Using the Thomson Jumping Ring to Study the Effect of Eddy Currents and Skin Depth on Ring Jump Height

2021 ◽  
Vol 59 (6) ◽  
pp. 459-461
Author(s):  
Patrick McCaughin ◽  
Lyle Ford
Keyword(s):  
1994 ◽  
Vol 116 (4) ◽  
pp. 720-725 ◽  
Author(s):  
Michelle Simone ◽  
John Tichy

A conducting body moving with respect to a magnet experiences lift and drag forces from the eddy currents induced in the conductor. The force on the conductor is dependent on the relative velocity between the conductor and the magnet. In this study, we investigate the force dependence on magnetic Reynolds number, a dimensionless indicator of velocity. The Lorentz equation is used to predict the force on the conductor, given the spatial dependence of the eddy currents and magnetic induction vector inside the conductor. Maxwell’s equations, which govern the electromagnetic quantities, are reduced to a single convection-diffusion equation for the magnetic induction vector inside the conducting body. An integral solution which satisfies the governing equation and boundary conditions is used to obtain the eddy currents and magnetic field. For our model, both lift and drag forces increase sharply with Reynolds number, reach a maximum, and decrease with increasing Reynolds number to an asymptotic limit. We also find that skin depth, the depth to which the eddy currents decay inside the conductor, decreases with increasing Reynolds number. The relevance to magnetically supported high-speed vehicles and magnetic bearings is discussed.


The induction of eddy currents in a conducting, non-magnetic plate containing a through-the-thickness crack is considered in the limit where the plate thickness ( h ) is small compared with the electromagnetic skin depth ( δ ). The plate is represented by a current sheet and the crack by a distribution of generalized current vortices. The current density for these vortices must differ from that of conventional hydrodynamic vortices so as to be consistent with Faraday’s law for self-induction. The vortex density, and hence the induced current in the cracked plate, can be obtained by solving a one-dimensional singular integral equation involving the current density for the uncracked plate which is assumed to be known. The case where the induced currents are produced by a probe coil, as in eddy-current non destructive inspection, is investigated in detail and a simple expression is derived for ∆ Z , the change in coil impedance due to the crack. The formalism is applied to the specific problem of an infinite plate containing a straight crack. Analytical results are presented for the limiting cases of long and short cracks, and a numerical method based on Gauss-Chebyshev quadrature is used for intermediate crack length. These theoretical results are compared with experimental measurements of ∆ Z and the thin-plate theory is found to be in excellent agreement with experiment (to within 5% or better) for values of h / δ up to 0.4.


1980 ◽  
Vol 41 (C1) ◽  
pp. C1-217-C1-218 ◽  
Author(s):  
G. Karczewski ◽  
M. Kopcewicz ◽  
A. Kotlicki
Keyword(s):  

2020 ◽  
pp. 54-58
Author(s):  
S. M. Plotnikov

The division of the total core losses in the electrical steel of the magnetic circuit into two components – losses dueto hysteresis and eddy currents – is a serious technical problem, the solution of which will effectively design and construct electrical machines with magnetic circuits having low magnetic losses. In this regard, an important parameter is the exponent α, with which the frequency of magnetization reversal is included in the total losses in steel. Theoretically, this indicator can take values from 1 to 2. Most authors take α equal to 1.3, which corresponds to the special case when the eddy current losses are three times higher than the hysteresis losses. In fact, for modern electrical steels, the opposite is true. To refine the index α, an attempt was made to separate the total core losses on the basis that the hysteresis component is proportional to the first degree of the magnetization reversal frequency, and the eddy current component is proportional to the second degree. In the article, the calculation formulas of these components are obtained, containing the values of the total losses measured in idling experiments at two different frequencies, and the ratio of these frequencies. It is shown that the rational frequency ratio is within 1.2. Presented the graphs and expressions to determine the exponent α depending on the measured no-load losses and the frequency of magnetization reversal.


2010 ◽  
Vol 130 (3) ◽  
pp. 258-264 ◽  
Author(s):  
Kazutaka Takahara ◽  
Jun Muto ◽  
Hiroyuki Nagahama

2007 ◽  
Author(s):  
Ray T. Ko ◽  
Shamachary Sathish ◽  
Jeremy S. Knopp ◽  
Mark P. Blodgett

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