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Background noise can also be reduced by reducing the strength of the
exciter field. With B
&
K exciters, this can be done by reducing the mains
supply voltage to the Field Coil Rectifiers. For non-B
& K
exciters the
manufacturer's instructions on reduced field strength operation must be
followed.
3.8.
OPERATION WITH REDUCED EXCITER FIELD
If operation is desired at very low vibratory levels, two methods are
available to shift the dynamic range of the system to a lower range of
acceleration levels:
a) A large mass can be added to the table. The same forces then cause
smaller motions without reducing the dynamic range.
b) The mains voltage to the Field Supply can be reduced. Since the same
current causes smaller deflections when the magnetic field is reduced,
the background level is lower but the dynamic range is substantially
the same. The amplifier mains voltage must remain at the nominal
value.
Operation at levels in the 0.001 g range is very difficult. Very good
instrumentation and a good seismic block system, to isolate the exciter from
building motions, are essential.
A flexible means of providing reduced voltage operation is to wire a
variable autotransformer in the power line to the exciter, providing a volt-
meter to monitor the exciter mains voltage. An alternative method is to set
the exciter transformers' primary taps for a nominal voltage greater than the
actual mains voltage used. The preferred source of power is via the amplifier
AC POWER EXCITER connector; thus avoiding the possibility of energiz-
ing the moving coil from the amplifier before supplying power to the exciter
to establish moving coil cooling.
For the
B & K
Type 4801 Exciter Body the mains supply can be in the
range -15% to
+
5% from a nominal value selected by appropriate wiring of
the transformers primaries. However, it is possible to operate the 4801 on
voltages considerably below nominal, using the method outlined in the
wiring diagram (Fig.3.12).
Reduction of the mains supply to an exciter field affects the HEAD
CONSTANT and CURRENT LIMii setting requirements, reduces the
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