BRUEL & KJAER System V - THE MAGNET Assembly; Field Coils; Secondary Bands; Stem

4.1. THE MAGNET ASSEMBL y
The magnetic system is double-ended, having two magnetic circuits supplying magnetic
flux to a single air gap. Except for the Split Hub, which is part of the Exciter Head, the
magnetic system is contained in the Exciter Body. The Magnet Assembly has two field
coils, two Secondary Bands, and three iron parts, the Sterri, the Centre Bowl and the Up-
per Wheel .
4.1 .1. Field Coils
The Lower Coil is clamped between the Stem and the Centre Bowl. The Upper Field Coil
is
clamped between the Centre Bowl and the Upper Wheel. Rubber spacers are fitted
above and below each field coil. These spacers provide a stift, heavily damped support
for the field coils, allowing for thermal expansion vet restricting relative motion due to vi-
bration.
4.1.2. Secondary Bands
An Inner Secondary Band surrounds the inner post of the Stem just inside of the Driver
Coil. An Outer Secondary Band is mounted on the inner surface of the lower part of the
Centre Bowl just outside of the Driver Coil. These are copper rings which are soldered in
place and nickel plated.
Induced currents in these shorted Secondary Bands keep the impedance of the Driver
Coil low. If these Secondary Bands we re omitted, the voltage needed to drive the exciter
at high frequencies would be greatly increased and a much larger amplifier would be
needed .
4.1.3. Stem
The Stem is the lower part of the Magnetic Assembly, the inner surface of the magneti-:;
air gap, and the guiding surface for the Split Hub. It contains the lower field coil. Slots
are cut into the center post of the Stem to provide clearance for the ribs of the moving
element skeleton. The lower magnetic circuit is form ed by the bowl shaped part of the
Stem, the lower part of the post of the Stem, and the lower part of the Centre Bowl.
4.1.4. Centre Bo w l
The Centre Bowl is the middle part of the Magnetic Assembly, the outer surface of the
magnetic air gap, and contains the upper field coil. The bottom of the Centre Bowl is
common to both the lower and upper magnetic circuits. The upper magnetic circult IS
formed by the Centre Bowl , the upper part of the post of the Stem, the Split Hub, and
the Upper Wheel.
4.1.5. Upper Wheel
The Upper Wheel is the upper part of the Magnetic Assembly, and forms the top of the
upper magnetic circuit. The Exciter Head is held rigidly to the upper surface of the Upper
Wheel by both magnetic forces and by hold - down clamps.
4.2. THE FIELD SUPPLY AND COOllNG ASSEMBLY
The Field Supply and Cooling Assembly consists of the Head and Field Cooling Fan, the
Blower Motor and the Field Supply.
27