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Induction Heating Glossary

Plain-language definitions of the terms used in induction heating and magnetic flux control.

A

Active power (Power) Pa
the amount of energy absorbed by circuit component per unit of time (1 second). Active power is measured in watts W or kilowatts kW. Active power is always less than or equal to apparent power. For example, if an inductor has a voltage of 50 V and current of 4000 A, its apparent power is 200 kVAs. Active power absorbed by the workpiece and the coil winding turns may be 30 kW, 80 kW or 120 kW depending on the Power Factor.
Apparent (total) power S
is the product of voltage and current associated with a certain component of the circuit. In induction heating, it is measured in kilovolt amperes, kVA. For example, a transformer with a primary voltage of 800 V and current of 500 A has an apparent power of 400 kVA. In direct current (DC) circuits Apparent power is the same as real Active power and the term “Apparent” is of no use. In alternate current (AC) circuits, especially in induction heating tank circuitry, energy oscillates back and forth with only a part of energy being absorbed in components in the form of heat or mechanical energy (motors). In this case Apparent power is always higher than Active power.

C

Channel or Tunnel coil
inductor made of two parallel conductors with currents of opposite directions. It may be single- or multi-turn. Part(s) is (are) located between the direct and return conductors. Widely used for heating parts continuously transported through the inductor. Magnetic controllers are recommended for this coil style.
Coil head
an “active” part of inductor that generates magnetic field for a part heating.
Concentrator
component of induction heating coil made of soft magnetic material designed for concentration and augmentation of magnetic field and resulting part heating in a desired area.
Core
a magnetic controller placed inside of the inductor for internal heating (ID inductor); b. magnetic controller placed inside of the tubular part for heating improvement; c. central (mainly magnetic) area of heated part’s cross-section.

E

Eddy currents
currents generated inside the conductive body by alternating magnetic field flowing through the body’s cross-section (i.e. by the field, “coupled” to the body). Eddy currents must be always closed so there must be a conductive path or “loop” for the current flow within the body. No closed loop – no current, no current – no heating. For example, there is no heating in a thin open ring placed into alternating magnetic field. If the ring is closed, there is heating. In induction heating of compact material eddy-current losses are considered as the major component of the total losses, with Hysteresis losses being a minor or totally negligible component.
Eddy currents losses
heat generated by eddy currents induced in a conducting body by an alternating magnetic field.
Electric current
is a measure of the electric charge (electrons and ions) flow. It could be compared to a water flow rate in a pipeline. The unit of current is called Ampere (A). A current in the induction heating coil may be as small as tens of amperes and as big as several tens of kiloamperes depending on application.

H

Hair-pin coil
inductor made of two parallel conductors and designed mainly for heating one side of a flat part. Both conductors are typically used for heating effect. Magnetic controller is strongly recommended for this coil style.
Hysteresis losses (HL)
are due to the “internal friction” of tiny magnetic domains in a magnetic material when they turn back and forth following the direction of alternating magnetic field lines. The contribution of HL in heating is typically small (less than 10 % at low frequency) and gets smaller as frequency rises. In non-magnetic materials (paramagnetic and diamagnetic), HL are zero. On the contrary, HL may be the main part of losses in magnetic flux controllers made of ferrites, laminations or magnetodielectric materials (magnetic composites).

I

I.D. coil
inductor for heating internal (Inner Diameter) surface of the part. Magnetic controller (core) is strongly recommended for this coil style.
Impedance Z
is the ratio of voltage and current. It is a parameter of the component of the electric circuit or of the whole circuit. The unit of impedance is called Ohm (W or Ohm). 1 W= 1 V/A.
Induction coil
see Inductor.
Induction heating
is a contactless heating method of bodies, which absorb energy from Alternating Magnetic Field generated by Induction Heating Coil (Inductor). There are two mechanisms of energy absorption: a. Eddy current heating (in electrically conductive bodies); b. Hysteresis heating (only in Magnetic materials).
Inductor
is a device generating alternating magnetic field for induction heating of parts. It contains one or several loops of conductors (typically water-cooled copper tubes), magnetic flux controller, constructive components and quenching rings (optional).

L

Leads
conductors for current delivery to the coil head from the source (transformer output, capacitor battery or generator). Leads may be made in the form of busses (busswork), coaxial cable, or water-cooled tubes.
Load (also Workpiece or Part)
part to be heated, placed in close proximity to the inductor.

