<s>
Doubly-fed	B-Algorithm
electric	I-Algorithm
machines	I-Algorithm
also	O
slip-ring	B-Algorithm
generators	I-Algorithm
are	O
electric	O
motors	O
or	O
electric	O
generators	O
,	O
where	O
both	O
the	O
field	O
magnet	O
windings	O
and	O
armature	O
windings	O
are	O
separately	O
connected	O
to	O
equipment	O
outside	O
the	O
machine	O
.	O
</s>
<s>
Doubly	B-Algorithm
fed	I-Algorithm
generators	I-Algorithm
are	O
another	O
solution	O
to	O
this	O
problem	O
.	O
</s>
<s>
With	O
its	O
origins	O
in	O
wound	O
rotor	O
induction	O
motors	O
with	O
multiphase	O
winding	O
sets	O
on	O
the	O
rotor	O
and	O
stator	O
,	O
respectively	O
,	O
that	O
was	O
invented	O
by	O
Nikola	O
Tesla	O
in	O
1888	O
,	O
the	O
rotor	O
winding	O
set	O
of	O
the	O
doubly-fed	B-Algorithm
electric	I-Algorithm
machine	I-Algorithm
is	O
connected	O
to	O
a	O
selection	O
of	O
resistors	O
via	O
multiphase	O
slip	O
rings	O
for	O
starting	O
.	O
</s>
<s>
Doubly-fed	O
induction	B-Algorithm
generator	I-Algorithm
(	O
DFIG	O
)	O
,	O
a	O
generating	O
principle	O
widely	O
used	O
in	O
wind	O
turbines	O
.	O
</s>
<s>
It	O
is	O
based	O
on	O
an	O
induction	B-Algorithm
generator	I-Algorithm
with	O
a	O
multiphase	O
wound	O
rotor	O
and	O
a	O
multiphase	O
slip	O
ring	O
assembly	O
with	O
brushes	O
for	O
access	O
to	O
the	O
rotor	O
windings	O
.	O
</s>
<s>
A	O
better	O
alternative	O
is	O
a	O
brushless	B-Algorithm
wound-rotor	I-Algorithm
doubly-fed	I-Algorithm
electric	I-Algorithm
machine	I-Algorithm
.	O
</s>
<s>
The	O
doubly-fed	B-Algorithm
generator	I-Algorithm
rotors	O
are	O
typically	O
wound	O
with	O
2	O
to	O
3	O
times	O
the	O
number	O
of	O
turns	O
of	O
the	O
stator	O
.	O
</s>
<s>
As	O
a	O
summary	O
,	O
a	O
doubly-fed	O
induction	O
machine	O
is	O
a	O
wound-rotor	B-Algorithm
doubly-fed	I-Algorithm
electric	I-Algorithm
machine	I-Algorithm
and	O
has	O
several	O
advantages	O
over	O
a	O
conventional	O
induction	O
machine	O
in	O
wind	O
power	O
applications	O
.	O
</s>
<s>
First	O
,	O
as	O
the	O
rotor	O
circuit	O
is	O
controlled	O
by	O
a	O
power	O
electronics	O
converter	O
,	O
the	O
induction	B-Algorithm
generator	I-Algorithm
is	O
able	O
to	O
both	O
import	O
and	O
export	O
reactive	O
power	O
.	O
</s>
