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Sorvall®
Centrifuges
Drive
system
circuitdrive
motor.
The
brake
circuit
is
designed
to
reduce
deceleration
time
by
approximately50%.
Automatic
braking
force
is
decreased
as
the
rotor
speed
decreases,
thus
minimiz¬ing
the
possibility
of
resuspending
the
test
sediment.
The
rotor
drive
has
two
modes
ofdeceleration
from
a
RUN
mode.
They
are:
(a)
free
coasting
and
(b)
dynamic
braking
(seebrake
circuit
In
Figure
3-8
or
3-22).
The
BRAKE
switch
SW109B
selects
the
mode
ofdeceleration.
With
the
BRAKE
switch
SW109B
in
the
OFF
position,
deceleration
is
freecoasting
to
a
stop.
When
the
BRAKE
switch
is
in
the
ON
position,
dynamic
braking
to
agradual
stop
occurs
as
the
centrifuge
enters
the
HOLD
mode.
115
Vac
is
appliedthrough
the
BRAKE
ADJUST
potentiometer
to
the
primary
winding
of
brake
transformerT201.The
brake
rectifier
CR201
converts
the
secondary
voltage
of
T201
to
a
dc
voltage
be¬tween
center
tap
and
contacts
7-4
of
relay
K208
on
PC
assembly
20126
and
motor
relayK202.
When
K202
is
deenergized.
contacts
8-9
are
closed
and
the
+5
Vdc
output
of
thebrake
rectifier
is
applied
across
the
field
winding
of
the
rotor
drive
motor.
At
the
sametime,
the
brake
resistor
R201
is
also
connected
through
contacts
8-9,
which
are
closedwhen
K202
is
deenergized
connecting
the
CR20
to
the
field
winding
of
the
drive
motor.Hence,
when
the
BRAKE
switch
is
in
the
ON
position
and
K202
is
deenergized,
the
drivemotor
acts
as
a
dc
generator
with
its
output
connected
to
a
2.7
ohm
load
resistor
R201.This
electrical
load
is
reflected
to
the
rotor
drive
linkage
as
a
mechanical
braking
force.The
BRAKE
ADJUST
R204
potentiometer
controls
the
output
of
the
field
strength
of
therotor
drive
motor.
The
potentiometer
provides
a
means
for
varying
the
deceleration
rateand.
consequently,
the
stopping
time
of
the
rotor
drive
motor.
The
brake
rectifier
circuitis
protected
from
ac
spikes
that
can
damage
the
rectifier
by
fuse
F301.4-5.
Brush
Warning
CircuitThe
brush
warning
circuit
consists
of
two
circuit
breakers.
CB1
and
CB2.
located
in
thebrush
warning
box
at
the
lower
front
In
the
base
of
the
centrifuge,
and
two
normallyopen
brush
switches,
operated
by
spring
tension,
located
in
the
drive
motor
brush
as¬sembly.As
the
brushes
wear
down
to
a
minimum
safety
length,
one
or
both
switch
contacts
willclose,
connecting
230
Vac
drive
motor
voltage
to
the
corresponding
circuit
breaker.
Thecircuit
breakers
are
set
to
trip
at
12
Vac
minimum.When
the
circuit
breaker
is
tripped,
the
115
Vac
circuit
between
the
RUN/HOLD
relayK205
and
the
ON
switch
(on
RC-5)
or
START
switch
(on
RC-5B)
energizes
thermal
relayK207.
This
relay
switches
ON
and
OFF
to
provide
an
Intermittent
voltage
to
the
brushindicator
lamp.
DS103.The
tripped
circuit
breaker
will
have
no
effect
on
the
centrifuge
operation,
while
thecircuit
is
activated
during
the
RUN
cycle.
The
115
Vac
circuit
will
be
interrupted
andwill
prevent
the
drive
enable
circuit
from
entering
the
RUN
mode
in
the
next
RUN
cycle.unless
the
brushes
are
replaced
and
the
circuit
breakers
are
manually
reset
(see
Figure8-6).4-13
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