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THE ELECTRICAL
EQUIPMENT
N
The consequent reduction in the dynamo field current
lowers
the dynamo output, and this, in turn,
weakens the
magnetic
flux
in the regulator core.
The
armature
therefore
returns
to its
original
position,
and
with
the
contacts closed the
dynamo
output rises again to its
regulated maximum. This cycle is then
repeated
and
an
oscillation
of the
armature
is maintained.
As the speed
of
the dynamo rises above
that
at which
the regulator
comes into
operation,
the
periods of con-
tact separation increase in
length
and, as a
result,
the
mean
value
of the
dynamo
output
undergoes prac-
tically no
increase
once
this
regulating speed has been
a ttained.
The
series
or current winding provides
a
compensation
on this system of control, for if the control were arranged
entirely
on
the
basis
Of
voltage there would be a risk of
seriously
overloading the dynamo
when the
battery
was
in
a
low
state
of charge, particularly if the lamps were
in
use
simultaneously.
Under
these conditions,
with
a
battery of low
internal
resistance, the dynamo
output
rises
and,
but for
the
series
wind
ing,
would
exceed
its
normal rating.
The
magnetism due to the
series
winding assists
the
shunt
winding, so that
when the dynamo
is
delivering a heavy
current into a discharged battery the
regulator
comes
into
operation
at a somewhat reduced voltage,
thus
limiting the output
acrordingly.
As s
hown
in
Fig.
N.5,
a
split
series winding is
used,
terminal 'A'
being con-
nected to the battery and
terminal
'AI'
to the
lighting
and
ignition
switch.
By means of a temperature compensation device the
voltage characteristic of
the
dynamo is caused
to
conform
more
closely
to
that of
the battery
under
all climatic
conditions.
In
cold
weather the
voltage required
to
charge the battery at a
given rate increases, whilst
in
warm
weather
the voltage
required
is
lower. The
com-
pensation device
is in
the
form of a
bi-metal
spring
located
behind
the tensioning spring
of
the
regulator
armature.
By
causing the operating voltage of
the
regulator to be
increased
in cold weather an
d
reduced
in bot weath<r the
bi-metal
spring compensates for
the
changing temperature
characteristics
of
the
battery
and
prevents undue
variation
of the charging current
which
would otherwise
oocur. The
bi-metal
spring
also
com-
pensates
for effects
due
to increases in
resistance
of
the
copper windings
from
cold to
working
values.
Cut~
The cut-out
is
an electro-magnetically operated switch
connected in the charging circuit between the dynamo
and
the
battery.
It
automatically
connects the dynamo
with
the battery
when the dynamo
output exceeds that
of the
battery and
disconnects
the
two
when
the
dynamo
output
falls
below that of the
battery,
and
so
prevents
4
2
4
678
A
Fig.
N.6
I
.
Regulator
moving
contacL
2.
Regulator
series
windin~:L
6.
Stop arm.
7.
A
rmature
ton su~
and
lllO\
-
ing
contact.
3. Regulator adjusting
screw.
4.
Cut·Ot:t adjuning screw.
8.
5.
Fixed
contact
blade.
Regulator
fixed
cont3C{
screw.
the battery
from discharging and
possibly
damaging
the
dynamo
windings.
The cut-out consists of an
electro-magnet
fitted
with
an armature which operates a pair
of contacts.
The
electro-magnt'l
employs
two
windings-
a
shunt
winding
of
many
turns
of fine
wire
and
a series winding of
a
few
turns of heavier-gauge
wire.
The contacts are
normally
held
open
and
are closed only
when
the
magnetic pull
from
the
armature
is sufficient
to overcome the tension
o
f
the
a djusting
spring.
The
shunt
coil
is
connected across the dynamo. When
starting,
the
speed of
the
engine
and thus the output
of
the
dynamo rises
until
the <lectro-magnet is
strong
enough
to
overcome the
spring tension and
close
the
cut-out
contacts. The
effect
of
the
charging
current
flowing
through
the cut-out windings creates
a
magnetic
field
in the
same
direction
as that produced
by
the shunt
winding.
This increases the
magnetic
pull on the arrna-
t!lfe
so that the contacts are firmly
closed
and
cannot be
separated
by
vibration.
When
the
speed of
the dynamo
falls to
a point
where
its output is lower
than
that of the
battery,
current
flows from
the
battery through
the
cut-out
series winding and dynamo in
a
reverse direction
to the charging
current. This
reverse current through
the cut-out
will produce
a
differential action
between
the two windings
and
partly demagnetize
the electro-
magnet. The
spring,
which
is
under constant
tension,
then
pulls the armature
away
from the
magnet
a nd
so
separates
the contacts
and opens the
circuit.
N.ll
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