Martin Ranlöf

Core Loss Prediction in Large Hydropower Generators: Influence of Rotational Fields

In this paper, we discuss the accuracy of several core loss models as compared to experimental data obtained from measurements on commercial hydropower generators. Our aim in this paper is twofold. First, we emphasize the problem of total core loss prediction by comparing the core loss figures obtained by different loss prediction schemes with measured no-load losses. Second, we examine the predicted net effect of bidirectional flux, i.e., the rotational loss. We show that the average degree of rotation in the stator core, and hence the rotational loss, is closely related to the stator slot geometry.

Reduction of Unbalanced Magnetic Pull in Synchronous Machines due to Parallel Circuits

This paper describes an analysis of the damping of unbalanced magnetic pull (UMP) in salient pole synchronous machines due to parallel stator circuits. Measurements of the UMP and currents in the parallel circuits have been performed on a 12-pole generator with the stator displaced relative to the rotor, creating a static eccentricity, and the stator winding connected as one circuit or two parallel circuits. Experiments were done at no-load conditions. Two numerical studies of the force reduction are also presented, one using a finite-element code and one using a permeance model. A good correspondence between measured and simulated forces is obtained. Results indicate that the reduction of UMP is strongly dependent on the direction of unbalance relative to the line of separation of the stator circuits. Eddy currents induced in the rotor during operation were found to reduce the standstill UMP with more than 20%.

Design and construction of a synchronous generator test setup

This paper describes the design considerations underlying the construction of a flexible synchronous test generator setup to be used for electrical, magnetic and mechanical measurements. Practical solutions to enable such measurements are also provided. Damper bar currents, stator winding currents, air gap flux density and mechanical forces can be measured for different rotor eccentricities as well as for different damper bar and stator winding configurations. A simple comparative test of the effect of inter pole damper connections on the slot harmonics during no load has been performed.

Form Factors and Harmonic Imprint of Salient Pole Shoes in Large Synchronous Machines

Abstract This article presents a review of the connection between salient pole shoe shapes and form factors that model saliency affects in synchronous machine design programs. Additionally, the impact of the pole shape on the harmonic contents of the air-gap flux density waveform is examined. The properties of different pole shoes are assessed with 2-D finite element analysis in the working environment of a large hydro-electric generator topology. The influence of magnetic saturation is particularly emphasized. Linear models for the calculation of form factors are established. It is concluded that the choice of pole shoe design typically must be a compromise between a high field and direct axis form factor and a low total harmonic distortion.

The rotating field method applied to damper loss calculation in large hydrogenerators

In this paper, we discuss key aspects of analytical rotating field models that can be used to estimate the damper losses in large hydrogenerators during steady load conditions. Model features of particular importance, like the saturation permeance and the structure of the damper circuit are described in greater detail. Damper currents extracted from the analytic model are found to be in good agreement with currents obtained from coupled field-circuit 2-D finite element calculations. The applicability of the model for damper current prediction during steady unbalanced load conditions is also demonstrated.

Finite Element Analysis of a Permanent Magnet Machine with Two Contra-rotating Rotors

Abstract Electrical machines with contra-rotating rotors find applications in direct-drive energy conversion systems and in ship propulsion systems that employ hydraulic machinery with two contra-rotating stages. In this article, the operating characteristics of permanent magnet machines with two contra-rotating rotors moving on opposite sides of a common stator core are studied via two-dimensional finite element analysis. Particular attention is devoted to the special flux distortion patterns that arise in contra-rotating machine topologies. The study suggests that the introduction of a screening iron core between the rotors provides for negligible flux distortion levels during contra-rotating synchronous operation of the two rotors.

Simulation of a radial flux permanent magnet generator with two contra-rotating rotors

In this paper, the no-load operational characteristics of radial-flux permanent magnet generators with two contra-rotating rotors are addressed. Three different radial flux machine topologies have been qualitatively studied through a sequence of stationary two dimensional finite element field solutions. It is shown that the peak value of the induced phase electromotive force during synchronous operation depend on the angular positions of the two rotors relative a fixed reference in the stator winding.

Comparison of field and circuit generator models in single machine infinite bus system simulations

This paper compares the transient response of a finite element salient pole synchronous generator model connected to an infinite network bus to that of a system model based on an equivalent circuit representation of the machine. The characteristics and operating conditions of the two models are made equal as far as possible by thoroughgoing initiation and by using the finite element model for estimation of the circuit model parameters. It is found that the circuit model underestimates the stiffness and damping properties of the machine in comparison with the electromagnetic model, which largely is believed to be attributable to inadequate representation of the rotor circuits. The results raise questions regarding some common applications of equivalent circuit models.