Operation; Selection Of Appropriate B & K Exciter Head - BRUEL & KJAER System V Instruction Manual

3 . O PER ATION
3.1. PRINCIPLE OF OPERATIO N
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
Po w er
Am p li f ier
1 - - - - - - - - - - - - - ,
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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
=
Magnetic 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 + m e
where m is the mass of the test object and me is the moving element mass .
3.2.
SELECTION OF AP PROPRIATE B & K EXCITE R 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 particular application. A brief summary of typical specifications is shown in
Table 3 . 1.
In Table 3 . 1 the first three lines list the force availabl e fro m each of 5 B
&
K Exciter
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