Sorvall RT/T6000D - Page 123

System
Descriptions:
SORVALL®
Centrifuges
Electrical
6-21.
Closed-Loop
Speed
Control
(refer
to
figure
6-19)
The
RT/T6000D
and
RT/T6000B
(SNs
9101800
and
above) centrifuges
use a
closed-loop
speed
control
design.
As
discussed
In
the
Main
Speed
PWM and the
Anticipation
Component
Voltage Equivalent
paragraphs
(6-19
&
6-20).
a
reference voltage
created
by
the desired
speed setting
potentiometer
is
constantly being compared
to the actual
RPM
signal
buffered
by
the
anticipation component voltage equivalent
(ACVE).
Whenever
the
ACVE
voltage Is
less
than the
desired set
speed
voltage,
the
main
speed
control
PWM
signal
applies power
to
the
motor.
The
motor accelerates
until
the ACVE
voltage
level
just
passes
over
the
set
run
speed
voltage
threshold
(as
measured at
the
main
speed
threshold
comparator inputs).
The
PWM
signal
is then
removed
momentarily,
causing
the
motor
to
slow
down.
Once
the
rotor
speed
falls
below the
set run
speed,
the
anticipation
circuit
begins generating
the
FWM
signal
again
to accelerate
the
motor.
This
process,
the
generation and removal
of
the
PWM
signal to
the
motor control
circuit, occurs rapidly
throughout
the
course
of the
run
to
maintain
the
rotor
at
set
speed
(±100
rpm).
Deceleration
occurs
when the
speed
potentiometer
is
changed to a
lower
speed
setting
than
the
speed
at
which
the
motor
Is
currently running.
The
main
speed
control
PWM
signal
is
removed (shut down)
and
the
rotor begins
to decelerate due to windshear
and
gravity.
Depending
on
the
amount
of
deceleration
that is
required,
an anticipation
voltage
offset
equal
to
that
amount is immediately
added
to
the
actual
rpm
voltage,
creating
an
anticipated component voltage
equivalent (ACVE).
As
the
rotor
speed approaches
the
newly
(lower) set run
speed,
the
speed
control circuit
begins
diminishing the
anticipation
voltage
offset; this minimizes
speed
overshoot.
If
the
run
is terminated
by
setting
the
speed
control
or
timer switch
OFF
(or
if the
set
run
time
runs out)
and
the brake switch is in
the
ON
position,
the deceleration
occurs
in the
brake
mode.
If
a
lower
rotor
speed
is selected
by
the
speed
control
knob
(but
the
run
Is
not
terminated),
the
rotor
will
coast
downward to
the
new
set
speed
without the
use
of
the
brake,
even
if
the brake
switch
is
ON.
6-22.
Open-Loop Speed
Control
(refer to figure
6-20)
The
RT/T6000B
centrifuges
(SNs
below
9101800)
and
RT/T6000
centrifuges have
an
open-loop
speed
control
design.
The desired
set
speed
Is
obtained
by
increasing
the
speed
control
pot,
thereby
allowing
the motoro
to
accelerate
while
observing
the
RPM
meter
indication.
The
user
continues to adjust
the
speed pot
until
the
desired
set
speed
Is
attained.
The
marks on
the
speed
control
knob
are
for
reference
only.
Deceleration
occurs
when the
speed
potentiometer
is
changed to
a
lower
speed
setting
than
the
speed
at
which the
motor
is
currently
running.
The
main
speed
control
FWM
signal
is
removed
(shut down) and
the
rotor begins
to decelerate
due
to windshear and
gravity.
Depending on
the
amount
of deceleration
that
is
required,
an
anticipation
voltage
offset
equal
to
that
amount is
immediately
added
to
the
actual
rpm
voltage,
creating an
anticipated
component
voltage
equivalent (ACVE).
As
the
rotor
speed
approaches
the
newly
(lower)
set run
speed,
the
speed
control circuit
begins
diminishing
the
anticipation
voltage
offset; this
minimizes
speed
overshoot.
6-24