INDUCTION HEATING : INFORMATION
History
Practical induction heating has been used since the 1920’s. Growth in the induction heating industry expanded very rapidly during World War II. Surface hardening, or case hardening, was one of the main growth areas during WWII. Military vehicles and weaponry using case hardening on axles and engine components could outlast those without case hardening. After the war, the technology improvements moved rapidly into the civilian sector as the demand for reliable automobiles increased. Today induction heating is used in a wide variety of industrial processes including: forging of metals, melting and casting, surface coatings, “cap sealing” for food products, metal surface hardening, bonding metal parts and the manufacturing of semi-conductors.
What is Induction Heating?
Induction heating is the process of heating conductors, (usually metals), by inducing an electric current to flow in the object to be heated. Current is induced into the object in the same manner that current is induced into the secondary of a transformer.
An alternating current is applied to the primary of a transformer, which creates an alternating magnetic field. The secondary of the transformer is located within the magnetic field. Faraday’s Law shows that an electric current will be induced into the secondary of the transformer. In induction heating, a coil of copper is wound around an object to be heated. The coil of copper can be compared to the transformer primary, and the object to be heated can be compared to the secondary of the transformer.
The object to be heated acts like a single turn secondary in a transformer. Additionally, the object acts as if the single turn secondary were short circuited. Thus, applying an alternating current to the induction coil induces a current into the object to be heated. Imagine how a short circuit secondary on a transformer would heat up if you connected power to the primary!
Induction heating equipment must create alternating currents at frequencies from 60 Hz to over 1 MHz. In the beginning, spark gap oscillators, motor driven generators and vacuum tubes were used to create the alternating current. Technology advanced and soon SCR, (Silicon Controlled Rectifier), based power supplies were used to replace older generators. Very large and powerful transistors are now used in power supplies for induction heating.
Components
An induction heating system is comprised of several major components.
- Power Supply (generates the high frequency current)
- Load Matching Station (matches the impedance of the coil to the power supply)
- Induction Coil (copper coil wrapped around object to be heated)
- Water Cooling (high power systems are water cooled to remove waste heat)
Applications
Induction heating is replacing many gas fired heaters because of the efficiency of producing product, superior quality of product, and zero pollution contribution by induction heating.
The following is a quick list of some applications where induction heating is used. The list is by no means complete since new induction heating applications are created every day. Also, the descriptions are not intended to be technically correct, but rather to describe the industries in which induction heating is used.
FORGING : Metal is heated to near its melting point, and then squeezed in a press into a new shape. Some products are bicycle cranks and axles, piston rods in automobile engines, axles, bearings, and high pressure pipe fittings.
MELTING AND CASTING : Metal is heated above its melting point, and then poured into a casting mold. Some products are art sculptures, engine blocks, and lamp posts.
COATINGS : Metal is heated and a coating is sprayed onto the surface of the metal. The coating melts and adheres to the metal. Some products include reinforcing bar, (rebar), for concrete structures (for corrosion resistance), and paint curing, (speeds up the drying process for paints).
FOOD PRODUCTS : This industry is known as “Cap Sealing”. The aluminum foil that you have to remove from your vitamin bottles, ketchup bottles, and all others has been melted into place, on the bottle cap, by induction heating. This provides you with the tamper evident bottles.
SURFACE AND CASE HARDENING : Any product that must be abrasion resistant is surface hardened during manufacturing. All engine and drive train products in automobiles are surface hardened. Pliers, scissors, wrenches, lawn mower blades, fish hooks, surgical instruments, fork truck forks, pulleys in elevators, and all bearings. BONDING : Metals are bonded together using a thermo set adhesive. Believe it or not, your automobile is glued together now. In many locations where spot welds once held body panels together, glue is now used, and is stronger than the old spot weld. Other applications are tool boxes, luggage, and cabinets.
MANUFACTURE OF SEMI-CONDUCTORS : A curious process called zone refinement may be used in the manufacture of semi-conductors. Highly pure silicon for semi-conductors is required for today’s integrated circuits. When silicon is melted, impurities are attracted to the molten area. Therefore, induction heating is used to melt portions of an ingot of silicon. The coil is moved so that the molten zone of silicon moves, and this carries the impurities along with the molten zone. Several passes of an induction created molten zone removes impurities from the ingot of silicon.