Nick Stranges

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. >

Instrumentation, testing, and analysis of electric machine rotor steady-state heating

It is not a simple task to measure temperatures on the rotors or rotor components of large induction and synchronous machines during steady state operation. The single greatest obstacle to obtaining this data is related to getting the data from the rotating element. Other problems are associated with instrumentation noise, sensor type and attachment. This paper describes a number of techniques that have been used by the authors and their associates over the last 30 years. Test and calculated temperatures are presented for one machine. Also, test data from a number of machines is presented that show for this particular population of enclosures and ventilation schemes the steady state temperatures at the design load can range from the value of the stator rise to greater than 70/spl deg/C higher.

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.

Methods for predicting rotational iron losses in three phase induction motor stators

Iron losses in electrical machines are difficult to predict. Discrepancies between tested and calculated no-load core losses are sometimes large. In induction motors, a large portion of the stator core is subjected to flux that rotates in the plane of the laminations. Losses due to rotating flux differ from those observed under alternating flux conditions. This paper describes direct approaches for coupling rotational iron loss measurements with finite element analysis (FEA) results that yield the distribution of rotational flux in induction motor cores. Using these methods, the prediction of no-load core loss can be made to include the effects of rotational iron losses.

Operation on unbalanced voltage: one motor's experience and more

Derating a motor for operation on an unbalanced voltage system is still much of a black art although National Electrical Manufacturers Association and International Electrotechnical Commission have generic derating charts for this application. Numerous papers have been written in an attempt to provide a theory for this operating mode and the losses that may be generated. This paper will discuss the instrumentation and testing of a four-pole 1120-kW induction motor that was tested at three different levels of unbalanced voltage. Results will be presented on the stator and rotor heating, the speed-torque and locked rotor performance, as well as noise, vibration and shaft voltage. The problem has also been analyzed using time-stepping finite-element analysis

The effect of surge testing on the voltage endurance life of stator coils

Surge tests with short rise times are used to ensure turn insulation integrity in dry vacuum-pressure impregnated (VPI) coils. The use of this test migrated from the evaluation of low voltage random wound stators and fully processed generator bars. The dielectric properties of dry (unimpregnated) VPI coils are not fully developed. Application of a high voltage transient electric field in uncured or partially processed coils may initiate insulation damage. This damage may shorten the life of the insulation system after complete impregnation and cure. This paper presents a series of laboratory tests and finite element simulations examining the applied test voltage and number of applied pulses as factors in determining the risk of using surges as a manufacturing proof test on unimpregnated coils. Potential failure initiation sites can be linked to the test voltage as a more significant factor than the number of pulses applied. The sympathetic voltage response in a single coil as a function of location within a winding is described for a 20-pole stator undergoing green surge testing as part of a quality assurance program. Voltage endurance testing and sample dissection data are presented to show that the voltage endurance test life of the coils after complete processing is shortened by the surge test.

How design influences the temperature rise of motors on inverter drives

Drives and electric motors are sometimes paired in order to improve efficiency and process control or to eliminate gears and reduce maintenance costs. Sometimes the addition of the drive to an existing installation has not been the success that was originally envisioned and substantial derating has occurred or the original motor or drive replaced. One industry standard for product certification requires that there be a 30/spl deg/C margin in the temperature rise if a motor intended for use on a drive is acceptance tested on sinusoidal power. The intent of this temperature margin is to allow for the additional heating losses due to inverter harmonics. Machine design, especially the ventilation, has a significant impact on the difference in temperature rise due to the increased harmonic losses. This paper discusses a number of different ventilation methods present in motors. The results of tests done on machines using the same load on both sinusoidal power and inverters are also presented.

Iron losses in AC machines due to rotational flux conditions

Core losses in rotating AC machines are largely caused by flux which rotates in the plane of the machine laminations. Reduction of rotational losses in motor cores could greatly lower AC machine operating costs. This paper discusses rotational losses and how they are produced in AC machines. Steel producers would like a standardized test for rotational losses in order to investigate the effects of composition and processing variables. This is necessary in order to produce low loss steels for motor applications. Motor manufacturers would like rotational loss data as a material characteristic in the same fashion that Epstein test results are provided for alternating losses. The development of analysis tools for determining the amount of rotational flux and degree of polarization in a machine core is also imperative. The authors have investigated a time harmonic finite element formulation utilizing Magnet 2D, a commercially available package.<<ETX>>

Predicting rotational flux distributions in three phase induction motor stators

A substantial portion of the stator in large induction motors is subjected to magnetic flux that rotates in the plane of the laminations. This flux is nearly circularly polarized at the roots of the stator teeth, and is elliptically polarized at the base of the stator slots. Iron losses due to rotational flux differs from that found under alternating flux conditions. This paper describes results obtained from two finite element approaches, a time-harmonic method and a time-stepped magnetostatic solution.

A comparison of two finite element methods for determining flux density polarization in induction motor cores

This paper compares two forms of the finite element method (FEM) for the purpose of studying rotating magnetic flux in machine cores. The authors compare the use of a series of time-stepped magnetostatic solutions and a single time-harmonic formulation. The development of analysis tools for determining the amount of rotational flux and degree of flux density polarization in a machine core is of great importance to machine designers. A significant portion of the total core loss in an AC machine is caused by flux which rotates in the plane of the machine laminations. This rotating flux results in a polarized flux density at the roots of the stator teeth and all along the inner periphery of the stator core. The total iron loss in the material caused by rotating magnetic flux may be much larger than the iron loss due to alternating flux. A method of determining the amount of rotational flux will aid the designer in predicting no-load core loss and locating potential hot spots in the machine core.