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BRUEL & KJAER System V - Operation; Selection of Appropriate B & K Exciter Head

BRUEL & KJAER System V
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3. OPER
ATION
3.1. PRINCIPLE OF
OP
ER
AT
ION
The
forces
generated
in
an
electrodynamic
exciter
are
due
to
the
interaction
between
an
alternating
current
in
a coil
and
a
magnetic
field.
This
is
illustrated
in
Fig .
3.1.
Test
Obj
ec
t
Signal
Source
Power
Am
p
li
f i
er
1-------------,
211056
Fig.3
. 1.
Principle
of
operation
of
an
Electrodynamic
Exciter
, is
the
current
in
the
coi',
B
the
flux
density
of
the
magnetic
field
and
F
the
force
gener-
ated by
the
coil .
The
magnitude
of
the
force
is
given
by
F = BIL
where
F = Force
in
newtons
B =
Ma
gnetic
flux,
in
webers
I =
Current
in
coil,
amperes
L =
Length
of
conductor
in
magnetic
field,
metres
The
current
for
the
exciter
is
provided
by a
power
amplifier.
A
signal
source
,
such
as a
sine-wave
or
random
generator,
is
connected
to
the
input
of
the
amplifier.
As
the
signal
is
fed to
the
coil,
a
fluctuating
force
and
hence
a
vibratory
motion
is
imparted
to
the
test
ob-
ject.
Of
course,
the
coil
itself
is
wound
about
a
moving
element
and
this
has
a
certain
mass.
Hence
the
acceleration
due
to
an
applied
force
F
will
be
F
a
m + me
where
m is
the
mass
of
the
test
object
and
me
is
the
moving
element
mass
.
3.
2. SELECTION
OF
APP
ROPRIATE
B & K EXCITER HE
AD
The
user
is
urged
to
read
the
detailed
specifications
for
each
of
the
currently
available
B & K
Exciter
Heads. He
should
select
the
Exciter Head
having
specifications
best
suited
to
his
p
articular
application.
A
brief
summary
of
typical
specifications
is
shown
in
Table
3 .
1.
In Table 3 .1
the
first
thr
ee
lines
list
the
force
a
vailabl
e
fro
m each
of
5 B & K E
xciter
13

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