Du Pont SORVALL RC-5 Service Manual page 97

Drive
System
Circuit4-6.
Speed Control
CircuitSon/all®
CentrifugesThe
speed
control circuit consists of a speed control potentiometer (R106), a
saturablereactor
(which
is also
part
of
the
drive
motor
circuit), a comparator-amplifier, a
transis¬tor
switch
Q5,
tachometer
generator
TG301
and
a
bridge rectifier circuit. When
relayK206
is
energized, a
dc
voltage
from the bridge rectifier
passes
across contacts 9-6.
Theinput
voltage
to the rectifier is
115
Vac; the
output supplies
approximately
150 Vdc
forthe
control winding of the saturable
reactor.A
slight decrease in
the
drive motor speed
and,
therefore, a reduction In
the
tachometergenerator
output voltage, causes
the
transistor switch duty cycle to be increased,
thusallowing
more current to flow in
the
control winding of
the
saturable reactor
and
morecurrent
to
flow
to
the
drive motor.
The drive
motor
then
accelerates to a preset
speed.The
rapid
ON/OFF action
provides a
virtually
constant motor speed at a
preselectedspeed
control
setting.L______--JFigure
4-5. Speed Control Circuit, Simplified
Schematic
Diagram4-16