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History of Induction Heating

Induction heating equipment for induction hardening, induction brazing, soldering, melting, forging, and other forms of heat transfer to metallic parts is made in a wide range of sizes to meet a great many heating requirements in industrial plants. These units are available at different output-power ratings and at various frequencies, each of which covers a certain range of induction heating applications.

There are four basic types of induction heating equipment used for inductively heating metal parts. These are: (1) the motor-generator set; (2) the spark-gap converter; (3) the vacuum-tube or electronic-type generator; and (4) the solid state medium and high frequency generator. In principle they are alike in that an inductor or heating coil surrounds the work to be heated, or perhaps is formed to suit the area requiring heat, so that with a flow of current induced to the work’s surface from the induction coil, an immediate heating action takes place. However, since the frequency of the current has a direct effect on the depth of heat penetration, one type of unit will perform certain types of heating operations better than others.

Motor-generator sets

For heat-producing current at the lower frequencies of from 2 to 9 kHz, motor-generator sets are available with power-output ratings of from 5 to 500 kW. At the frequencies covered by these units a deep penetration of heating takes place. These sets therefore are widely used for the heating of parts where deep hardening is required, or for the through heating of bars, such as those required for forging. When in operation, the motor-generator runs continuously, most usually at a speed of 3,600 rpm. This is very costly and can be reduced significantly with solid state technology. One of the earliest means of producing high-frequency current is by the spark-gap-type converter. Units of this type are made in sizes from 5 to 50 kW input power range, and at operating frequencies of from 25 to 250 kHz. This type of set is not feasible in large-sized units and, while rated by its input power, it has an output efficiency of approximately 50 percent, so that the power output rating comparable to other units would be from 2 1/2 to 25 kW. In order to obtain correct efficiency, a series of spark gaps must be accurately adjusted. Power consumption is low during stand-by periods, as compared with the motor-generator type set.

Vacuum-tube oscillators

The electronic-type generator, using vacuum tubes for power transmission and oscillation, has perhaps the broadest scope in the induction heating field. Generators of this type are available in ranges from 1 to 400 kW output power, and with frequencies of from 100 to 1,000 kHz, or more if desired. Their broadest use, however, is in the 5 to 50 kW output range, at frequencies of 150 to 500 kHz. In operation, they are silent and use little power during stand-by periods or when required only for heating of the tube filaments. Maintenance requirements are exceptionally low, especially when compared with the spark-gap-type sets, which depend on gap settings for correct power transmission. The power tubes render a comparatively long service life, of 10,000 hr. or more, for average applications. Units of this type provide a constant and uniform power output throughout their tube life.

Selection of induction heating equipment

Most induction heating equipment can be used continuously for single-purpose operations and are flexible enough to permit change-over from one operation to another. They can be set up in a production line and operated as any other machine for a single purpose, or they can be installed in a central location within a plant and arranged to handle a multitude of heating operations.

For general-purpose induction heating applications, it should be remembered that any one type of unit is limited to work pieces within a given size range. In other words, a generator of 10 kW cannot handle the induction hardening of a large component, say for example a gear 10 inches in diameter with a 1-inch face. Such a unit would very likely be too large for delicate parts, like those required for instruments, or where a very small area may require heating, unless such parts can be handled in multiples in order to distribute the available power over a larger mass.

In attempting to surface-heat parts that are oversized for the available power of a generator, the advantages of rapid induction heating are lost. The absorption of heat will be slow and sometimes will dissipate itself into the inner area before the desired temperature is reached on the outside surface. In other cases the proper heat eventually may be attained, but much of the energy will go deep into the surface of the part, largely by conduction, possibly resulting in excessive distortion, indicating that more power energy is needed.

The selection of induction heating equipment, therefore, resolves itself to analysis of the mass or weight of the material to be heated and the temperature rise required, in relation to the available output power of the generator. It must also be determined whether a deep or shallow heated surface is required, which in turn governs the frequency.

If a 2-inch steel shaft requires through heating on one end for forging, a low frequency of 9.6 kHz would be appropriate. If, however, a surface hardness of 1/32 to 1/16 inches on this same shaft is required, a higher frequency of 200 to 500 kHz would be preferable. For the surface hardening of a pinion, say 5 inches o.d., with a 3/4 inch face, and 10 pitch, requiring a quick, shallow heat layer around the contour of the teeth, a high frequency would be preferable to one of 5 to 10 kHz, which most likely would penetrate a heat band through the entire tooth form.

