Sorvall RT/T6000D - Page 120

SORVALL®
Centrifuges
System
Descriptions:
Electrical
full
wave
rectifiers and
stabilized
by
the
100
VDC
Zener
Diode before
being passed
to
the
Slow
Start
Opto-Isolator.
Because
the
Slow
Start Opto-Isolator is now enabled,
each
LOW
pulse
of
the
Slow
Start
PWM
signal turns
on
the
Slow
Start
Opto-Isolator.
This
allows
the
voltage
to
pass
through to
the
Interlock
Opto-Isolator
(which is also enabled when
all
Interlocks are
satisfied) and
go
on to
charge
the
SCR
Control
Capacitor.
As
the
potential
on
the
SCR
Control
Capactor
increases (to
approximately
8-lOVdc).
the Silicon Unilateral
Switch
(SUS)
breaks
down and
allows the
charge
to
be
dumped
into the
gates
of
both SCRs
(A
and
B).
The
SCR with the
positive
power
cycle
on
the
anode
side
gates
ON
and
passes
the
voltage
through to
the
motor.
At the
next zero crossing
event,
that
SCR will
then
turn
OFF.
When the
duty cycle
of the Slow Start
FWM
signal
is
low.
the
amount
of
power
being
applied
through
the
SCR
to
the
motor is minimal.
This
is
because
the
charge
rate
on
the
SCR
Control
Caplcltor
is diminished,
causing
the
SUS
to break
down late in
the
power
cycle.
The result is
that
only a
small
portion
of
the
power cycle
gets
through
to
the
SCR.
At the
zero crossing
event (reference
the
secondary
of
the
Transformer
Tl 24 Vac
tap),
the
Zero Crossing
Opto-Isolator turns
ON
and
discharges
any
residual
voltage
left
on
the
SCR
Control
Capacitor
to
a
0
Vdc
level;
this is
done through
the
Discharge Resistor,
Zero
Crossing
Opto-Isolator.
and Blocking
Diode.
This
ensures
that the
SCR
Bridge fires
consistently
from
one
half
cycle
to
the
next.
As the Slow
Start PWM
signal increases
in
duty
cycle,
the
SCR
Control
Capacitor
charges
more quickly
during
the
power
phase:
this
causes
the SUS
to
breakdown earlier
in the
power cycle,
thereby
allowing
more
power
to
be
passed
through
to
the
motor.
Part Of
Trafisfomer
6-16.
Slow
Start
PWM
(Pulse
Width
Modulation) Circuit