The authors attempt to identify the various causes of stator and rotor failures in three-phase squirrel cage induction motors. A specific methodology is proposed to facilitate an accurate analysis of these failures. It is noted that, due to the destructive nature of most failures, it is not easy, and is sometimes impossible, to determine the primary cause of failure. By a process of elimination, one can usually be assured of properly identifying the most likely cause of the failure. It is pointed out that the key point in going through this process of elimination is to use the basic steps of analyzing the failure class and pattern, noting the general motor appearance, identifying the operating condition at the time of failure, and gaining knowledge of the past history of the motor and application. >

Cause and analysis of stator and rotor failures in three-phase squirrel-cage induction motors

During the past two decades, tremendous improvement has been made in the design and manufacture of stator windings. This has been accomplished primarily through the development of improved insulation materials and treatment processes. As a result, the expected life from a thermal, dielectrical, mechanical, and environmental standpoint has been significantly increased. The rotor design and manufacturing remains basically unchanged. A survey of warranty data and service facilities suggests that rotor failures now account for a larger percentage of the total induction motor failures. The majority of these failures are caused by various stresses which act on the rotor assembly. These various stresses and how they affect the life of the motor and contribute to premature failure are discussed.

Rotor Failures in Squirrel Cage Induction Motors

A brief design review of permanent magnet synchronous motors has been presented. A procedure has been developed to predict the steady state and dynamic performances of a brushless permanent magnet synchronous motor. Finite element analysis has been combined with a lumped parameter circuit model in order to provide satisfactory engineering information. To this end, two coordinated steps are involved. One is to develop a unified lumped parameter circuit model for both steady state and dynamic analysis. The second step is to extract the individual lumped parameters from finite element solutions based on corresponding equivalent circuits, each with a pre-determined topology. The proposed techniques have been experimentally verified in a laboratory permanent magnet synchronous motor.

Analysis of brushless permanent magnet synchronous motors

The modern synchronous reluctance motor drive is illustrated as a new incoming technology solution. High anisotropy motor design is briefly summarized. The obtainable performance in terms of torque-per-volume is then compared to that of brushless and induction motors. With reference to traditional vector current control, the peculiarities of the synchronous reluctance motor drive are shown. Moreover, the effect of some second-order control aspects are evidenced, together with the possible advantages of adoption of flux-observer based schemes. Finally, both research and industrial trends in this field are summarized. >

The synchronous reluctance solution: a new alternative in AC drives

The synchronous reluctance solution: a new alternative in A.C. drives

Characteristics of Induction Motors with Permanent-Magnet Excitation

The development of the idea of magnetic reluctance is sketched, and the mathematical and experimental methods of determining it are first discussed. On the theoretical side it is the theory of complex quantities which leads to numerical results. The electric field of an electric generator is next analyzed and made to depend approximately upon the reluctance of two-dimensional magnetic fields. This study indicates as preferable the one shown in Fig. 6 at the left. The reluctance of these various magnetic fields is determined by experiments made on sheets of high-resistance metal cut in suitable forms, and by mathematical computation. These results are put into charts and formulas convenient for the use of those interested in electrical design. The results of the tests are proved to be more accurate than most of those already published. In particular, present leakage flux calculations are most in error. Of more general interest are the plates of the shape of the magnetic lines of force in the various parts of electrical machinery. In particular, Figs. 42 to 48 include some new results. The flux lines in the corners of transformers and induction motors are there shown, and the exact solution to the much-discussed sine-wave alternator problem is there given. The paper is divided into (I) Introduction, (II) Historical Development, (III) Description of Experiments, (IV) Accuracy of Results, (V) Conclusions. There are two appendices giving some mathematical details, for reference purposes.

The Reluctance of Some Irregular Magnetic Fields

The deep-bar rotor may be defined as a single-cage rotor in which skin effect plays a prominent part. With rotor slots nearly one inch deep, skin effect reduces starting current and increases starting torque so that in many ratings this type of motor has superseded the older shallow cage motor. This paper will endeavor to supply the theory of the tapered bar and of bars with a tapered section.It also will endeavor to improve the theory of the round bar. A graphical method for computing the impedance of any shape of bar will be given.The formulas developed indicate that further improvements should result if the bar is somewhat enlarged near the slot opening.It will be hown that: 1) The tapered bar is superior to the rectangular bar, provided the narrow portion is near the top. 2) Further improvements should be sought among bars of compound shape. 3)The slot constant,for a round slot is close to 0.72when running.

A Contribution to the Theory of the Deep-Bar Induction Motor

THIS SUBJECT is timely because of the recent improvement of reluctance motors to a point where they have considerable sales volume. The pull-in criterion is related to the maximum inertia that can be pulled into step for a given frame size in the rating set on that frame, expressed as a graph or by an equation. This paper derives this equation. A reluctance motor is a synchronous motor without rotor excitation. The pull-in criterion for synchronous motors with d-c excitation was studied by Edgerton and Fourmarier,1 and by Shoults, Crary, and Lauder.2 Although their results cannot be applied to the reluctance motor directly, they were found useful in checking certain steps.

Pull-in criterion for reluctance motors

The chief topic in this paper concerns field structures in synchronous machines which simulate an anisotropic material, which have a greater permeance in the direct axis than in the quadrature axis. Superficial anisotropy is also considered alone and in combination with internal anisotropy. It is shown that an increase in reluctance torque may be secured by due attention to both superficial and internal construction. No new method of securing superficial anisotropy is given but, in a brief discussion, a short method of estimating it is proposed. The internal structure proposed for analysis, is a slitted rotor core simulating an anisotropic material.

John F. H. Douglas

Papers

Characteristics of Induction Motors with Permanent-Magnet Excitation

The Reluctance of Some Irregular Magnetic Fields

A Contribution to the Theory of the Deep-Bar Induction Motor

Pull-in criterion for reluctance motors

The Theory of Anisotropic Field Structures in Synchronous Machines [includes discussion]