test conditions

The stray load losses of polyphase squirrel-cage induction motors have hitherto rarely been measured at rated voltage and full-load, owing to the great difficulties in achieving the required measuring accuracy; instead, mostly the reverse rotation test is used. This paper primarily deals with the question whether the reverse rotation test yields representative results. It is also examined to what extent it is possible to predetermine the stray losses by means of a series of new investigations on space harmonics of flux density and harmonics of rotor current. For this purpose the stray losses of a four-pole 11 kW polyphase induction machine with semi-closed and open stator slots as well as rotors with a varying number of unskewed and skewed slots are measured under no-load and full-load conditions and in the reverse rotation test and computed for machines with unskewed slots.

Effects of Slotted Hub and Casing Treatments on Compressor Endwall Flow Fields

An experimental investigation was carried out to examine the influence of a rotating slotted hub treatment on a compressor stator passage endwall. Detailed mapping of the three-dimensional unsteady velocity field near the hub was done for a smooth hub and a hub having axial skewed slots. With the smooth wall, the velocity measurements indicate that a large region of blockage occurs near the rear of the blade passage. This blockage is seen to be associated with the hub endwall rather than either the suction surface or the pressure surface of the blade. With axial skewed slots, the blockage is markedly decreased, the stalling flow coefficient is reduced, and the peak pressure rise greatly increased. Due to the axial (rear-to-front) flow in the slots, there is a region of fluid removal (from the endwall) near the rear of the passage and a strong fluid injection, or jet, near the front. The data appear to show that this flow removal is an important part of the effect on stall margin found with hub and casing treatment. Furthermore, the data indicate that it is the (dissipation of the) high velocities in the jet that is the prime source of the efficiency reduction associated with this treatment.

Flow Phenomena in Compressor Casing Treatment

An axial skewed slot casing treatment has been tested over the tips of an isolated low-speed rotor with a hub-tip ratio of 0.4. An improvement in stall margin of 22 percent at nominal clearance was obtained. Detailed measurements of the loss pattern downstream of the rotor were made with a hot wire. Measurements were also made in the blade passage with a traversing gear moving with the rotor and in the treatment slots themselves using hot wires. The tentative conclusion is that unsteady effects in the slot are of secondary importance. Of primary importance is the selective removal of high absolute swirl, high loss fluid from the endwall near the trailing edge of the pressure surface of the blade, and reintroduction of this, with absolute swirl direction reversed, near the blade leading edge.

Simple rules for the elimination of cogging torque in squirrel-cage induction motors

Cogging torques can be made negligibly small by a correct choice of design parameters for induction motors. The method of analysis has been described in earlier papers, and it is shown here that simple rules can be derived for the effective elimination of the cogging tendency without recourse to slot skewing. The rules are not so restrictive that they cannot be applied to most machines in conjunction with other criteria for the choice of slot numbers.

Effect of skewing slots on flux distribution in induction machines

Although skewing is practically the rule for all but the largest induction motors, its effect on the flux distribution has received little attention. It is shown by experiment that the effect of skewing can be very significant and results in a poorer performance. Some of the advantages which are commonly believed to accrue from the use of skew are fictitious if a proper choice of slot combination is made; these are discussed.

Effect of skewing slots on flux distribution in induction machines

Although skewing is practically the rule for all but the largest induction motors, its effect on the flux distribution has received little attention. It is shown by experiment that the effect of skewing can be very significant and results in a poorer performance. Some of the advantages which are commonly believed to accrue from the use of skew are fictitious if a proper choice of slot combination is made; these are discussed.

Influence of Slot Skewing on the Characteristic of the Squirrel Cage Induction Motor

Influence of Slot Skewing on the Characteristic of the Squirrel Cage Induction Motor

Skewed slots in induction motors

INDUCTION MOTOR slots are usually skewed to prevent “cogging” and noise on load, but indiscriminate skew irrespective of number of poles, especially when this number is large, leads to very large reduction of short circuit current and motor performance. The reduction of short circuit current is due not so much to increase of leakage reactance as to an unbalanced component of mutual flux.