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properly set, assuming the exciter cooling system is otherwise operating
normally. The correct setting for the 4801 system of the CURRENT LIMIT
is given in Fig.3.13.
Adjustment of the HEAD CONSTANT control on the amplifier front
panel is necessary to maintain calibration of the DISPLACEMENT LIMIT
protection (Fig.3.14 for B & K Type 4801). The circuit integrates the
exciter moving coil voltage to obtain an analogue of displacement, and the
proportionality between voltage and displacement depends upon mains volt-
age.
I
Head Constant
Nominal
Head Constant
4
2
0
I\
\
\
'
r\
\
"'
~
~~
r--
t---
0
10
20
30
40
50
60
70
80
90
100
110
Mains Voltage/Nominal Mains Voltage
%
2 7ftU9
Fig.3.14.
HEAD CONSTANTv. Mains Voltage
The Exciter Interlock protective circuitry will normally shut down the
signal to the driver coil in the event of an exciter cooling system failure
caused by power loss or a stalled motor. With reduced voltage this protect-
ive circuit must be by-passed, (EXCITER INTERLOCK switch to "Bypass")
so the operator must check that the cooling is operating. Note that at
voltages below 25% of normal, forced cooling is not required for either the
moving coil or the exciter body components, so protection against cooling
27
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