5 Basics of Induction Heating Coil Design
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 conductors made of copper tubing that is readily 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.
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 brazed.
Coils transfer energy from the power supply to the workpiece by generating an alternating electromagnetic field due to 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.
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Designs & Frequency Selections
Induction coil design has a major impact on process efficiency and final part quality, and the best coil design for your product largely depends on your application. Certain coil designs tend to work best with specific applications, and a less than optimal coil-application pairing can result in slow or irregular heating, higher defect rates, and lower quality products.
Designing an induction coil for your application
Start with understanding where the heat needs to be generated in the part to perform the process, and then design the coil to achieve the heating effect. Similarly, frequency selection will depend on the induction heating application you'll be using for your part.
Before designing your induction coil, consider these three factors along with your induction application:
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Part motion relative to coil - Several applications rely on part movement with the help of conveyors, turntables, or robots. A properly designed induction coil incorporates these individual handling requirements without the loss of heating efficiency.
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Frequency- Higher frequencies are used for applications like brazing, soldering, annealing or heat treating, where surface heating is desired. Lower frequencies are preferred for applications requiring through-heating of the parts to the core like forging and die heating.
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Power-density requirements- Higher power densities are required for short cycle heating applications requiring high temperatures. Higher power densities may also be required to keep the hot zone confined to a small area, reducing the heat affected area.
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Calculate Coupling Distance
Coupling is the transfer of energy that occurs in the space between the heating portion of the coil and the workpiece. So, coupling distance is how big that space needs to be to balance efficiency and manufacturing requirements.
Generally, distance increases with the diameter of the part, typical values being 0.75, 1.25, and 1.75 inches (19, 32 and 44 mm) or billet-stock diameters of approximately 1.5, 4 and 6 inches (38, 102, and 152 mm), respectively.
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Improve Heating Uniformity via Coil Design
Magnetic flux tends to concentrate toward the center of the length of a solenoid work coil. This means the heating rate produced in this area is generally greater than that produced toward the ends. Further, if the part being heated is long, conduction and radiation remove heat from
the ends at a greater rate. The coil can be modified to provide better heating uniformity along the part length. The technique of adjusting the coil turns, spacing, or coupling with the workpiece to achieve a uniform heating pattern is sometimes known as 'characterizing' the coil.
- Coil characterization
- Six other common ways to improve heating uniformity
- Heating two separate areas on a part
- Heating tapered parts
- Heating a part with secondary fabrications
- Heating a variety of different parts with one coil
6 ways to improve uniformity of heating...
Solenoid Coil Designs & Calculations
The type and design of the induction coil determines how effectively and efficiently a workpiece is heated. 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 brazed.
The helical solenoid coil is the most ubiquitous induction coil design. It provides a wide range of heating behaviors since the part or heating area is located within the coil, in the area of greatest magnetic flux. Flux lines in a solenoid coil are concentrated inside the coil, providing the maximum heating rate at that location.
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Induction heating has considerable advantages over other common heating methods in specific applications. Is yours one of those applications?
Induction heating offers many benefits to manufacturing processes because it is a rapid, energy-efficient, flameless method of heating electrically conductive materials. A typical system involves an induction power supply, workhead with a copper coil and a chiller or cooling system. Current flows through the coil to create an electromagnetic alternating field. When a conductive part is placed inside the coil, current is induced to run through it. Current flow combined with the resistance properties of the conductive part results in heat generation.
10 Selection factors to consider
Your Part's Material
Induction directly heats conductive materials such as metals. Nonconductive materials are sometimes heated with a conductive susceptor. Due to hysteresis, magnetic materials are heated more easily than nonmagnetic materials; consequently, nonmagnetic materials often require more power. Metals with high resistivity like steel heat quickly, while low-resistivity metals like copper or aluminum require more heating time.
Depth of Heating Penetration
The induced current will be most intense on the surface of your part. In fact, more than 80% of the heat produced in the part is produced on the 'skin,' or surface. Consequently, larger parts and parts that require through-heating take more time to heat than those that are thin or small.
Operating Frequency
Lower-frequency, higher-power systems are generally suited for heating larger parts that require through heating. Lower-power, higher-frequency systems are often the right choice for surface heating. As a general rule, the higher the frequency, the shallower the heating of the part.
Applied Power
The output power of your induction heating power supply determines the relative speed at which your part is heated. The mass of the part, rise in temperature and heat losses from convection and conduction need to be considered. Often, the induction equipment manufacturer can help you make this assessment.
Rise in Temperature Required
Induction can generate a significant change in temperature, but, generally speaking, more power is needed to accommodate a significant temperature change and will impact your power-supply choice. The rate of temperature change also affects your power-supply choice. The faster the rate of change, the more significant the power requirement.
Coil Design
Your coil, which is generally water-cooled and made of copper, needs to follow the shape of your part and take the variables of your process into account. An optimal coil design will deliver the right heat pattern to your part in the most efficient way. A poorly designed coil will heat your part more slowly and deliver an improper heating pattern. Flexible coils are now available and work well with large parts and unique part geometries.
Coupling Efficiency
The part being closely coupled with the coil elevates the flow of current, which increases the amount of heat generated in the part. Coupling enables faster and more efficient heating, which can increase manufacturing efficiency. Poor coupling has the opposite effect.
Your Facility and the Footprint
Induction requires cooling from a chiller or cooling system. Lower-power systems generally require a compact water-to-air heat exchanger, while a higher-power system may require a larger water-to-water heat exchanger or chiller. Additionally, you will need space for the induction heating power supply and workhead. Generally speaking, an induction system will save considerable space over an oven, especially when you consider that the workhead can be placed a significant distance away from the power supply. Of course, you also need to be sure your facility can handle the amount of power the system requires.
Additional Heating Requirements
Will you need to measure and store heating data? Some induction solution providers can offer a full system that includes an optical pyrometer and temperature-monitoring software so such data can be recorded and stored. A comprehensive solution can lead to a smooth installation and start-up.
Industrial Expertise
Many induction manufacturers have expertise with certain applications, and if they've worked with your application, it will provide peace of mind. Additionally, some providers offer laboratory testing and a tailored system recommendation based on your heating requirements. This type of service takes the guesswork out of selecting a system and helps you account for the aforementioned factors.
In conclusion, induction heating has considerable advantages over other common heating methods. Buying the right system for your application's needs and ensuring it meets any needs you may have for it in the future will help your company fully benefit.
Note: This article was originally published as a cover feature in Industrial Heating (which has ceased publishing) via a submission from Ambrell.
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