Representative of the motor-generator-type set is a unit with an output rating of 20 kW. The motor and the generator are located in the base, into which also are assembled the control board and the rectifiers. The upper section houses the output transformer, capacitors, and timer. At the front panel are located the two connectors for the heating coil while at the front is located a suitable splash guard around the workpan. The unit has a single outlet station and is arranged for progressive hardening of the outer surfaces of steel shafts. The water quench is supplied through the heating coil, and is controlled by a timer which operates a solenoid value.

This type of induction heating equipment is also made with two or three work stations and may be equipped with rotatable-type transformers so the radial position of the heating coil can be adjusted as needed to accommodate certain types of work pieces. The transformers are of the iron-core type and have water-cooled primary and secondary.

When larger motor generators are required, they are usually located remotely from the hardening or heating station. An example would be a 800-hp, 500-kW alternator used for the treatment of tractor gears. The motor and the generator both are totally enclosed and are hydrogen cooled. The generated current is 9.6 kHz. The unit includes four banks of capacitors to improve the power factor. It also includes automatic-control equipment in three steps for regulation of the current during the heating cycle. This compensates for the transformation taking place as the metal being heated passes its critical temperature and loses its magnetism.

Spark-gap circuit

In many ways, this circuit resembles the spark-gap transmitters of early radio development. This circuit has been found effective for generating frequencies in the range from 100 to 200 kHz and is used more broadly in units having power output ratings ranging from 2 to 15 kW.

Water-cooled spark gaps with tungsten disks about 1-inch in diameter are used with as many as 30 gaps connected in series of higher powered units. This allows the distribution of heat over a greater area, readjusted to a spacing of 0.003 to 0.004 inches about every 25 to 50 working hours, to maintain peak efficiency and to keep the heating time uniform.

Each unit will usually have a few air-cooled gaps in series with the water-cooled gaps to ensure starting of the spark even though condensation on the water-cooled gap shorts them out for a time after the cooling water is turned on. A few minutes of operation will usually heat the gaps enough to drive off this external condensation of moisture. With extreme moisture present, however, the water used for cooling will require heating to a temperature that will overcome condensation.

The induction coil and tank condensers form a parallel-resonant tank circuit which is, in turn, connected into a series-resonant circuit across the spark gap. Tuning involves adjusting the series tuning coils from maximum output current in the work coil, as determined by a radio-frequency ammeter, loosely coupled to the output circuit.

Maximum output current is obtained only when the series-resonant circuit is tuned to the same frequency as the output tank circuit. If a particular frequency is required, the series-tuning coils can be set for this frequency rather than for resonance if there’s sufficient reserve power to permit operating the tank circuit off its resonant frequency.

Induction heating coils used with spark-gap sets usually are limited to those made of flattened copper tubing, shaped to provide the necessary number of turns either around or inside the object to be heated. The majority of coils have from 2 to 10 turns. Cooling water is forced through the work coil and its connecting leads to prevent overheating during operation, since a coil may carry up to 1,000 amps of radio-frequency current.

In spark-gap units, the radio-frequency output power can be switched on and off by a remote-control magnetic switch, operated by an automatic timer, foot switch, or small pilot switch. The work coil has only a few turns and the high-frequency circuits are balanced to give zero potential to ground at the center of the coil; hence the work coil can be touched by the operator while power is on without harmful effects. Mica blocking capacitors prevent the high voltage across the spark gaps from reaching the work coil.

There is likely to be some radio-frequency radiation from the work coil and its leads. The radiation from the spark gaps is usually spread over a wide portion of the frequency spectrum, but the energy at any particular frequency may travel only a short distance. In some instances where interference is observed, because of a peculiar combination of circumstances, the radiation can be eliminated by placing a metal screen completely around the unit.

A typical unit is usually attached to a suitable worktable for brazing or hardening purposes. The entire assembly is mounted within a wooden frame so that stray high-frequency currents will not be absorbed or otherwise dissipated.

A 20 kW converter of the spark-gap type and a furnace capable of melting up to 17 lb. of metal is typical of this type of induction heating equipment. The converter consists essentially of a high-reactance transformer, a discharge gap, and a bank of capacitors connected to the furnace coil. The frequency will vary generally between 25 and 40 kHz, depending upon the size of furnace used. Single-phase alternating current, usually 220 or 440 volts, is required. The power efficiency will average about 50 percent. The transformer is fitted with a high reactance and is contained in a non-magnetic, oil-filled, water-cooled case.

