What is Induction Heating and How Does it Work

Induction heating is a process where electrically conductive material is heated when it's placed within a dynamic magnetic field without touching the inductor. It is a simple and cost-effective heating process that delivers fast and consistent heat when compared to other conventional heating processes used for preheating and stress-relieving welds. Heat is generated by circulating electric current as it is placed on the magnetic field(electromagnetic induction). To develop heat, the resistance of the material must be low(metals) and the voltage must be high. For example, metals with high resistance like iron will heat up much faster than low-resistance metals like copper.

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Heat is generated through resistance losses and hysteresis losses when the induced electrical current flows. Hysteresis losses mainly occur in ferromagnetic materials when they are magnetized and demagnetized.

Induction heating is mostly used in industrial processes when manufacturers want to change the physical properties of metals(bonding, hardening, and softening). During the induction heating process, there are no residual combustion emissions because metals are not heated with fire and smoke. Also, the rate of heat transfer is regulated and steady during the process with minimum heat loss. Unlike traditional heating processes(flame, resistance heating...), induction heating is easy to set up, fast time to temperature, safer, and also it is more efficient, accurate, and uniform.

How does Induction Heating work

We discussed in the paragraph above what’s induction heating, now let's talk about how induction heating works. When electrical current flows through a copper conductor, it produces a magnetic field around the conductor. The direction of the electric field depends on the direction of the electrical current using the right-hand thumb rule. 

The more current that passes through the conductor, the bigger and stronger the magnetic field will be around the conductor. When the electrical current flow in the conductor is changed to the opposite direction, also the magnetic field changes. Passing an alternating magnetic field through a conductive material generates localized electrical currents within the metal. The generated electrical currents are called Eddy currents. The stronger the magnetic field, the more Eddy currents are generated.

Metals have a certain amount of electrical resistance, and the Eddy currents circulate against the resistance of the metal which causes the metals to heat up. This process is called Joule heating, and it is responsible for the generation of most of the heat. 

The electrical resistance of the conductive material that is being heated, plays a major role in the heat that is generated. For example, metals with a low resistance value require more Eddy currents to heat than metals with a high resistance value. While heating ferrous materials, hysteresis losses should be taken into consideration. This occurs due to the material's resistance to a changing magnetic field. Hysteresis losses generate less heat than Joule heating but still significantly contribute to the total heat within the material. 

Also, the magnetic properties of the conductive material that's being heated play a big role in the amount of heat that is generated. For example, magnetic materials such as iron generate more heat due to hysteresis losses, while non-magnetic materials like copper or aluminum won't generate any heat due to hysteresis. 

Eddy currents produce heat at the surface of the part which is directly next to the heating coil. The heating depth is determined by how fast the alternating field switches back and forth through the material. The remainder of the part's thickness is heated from conduction from the part.

Components of Induction Heaters

An induction heater consists of 4 main parts:

1. Induction heating coil
2. Workpiece
3. Power supply
4. Component circuit

Different Induction Heaters are available on our website:

• Portable Induction Heaters
• Yoke Style Heaters
• Precision Handheld Heaters
• Induction Heating Generators
• Betex Cone Heaters

Also, you can check our Youtube channel to watch how various induction heaters function and are applicable. 

HISTORY OF INDUCTION HEATERS

Michael Faraday was the first to discover induction heating in using a battery and two copper wires wrapped around an iron core. However, the first time it was implemented with success was around 100 years later, in the year in England, where the first induction melting system had been installed by EFCO. With the need for a reliable and fast process to manufacture metals for engine parts during the second world war, the technology of induction heating advanced swiftly. As the focus shifted toward lean manufacturing and enhanced quality control, the technology of induction heating was rediscovered with the development of controlled induction power supplies.

Check out our post What Are The Benefits of Bearing Induction Heaters?

Feel free toContact Us if you have any questions, need more information or if you are interested in purchasing an induction heater.

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In this video I show you how to cryogenically case harden your low carbon steel parts with the induction heater. What you end up with is a material that has a ductile inner core with a hard outer shell. Parts that require impact resistance as well as abrasion resistance are great candidates for case hardening. Low carbon steel does not harden well on its own. When heating the part up and submerging it in a carbon compound, carbon gets impregnated into the surface of the material. This results in a hard outer surface.

