inductor

Definition of inductor: an electrical component consisting of a coil of wire used to introduce inductance effects into a circuit by storing energy in its magnetic field

 An inductor is a coil of wire either hollow, or wound around some ferrous (magnetic) material. When current flows through the coil a magnetic field is produced. When the current stops flowing the magnetic field collapses. If the coil is connected to a d.c. supply, a steady current will flow, and the opposition to the flow will be mainly due to the resistance of the wire used to make the coil.

small inductor coil

However, at the moment when the current is switched on or switched off the rising or falling magnetic field also opposes the flow of current. This means that if an alternating supply is connected to the coil the opposition to the flow is greater than that due to the resistance alone. The amount of opposition to a.c. depends upon the wire used, the number of turns, type of material inside the coil etc., and the effect is known as the inductance of the coil. Inductance is measured in henrys, and a small inductor may have a value of, say, 10mH. There is also some capacitance associated with the coil, and this too affects the way it behaves with a.c. So the whole effect due to the resistance of the wire, the inductive effect and capacitive effect is summed up by referring to the impedance of the coil. Impedance is the total opposition to a.c. and will depend upon the a.c. frequency. You will probably know that one of the important loudspeaker measurements is its impedance. If your amplifier has an output impedance of 8 ohm, then you need a speaker of 8ohm impedance if you wish to extract the maximum power.

                    Inductors are often used to reduce voltage spikes in a circuit – in fact you often see ferrous material wrapped around mains leads or other leads associated with computers, video recorders etc. Inductors are also used in radio tuning, and combined with capacitors can form a “tuned circuit” i.e. one which resonates with a particular frequency – to tune in your favorite radio station for instance.

 Many types of cores are commonly used in inductors. The simplest core is basically nothing, or air. Any core consisting of non-magnetic material behaves essentially the same as air. Most commonly used inductors, however, use some type of magnetic material in the core. This tends to concentrate the inductor’s magnetic field inside the core and increases the effective inductance. While a magnetic core can provide greater inductance in a given volume, there are also drawbacks. A magnetic core can contain only a limited magnetic field. As you increase current in a magnetic core inductor, the magnetic field increases. At some point, further increasing the current no longer produces an increase in the magnetic field. At this point, the core is said to be saturated, a condition that generally is undesirable.

Electrical Inductance Calculator

 Calculations include inductance of single-layer circular solenoid of round wire, multi-layer circular solenoid, flat spiral, circular solenoidal current sheet, N-turn circular loop, straight round wire, circular toroid with circular winding, circular torus ring with rectangular winding, single-layer square solenoid, single-layer rectangular solenoid, multi-layer square solenoid, and wire gauge calculation. 

 Usage:

  On startup, the following initial menu is displayed:
  Geometry of coil:

• 1 Circular solenoidal current sheet
• 2 Straight round wire
• 3 N-turn circular loop
• 4 Circular toroid, circular winding
• 5 Multi-layer square solenoid (low precision)
• 6 Circular torus ring, rectangular winding
• 7 Multi-layer circular solenoid
• 8 Single-layer circular solenoid of round wire
• 9 Single-layer square solenoid
• 10 Single-layer rectangular solenoid
• 11 Wire gauge calculation
• 12 Select dimensions in inches or centimeters
• Choose geometry (1) ?

  Select the desired coil geometry from the menu. Enter mean diameter, length, and thickness of winding, and total number of turns, as prompted. The program will display the inductance in henrys, then loop back for more input. "Mean diameter" is the arithmetic average of the inner and outer diameters of the winding; for a single layer coil it is measured to the center of the wire. "Thickness," for a single layer coil, is the wire diameter (see below for the correction for empty space in the winding).
  For each item the previously entered value is displayed in parentheses and will be kept if you just hit the carriage return key.
  Item 11, wire gauge calculation, prompts for the wire gauge and the dimensions of a rectangular area to be filled. It displays the wire diameter and the number of turns that will fit into the indicated space.
  The formulas for a multi-layer circular solenoid were obtained from National Bureau of Standards publications and are very accurate. They work for arbitrary winding thickness and length. A flat spiral disc coil is obtained by setting the length = 0.
  Also very precise is the NBS formula for a helical solenoid of round wire. It models the size of the wire and the nonuniform current density inside the wire. The only low precision formula included is the one for a multi-layer square solenoid; it is an approximation from the CRC handbook.
  In the case of a single-turn loop or a straight piece of wire, a skin effect correction is computed. This requires that you enter the frequency of operation. The wire is then assumed to have the conductivity of copper.
  If the wire diameter or winding thickness is not explicitly requested, the coil is modeled as a zero thickness current sheet. Except for the circular solenoid of round wire, the formulas assume uniform current density throughout the winding, modified only when skin effect is included.
  In the cases that assume uniform current density, there is no correction for empty space in the winding. An approximate correction for close-wound coils is (Rosa, 1906)
  dL = 0.00097 d N
  where d is the mean diameter of the winding, in centimeters, and N is the total number of turns. This correction, in microhenrys, is added to the inductance.
  All calculations assume both the core material and the wire are non-magnetic.