M

Magnetic (flux) controller
generic name for magnetic devices used for concentration or modification of induction heating coil magnetic field. Depending on application and major produced effect it may be called Concentrator, Core or Shield.
Magnetic field
is a specific type of physical field. It is distributed in space and can vary in time according to a variation of the source. Electric currents or permanent magnets are sources of magnetic fields.
Magnetic field energy
is an energy associated with magnetic field. It is distributed in the space around the conductors carrying currents, which generate the magnetic field. For alternating current, the energy of the magnetic field transforms continuously into the electric energy of the coil circuitry and back. The energy oscillates between electric and magnetic fields (electromagnetic energy) with double the frequency of the current. Conductive bodies absorb a certain part of the oscillating energy during each cycle of oscillation. Absorbed energy is described by Active Power. The part of energy returning to the coil is described by Reactive Power. The unit of magnetic field energy (as any other energy) is Joule (J). 1 J = 1 Watt-second. In industry kilowatt-hours (kWhrs) are more common unit. 1 kWhr = 3,600,000J.
Magnetic field lines
or lines of induction (B) help to visualize the magnetic field distribution. The magnetic field is stronger where the density of magnetic lines is higher. Magnetic lines are always closed around the source. They are similar to the contours of laminar water flow inside a close-loop pipeline. In AC systems such as induction devices, magnetic line pattern depends on an instant inside the cycle of current alternation.
Magnetic field strength (H)
is a measure of the magnetic field driving force intensity. It may be compared to a pressure gradient in flowing water. The unit of magnetic field strength is A/m or A/cm. Another unit Oersted, Oe, is also used in technical literature. 1 A/cm = 1.26 Oe.
Magnetic flux density (American also “Induction”)
is a measure of magnetic field intensity (flux density). It is a vector and may be compared to a local speed vector in the case of a flowing liquid. The unit of magnetic flux density is called Tesla (T). 1T = 1Wb/m2 = 10,000 Gs (Gauss).
Magnetic flux, F (phi)
is a measure of magnetic field flow. Magnetic flux may be compared to a flow rate of liquid. The driving force of magnetic flux is the current, or more precisely, ampere-turns of the coil. Magnetic flux flows around the coil turns and its path must always be closed. The unit of magnetic flux is called Weber (Wb).

O

OD coil
inductor (cylindrical or oval) for heating external (Outer Diameter) surface of the part.

P

Pan-cake coil
inductor in the form of flat spiral designed for heating one side of flat body. Magnetic controller is strongly recommended for this coil style.
Permeability
In a linear media the ratio B/H is an unequivocal parameter of the material property and is called absolute permeability of the material. We also can define relative permeability by calibration “permeability of vacuum (or air) is equal to one”. Relative permeability of all non-magnetic materials is one. Permeabilities of ferromagnetic materials may be as high as some tens of thousands and depend on material nature and magnetic field intensity (non-linearity). The higher the permeability, the less driving force (magnetic field strength or inductor current) is required to push the same magnetic flux through the material.
Power factor or cos j (cosine phi)
is a ratio of active and apparent power. It can be in a range 0< cos ?< 1. In practical induction systems cos ? is usually in a range 0.05 – 0.7, higher the better. Power factor in induction systems is approximately equal to inverse value to Quality factor.

Q

Quality factor Q
is the ratio of reactive power to active power, kVAr/kW. This term came from radio where active power was energy loss and high value of Q corresponded to high quality circuit components. In induction heating low value of Q is desirable.

R

Reactive power Pr
is a part of apparent power that is reflected from the coil back to supplying circuitry. Units of reactive power are VAr (voltamps reactive) or kVAr (kVAR).
Reluctance (magnetic resistance)
an analog of electrical resistance of the electric circuit or its component. In an electric circuit, voltage (driving force for current) pushes a current (flow or yield) through the resistance. In a magnetic circuit, ampere-turns of the coil (driving force for magnetic flux) push magnetic flux through the reluctance of a certain area. Introduction of magnetic bodies (controllers) into the magnetic circuit reduces reluctance of corresponding area and decreases the current demand for production of the same flux value or increases generated flux for the same induction heating coil current.

S

Shield (magnetic)
type of magnetic flux controller designed for reduction or elimination of unintended induction heating of machine or part’s component.
Single-Shot coil
channel-type induction heating coil for heating rotating shafts and rods. Magnetic controller is strongly recommended for this coil style for efficiency improvement and temperature distribution control along the heated part length.
Split-and-Return coil
inductor of three parallel conductors designed mainly for local heating on one side of a flat part. Central conductor is used for heating; two side conductors are used for coil current return. Magnetic controller on a central conductor is strongly recommended for this coil style.
Style (Type) of inductor
principal configuration of the induction heating coil head that allows to attribute it to a certain type of inductor (OD, ID, Oval, Hair-pin, Split-and-Return, Pancake, Transverse Flux, Vertical Loop, Transformer, Tunnel or Channel etc.)

T

Transformer inductor
inductor in a form of transformer with a load placed into a gap between the poles or (in the case of tubular part) forming a short-circuited secondary winding of the transformer.
Transverse Flux coil
induction heating coil for heating thin flat bodies (strips, foils, thin slabs). Consists of one or more pairs of hair-pin or split-and-return coils with the same current polarity, located on both sides of the part.
Tunnel coil
see Channel coil.

V

Voltage
is an electromotive force or a driving force of the current. The voltage provided by power supply (battery, mains, high frequency generator etc.) drops on the components of the electric circuit. The voltage and voltage drops could be compared to the water pump pressure and pressure drops in the pipeline. The unit of voltage is called Volt (V). The coil voltage can vary in a wide range from several tens volts (small single-turn coils) to several kilovolts (multi-turn coils of big melting furnaces).

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