The capacitors are high-voltage, heavy-duty units, and each is self-contained in a sealed tank and adapted for individual water cooling. The discharge gap comprises a mercury pool, two specially tipped copper electrodes, and an atmosphere of hydrogen, contained in a water-cooled chamber. The hydrogen is maintained in the gap by a pressure governor. Control of power is obtained by turning a hand wheel that regulates the volume of mercury in the discharge-gap chamber.

The furnace comprises an electrical conductor that is hollow and wound in a helix close to the charge to be heated. High-frequency current passed through the coil creates an electromagnetic field about it, which induces current in a charge for heating or melting, or for stirring it after it is melted. As the electro-magnetic field passes through the nonconductors, it is possible to heat or melt a charge without heating any of the surrounding material, except as it may be heated indirectly by the charge itself.

With the installation, a charge of 15 to 17 lb. of steel can be melted in a non-conducting crucible in about 50 minutes. A copper charge of the same size requires about 50 minutes also, but when melted in a conducting-type crucible it requires only about 18 minutes.

Tube-type oscillator

This is a vacuum-tube-type generator with a typical output rating of 20 kW. The two output leads are on the right-hand side. Only one manual control requires adjustment in connection with the operation of this generator.

The unit consists of two basic sections: the power-supply section and the oscillator section. The induction heating equipment converts ordinary power-line frequencies to frequencies of the order of 375 kHz. The input energy is fed to a plate transformer in the power-supply section, which raises the voltage to approximately 10,000 volts. This 10,000-volt alternating current is then fed to a four-tube, bridge-type rectifier that converts the alternating current to full-wave, rectified direct current at approximately 9,000 volts average value. The high-voltage direct current feeds the oscillator section through choke coils and by-pass condensers which prevent the feedback of high-frequency energy into the power source.

The oscillator section consists of water-cooled, three-electrode vacuum tubes, a tank circuit made up of a tank condenser and a tank-coil inductance, and a coil for feeding back high-frequency energy to the grids of the three-electrode vacuum tubes. The direct current from the power-supply section is applied across the plate and filament of the oscillator tubes, and the flow of current in these tubes is controlled by the action of the grid. The oscillator tubes feed high-frequency energy through a blocking condenser, which prevents the passage of direct current but permits the passage of high-frequency current to the tank condenser and tank inductance. The tank circuit is designed so that it has a natural resonant frequency of approximately 375 kHz. When this circuit receives a direct-current impulse, its natural tendency is to oscillate at its basic frequency in exactly the same manner as a pendulum oscillates when it has received a mechanical impulse.

When the tank circuit oscillates, alternating current flows in its component parts. This current produces a magnetic field that may be picked up by a coupling unit, and a small portion of the available energy is returned to the grids of the oscillator tubes. This grid energy causes the tube to periodically “block” and “fire” in synchronism with the resonant tank circuit. Thus, impulses of energy are fed to the tank circuit at exactly the right moment so that the oscillations are continuous and do not decay with time as the motion of a pendulum decays after having received only one mechanical impulse.

The circulation of this high-frequency current in the tank current makes energy available for use in various applications. With induction heating equipment, it’s usually desirable to circulate a large high-frequency current in a water-cooled “work coil”, which surrounds the work being heated. If a step-down transformer is interposed between the inductance of the tank circuit and the work coil, it’s possible to increase the current available in the work coil and at the same time reduce the voltage applied to it. In order to obtain optimum heating results with this type of induction heating equipment, it’s necessary to have the electrical impedance of the work coil match the terminal impedance of the generator. This is accomplished by having several types of transformers designed so they cover the usual impedance range of the various work coils encountered in industrial applications. These transformers permit the circulation of very heavy currents at very low voltages, which in turn make it possible to heat many objects with a simple, single turn of copper tubing or a single-turn copper block.

The filaments of the rectifier and oscillator tubes are energized from small-filament transformers. Power is applied to the work by energizing the plate circuit of the oscillator by a push-button control that is located adjacent to the working position. A timer, which may be set at will to any desired value, is incorporated in the generator. This timer automatically shuts down the equipment as soon as the predetermined heating cycle has been completed. Pushing the button restarts the timer and the operation is automatically repeated.

Several protective devices are incorporated in the equipment. In the event of water failure to the oscillator tubes, one of two devices will function: 1) a flow switch that operates on gallons per minute flow only, and 2) a pressure switch that operates on pounds per square inch only. A reduction in flow of water or pressure will automatically trip these devices. An overload relay is installed in the plate circuit so that, if excessive power is drawn from the equipment, it will be tripped automatically from the line. A time-delay relay is installed in the equipment to prevent application of plate voltage before the filaments of the tubes are sufficiently heated upon starting the equipment.

