Electronics-World-1959-05

RF
GENE RATOR
METAL
ROD
WORK
COIL
MAGNETIC
FLUX
Fig. 5. Basic relation between r.f. generator,
coil, and the rod to be heated.
WORK COIL
RF
GENERATOR
RESISTANC
OF METAL
CURRENTS IN
METAL ROO
SELF-
INDUCTANCE
Fig. 6. Cross- section of currents in rod.
In addition to the few examples outlined
here, there are a host of special
purpose applications where induction
heating is often the only method which
permits the efficient production of a
particular metal part. Whenever metal
must be heated, induction heating
offers a rapid, efficient, and easily controllable
source of heat. The basic
reason for this lies in the principle of
induction heating -the heat is generated
electronically, directly in the
work piece itself.
How It Works
The device that generates the power
for the induction heating process is
very similar to a radio transmitter. It
takes low -frequency power from the
power line and converts it into a high -
frequency signal. In an ordinary radio
transmitter the signal is sent out over
the antenna and radiates through the
air. In addition to the power that is
radiated, a certain amount of power is
lost because none of the components
is "ideal." Thus we know that in a
power transformer there are losses due
to hysteresis and eddy currents. To
keep the latter to a minimum, laminated
rather than solid steel cores are
used. In addition there are losses in
capacitors and coils as well as in the
purely resistive circuit elements. In a
radio transmitter the ratio of input
from the power line and the antenna
output is an indication of its efficiency.
In induction heating equipment, the
amount of radiated energy is kept to a
minimum and the losses, concentrated
on the work piece, are a measure of its
efficiency. Here the eddy current and
hysteresis losses are utilized to heat up
the work piece. The r.f. energy is concentrated
in the metal by means of a
work coil which is designed to fit the
particular piece to be heated.
Fig. 5 shows the basic relationship
between the r.f. generator, the work
coil, and a steel rod which is to be
heated. The generator puts a current
Fig. 7.
Depth of penetration for metals.
through the work coil and this current
sets up a magnetic flux. The alternating
magnetic flux, in turn, sets up a
voltage to be precise, a counter -electromotive
force -which causes a current
to flow in the metal. This is the
eddy current and, depending on the
type of metal and the frequency used,
Fig. 8. Photo above shows the front panel
of a 7.5-kw. induction heating generator.
Note simplicity of the controls required.
more eddy current tends to flow on the
outside of the metal than in the inner
core. This characteristic. called "skin
effect." is used to regulate the depth
of heating by proper frequency choice
and is especially useful in such applications
as surface hardening. In magnetic
materials there is a secondary
heating effect due to hysteresis losses
but these are relatively small and are
not usually considered in calculations
of heating efficiency.
To show how the skin effect works
for the smooth rod used as an example
in Fig. 5, a simplified electrical presentation
of the currents in the metal is
shown in the cross -section drawing of
Fig. 6. This shows that the current
flowing in each circular path sets up a
flux opposing the work coil flux, thereby
acting as an electromagnetic shield
for the material inside it. For this
reason the flux in the inner concentric
ROUND
RECTANGULAR
FORMED
AREA TO BE
- HARDENED'
AREA TO BE
HARDENED
PANCAKE
SPIRAL -HELICAL
INTERNAL
(A)
(B)
;D
`fID
EXTERNAL COIL
FLAT OR PANCAKE
(C)
INTERNAL COIL
Fig. 9. This illustration shows typical work coils that are
employed in induction heating equipment. (A) shows some of
the common shapes used. (B) illustrates how special shaping
is used to fit the metal sections that are to be heated. (C)
shows placement of work coils with respect to work pieces.
May. 1959
37