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multiple exciters at low frequencies if the same motion is desired from each
exciter, such as the use of one exciter to push and another to pull on a
model of a bridge.
The CURRENT LIMIT circuitry operates with a 50 second time con-
stant. This can be reduced to 2.5 seconds by removing an internal shorting
link from printed circuit card ZL
0028
(Fig.3.9).
For non-B & K vibration exciters the allowable moving coil current must
be ascertained from the relevant manufacturer and the CURRENT LIMIT
control set accordingly, so that full protection is available.
Set the DIRECT CURRENT OUTPUT control to 5.0 and the AMPLI-
FIER GAIN control fully anti-clockwise to the "Reset"
position.
On the rear panel set the EXCITER INTERLOCK to "In" for the B
&
K
Type
4801
system and to "Bypass" for any other manufacturer's shaker.
The SIGNAL REFERENCE control is set up as required, although normally
this will be on "External". The PHASE switch can be
"0°"
or
"180°"
for
single shaker applications and as dictated by circumstances in multiple
shaker systems.
3.3.2. Switch-on
The POWER switch on the
2707
has three pos1t1ons "Power Off",
"Power On", and "Load On". In the "Power On" position, AC mains
supply is applied to the amplifier and the field coil rectifiers in the Exciter
Body. The moving coil is open circuit at this time otherwise the e.m.f.s
induced in the moving coil by the sudden build-up of field would cause a
severe transient at the Exciter table. The switch also ensures that the small
switch-on transients due to the amplifier have had sufficient time to die
away before the load is connected. To avoid damage to the test specimen,
exciter or amplifier, a pause of one or two seconds duration must be made
in the "Power On" position before switching to "Load On".
When the POWER switch is at "Power On", the Amplifier and
Exciter
Body cooling fans and the scale lamps are also energized. The meter
indicat-
ions should be zero.
Turn the POWER switch to "Load On". The meter indications of AC
quantities will still be zero. Turn the RANGE ARMS switch to±
10
A and
18
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