I am indebted to my colleague, Mr Tony Grainger, for the
JavaScript program which performs the calculations for this
page.
How to use this page:
An electromagnet consists of 2 basic parts;
an electric circuit (or coil) and
a magnetic circuit (or flux path).
An electric current in the coil induces a magnetic current (flux) in the magnetic circuit.
The program on this page brings together all the salient features of
both these basic parts and makes detailed calculations to inform your electromagnet
design.
Feel free to experiment.
You can type any numbers you like into the
input fields. The program will immediately make all the relevant calculations and present them in the "
Calculated Results"section.
At the same time both the geometry and the dimensions for
electromagnet cross section drawing will be automatically updated
.
Additional Calculated Results can be displayed if desired by clicking on the "
Details +/-" button. These additional results will not usually be needed but may be useful for advanced designers.
To reset the program to the default numbers just refresh (or reload) this web page.
The calculations assume that the coil is made from
copper wire (resistivity = 0.0171 Ωm at 20ºC) and that the magnet body is made from
mild steel SAE 1020.
The relationship between the magnetisation in the steel and the field strength
is non linear and cannot be represented by an equation.
The calculations on this page use a table lookup followed by an
iterative method to obtain accurate results for the flux density
pertaining to the particular magnet configuration.
MAGNETISATION DATA:
Magnet End Configurations:
Two common ways of finishing the end of a Magnabend magnet are shown
below:
In the first picture the outer poles of the magnet are longer than the
middle poles, and in the second picture all the poles are the same
length. (The configuration in the second picture provides
better
clamping near the ends of the magnet).
When entering dimensions into the calculator above the effective magnet
length is the length of
the middle poles.
How
Much Clamping Force is Needed?
The
Electromagnet Design Tool at the the top of
this page provides a means for calculating the
clamping force
obtainable from just about any magnet configuration, but it does not
provide information about how much force is needed for a given
bending job.
The bending force needed depends on:-
- The length of the bend, L
- The thickness of the sheet, t
- The tensile strength of the material, σ
- The width of the flange being bent, w.
- And the bend radius of the bend, r
The bending force can be expressed mathematically:
If the sheet length, sheet thickness, flange-width and bend radius are all
expressed in metres, and the tensile strength of the sheet material is
expressed in Pascals (Newtons per square metre), then the bending
force, F, will be in
Newtons per metre of length.
That unit can be divided by 9,810 to produce tones per metre which matches the unit for "Clamping per metre" in the calculator at the top of this page.
k is a dimensionless
constant. By measuring values for a known marginal bend we can derive
an
empirical
value for
k that will apply to
bending with a Magnabend. The value of
k will be only approximate
as the the bending force also depends on the tendency for the sheet
material to work-harden during bending and on other factors such as the
frictional value on the surface of the Magnabend.
Here is a handy table of wire sizes that will encompas most gauges that are
likely to
be needed for a magnet coil.
Preferred
wire
diameters |
Wire cross
sectional area.
|
Kg
per
Km |
Nearest
SWG
gauge |
Nearest AWG gauge |
2.00 mm |
3.14
mm2 |
27.9 |
14 |
12 |
1.80 mm |
2.54 mm2 |
22.6 |
15 |
13 |
1.60mm |
2.01 mm2 |
17.9 |
16 |
14 |
1.40 mm |
1.54
mm2 |
13.7 |
17 |
15 |
1.25 mm |
1.23 mm2 |
10.9 |
18 |
16 |
1.12 mm |
0.985 mm2 |
8.76 |
19 |
17 |
1.00mm |
0.785mm2 |
6.98 |
19 |
18 |
0.90 mm |
0.636 mm2 |
5.66 |
20 |
19 |
0.80mm |
0.503 mm2 |
4.47 |
21 |
20 |
0.71 mm |
0.396
mm2 |
3.52 |
22 |
21 |
0.60 mm |
0.283 mm2 |
2.51 |
23 |
22 |
0.56 mm |
0.246 mm2 |
2.19 |
24 |
23 |
0.50 mm |
0.196mm2 |
1.75 |
25 |
24 |
0.45mm |
0.159 mm2 |
1.41 |
26 |
25 |
0.40mm |
0.126 mm2 |
1.12 |
27 |
26 |
0.37mm |
0.110mm2 |
0.98 |
28 |
27 |
See also the
Coil Calculator.