Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Electric induction heating
has been used in manufacturing processes since shortly after Benjamin Franklin invented
electricity. Of course I jest about Franklin; he didn't invent electricity but discovered
that lightning was a form of electrical discharge. One of the most energy-consuming
forms of induction heating is that used by
Alcoa for smelting aluminum. Beyond that are
many thousands of processes ranging from forming, tempering, and joining metal parts
to cooking food and curing adhesives. Both Tocco and Ajax-Northrup, now
brands of equipment are featured in this 1955 article which appeared in Popular
Electronics magazine. Some processes work by directly inducing a high current
in the primary target object - usually metallic - being treated. Others induce the
current in a secondary object like a metallic cooking container which then warms
the target object through heat conduction process. Ajax Tocco has a pretty awesome
video providing an overview of their capabilities - including some applications
you probably never knew were performed via induction heating. They also have a slew
of videos for individual processes.
Current following through a conductor sets up magnetic field
(a) which may be concentrated by bending the conductor to form a loop (b), or by
providing a core of magnetic material (c).
In the first part of this two-part story, the Editors of Popular Electronics
report on modern advances in the scientific use of induction heaters by industry.
Without heat, man never could have smelted metals and built the great civilization
of today on its foundation of steel, copper, tin, and other metals. For many, many
centuries, heat was intimately associated with fire, but in comparatively modern
times, the development and use of electrical power has led to heat without fire.
When an electric voltage is applied to a conductor, the conductor offers a resistance
to the flow of current and the current, in overcoming this resistance, produces
heat. The amount of heat produced depends on both the resistance of the material
and the amount of current.
However, even with the development of electric heaters, heat had to be produced
before it could be applied. But today in factories and plants all over the world
heat is being used without initial generation. Instead, the heat is generated within
the material being processed. Two kinds of heaters are in use: induction heaters,
used with conductors such as iron and steel; and dielectric heaters, used with insulators,
such as wood and plastics. Induction heaters will be discussed now. In a subsequent
issue of Popular Electronics dielectric heaters will be covered.
Principles of Induction Heating
When electric current flows through a conductor, heat is produced. In addition,
a magnetic field is set up around the conductor. This magnetic field may be concentrated
by bending the conductor in loops to form a coil and by providing a core of magnetic
material. The intensity of the magnetic field depends on three factors: the amount
of current in the conductor; the number of turns in the coil; and the type of core
A magnetic field will couple the primary and secondary windings
in a transformer.
The effect of the electromagnetic field produced by loading the
coil is shown above.
If an alternating current flows through the conductor, a changing magnetic field
will be produced which will build up to maximum intensity, collapse, and build up
to maximum intensity again, but with reversed polarity. If this varying magnetic
field is brought near a second conductor, a current will be induced in the second
conductor. This principle is used in transformers. Two or more coils are placed
fairly close together. An alternating current is passed through the first conductor,
or primary winding, and this current produces a magnetic field which induces currents
in the secondary windings.
When the primary coil surrounds a relatively solid conducting mass, the induced
currents are not channeled as in a transformer, but flow like eddies in many directions.
Since the solid mass acts like a short circuit, these eddy currents may be large,
causing considerable heat to be generated within the mass. It is on this "transformer"
action that induction heating is based.
Where the core is a magnetic material, such as iron or steel, it is rapidly magnetized,
demagnetized, and remagnetized in the opposite direction. The individual molecules
change position with each magnetization change, resulting in considerable molecular
"friction" and producing additional heat. The molecular "friction" which keeps the
molecules of the material from changing position easily is known as hysteresis.
In a conventional transformer, the heat produced by eddy currents and hysteresis
represents a power loss. Although induction heating equipment is designed to produce
such heat in the material being processed, iron core power transformers are designed
to keep such heating to a minimum. To accomplish this, the cores of such transformers
are made of thin laminations of steel rather than a solid piece. The thin laminations
have a comparatively high resistance, keeping eddy current losses at a minimum.
As the frequency of the alternating current is increased, the induced currents
tend to flow nearer the surface of the conductor. At fairly high frequencies, the
induced currents may be concentrated in a very thin "skin" right at the surface.
This is known as skin effect. Since heating depends to a large extent on these induced
currents, induction heating is especially valuable in heating only the surface of
a piece of work, such as a bearing or a gear face.
Types of Induction Heaters
Commercial induction heaters consist of two major components: the alternating
current power source or "generator" and the work coil which is coupled to the load
and serves to change the alternating current to a varying magnetic field. The frequencies
used for induction heating range from 25 cycles per second to several megacycles.
Where commercial line frequencies (25-60 cps) are involved, electrical energy may
be obtained directly from the power lines. But where higher frequencies are required,
there are three basic types of "generators" widely used by modern industry: the
motor-generator set; the spark-gap oscillator; and the vacuum-tube oscillator.
The Motor-Generator Set: A motor-generator set is an
electric motor driving a high frequency generator.
This Tocco heating station with inductor and fixture is used
for brazing hydraulic cylinders and cylinder cap assemblies.
Due to practical limitations on generator size and speed, output frequencies
of 1000, 3000, and 10,000 cps are the most popular and the majority of commercial
induction heater motor-generators are designed to supply power at one of these frequencies.
