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Refrigeration
System
Sorvall®
CentrifugesNOTEK203
is
found
on
some
of
the
old
RC-5
centrifuges
and
is
bypassed
in
anew
modification.The
condenser
fan
motor
is
not
equipped
with
external
components
toincrease
starting
torque.When
the
compressor
motor
is
operating,
exciter
currents
in
the
RUN
and
START
wind¬ings
are
considerably
out
of
phase
due
to
the
capacitive
reactance
of
START
capacitor
Cgand
RUN
capacitor
C,.
connected
in
parallel
with
relay
Kp
(deenergized).Exciter
currents
in
the
START
windings
lead
the
currents
in
the
RUN
windings,
produc¬ing
a
high
motor
starting
torque.
When
the
motor
speed
increases
sufficiently,
acounter
voltage
induced
in
the
START
windings
will
energize
potential
relay
Kp.
STARTcapacitor
Cg
is
then
disconnected
from
the
circuit
by
relay
Kp.A
phase
angle
decrease
is
caused
by
the
capacitive
reactance
of
RUN
capacitor
C,.
whenthe
compressor
motor
is
at
normal
running
speed.
Current
in
the
RUN
winding
lags
theline
voltage
in
phase.The
potential
relay
coil
is
wired
across
the
start
winding
of
the
compressor
motor
andsenses
voltage
changes
across
the
winding
when
the
START
winding
voltage
increaseswith
motor
speed.
As
the
voltage
increases
to
the
relay
pickup
value,
the
relay
armaturepulls
up.
opening
the
relay
contacts
and
deenergizing
the
START
winding.
After
thisswitching
function,
there
is
sufficient
voltage
Induced
in
the
START
winding
to
hold
therelay
coil
energized
and
the
relay
contacts
open.5-4.
Refrigeration
ControlThe
compressor
circuit
permits
the
compressor
and
condensing
fan
to
run
simultane¬ously
in
the
HOLD
mode
for
a
maximum
of
15
seconds
ON
and
2
minutes
OFF
until
thetemperature
reaches
the
red
pointer
on
the
temperature
Indicator.
At
that
point,
boththe
compressor
motor
and
the
condensing
fan
will
simultaneously
stop
operating
for
aminimum
of
2
minutes
and
will
only
restart
to
regain
the
desired
temperature.
In
theRUN
mode,
the
circuit
permits
the
condensing
fan
to
run
continuously.
The
compressorwill
run
only
until
the
temperature
Indicated
by
the
blue
set
pointer
is
attained
on
thetemperature
indicator.
Then
the
compressor
will
stop
operating
for
a
minimum
of
40seconds
and
will
not
cycle
ON
until
it
receives
a
signal
for
additional
cooling.In
the
HOLD
mode.
the
motor
drive
enable
circuit
is
open,
and
the
condensing
fan
relayK203
has
not
been
energized.
When
K2Q3
is
deenergized.
the
230
Vac
which
powers
thecondensing
fan
motor
finds
a
path
through
compressor
relay
K201,
across
contacts
2-1.Since
K201
is
only
energized
at
a
minimum
of
2
minute
intervals,
the
compressor
andcondensing
fan
will
operate
during
that
2
minute
Interval.In
the
RUN
mode,
relay
K203
is
energized
and
the
path
to
the
condenser
fan
motor
is
nolonger
through
K201.
Relay
K201
is
now
only
energized
for
a
minimum
of
40-secondintervals.
Therefore,
the
compressor
cycles
ON
and
OFF
as
required,
but
the
condens¬ing
fan
operates
continuously.5-4
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