Cable wrap magnetisation

Wrapping the output cable of a portable unit several times around a part turns the cable into an encircling coil, producing a longitudinal field along the wrapped axis. It is the standard way to magnetise a shaft, a pipe section or a large weldment in the field, where no fixed coil bench exists. Because the turns lie directly on the part, the arrangement is a high fill factor coil: the cable is essentially in contact with the surface, so the flux does not have to spread across an oversized bore.

The field is set by ampere-turns, so wrapping more turns lets a small portable set reach the required magnetomotive force at a current it can actually deliver. Five turns at 1 000 A is the same 5 000 ampere-turns as one turn at 5 000 A, which no portable unit will supply. The demagnetising field from the part's own end poles still governs how many ampere-turns are needed, hence the length-to-diameter ratio in the formula and its 2 to 15 validity band.

The usual field mistake is treating one wrap as covering the whole part. A coil's field is only usefully strong for about 230 mm (9 in.) either side of the windings, so a long shaft must be wrapped, shot, unwrapped and re-wrapped further along with overlap. Turns should be kept tight against each other rather than spread out, since a spread wrap behaves like several weak coils instead of one strong one.

Cable heating is the practical limit. Wrapped cable cannot shed heat the way a straight run can, so keep shots short, respect the duty cycle of the set, and check the cable rating against the calculated current before starting.

L/D ratio = part length / part diameter, limited to the range 2 to 15
NI = 35 000 / ((L/D) + 2)   ampere-turns, ±10%  (cable wrap is high fill factor)
I = NI / N
Cable per wrap = N x pi x (part diameter + cable diameter)
Effective field reaches about 230 mm each side of the wrap