# MiscEl: Air coil

 Back to MiscEl main page Write a message to author This page is used to calculate different types of air coils. For best result always use the formulas without (...) after.

The basic formula to calculate inductance is: inductance=permability*turns*turns*area/length, the problem is that the result are not very exact when compared to a real coil. To give results that are closer to real world values some other formulas was made (Wheeler, Blakey) for specific coil geometries. They are simple and give better results than the basic formular. But even better formulas has been made, they are not simple, but the result are usual very close to the correct value (This program are using these formulas).

### Geometries supported

• Flat spiral, good result
• Flat spiral (Simple), simple formula
• Loop circular (Simple), can be used for a loop with one or more turns
• Loop rectangular (Simple), can be used for a rectangular (or square) loop with one or more turns
• Multi layer, advanced calculation of a multi layer coil
• Multi layer (Blakey), a classic and simple formula for a multi layer coil
• Multi layer square (Simple), a simpel formula for a multi layer square coil
• Single layer, advanced calculation of a single layer coil
• Single layer (Current sheet), assumes that the current is evenly distributed around the coil
• Single layer (Wheeler), a classic and simple formula for a single layer coil
• Single layer rectangle, a rectangluar coil with one layer
• Single layer square, a square coil with one layer
• Wire, inductance of a piece of wire
• Wire pair, inductance of a two wires (Remark: "Side" is distance between wires)
All names with (...) after has lower precision or some restrictions.
When the graphic shows two yellow squares, one is a look from the end of the coil, but without the turns illustrated.

### Input values

 Turns Number of turns on coil Length Length of coil Diameter Řmin Inner diameter of coil Diameter Řmax Outer diameter of coil Wire diameter Wire diameter, use Wires page to convert units Bundle diameter Diameter of wire bundle on a "circular loop" coil Side One of the sides on a square/rectangular coil Side 2 The other side on rectangular coils Winding thickness Thickess of winding layer Frequence Frequence used on coil/wire, used to compensate for skin effect Fill factor Defines how tight the wire is wound (100 means no space for air or isolation)
Note: All measurement inputs, except wire diameter, are in meters.

### Output values

 Inductance The estimated inductance, the error is depending on formula, how exact the coil is constructed, any iron in the vicinity and frequence Coil length Lenght of coil, this is the same value as the input value "Length" Avg. coil diameter Average diameter, this is diameter_Řmin+winding_thickess Avg. side Average side, this is side+thickess of copper Avg. side 2 As above, but for long side on rectangles Winding thickness The thickness of the windings Wire length Total length of wire use for this coil Resistance Resistance in wire Estimated weight Weight of wire

### Hints

• Fill factor is only used in design
• In design use "Fill factor" (on config page) to change the length of the coil
• Keep an eye on the copper mass. If it show a couple of Kg, it is a BIG coil
• Most formulas do not warn when using invalid geometries
• Wire pair: "Side" is distance between wires, "AvgSide" is distance between wire centers
• "To analyze" button only copy visible digits. If "Fill factor" is high, analyze may think there is to much copper and mark one of the dimensions in red
• "To analyze" button only copy visible digits. This may change the inductance a little
• It is possible to leave "Wire diameter" empty in "Analyze" for some geometries, it will not change the result
• Small changes to the dimensions of a coil may give big changes to the inductance
• To enter a value in inch, use the calculating ability of the input field. i.e. 3 inch is entered as "3*inch". This constant comes from the Calculator const page (This does NOT work for the wire diameter field, it expects input in mm, not meter)

### Other pages

• For calculation charge/discharge curves use Charge curve page.
• For calculation impendance of a inductor use Ohms law page.