2.4.5 Grounding
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2.4.5 Grounding
A ground connection (tapped hole) is usually available at the rear of the
electromagnet frame. This ground point is provided for customers who would like to
use the electromagnet frame as a signal ground or will be bringing hazardous live
voltages near the electromagnet and would like to make it an electrical safety
ground. Please verify suitability for such a function and compatibility with local and
national electrical codes before making ground connections. Scrape off excess paint
near the connecting screw to ensure a good electrical contact with the bare steel of
the electromagnet frame.
2.4.6 Final Check-Out
When all of the connections have been made, the system should be tested to be sure it
is operating correctly. The settings for the magnet water should be checked to verify
that they are correct for the configuration which has been installed (section 4.13).
The maximum current setting for the magnet should be set also (section 2.5.2 and
section 4.12.1).
2.5 Electromagnet
Operation
This section provides a brief description of the typical operation of an electromagnet.
For operation of the Model 648, refer to Chapter 4.
2.5.1 Air Gap and Pole
Caps
The first step in setting up a magnet for operation is to select the proper pole caps and
adjust the air gap. These parameters are determined by the size and shape of the
sample, and the connections that must be made to the sample. Generally, a smaller
pole face provides a higher field within the air gap. A smaller air gap also provides a
higher field. The pole faces must be selected to accommodate the size of the sample
being tested. The air gap is selected based on the size of the sample and the other
equipment being used. The curves for field versus current for various air gaps and pole
cap sizes for the Lake Shore Model EM7-HVA are shown in FIGURE 2-6 through
FIGURE 2-7. It also shows that these parameters are not linear. This must be taken
into account when operating an electromagnet. To obtain linearity, it is necessary to
operate the magnet and power supply under field control(section 2.5.3).
2.5.2 Maximum
Current and Power
The Model 648 was designed to operate with a magnet load resistance of 0.50
)
, but
will provide full current output within a resistance range of 0.41
)
to 0.55
)
. The
Model 648 will also work outside of the previously mentioned resistance range, but
will be limited by two factors: internal power dissipation and compliance voltage.
FIGURE 2-5 plots the maximum current output versus a resistance range of 0.00
)
to
1.00
)
.
Also to be taken into account, the resistance of a magnet will rise with a rise in
temperature. The power dissipated in the magnet is given by: P = I
2
R. If the current
remains constant, the power dissipated will rise proportionately with the rise in
resistance.
