Civil Engineering Reference
In-Depth Information
high due to low resistance, the overall power (and therefore energy delivered in a
given time) is low.
10.2 AC Versus DC Current
All previous analyses in this topic have been for application of direct current to
assist forming, but alternating current (AC) is worthy of consideration, as it is
hypothesized to produce the same effect. Additional considerations with the use of
AC are discussed below.
10.2.1 Energy Analysis
Energy use for the AC case with power factor
cos
(φ)
can be evaluated using the
following relationship (
φ
is the phase angle between current and power). This
equation uses an expression for average power, relevant to AC circuit analysis.
E
EAFAC
=
E
EAFAC,mech
+
E
EAFAC,elec
ε
f
σ
EAFAC
d
ε
+
VI
cos
(
φ
)
d
t
E
EAFAC
=
0
t
f
sin
2
ω
t
d
t
ε
f
(10.15)
0
E
EAFAC
=
σ
EAFAC
d
ε
+
V
m
I
m
cos
(φ)
t
f
0
ε
f
σ
EAFAC
d
ε
+
V
m
I
m
cos
(φ)
2
E
EAFAC
=
0
where
V
m
and
I
m
are the nominal voltage and current values, respectively.
10.2.2 Skin Effect
One consideration for use of AC power is the skin effect, a phenomenon observed
in AC conductors where the current density is higher at the surface of the conduc-
tor than at the center. Though not typically an issue in well-designed AC circuitry,
when AC is applied to EAF, there can be process behavior not captured in the pre-
viously described models.
