How Many Ampere Turns are Needed?
Steel exhibits a saturation magnetisation of about 2 Tesla and this sets a fundamental limit on how much clamping force can be obtained.
From the above graph we see that the field strength required to get a flux density of 2 Tesla is about 20,000 ampere-turns per metre.
Now, for a typical Magnabend design, the flux path length in the steel is about 1/5th of a meter and therefore will require (20,000/5) AT to produce saturation, that is about 4,000 AT.
It would be nice to have many more ampere turns than this so that saturation magnetisation could be maintained even when non-magnetic gaps (ie non-ferrous workpieces) are introduced into the magnetic circuit. However extra ampere turns can only be gained at considerable cost in power dissipation or cost of copper wire, or both. Thus a compromise is needed.
Typical Magnabend designs have a coil which produces 3,800 ampere turns.
Note that this figure is not dependent on the length of the machine. If the same magnetic design is applied over a range of machine lengths then it dictates that the longer machines will have fewer turns of thicker wire. They will draw more total current but will have the same product of amps x turns and will have the same clamping force (and the same power dissipation) per unit of length.