94
DC
machines
ω
Φ
Q.
co
ω
CL·
co
)
Reduced field current
Normal field current
(b)
fa A
Figure
6.34
Field winding
Bypass resistor
Figure
6.35
normal although
a
larger speed range
is
achievable using specially
designed motors.
Since steel saturates
at
less than
2
tesla, increasing
the
field cur-
rent
and
hence field strength
as a
method
of
speed reduction
has
very limited applications. This must
be
done by reducing the voltage
on
the
armature.
If
the field
and
torque remain constant,
the
arma-
ture current
and
volt drop will remain constant.
Since
E
=
V-
I
a
R
a
,
as the
applied voltage
V is
reduced
so the
speed will fall
in
proportion
in
order
to
satisfy
the e.m.f.
equation.
Figure
6.34
shows