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INDUCTION HEATING

For industrial inductive heating applications

GENERAL

   TN-Design   Applications   Advantages
Induction heating is used since more than 50 years for comercial applications, but nevertheless it is not well known to the wide public.
For induction heating the working piece must be electrically conductive such as metal or semiconductive as silicon. The highfrequency-energy is transmitted by way of an induction coil to the workpiece. The induction coil or simple "inductor" is geometrically adapted to the shape of the workpiece, in most applications it consists of a bent cupperpipe and is water-cooled.
Due to technical advances (semiconductors, transistors) the size and weight of present-day equipment could be minimized about to about 10% (with respect to previous models). The compact design, small weight and flexibility of the equipment and its advantageous price makes it possible to use induction heating for many applications.
In the following we try to show you some of the most frequently used ranges of activities, and at the same time help you to imagine the use of induction heating for your own purposes. There are a lot of various possible applications, and with the advances of induction heating many existing processes could also be improved with this technique!


What is induction heating?

Induction heating is basically different with respect to most other heating processes. The heat develops in the workpiece itself, there is no need for a transmission medium (such as air, water or a mechanical connection) The electrical energy is transmitted to the workpiece trough a magnetic field.

The alternating current flowing through the induction coil generates an alternating magnetic field that produces a current in the workpiece. The electrical energy through the coil is converted to magnetic energy, and this magnetic energy is converted back to heat in the workpiece. The current density in the workpiece follows the "Skin-effect", a phenomenon, whereby most current flows on the workpiece surface. Towards the center the current density decreases exponentially. In the center of the workpiece the current flow is nearly 0.

The heating power (P) transmitted to the workpiece is:

The penetration depth of the current is:

The formula is adapted to the case that the workpiece diameter is at least double the penetration depth of the current.

Three inductors are connected in line (equal current for every coil). The equation D > 2 x d is only valid for the first coil (left side). The workpiece in the last coil (right side) is not heated, because D < 2 x d.

The relation between heat energy, workpiece-temperature and heat-up time is:


The energy provided to the induction coil is much greater than the energy transformed to heat in the workpiece. High currents in the induction coil develop large losses in the copper (induction coils are mostly made up of copper-pipes). Therefore the maximum efficiency of a cylindrical coil is about 90% (workpiece made of steel) and about 40% (workpiece made of copper). The alternating current is produced by an RF generator with an efficiency of 90 to 95% (coil power/line power).


TN-Design

   General   Applications   Advantages
The static frequency converter is designed for different induction heating and melting processes. The compact, nearly totally enclosed design allows to use the converter in industrial ambient conditions, also in case of difficult locations. All components are contained in one case, ready to connect for power and cooling water supplies.

The handling and indicating devices are located on the front side of the generators. The clearly arranged buttons, status lamps and status indications allow easy handling.

The reliability of the converter is ensured with a 2-step supervising system for all relevant electrical and thermal working-states. The technique of the frequency conversion is based on a parallel-oscillating circuit on the working side with power capacitors and an induction coil. The values of inductance and capacitance of this device determines the working frequency of the generator. The load-oscillating circuit is connected to a power source. The power source comprises a 6-pulse rectifier, chopper, ferrite-transformer and smoothing-device.

figure 1

With the use of a mains transformer there is a potential-separation between the mains and the working circuit. Due to this design there is no short-circuit in case of an earth link at the induction-coil. The switching of the H-bridge is achieved with the switching of the chopper. Due to this solution it doesn't matter how often and under which power states the high frequency is switched on or off. There are no mechanical switches employed.



Applications/advantages of induction heating

   

From the wide range of possible working constellations with middle and highfrequency converters for induction heating some typical applications:


Applications

   General   TN-Design   Advantages
  Application Advantages of induction
1 Stretching metals
Shrinking rings, cases and wheels. Sections are warmed up evenly. With natural cooling or contact cooling from touching the colder workpieces, there results a mechanical contact with colder workpieces (e.g. installing roller bearing rings onto the axle).

