Page 136 - Electromechanical Devices and Components Illustrated Sourcebook

98 Electromechanical Devices & Components Illustrated Sourcebook

Eddy Currents As the magnet moves, it induces eddy currents into the
plate. When the currents are present, they can work against the
If a conductor is moved through a magnetic field, an electri- magnet’s field and the plate becomes momentarily magnetic.
cal charge is induced. Any conductor will harbor eddy cur- As soon as the magnet motion stops, the eddy currents dimin-
rents. Eddy currents are complete electric circuits that reside ish and the plate resumes its nonmagnetic characteristics.
entirely in the conductor, as shown in Figure 5-56. Because One common use of eddy currents is as shock absorbers or
the currents are contained within the conductor, they repre- dampers. Figure 5-58 shows a permanent magnet eddy cur-
sent a short circuit. Two things result from eddy currents, first rent damper. The horseshoe magnet may be hung from a pen-
the conductor will heat up in direct proportion to the amount dulum and the plate is fixed. If the pendulum motion is slow,
of power being dissipated. Second, the energy that goes into it can move freely. As its speed increases, progressively
the eddy currents represents a loss. In wires, eddy currents do stronger eddy currents are induced into the plate and the
not represent a significant problem. However, in the cores of motion is damped.
magnetic components such as transformers, solenoids, and
motors, this loss can be problematic. To minimize the path
that eddy currents can be generated in, most AC magnetic
components use laminated cores. The laminated plates pre-
sent a single magnetic mass and, at the same time, are electri-
cally isolated. The thin plates reduce the effective path and
eddy currents are minimized.
Moving Horseshoe
Magnet

− − − + Eddy Currents
+ + Pole Pieces

+ − Conductive Plate Fixed Aluminum
− + + Plate
−
Figure 5-58 Permanent Magnet Eddy Current Damper
Figure 5-56 Eddy Currents

An interesting demonstration of the effects of eddy cur-
rents is shown in Figure 5-57. Place a nonmagnetic, conduc- Figure 5-59 shows an electromagnet eddy current damper.
tive plate (aluminum or brass) on a table. Set a horseshoe In this manner the effects of the damper can be switched on
magnet on the center of the plate. Slide the magnet back-and- and off. Additionally, by varying the magnetic field, the
forth very slowly. You will notice that there is no restriction to strength of the eddy currents can be controlled and variable
the movement of the magnet. Now move the magnet very fast damping can be utilized.
and you will notice that there is a great deal of restriction to
the movement. In fact you will most likely have to clamp the
plate down to keep it from moving with the magnet.

Coil

Horseshoe Magnet
Moving
Electromagnet

Nonmagnetic Frame
Metal Plate

Fixed Aluminum
Magnet
Motion Plate
Poles
Figure 5-57 Eddy Current Demonstration Figure 5-59 Electromagnet Eddy Current Damper