5 Must-Have Features in a industrial induction heating machine

Induction coil design can have a major impact on part quality, process efficiency, and manufacturing costs. How do you know if your coil design is best for your part and process? Here are some induction coil basics and five tips to optimize your design.

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How Induction Heating Coils Work

The induction coil determines how effectively and efficiently a workpiece is heated. Induction coils are water-cooled copper conductors made of tubing that is formed into the shape of the coil for the induction heating process. Induction heating coils do not themselves get hot as water flows through them.

>> Read our comprehensive guide to designing induction coils

Work coils range in complexity from a simple helical- or solenoid-wound coil (consisting of a number of turns of copper tube wound around a mandrel) to a coil that is precision machined from solid copper and then brazed.

Coils transfer energy from the induction heater power supply to the workpiece by generating an alternating electromagnetic field from the alternating current flowing in them. The coil’s alternating electromagnetic field (EMF) generates an induced current (eddy current) in the workpiece, which generates heat due to I Squared R losses (core losses).

The current in the workpiece is proportional to the coil’s EMF strength. This transfer of energy is known as the transformer effect or eddy current effect.

Transformers & Induction Coils

Because coils use the transformer effect, characteristics of transformers can be helpful in understanding coil design. The inductor is similar to a transformer primary, and the workpiece is equivalent to the transformer secondary (assumed to be a single turn).

There are two important features of transformers that impact coil design:

• Efficiency of coupling between the windings is inversely proportional to the square of the distance between them

• (Current in the primary of the transformer * # of primary turns) = (current in the secondary * # of secondary turns)

Five conditions to keep in mind

1. Higher flux density near the heating area means a higher current is generated in the part

The coil should be positioned as close to the part as possible with the largest possible number of magnetic flux lines intersecting the workpiece at the heating point. This allows for maximum energy transfer.

2.  The greatest number of flux lines in a solenoid coil are toward the center of the coil

The flux lines are concentrated inside the coil, providing the maximum heating rate at that location.

3.  The geometric center of the coil is a weak flux path

Flux is most concentrated closer to the coil turns themselves, and decreases with distance from the turns.

If a part were placed off center in a coil, the area closer to the coil turns would intersect a greater number of flux lines and thus be heated at a higher rate. The area of the part away from the copper coil experiences less coupling and would be heated at a lower rate.

This effect is more pronounced in high-frequency induction heating.

4.  The magnetic center of the inductor is not necessarily the geometric center.

At the point where the leads and coil join, the magnetic field is weaker.

This effect is most pronounced in single-turn coils. As the number of coil turns increases and the flux from each turn is added to that from the previous turns, this condition becomes less important.

Due to the impracticality of always centering the part in the work coil, the part should be offset slightly toward this area in static heating applications. If possible, the part should be rotated to provide uniform exposure.

5.  Coil must be designed to prevent cancellation of the magnetic field.

If opposite sides of the inductor are too close, the coil does not have sufficient inductance required for efficient heating. Putting a loop in the coil at the center will offset this effect. The coil will then heat a conducting material inserted in the opening.

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Ambrell designs and manufactures coils for its power supplies, and even for non-Ambrell systems when provided with a drawing. Have questions about induction heating coil design?

Read our comprehensive guide to designing induction coils

Induction heating has been around ever since Nicola Tesla discovered and harnessed the phenomena. His industrial applications made much of the modern world we know today possible. So, what exactly is Induction heating? For most applications it is the use of an alternating magnetic field on an electrically conductive part. The magnetic field pushes and pulls electrons within the material. The energy used to do this is dissipated as heat within the part. When properly controlled through coil design and current control, this can produce precise, repeatable and predictable results. This can be seen in modern induction stoves in many kitchens around the world. These stoves require a special pot with a bottom that can accept the magnetic field.  Precise current control through an engineered coil provides even controllable heat with a nearly infinite temperature control. 

But did you know that Induction heating has solved many industrial challenges as well? 

#1 Removal of worn-out boiler tubes from a tube sheet. 

The issue: In large industrial boilers as found in many power plants water is fed in to a large header then up through small tubes to another header where the heater water or steam is gathered and sent to be used other places in the power plant. These tubes degrade over time and must be replaced. Within an industrial application these headers my be 3” or thicker and 48” in diameter with ’s of 2” diameter holes where the tubes are inserted and then rolled and expanded into the holes. In the past skilled technicians used cutting torches to slice the tube and then beat it out with chisels. Many times, this resulted in damage to the tube sheet that required welding and machining prior to fitting and rolling in new tubes. 

#2 High Temperature Testing/Thermal Cycling

#3 High Speed Curing of Coatings

The solution: custom built induction machines were used along with engineered coils to provide heat to the parts prior to coating them. For powder coated parts these bars pass through the powder booth shortly after the induction heating. This adds enough heat to melt the powder and adhere to the parts consistently and evenly and since the heat is so fast, its only on the outer surface causing a rapid cooling after the coating adheres. This greatly reduces the production line length. The same can be done for painted on products, force curing the paint shortening the overall line length and removing a source of heat and CO2 within assembly plants. 

#4 Thermal Expansion

The issue: Large heavy shrunk on parts of a Turbine or Generator rotor need to be removed to repair or replace the part or components underneath. In the past Electrical resistance heating or flame rings were used to heat and thermally expand large components for removal. Electric resistance has limitations in its total heat input and flame rings can severely affect the metal or burn adjacent components and combustion byproducts are poisonous to humans.  

The solution: Custom designed portable induction heating equipment and flexible coils are used to apply the magnetic fields and heat in the appropriate places to thermally expand these components for removal.  This is not limited to the power generation industry. Applications can be found in Steel Mills, Paper Mills and Sugar Plantations to name just a few.

#5 Disassembly of Rusted Nuts and Bolts

The issue: In many industrial places bolted flanges are common. These flanges are outside and exposed to the weather and may become corroded or rusted to the point where they cannot be removed by simple torquing methods. 

The solution: Induction coils can be wrapped around these rusted and frozen nuts. Then when the heat is applied two things happen. The rapid expansion helps to break the bond formed by the iron oxide and the second is the further degradation of the rust into finer particles. These smaller particles no longer lock the nut to the threads allowing for the mechanical turning and removal of the nuts. 

Note: These are just a few of many industrial applications for Portable or Stationary induction heating machines. New applications are found every day.

Autor: Jeff Weinacker, President Mobile Induction Solutions, at Thermo International.

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