Eddy Current Brake Research Articles

Influence of different magnetic field profiles on eddy- current braking

The formulas usually employed to describe the forces acting during eddy-current braking contain only one numerical value for magnetic-field strength at the site of the eddy-current disk, namely, its maximum or average value above the magnetic poles. The author investigates the influence of different magnetic-field profiles at the site of the eddy-current disk on braking force. For this purpose a number of magnetic systems using barium-ferrite magnets, which supplied different magnetic fields in the eddy-current disk, were developed. These fields varied from a nearly straightforward sinusoidal shape to fields which existed practically only at the poles, beyond which they dropped very steeply. The braking forces measured at these systems are compared with calculated braking forces. In the calculations a formula which establishes a relationship between the measured magnetic-field profile and the braking force is used. The formula was developed for a rectangular, a cosine and a cosine-squared distribution of magnetic-field strength perpendicular to the measured magnetic-field profile, allowing for the boundary conditions prevailing at the edge of the eddy-current disk.

General theory of eddy-current couplings and brakes

The general steady-state theory of eddy-current coupling is extended and new material included. The differences in operation between salient-pole and inductor-type eddy-current couplings are shown and the limitations of inductor couplings emphasised. The peak torque of the inductor coupling is shown to be roughly half that of the Lundell machine and to occur at a much higher slip, giving considerable difference in low-slip performance. Tests taken on an experimental model, which are given in some detail, are shown to verify the theory. The appendix discusses the optimisation of eddy-current couplings of all types, based on the theory given in the paper. This gives the best airgap configuration for maximum low-slip torque, taking into account saturation and the presence of a parasitic airgap. The best pole shapes for torque production are also derived.

An Experimental and Theoretical Study of Eddy-Current Couplings and Brakes

The steady-state theory of eddy-current couplings with homogeneous ferromagnetic drums is derived and shown to be valid by tests on an experimental coupling. The effect of machine parameters on peak-torque conditions and a normalized torque-speed relation are given. The flux-density distribution across the pole face under load was measured and correlated with measured torque and speed. Measurements of drum current-density and flux-density distribution through the drum thickness are also given.

Design of an eddy-current brake for a sodium-cooled reactor

Eddy-current brakes have proved very effective for throttling sodium flow in a sodium-cooled nuclear power reactor, thus maintaining a constant temperature gradient for afterglow conditions. Design steps and calculations are presented.

Strong Eddy-Current Applications of Permanent Magnets

Small eddy-current brakes for kwh meters have a practically linear torque-speed characteristic, because the induced current increases proportionally to the low relative speed between eddy-current disk and magnetic poles. In bigger designs a considerable reaction of the induced-current field shifts the eddy-current paths in the direction of movement, so that the electrodynamic force between current and magnetic poles deviates from the linear relation, reaches a maximum at a critical speed, and decreases asymptotically to zero at infinite speed, whereas the dissipated power becomes a maximum. Some investigations were made into the behavior of permanent magnets with a water-cooled setup with p.m. rotor and an air-cooled device with p.m. stator, both of cylindrical shape with 11.8 in. effective diameter and an axial length of 5.5 in. and 6 in., respectively. Maximum torques of 410 ft-lb and 304 ft-lb were obtained. Pole pieces may sometimes increase the static flux, but do not always improve the dynamic behavior. The ideal solution might be obtained with a high-induction magnet without pole pieces, but the induction in available materials is limited by the demagnetizing effect on the free pole top. Possible applications could be found in a retarder for heavy trucks in mountainous districts and in a flexible coupling, slipping with decreasing torque when overloaded.

An eddy-current braking crane-hoist controller with variable brake excitation

An a-c crane-hoist controller for a wound-rotor induction motor and an eddy-current brake is described. The brake serves as an artificial load for the motor. Emphasis is placed on the desirability of using speed-responsive excitation for the brake to cause its torque to increase so rapidly with speed that flat speed-load characteristics are obtained. A magnetic amplifier responsive to a signal voltage taken from the secondary circuit of the motor is a simple and reliable means for controlling the brake excitation.

Eddy-current phenomena in ferromagnetic materials

INTEREST in eddy currents in solid iron masses has kept pace with the development of electromagnetic devices generally.1 The first problems arose in the design of eddy-current brakes for flywheels. Then the use of iron wires for telephone lines and iron rails for the supply of power to a-c locomotives led to new problems. It was recognized that the saturation of the iron was an important factor, and many authors have presented their theories to take saturation into account. Following the appearance of Rosenberg's work,2 others <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> ‾ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> have made an academic problem of substantiating it or elaborating upon it. Recently the damping effect of eddy currents in the solid yokes and pole pieces of d-c machines has become important. Still more recently, it has been recognized that the saturation effect is important in computing the core losses in thin steel or alloy sheets.

Rise of flux due to impact excitation: retardation by eddy currents in solid parts

In designing machines and apparatus with solid or partly solid magnetic circuits, such as eddy-current brakes, clutches, impact exciters, and rolling-mill motors, flux/time curves have often to be predetermined to secure a desired rapidity of response. The prediction has hitherto been uncertain, especially in the absence of test results on similar machines, and expensive changes of design, introducing laminated poles and yokes have sometimes been resorted to where the standard design or a simple modification of solid sections would have sufficed.The paper considers the rise of the total flux as retarded by eddy currents and shows that for this purpose all eddy-current paths can be represented by a single fictitious damping-ring whose resistance and leakage reactance are calculated from first principles. Simple expressions for the usual shapes of iron section are developed, and the test results which are given provide agreement between theory and experiment.For magnetic circuits which consist of solid iron in parts only and with fairly large air-gaps as in d.c. machines and alternators, it will generally suffice to take account only of the resistance of the fictitious damping-ring and to ignore its leakage reactance. A graphical procedure may then be used for obtaining the flux/time curve very rapidly. The error thus introduced makes the response appear rather less rapid than it is in reality.Where greater accuracy is needed, the leakage reactance of the substitute damping-ring has to be introduced and the solution found by solving transformer equations for successive parts of the characteristic.The substitute damping-ring, as a theoretical device, should prove helpful in solving other problems involving eddy currents by converting the system into a transformer with short-circuited secondary.

Eddy current brakes

A study of the properties of eddy current brakes suggests some lag effect in the action of eddy currents which increases with frequency. Using this conception, a theory is given to account for the results obtained and to evolve a convenient mode of presenting them graphically.

The eddy current brake for testing motors

The eddy current brake for testing motors

Discussion on “The eddy current brake for testing motors”

Discussion on “The eddy current brake for testing motors”