Table of Contents
Advertisement
generally used to offset the static deflection of the flexures to re-centre the
table, thus the full displacement capability is unaltered. When the DC
centring signal is used to give a static displacement to the test specimen, the
mean table position will not be at the centre of travel. This offset is not
accounted for by the displacement sensing circuitry and must be kept in
mind when setting the DISPLACEMENT LIMIT control.
4.5.3. Temperature
Abnormal load conditions, failure of the heat sink cooling fan, or high
ambient temperatures could result in output transistor junction temper-
atures in excess of design limits, and subsequent transistor failure. To
prevent damage to the output transistors under these conditions the
TEMPERATURE SCR will trigger, removing the inputs and lighting the red
front-panel lamp labelled TEMPERATURE.
The heating effect of the drive current is a function of both current and
frequency. The drive current in the useable frequency range, as limited by
the TEMPERATURE circuitry, is shown in Fig.4.8. The TEMPERATURE
protective circuitry is calibrated to trigger at 14 A RMS load current, at a
frequency of 5 Hz, in an ambient of 25°C (77° F). This gives a trip margin
over the rated output current, thus allowing the shaker system to be oper-
ated at higher temperatures than 25°C. A rise of 20°C (36° F) ambient
temperature reduces the tripping current by about 10% (i.e. to 12.6 A).
100
50
RMS Output
Current
lAmps.
20
10
)
.1
. 1.
" Current"
L1m1ted
RMS Output Current
~
--
--
~
10
20
I
.1 . .
"Temperature"
L1m1ted
RMS Output Current
-
~
~
-
..,.......
--
iiT
f - - -
50
100
200
500
1000
Frequency 1Hz)
27fOfO
Fig.4.8. Output Current as limited by TEMPERATURE protective circuitry
39
Advertisement
Table of Contents
