Sorvall RT/T6000D - Page 117

System
Descriptions:
Electrical
SORVALL®
Centrifuges
6-16.
SCR
Phase
Angle
Control
Circuit
(refer
to figure
6-14)
The
firing
angle
of the SCR
bridge is
controlled
by
a
silicon
unilateral
switch
(SUS).
The
gating
capacitor
will
be
charged
to the breakdown voltage
of
the
SUS.
which
is
between
8
and
10
Vdc.
At
breakdown,
the
charge
on
SCR
gating
capacitor
is dumped into
the
gates
of
both SCRs
in
the
Motor
Control
SCR
Bridge circuit.
Since
only
one
of
the
two
SCRs
can
pass
a
positive
voltage
at
a
time (dependent on
which
one
sees
a
forward
bias
from
the
ac
line),
the SCR
seeing
the
forward bias
from the
current
ac
cycle
will
turn
on.
This
then
passes a
rectified dc
voltage
to
the
motor.
At
zero
crossing,
the
zero
crossing opto-coupler
turns on
and discharges
any
voltage
left
on
the SCR
gating
capacitor
to
a
zero
potential.
During
the
second
half
of the
ac
cycle.
SCR
gating
capacitor
is charged
again
to
the
breakdown
voltage
of
the
SUS.
and
the
charge
is
again
dumped
into
both
SCR
gates.
This
time.
however,
the other
SCR
passes
the rectified dc
voltage to
the
motor.
This
process
continues
twice
every
cycle
(50 or
60
Hz)
as long as
the
centrifuge
is
in the
RUN
mode.
Figure
6-14.
SCR
Phase
Angle
Control Circuit