Power output for a single unit may range from less than 10 kw. to more than 1200
kw. Where higher powers are required, a "bank" of individual units may be used.
Most commercial units have efficiencies running from 60 to 90 per-cent. Motor-generator
sets are used where large amounts of relatively low frequency power are required
for such applications as forging, melting, and deep hardening.
The Spark-Gap Oscillator: The first high frequency
generators used for induction heating were spark-gap oscillators. Dr. Edwin F. Northrup,
one of the pioneers in the induction heating field, did most of his work in the
early 1900's, when spark-gap oscillators were widely used for "wireless" communications.
Although vacuum tube oscillators have replaced spark-gap units in the communications
field, spark-gap oscillators are still extensively used by industry.
In a spark-gap oscillator an inductance coil is connected in series with a spark-gap
and both are connected across a large capacitor. A high a.c. voltage is applied
to the capacitor. Normally, the spark-gap is non-conducting and acts as an open
circuit. The voltage charge of the capacitor builds up to near its peak value, at
which point the gap breaks down and effectively closes the connection between coil
and capacitor, forming an oscillatory tuned circuit. Very heavy currents surge back
and forth between the coil and capacitor, as the capacitor is alternately charged
and discharged, at a frequency determined by the inductance and capacity values
of the components. With such current surge, the amplitude drops slightly, as energy
is dissipated in the circuit and in the load, until there is no longer sufficient
energy to maintain the spark. At this point the spark dies out and the gap again
acts like an open circuit. The output of a spark -gap oscillator is thus a series
of high frequency pulses.
Spark-gap oscillators are generally designed to operate in the frequency range
of from 20 to 500 kc. Most manufacturers rate them by power input rather than by
power output. Standard spark-gap oscillators are made with power ratings up to about
40 kw., and down to about 2 kw. With a wide frequency range and low to moderate
output powers, spark-gap oscillators are well suited to such applications as melting
and forging small to moderate sized pieces.
Feed rolls push bar stock through a set of Tocco induction coils
which heat the stock to a temperature of about 2350 degrees F.
This huge Ajax-Northrup heater coil is used for the hot pressing
of carbide steels. Portable units are also available for this purpose.
The Vacuum-Tube Oscillator: Except for the power ratings,
the oscillator circuits are similar to the circuits used in radio transmitters.
But there is a difference between a high-power induction heater and a high-power
radio transmitter. Broadcast transmitters generally employ a low to moderate power
oscillator, followed by several stages of amplification. Induction heaters, or the
other hand, use high power oscillators directly.
The plate (output) and grid (input) circuits of a vacuum-tube amplifier are coupled
so that part of the output energy is fed back to the input to overcome circuit losses
and to start and sustain oscillation. A tuned circuit is incorporated in either
the plate or grid circuits (or both) to establish the operating frequency. The operation
of a vacuum-tube oscillator is similar to that of a spark-gap unit, but with a vacuum
tube replacing the spark-gap as a source of power pulses. However, the high frequency
output is obtained as a continuous wave rather than as a series of pulses with a
comparatively low repetition rate.
Vacuum-tube power oscillators are used at frequencies of from 150 kc. to one
mc. and higher. Commercial units are available with power ratings from a few hundred
watts to hundreds of kilowatts.
Epimetheus and Prometheus were assigned the task of providing
man and animals with faculties necessary for their preservation. Epimetheus was
to do the actual fitting, with Prometheus supervising. Claws were bestowed to one
animal, protective armor to another, and wings, fangs, and special coloring to others.
But when Epimetheus came to man, he had exhausted his store of gifts and his orders
had been to make man superior to all creatures. When Prometheus learned of this,
he went to heaven to light his torch at the chariot of the sun and he brought fire
to mankind. Zeus was enraged for man was now able to do things which had only been
done by gods. And Prometheus was bound to a rocky cliff for all eternity because
he dared to bring the power of fire to man. - Greek Mythology
Induction Heater Applications
Induction heaters are used in the metal working industries and for soldering
the seals of canned foods, hardening machine parts, annealing sheet metal products,
brazing fittings, and melting small lots of metals in laboratories developing new
alloys. In many cases induction heaters are used to supplement other heating techniques
or to do a job faster and more efficiently than older methods. However, induction
heating is frequently used in applications where no other heating method has been
One example of this type of application is in vacuum heating and melting, where
metals are processed in a vacuum. Another example is in the vacuum-tube manufacturing
industry. During the evacuation process, the metal elements of vacuum tubes must
be heated to high temperatures to drive out gases which might later shorten the
service life of the completed tube. Since the tube electrodes are surrounded by
a glass envelope, a direct method of heating is not practical, but induction heating
may be used when the coil is placed around the outside of the tube's envelope.
Many present-day applications of induction heaters seem almost like "black magic."
A person can place his hand in some types of induction furnaces and, if the furnace
were not charged with a load of metal and if he wore no ring, he would feel no heat.
Yet this same furnace could melt a piece of steel in minutes.
One problem in melting some types of metals is finding a furnace lining which
will not melt itself under the intense temperature. With induction heating it is
possible to float a "charge" of metal in mid-air during the heating and melting
process, then to release the melted ball at will! This has already been accomplished
in laboratory experiments.
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