Inductors can be classified on the basis of nature of value and type of core used. Different type of inductors based on the type of core is:
AIR CORE INDUCTORS: Air core coils are wound on formers to provide inductance of a few micro-henries. They are used at radio frequencies in tuning coils, interstate coupling coils, etc. The main requirements of such coils are:
(1) Stability of inductance under all operating conditions.
(2) High ratio of inductive reactance to effective loss resistance at the operating frequency.
(3) Low self capacitance.
(4) Reasonable size and cost.
(5) Low temperature coefficient.
IRON CORE INDUCTOR: Low frequency inductors are normally iron core inductors. These are generally large, both in inductance and in physical dimensions. Choke and reactor are other name of such inductor. They core materials generally used are silicon-iron laminations and grain oriented silicon steel. Windings are usually of enamelled solid copper wire with interlayer insulation and impregnated with suitable materials.
POWDER CORE INDUCTORS: Powder core inductors are used at much higher frequencies then iron core inductors. The technique of reducing eddy current loss by dividing the core into smaller segments, account for use of the "powder iron core". The iron dust in these cores is ferrite (a mixture of ferric oxide and other substance such as nickel and cobalt). The magnetic powder mixed with an insulating binder material so that each material particle is electrically insulated from one another. The magnetic powder and insulating binder substance are moulded to fit into the coil.
FERRITE CORE INDUCTORS:
They are usually pot cores (cores consisting of an outer cylinder with closed ends, the magnetic path being more or less completed by central cylindrical core. The winding is placed in annular space. An air gap is produced in the centre core and by choosing a suitable length for this gap the properties of the pot core may be arranged.
Variable inductors give variation in value of inductance. In these inductors iron core or ferrite slug can be moved in and out of the coil winding. Variation in inductance can also be achieved by providing taps on the coil, but this arrangement will not give continuous inductive values. Inductance can also be changed by adjusting the air-gap in some cores.
Depending upon the inductor construction, they are divided in the following categories:
BOBBIN WOUND INDUCTORS:
Bobbin wound inductors refer to a type or method of construction of winding inductors, chokes and reactors. In these inductors coils are wound independently of the core. The coil must hold it shape or form until the coil is assembled on to the inductance core. One ordinary technique of doing this is to wind the coil into a bobbin. The bobbin is a preformed reasonably rigid part. The bobbin material is usually an insulating material hence it can provide electrical isolation between the coil and the neighbouring core material. Multisession bobbins are available to provide increased electrical isolation between coil windings. Bobbin wound inductors are used in a variety of applications hence bobbins are made from a variety of materials: plastic, phenol, glass, Teflon and others. Most bobbins are moulded. Some are fabricated. Bobbin designs for bobbin wound inductors often provide terminals, pins, and mount pads to ease wire termination and to facilitate printed circuit board mounting. Bobbin wound inductors can also use a variety of core materials: laminated or taped wound silicon steel alloys, nickel iron alloys, cobalt alloys; powered irons and nickels; ferrites; air core; etc.
(2)TOROIDAL INDUCTORS: Toroidal inductors are the high performers among inductors. They offer the smallest size and lower electromagnetic interference. Their windings cool better because of the proportionally larger surface area. A 360 degree wound Toroidal transformer has a high degree of symmetry. Its geometry leads to near complete magnetic field cancellation outside of its coil. Hence the Toroidal inductor has less EMI when compared against other inductors of equal power rating. Windings that are less than 360 degree exhibit more EMI. Toroidal coils are wound directly onto a Toroidal core. The core may be coated or boxed to insulated it form the coil windings. Toroidal inductors with a round core cross section are better performers than Toroidal inductors with a rectangular cross section. The cancellation is more complete for the round cross section. Toroidal inductors can be utilized in any of the inductor application that can accommodate its shape. Although usable Toroidal inductors are not always practical for some applications. Gapped Toroidal inductors usually require that the gap will be filled with some kind of insulating material to assist the winding process. This is an extra expense. Powered cores have an effective distributed gap. These are usually preferred over a field gap because of lower cost and reduced gap losses. Some printed circuit boards are space critical. Mounting a Toroidal inductor flat on the board may take up too much previous board area. Some applications also have registered height so the Toroidal inductor cannot be mounted vertically. Toroidal inductors cores are available in many materials: silicon steel, nickel iron, molypermalloy powder, iron powdered, amorphous, ferrites, and others. Silicon steel and nickel iron are available as taps wound cores or laminated pieces. Non-magnetic triodes are also available to makes air core Toroidal inductors.
SURFACE MOUNT INDUCTORS: Surface mount inductors can be classified in several ways: by power rating, by type of application, by type of construction, and other. "Surface mounts inductors" refer to a type of construction that permits attachment of surface mount inductors to a printed circuit boards (PCB). Historically, inductors and other circuit devices have been mounted on PCBs using "pin-thru" technology. Inductor wires are terminated to pin-type terminals. Holes are drilled in the PCBs copper circuitry to accommodate the transformers pins. The inductors pins are inserted through these holes and then soldered to the copper circuitry. Engineers have developed solder pastes, adhesives, and assembly processes that permit attaching inductor terminals to solder directly to copper circuitry surfaces hence the term surface mount inductor. This process removes the requirement to drill holes for the pins, thereby reducing the cost to manufacture a PCB. Surface mount inductors are usually wound bobbins, but are also available as Toroidal coils. The Toroidal coil is mounted on a "header" equipment with surface mount inductors, come in a variety of materials: Plastics (or headers), used with some bobbins and headers are "self leading". The winding wire is also used to form the surface mount terminal by looping the wire under a performed flat edge thereby forming a reasonably flat terminal area. Like other inductors, surface mount inductors can use a variety of core materials: laminated or taped wound silicon steel alloys, nickel-iron alloys, cobalt alloys; powdered irons and nickels; ferrite; air core; and/or core cobalt alloys; powdered irons and nickels; materials processed for square loop or round loop properties; and others.