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Sorvall®
CentrifugesRefrigeration
System5-6.
Overtemperature
Detection
CircuitThe
overtemperature
detection
circuit
consists
of
a
differential
amplifier
and
a
relaydriver.
It
functions
to
force
the
system
out
of
the
RUN
mode
and
into
the
HOLD
mode
inthe
event
a
malfunction
occurs.
The
temperature
control
circuit
allows
the
temperatureto
rise
3°C
above
the
red
temperature
set
pointer.When
the
chamber
temperature
becomes
the
same
as
that
indicated
by
the
red
setpointer,
the
photocell
is
blocked
and
its
resistance
goes
high.
The
overtemperature
po¬tentiometer
R58
is
calibrated
to
cause
the
differential
amplifier
to
drive
the
relay
driverQ20
into
saturation
when
there
is
a
3°C
temperature
differential
between
the
blackpointer
and
the
red
pointer.
Relay
K4
is
then
energized,
which
causes
triac
switch
CR20to
open.
This
deenergizes
the
coil
of
the
RUN-HOLD
relay
K205
forcing
the
system
toswitch
into
the
HOLD
mode.The
compressor
operation
is
similar
to
that
of
the
HOLD
mode.
with
one
exception:
thetemperature
control
is
programmed
through
the
normally
closed
contacts
5-8
of
relayK205
to
operate
from
the
blue
pointer
on
the
temperature
indicator.
The
compressorand
motor
is
a
hermetically
sealed
unit
equipped
with
an,
internal
thermal
line
breakoverload
protection
that
cannot
be
bypassed.
This
relay
disconnects
the
START
andRUN
windings
in
the
event
of
excessive
motor
temperature
resulting
from
high
currentdraw.RED
<>SET
POINT
[—-<
R42CAL
<CR20RUN-HOLDRELAYK205115
VacFigure
5-6.
Overtemperature
Detection
Circuit.Simplified
Block
Diagram5-11
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