- Rapid-ready status
- Short heating-up cycles
- No "hot spots"
- Accurate controllable and high reproductibility heating process
- small heat losses to environment
2 Blue annealing of steelstrips, saw blades,.....
Corrosion protection by surface oxidation of the steel strips. Blue annealing is feasible also with hardened steel, thereby no hardness waste occurs. Heating up process with workpiece-rates (linespeed) of 10 to 100 m/min.
- Rapid-ready status
- No developing of hot exhaust gases as during the indirect flame heating
- No developing of ecologically harmful materials as with chemical blue annealing.
- Accurate dosage and reproductibility of the heating-up process (with line speed feedback our generators keep surface temperature of steel sheet stable in case of line speed changes)
3 Hardening
Today induction hardening is the most economical procedure for hardening of a section of the workpiece surface.

- Rapid-ready status
- Very short handling time
- No scaling
- Subsequent treatments (leveling, sanding off and annealing) are not necessary
- Continuous hardness quality
4 Heat-connecting:
- Soldering
As compared with furnace and flame, there is the following relation for the heating-up rate of the workpiece: 1 : 200 : 1000 (furnace, flame, induction)
- Welding
Longitudinal welding at round and square tubes from steel, brass and light alloys

- Rapid-ready status
- High soldering/welding speed
- Economic also for larger series
- Continuous quality of soldering/welding
- Small energy costs during breaks and standby
- No direct contact to the workmaterial ->no wear through contact
5 Elimination of stress in metals
- Pre-/postheating in welding processes - Prevention of stress-/hardening zone-developing by proportioned heating
- Soft-annealing (reduction of hardness)
- Recristallization-annealing. Reduction of stress after cold-forming/-drawing


- Rapid-ready status
- Partial annealing without heat drift into surrounding zones
6 Heat up of slugs
before warm-forming in pressing and cutting machines

- Rapid-ready status
- Clean workpieces
- Small edge carbonization
- Constant forging temperatures
- Fewer breaks in comparison with the air furnace
- Increased transition-solidity and notching impact strength

7 Melting
- Melting of metals:
Al, Au, Ms, Cu and Ag in graphite-crucibles. Nickel and steel in aluminium-oxide crucibles. Platin and palladium in zirkodium-oxide crucibles
- Outmelting of cores
(e.g. in automotive- industrie to make intake manifolds)

- Rapid-ready status
- Accurate temperature control
- Elektrodynamic mixing of melting materials
- Short melting times
8 Miscellaneous
- Melting of non-metallic materials (e.g. glass)
- Epitaxy machines for semiconductor-production
- Sintering (making compounds)
- Tool-heating for plastic machines
- Inductive cover gluing (e.g. yogurt cover)

 

Advantages of induction

   General   TN-Design   Applications
1. Accuracy of dosage
   This is the most important advantage of induction heating. The supplied heat may be adjusted independently and in closest boundaries with respect to the fluctuations of the current supply or the like. Any temperature gradients can be copied by application of the pyrometer (measurement of the temperature by infrared radiation) and a temperature regulator.

2. Max. productionspeed
   The heating-up energy can be supplied as fast as the material permits. In comparison to radiation heating this causes an increase of the production rate by a factor of up to 1000.

3. Heating up of objects otherwise unattainable:
   Metal parts in plastic/vacuum/water/wood etc.

4. Small space requirement
   In comparison to conventional heating methods

5. Improvement of working-conditions
  - No development of dirt and smoke
  - Low heat radiation because the heat is generated in the workpiece itself

6. High efficiency
  - No heat losses
  - No heat transfer to the environment

7. Uniform quality
   Since the characteristics mentioned above cause a local delimitation of the heating process, exact dosage, large operating speed and no deformation of the workpiece, the products are characterized by uniform continuity and quality.

We also produce installations for special applications. Please contact us for more exact information!


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