Miscellaneous induction heating equipment

With somewhat larger induction-melting installations, power is obtained from a motor-generator set located directly behind the control panel. The circuit-breaker ammeter, voltmeter, power-factor meter, wattmeter, and rheostat handles for voltage control are mounted on the panel. This installation is used for melting steel alloys and it operates at 2 kHz 60 kW power. The furnace handles 100 lb. of metal, which can be melted in approximately 30 minutes.

The furnace is built as a tilting unit and is provided with flexible cable leads, which enable power to remain on during pouring. This is advantageous when several castings are poured from one heat, since the molten metal remains at correct pouring temperature regardless of the time interval between castings.

Another type of vacuum-tube induction-heating generator has six rectifier tubes and two oscillator tubes and is rated at 20 kW output. It typically uses a three-section generator capable of delivering 100 kW at a frequency of 200 kHz. The section on the left contains the rectifier portion of the circuit. The center and right-hand sections contains the power-oscillator tube and oscillator circuit. This unit furnishes the high-frequency power for the flowing of tin plate.

Low-frequency heater

This type of induction heating equipment is used for the expansion of steel parts that are to be shrunk onto shafts, such as gears onto fly wheels. This type of induction heating equipment operates on a relatively low cycle of 50 or 60 Hz and usually is applied to the heating of parts up to 500 or 600˚F. This is comprised of iron-core transformer which is opened and closed by means of a pneumatic cylinder to facilitate insertion and removal of the gear or part to be heated. The transformer in this case is the primary of the circuit, where as the gear itself forms a one-turn secondary winding.

The induction heating equipment typically used has a rating of 15 kVa and accommodates rings from 12 to 36 inches in diameter. The heater is provided with an automatic timer so the cycle can be duplicated once the proper heating time has been established.

Another low-frequency induction heating equipment is a single-station heating unit of the motor-generator type, as used for forging. The steel bar is usually heated to 2200˚F at one end. The heating unit is located adjacent to an upsetting press, so that the heated bar can be transferred quickly from one to the other.

A similar heater may have two heating crucibles. These furnaces are usually loaded manually but otherwise are controlled automatically with regard to the time and heating cycles. An end of a 2 inch bar can be heated to 2200˚F in approximately 2 minutes, and the power required for heating is from 0.2 to 0.25 kwhr, per pound of metal heated.

Solid State induction heating equipment

Today we have a wide range of induction heating equipment that utilizes various forms of solid state power supplies, from SCR, IGBT and MOSFET power circuits.

SCR- or Thyristor-based induction heating equipment is normally used in large power ratings for mass heating applications. These devices are considered inexpensive when compared to IGBT and MOSFET based induction heating equipment. Usually they are used in induction heating equipment to produce from 3 kHz to 30 kHz.

IGBT-based induction heating equipment is typically used for medium frequency applications that will range generally from 10 kHz to 90 kHz. This range is ideal for most heat treating induction heating equipment and is used for gear hardening, camshaft hardening, crankshaft hardening and most heat treating applications that require 1.5 mm to 3 mm case depth.

MOSFET-based induction heating equipment is used for high frequency applications generally in the 100 to 400 kHz range. Some companies offer equipment operating in this frequency range that use IGBT devices but it’s not operating at a true fixed high frequency range. It is achieved by using an internal ringing circuit to get to the higher frequency level and is not consistent or reliable. MOSFETS are the most reliable devices for true high frequency induction heating equipment. High frequency induction heating equipment is used for heat treating applications that require hardened depth areas of 0.5 mm and greater.

Some more advanced induction heating equipment can operate at 1 MHz. This frequency range is ideal for certain food and drug packaging induction heating equipment. It is also used for selective heat treating of very small parts.

Another relatively new use of solid state induction heating equipment is for simultaneous dual frequency heating (SDF). The process essentially consists of using two power circuits that are operating at medium and high frequency respectively and sharing a common output transformer. Each power circuit is individually controlled to select the proper mix of power and frequency. Again, some companies use IGBT devices for both medium and high frequency outputs, but the question of reliability still remains a concern. The use of IGBT or MOSFET devices for medium frequency applications and MOSFET devices for high frequency applications is best for developing very reliable and repeatable SDF induction heating equipment.