GMW 3470, User Manual

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Section 4
OPERATION
General
The magnet operates as a conventional electromagnet.
1. Adjust the poles to the desired gap with the poles approximately symmetrical about the center
magnet line.
2. Adjust the cooling water flow to about 1 liters/min (0.26 USgpm) for the 3470. For operation at
less than maximum power the water flow may be correspondingly reduced. Note that the inlet
water temperature will determine the actual flow rate required. The above specified flow rates
were determined with a water inlet temperature of approximately 18°C.
3. Turn on the power supply and increase the current until the desired field is reached.
Calibration
The induction curves may be used to estimate the field in the air gap to within four or five percent.
More accurate field determination may be obtained by deriving experimentally a calibration curve for
the particular pole and air gap combination being used. Magnetic hysteresis in the yoke and poles can
cause an error of 30 to 70G (3 to 7mT) with an arbitrary application of such a calibration curve. This
effect may be reduced to less than one percent by following a prescribed 'current setting schedule'
designed to make the magnet 'forget' its prior magnetic history. The schedule should of course be used
both in establishing the calibration curve and in its subsequent use. A possible schedule would be:
From zero current, increase to maximum current and reduce again to zero current. Increase again to
maximum current and reduce to the current to give the desired field setting. Approaching the desired
field from a higher setting will typically produce better field uniformity. This is because the field
changes at the pole edges will normally lag the field change at the center thereby helping to
compensate the radial decrease in field.
Greater precision in setting up the calibration curve will be achieved with the use of a digital
teslameter and by making a numerical table. This table used with an interpolation routine will
eliminate the error associated with reading a graph.
In any event, three points need to be remembered:
1. A calibration curve or table is only as good as the precision employed in generating it.
2. The field is defined only at the point it is measured. It will generally be different at a different point
in the air gap. For example, the induction curves refer to the field on the pole axis and at the center of
the air gap (median plane).
4-1
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