<s>
The	O
process	O
of	O
generating	O
a	O
magnetic	O
field	O
by	O
means	O
of	O
an	O
electric	O
current	O
is	O
called	O
excitation	B-Algorithm
.	O
</s>
<s>
The	O
flexible	O
excitation	B-Algorithm
of	O
a	O
rotating	O
electrical	O
machine	O
is	O
employed	O
by	O
either	O
brushless	O
excitation	B-Algorithm
techniques	O
or	O
by	O
the	O
injection	O
of	O
current	O
by	O
carbon	O
brushes	O
(	O
static	O
excitation	B-Algorithm
)	O
.	O
</s>
<s>
Except	O
for	O
permanent	O
magnet	O
generators	O
,	O
a	O
generator	O
produces	O
output	O
voltage	O
proportional	O
to	O
the	O
magnetic	O
flux	O
,	O
which	O
is	O
the	O
sum	O
of	O
flux	O
from	O
the	O
magnetization	O
of	O
the	O
structure	O
and	O
the	O
flux	O
proportional	O
to	O
the	O
field	O
produced	O
by	O
the	O
excitation	B-Algorithm
current	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
excitation	B-Algorithm
current	O
the	O
flux	O
is	O
tiny	O
and	O
the	O
armature	O
voltage	O
is	O
almost	O
nil	O
.	O
</s>
<s>
Modern	O
generators	O
with	O
field	O
coils	O
are	O
usually	O
self-excited	B-Algorithm
;	O
i.e.	O
,	O
some	O
of	O
the	O
power	O
output	O
from	O
the	O
rotor	O
is	O
used	O
to	O
power	O
the	O
field	O
coils	O
.	O
</s>
<s>
Self-excited	B-Algorithm
generators	O
must	O
be	O
started	O
without	O
any	O
external	O
load	O
attached	O
.	O
</s>
<s>
a	O
shunt	O
,	O
the	O
simplest	O
design	O
,	O
uses	O
the	O
main	O
winding	O
for	O
the	O
excitation	B-Algorithm
power	O
;	O
</s>
<s>
an	O
excitation	B-Algorithm
boost	O
system	O
(	O
EBS	O
)	O
is	O
a	O
shunt	O
design	O
with	O
a	O
separate	O
small	O
generator	O
added	O
to	O
temporarily	O
provide	O
an	O
energy	O
boost	O
when	O
the	O
main	O
coil	O
voltage	O
drops	O
(	O
for	O
example	O
,	O
due	O
to	O
a	O
fault	O
)	O
.	O
</s>
<s>
This	O
may	O
be	O
a	O
battery	O
,	O
a	O
house	O
unit	O
providing	O
direct	O
current	O
,	O
or	O
rectified	B-Algorithm
current	O
from	O
a	O
source	O
of	O
alternating	O
current	O
power	O
.	O
</s>
<s>
Even	O
small	O
portable	B-Algorithm
generator	I-Algorithm
sets	I-Algorithm
may	O
occasionally	O
need	O
field	O
flashing	O
to	O
restart	O
.	O
</s>
<s>
The	O
critical	B-Algorithm
field	I-Algorithm
resistance	I-Algorithm
is	O
the	O
maximum	O
field	O
circuit	O
resistance	O
for	O
a	O
given	O
speed	O
with	O
which	O
the	O
shunt	B-Algorithm
generator	I-Algorithm
would	O
excite	O
.	O
</s>
<s>
The	O
shunt	B-Algorithm
generator	I-Algorithm
will	O
build	O
up	O
voltage	O
only	O
if	O
field	O
circuit	O
resistance	O
is	O
less	O
than	O
critical	B-Algorithm
field	I-Algorithm
resistance	I-Algorithm
.	O
</s>
<s>
Brushless	O
excitation	B-Algorithm
creates	O
the	O
magnetic	O
flux	O
on	O
the	O
rotor	O
of	O
electrical	O
machines	O
without	O
the	O
need	O
of	O
carbon	O
brushes	O
.	O
</s>
<s>
The	O
concept	O
was	O
using	O
a	O
rotating	O
diode	O
rectifier	B-Algorithm
on	O
the	O
shaft	O
of	O
the	O
synchronous	O
machine	O
to	O
harvest	O
induced	O
alternating	O
voltages	O
and	O
rectify	O
them	O
to	O
feed	O
the	O
generator	O
field	O
winding	O
.	O
</s>
<s>
Brushless	O
excitation	B-Algorithm
has	O
been	O
historically	O
lacking	O
the	O
fast	O
flux	O
de-regulation	O
,	O
which	O
has	O
been	O
a	O
major	O
drawback	O
.	O
</s>
<s>
Modern	O
rotating	O
circuitry	O
incorporates	O
active	O
de-excitation	O
components	O
on	O
the	O
shaft	O
,	O
extending	O
the	O
passive	O
diode	O
bridge	O
.	O
</s>
<s>
Moreover	O
,	O
their	O
recent	O
developments	O
in	O
high-performance	O
wireless	O
communication	O
have	O
realized	O
fully	O
controlled	O
topologies	O
on	O
the	O
shaft	O
,	O
such	O
as	O
the	O
thyristor	O
rectifiers	B-Algorithm
and	O
chopper	O
interfaces	O
.	O
</s>
