Sorvall RT/T6000D - Page 118

SORVALL®
Centrifuges
System
Descriptions:
Electrical
6-17.
Delayed
Start
Circuit
(refer
to figures
6-15)
The
delayed
start
circuit
allows
the
service engineer
to minimize
the
time between the
Initiation
of
a
start
sequence
and
the
beginning
of
motor
rotation.
The time is minimized
by
calibrating
the start
delay
potentiometer.
When the
Instrument
Is
first
powered
up
(with the Timer (SW2)
and
Speed
Control
Switches (SW6)
in
the
OFF
position),
the
Precharge Capacitor charges to a
level
set
by
the
Start
Delay
Adjustment
potentiometer. Because
the Start
Delay Comparator
output
at
this
time is
HIGH.
the
Pre-Charge Opto-Isolator is turned
ON,
allowing
the
Pre-Charge
Capacitor to
charge
quickly
through
the
Pre-Charge
Opto-Isolator
and Resistor.
This
pre-
charge
level is determined
through
calibration
procedures
and ends
up
with
a pre-charge
level
just
below
the Slow Start
Compartor
threshhold.
Both
Timer
(SW2) and
Speed Control
(SW6)
switches
are
also
open,
thereby preventing
+12
Vdc
from
passing
through
to
the Run/Brake
Relay
Kl
Comparator.
Due to
the
reference
set
up
on
this
comparator,
its
output
at
this
time is also LOW.
This LOW
level
signal,
after
it
is
inverted,
keeps
a ground potential
off
of the
Run
/
Brake
Relay
K 1
coil,
thereby
keeping
the
motor
control circuits
de-energized.
When the
Timer and Speed Control
Swithes
are placed
in
positions
other
than
OFF,
+12
Vdc
passes
through
them
on
the
way
to
both
comparator
inputs.
A
RUN
enable
delay
is
created
by
the
reslstive/capacltlve
network
at
the front end of the
Start
Delay
Comparator.
When the
charge
on
the
Run Enable
Delay Capacitor
passes
the threshhold
of the
comparators,
the
outputs
of the
comparators
change
state
from
a
LOW
to
a
HIGH.
For
the
Run/Brake
Relay
Kl
comparator,
this
change
of
state ultimately
provides
a
LOW
ground return
for
the +24 Vdc
potential to
pass
through
the
Run/Brake
Relay
Kl
coils,
which
then
energizes it.
For
the
Start
Delay
Comparator.
Its
LOW
output
ultimately
turns
off
the Start
Delay
Opto-Isolator.
This
allows the
Pre-Charge
Capacitor
to
continue
charging
towards
a
+12 Vdc
level
through
the
Slow
Start
Resistive
network;
the
network
includes the
Slow
Start Rate
Adjustment Potentiometer
used to
vary
the
charging rate
level
on
the the
capacitor.
This
charging
rate level
is
called
the
Run
Signal. As
this
Run
Signal
level
increases in
potential,
it
exceeds
the
Slow Start
Comparator
threshhold
input
with
respect
to
the
Triangle Waveform Generator
input.
As
a result,
the
comparator
output
begins
producing
the Slow
Start
PWM
(Pulse
Width
Modulated)
signal
used to drive the
motor
contol
circuitry.
As shown in
the
graph
on
the
lower
right
of
figure 6-15.
the
triangle
waveform
signal
A
is
constantly
running
and
being
applied
to the
input
of
the Slow Start
Comparator.
The
Run Signal
B first
starts out below
the
lowest
point
of
A.
This
causes
the Slow
Start
PWM
signal
C
to be
at
a
constant
HIGH
level.
As
the
Run Signal
begins
to
Increase
in
amplitude,
it
begins
to
pass
over
the
threshold
as compared to
the
triangle
waveform.
Each time
this
level
is
exceeded,
the
comparator output
goes
LOW.
When
the
triangle waveform
increases
again
and
exceeds
the
Run
Signal's
present
level,
the
output returns
to
a
HIGH
state.
This
alternating
Low and High Slow Start
PWM
signal
controls
the
motor
SCR
trigger
circuitry.
As the
Run
Signal
B
increases
in
potential,
the
Slow Start
PWM
signal
Increases
in
duty
cycle,
thereby
applying
more
power
to
the
motor.
This
process
continues
until the
motor
speed exceeds
500
rpm:
the
Slow Start
PWM
circuit is
then disabled
and
the
motor control
circuit
is
turned
over
to
the
Main
Speed
Control
PWM
circuit
6-19