Sorvall RT/T6000D - Page 116

System Descriptions:
Electrical
SORVALL®
Centrifuges
6-15.
SCR
(Silicon
Rectifier)
Bridge
Circuit
(refer
to figure
6-13)
An
SCR
power
bridge is
composed
of
two
S4015L
silicon controlled
rectifiers and
two
D4015L
rectifiers.
These rectifiers
are
rated for
15
amp
RMS
minimum.
Avaristor Is added
to
this
circuit
to
protect
to
protect
the
entire bridge
from
Inductive transients generated
by
the
motor armature
and
field
windings, as
well
as
from
the
2
ImH
smoothing
choke
L2.
Another
D4015L
rectifier in this circuit
acts
as a
freewheel
diode.
The
ac
power
cycle,
appearing
at
terminal block
TB1/TB2*
pins
5
and
3,
passes
through
the
SCR
bridge
in
the
following
manner:
Assuming
a
positive
half
cycle
of the
ac
cycle
is present
onTB 1 /TB2
pin
3.
this
positive
signal
is
applied
to
the
anode
side
of the
SCR
"A"
and
the
cathode side
of
blocking diode
"A."
Since
diode
"A"
Is
now reverse
biased
it will
not conduct and
allow
the
positive
signal
to
pass
through.
When
the
SCR
phase angle
control circuit
level
exceeds
the
Silicon
Unl
Lateral Switch
(SUS),
both SCRs (SCR
"A"
and
"B")
will
turn
ON.
Since
the
positive ac signal
is
at
the
anode
side of
SCR
"A,"
the
positive signal
is
passed
through
when
SCR
"A"
is
gated
on.
This
allows
it
to
go
through smoothing
choke
L2
and
through
the
motor
armature
and
field
before
coming
back to
the
junction
of blocking
diodes
"A"
and
"B."
The
positive signal
forward biases blocking
diode
"B,"
allowing
it
to return to
its
source at
TB1
pin
5.
At
zero crossing.
SCR
"A"
turns off.
During
the
second
half
of the
ac
power
cycle,
the
polarity changes
at
TB 1
/TB2
pins
5
and
3;
this
puts
the
positive signal on
TB1/TB2
pin
5.
This
level
is
applied
to
SCR
"B."
When the
SCR
trigger
control circuit level exceeds
the Silicon
Uni
Lateral Switch
(SUS).
both
SCRs (SCR
"A"
and
"B")
will
be
turned
ON
again.
Since
the
positive ac
signal
is
now
at
the
anode
side
of
SCR
"B,"
the
positive
signal
is
passed
through when
the SCR
is
gated
on,
allowing
it to
go
through smoothing
choke
L2
and
through
the
motor armature
and
field
before
coming
back
to
the
junction
of blocking diodes
"A"
and
"B."
The
positive
signal
now
forward
biases
blocking diode
"A,"
allowing
it
to
return to
its
source
at
TB1/TB2
pin
3.
At
zero crossing,
SCR
"B"
turns off.
This
cycle
continues
on
for each
half
cycle,
producing
an
effective
positive dc
level
at
TB 1
/
TB2
pin
2
and
a
negative
dc
level at
TB1/TB2
pin 6.
The
resulting
pulsating,
rectified dc
level
is
passed
through
the
motor brushes, armature,
and
field
windings
to turn
the
motor.
Figure
6-13.
SCR
(Silicon
Rectifier) Bridge
Circuit
RT/T6000B/D
centrifuges
SN 9101800
and
above have a
terminal
block labeled
TB1;
all
other centrifuges
are
labeled
TB2.