R.D. Findlay

Electro-mechanical analysis of the audible noise of an inverter-fed squirrel-cage induction motor

The audible noise emitted by a three-phase squirrel-cage induction motor fed by a frequency inverter is determined by the electromagnetic and mechanical behavior of the components of the drive. A computerized analysis method is described which uses theoretical as well as experimental data for predicting the audible noise spectrum components. Starting from the inverter voltage and the motor data, the electromagnetic forces are predicted and analyzed. The mechanical natural frequencies are considered, either in a theoretical (finite-element calculation) or in an experimental (modal analysis) methodology, in order to predict the vibrational behavior of the machine.<<ETX>>

Losses due to rotational flux in three phase induction motors

This paper discusses rotational losses and how they are produced in the core materials of induction motors. These losses are largely caused by flux that rotates in the plane of the machine laminations. This suggests that steel specification for applications to rotating machines should be given in terms of rotational loss data as a material characteristic, in much the same fashion as Epstein test results are provided for alternating losses. If a standardized test for rotational losses were to be used, steel producers could rationally investigate the effects of composition and processing variables. This is necessary in order to produce low loss steels for motor applications. Reduction of rotational losses in motor cores could significantly lower AC machine operating costs and contribute to the growing interest and design of high efficiency induction motors. The paper describes a test procedure for determining rotational losses in a sample. It then compares the results with standardized tests from an Epstein test procedure. It is seen that there are significant differences in loss results obtained for the rotational test versus the alternating current test. The authors have investigated a time harmonic finite element formulation utilizing Magnet 2D, a commercially available package. The paper includes a brief analysis of a typical problem using this tool. >

Stray Losses in Induction Machines: Part I, Definition, Origin and Measurement

The state of the art of stray load losses in induction machines is discussed with respect to definition, origin and effects, and measurement techniques. The historical search for a definitive evaluation technique for these losses is discussed together with the recognized criticisms of each approach. It is noted that although this subject has been the object of intensive investigations for more than seventy years there is still very little agreement on either the origin or evaluation of stray load loss. This paper discusses the major works in the area with the objective of delimiting the problem so that a rational approach for future work may be promulgated.

A Procedure for Illustrating the Effect of Variation of Parameters on Optimal Transformer Design

A procedure has been developed to illustrate the effect of parameter variation on the design of transformers in order to achieve minimum cost of production. It is shown that for a specified value of reactance there is a unique value of coil height for the three-in-line, three-phase transformer studied. With the choice of coil height established, there is also a unique value of primary turns yielding a minimum cost configuration for a specified flux density in the core leg, and a specified current density in the windings. The procedure illustrates also that there are many possible designs within a very small increment of cost. Hence standard wire sizes, steel gauges, and punch limitations can be accommodated. The process can be programmed on a microcomputer to illustrate the design procedure. In this form it is readily taught in option courses on power principles to senior undergraduates.

Some commentary on the choice of rotor bar material for induction motors

Almost any metal can be used in the design of the squirrel cage of an induction machine. Obviously lead is not used because of its low melting point and magnesium because of its low flash point. Others are rejected because of the cost, lack of strength, resistance, thermal conductivity, or manufacturability including ease of joining. Basically, the choice fails to two materials, aluminum and copper and their alloys. For lowest cost of manufacture, especially for mass production, aluminum surpasses copper. In order to determine which of the two materials is the best takes the combined skills of an electrical, a mechanical and a metallurgical engineer. This paper discusses some of the considerations in the choice of rotor bar material. >

Comparison of techniques for measurement of shaft currents in rotating machines

Irregularities in the magnetic circuits of motors may result in spurious voltages that lead to shaft currents through the shaft, bearings, bearing supports and closing through the machine framework. The IEEE Standard Test Procedure for Polyphase Induction Motors and Generators discusses the shaft current and presents a measurement method for recording either the voltage across the ends of the shaft or the current. This paper discusses an alternative measurement approach and its application to the identification of shaft current in a large induction machine. Procedures were developed for measuring the shaft current. The procedures include the shunt current method and the measurement using the Rogowski coil. Only the Rogowski coil measurement yields accurate measurements of shaft currents whereas the other method either yields inaccurate measurement or may result in other problems for the machines integrity. The theory and justification for the superiority of the Rogowski coil method is presented along with supporting test data.

Measurement of rotational iron losses in electrical sheet

A large portion of the stator core of a rotating electric machine is subjected to magnetic flux that rotates in the plane of the laminations. Iron losses under rotating flux conditions can differ considerably from those due to alternating fields. It has long been suspected that this difference may be responsible for some of the discrepancy between predictions of no-load core loss at the design stage and values obtained by factory test. At present, no international standard exists for determining iron losses due to rotational flux. This paper describes an apparatus capable of making such measurements and briefly reviews the state of the art in this area of research. Measured loss curves are given for seven grades of nonoriented (NO) electrical sheet.

Sparse formulation of the transient energy function method for applications to large-scale power systems

The authors present a sparse-matrix formulation of the transient energy function method, to analyze the transient stability of large-scale power systems. The proposed formulation preserves the original network structure and avoids network reduction. Consequently, all matrices used in the calculations are very sparse, leading to a significant saving in computational time as compared to the reduced-network-formulation approach. The proposed solution can handle very-large-scale power systems which are beyond the scope of the current reduced-network techniques. The sparse formulation is applied to several practical utility systems of up to 300 generators and 1724 buses. It is concluded that the results obtained demonstrate the superiority and potential of the sparse formulation in the stability analysis of large-scale power systems. >

Shaft current in AC induction machine: an on line monitoring system and prediction rules

This paper looks at a method of on-line shaft current monitoring system. Fault conditions such as air gap eccentricity, slot harmonics and rotor broken bars or end ring result in asymmetrical flux pattern around the shaft, hence resulting in shaft voltage. Bearing failures can occur when the voltage level exceeds the threshold of the oil film. The importance of the oil film is described. The proposed method of shaft current is noninvasive, reliable, portable and low cost. Causes of the shaft current are described and prediction rules introduced.

Parasitic torques in saturated induction motors

A method for analyzing the performance of induction machines for parasitic torques is described. Such torques arise from the interactions of fundamental and harmonic components of air flux density due to geometrical considerations such as slotting, and saturation effects. Parasitic torques arise under balanced sinusoidal drive conditions. However, they are greatly exacerbated under nonsinusoidal drive conditions. The technique described for analysis is applicable to both cases. >