Gears, Firing pins, Engine Camshafts, Lock Shackles, Security Fasteners, and Self Drilling Screws are all commonly case hardened. The method of case hardening used in the video is also known as carburizing with a cryogenic treatment. Liquid nitrogen is usually used but I used a isopropyl alcohol dry ice bath. Liquid nitrogen would provide even more benefits by converting even more retained austenite to martensite. Also, if you don't mind the darkness of the part after the hardening process, it's best not to polish it for even more rust protection. The method used in the video can be repeated multiple times until the desired hardness is achieved.

Induction is particularly good at case hardening. This is because of the skin effect discussed in the next step. The same high frequency skin depth that is seen in the working coil is also seen in the part being heated. The higher the frequency, the more the current is flowing on the outside of the material. For steel in this case, the current is only flowing about 6 thousandths of an inch deep. This is perfect for case hardening.

This process should only be used with low carbon steel so I'll show you a way to determine what type of steel you have.

In this video, I'll show you an easy way to determine the carbon content of your steel parts by observing the spark profiles. You can use this technique to find which of your parts would benefit from the case hardening method used in my last video. This is not the most accurate way of determining the composition of the steel, but many welders use this technique when welding unknown materials and is a good test for most non-critical jobs. This technique is known as spark testing.

In this video, I make a pancake coil attachment and boil water in about 10 seconds. It works by pulsing the DC at a high frequency(about 160kHz). This creates a constantly changing magnetic field that induces eddy currents in the cast iron skillet. Induction heating is more efficient than many other methods because there is less wasted heat. For example, induction cooktops are more efficient than electric and gas stoves.

The use of my wire choice is quite important here. I used what is known as Litz wire. Litz wire consists of many individually insulated wires. It is the preferred wire for use in high frequency application due to the skin effect.

The skin effect is the tendency of current to flow on the outside of a conductor at high frequencies. The higher the frequency, the shallower the skin depth will be. Using Litz wire will result in much more efficient working coils for your system. Not only does it transfer the energy more efficiently, it also generates less heat of its own. If you have 10amps flowing through 260 strand Litz wire, that's only 38mA of current in each wire. 10amps through a single conductor will generate substantial heat depending on the size.

If you do not want to use Litz wire, the next best thing is copper tubing. It has more surface area for the current to flow so it is more efficient than an equivalently sized solid core wire.

This induction heater works by pulsing the DC at a high frequency(about 160kHz). This creates a constantly changing magnetic field that induces eddy currents in conductive materials near the coil. Ferromagnetic materials will also heat up due to hysteresis. This is the heat produced from rapidly changing magnetic fields in the material. Induction heating is more efficient than many other methods because there is less wasted heat. For example, induction cooktops are more efficient than electric and gas stoves. I'll be making more attachments and demonstrating more practical uses for inductive heat.

It's a common misconception that induction heaters don't work for aluminum and other non-ferrous metals. This is not true. This induction heater will heat up aluminum and copper just fine, but not as fast as ferrous metals. This is mainly due to their lower resistivity. The higher the resistivity of the conductive material, the more it generates heat when current is passed through it. The higher resistivity of most ferrous metals coupled with the heating due to hysteresis makes them ideal for induction heating, but they are not the only metals affected by induction.

The three main components of the machine are the power supply, the ZVS driver, and the working coil. The ZVS driver,(Zero Voltage Switching) was invented by Vladmiro Mazilli. It uses resonant zero voltage switching (also know as ZVS) to pulse the power supply very rapidly. This means the MOSFET's are designed to switch (on or off) when the voltage across them becomes zero. Since the MOSFET's switch when there is no voltage across them, they generate very little heat. The main source of heat is caused by the MOSFET's internal resistance and the capacitors constantly charging and discharging. The circuit is fairly simple consists of just two MOSFETs, 6 capacitors, and a few resistors and diodes. You can build the circuit yourself, but it is cheaper to buy a premade board.

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