Magnetic Field Coil

01

How much axial field per kiloampere

Eaton's patent US 5,691,522 (Schulman and Slade) states the job of the field: force an initially columnar high-current vacuum arc to become diffuse and spread through the gap, so the anode becomes a passive collector of diffuse current, which gives the device its interruption ability.

The patent uses a minimum acceptable field as the design criterion. At peak current Im in kiloamperes, the axial field Ba in millitesla over the larger part of each contact face should satisfy 5 × Im > Ba ≥ 3.2 × (Im − 9). A field larger than needed adds complexity, cost, impedance, heat and eddy currents, so the aim is the bottom of the window, not the top.

It checks the rule against a published slotted-cup design (contact radius 41.5 mm, gap 15 mm, six slots): 4.2 mT per kA at the gap centre from both contacts, above that design's own stated minimum of 3.5 mT per kA. Its own one-sided example is rated for about 51 kA peak with a gap up to about 12.8 mm.

Editorial calculation from the patent's relation, using its own √2 conversion from rms; the rated currents are illustrative. The patent ignores radial variation of the field; ask for the measured or modelled field at the gap centre and at the contact face, at full gap and at peak current.

02

Impedance, heat rise and eddy-current lag

The same patent lists what the coil costs. AMF contact assemblies are harder and dearer to make than plain contacts, add impedance against the goal of a low-impedance interrupter, and so add heat rise during normal current conduction. Part of that heat comes from eddy currents that the alternating field induces in the conducting parts; those eddy currents also cut the net field and add a phase delay between field and current, and the patent notes that methods for suppressing them can add complexity to the contact geometry.

Eaton's answer is asymmetric: an AMF assembly behind only one contact, built as an annular coil of N segments carrying equal branch currents, with a small leakage path. That lowers complexity, cost, impedance, heat and eddy currents against designs that put a coil on both sides. The trade is that the field is no longer symmetric about the gap centre; it falls steadily with distance from the coil side.

Ask for the coil's contribution to the interrupter's main-circuit resistance, the temperature rise at rated normal current, and the phase lag between field and current at the expected current-zero.

03

Keeping the contact centre from burning

Hitachi's patent US 4,430,536 (Kurosawa et al.) describes the coil electrode in detail and a failure it can cause.

Each composite electrode carries a main contact with a coil electrode behind it: arm conductors take current radially out from the central rod and circumferential conductors turn it into loops concentric with the rod. The two electrodes are mated so the fields of the radial currents, which flow in opposite directions on the two sides, cancel, leaving the axial field from the loops, which run the same way on both sides. The field disperses the arc into countless fine fibre-like segments.

The problem the patent addresses is that with a strong parallel field the arc can stop spreading to the rim and collapse onto the centre of the contact face, which is then badly fused and the interruption fails. Its fix is a projecting piece of high-conductivity material at the centre of the contact: eddy currents in it partly cancel the field there, so the arc moves outward to where the field is stronger.

Ask how the field is distributed radially across the contact face and whether the contact centre is shaped or slotted to control it.

04

An external surrounding coil

A coil wound around the outside of the interrupter, as drawn in the illustration, is the arrangement studied by Jadidian, Niayesh and Hashemi (University of Tehran) in J. Plasma Fusion Res. Series vol. 8 (2009).

The paper notes that AMF is used across vacuum interrupters to keep the arc diffuse, and that a stronger field extends the multiple-arc (diffuse) mode to higher currents. Fields in earlier work stayed below hundreds of millitesla, with about 1.2 T as the highest reported; the obstacle to more is the size of the capacitor banks needed to drive a separate coil.

The authors propose a helical inductor tightly surrounding the chamber, driven by cascaded flux-compression generators fed from the fault current itself through current transformers with a 1:1 ratio used only to separate the circuits. In their simulation a 150 kA fault current is multiplied to about 1 MA, giving a field of the order of 10 T inside the chamber, and arcs up to 200 kA stay diffuse. This is a modelling study, not a tested product.

Editorial reading: an external coil must push its field through the interrupter's metal end caps and shields, where eddy currents oppose it, which is the same lag and loss problem described above. Treat a surrounding-coil design as special; ask for test reports at rated short-circuit current rather than simulated field values.

05

What to ask about an AMF coil or AMF contact assembly

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Give the rated voltage, rated short-circuit breaking current (rms and peak), rated normal current and the open contact gap. Ask for the axial field per kA at the gap centre and contact face at peak current and full gap, the coil arrangement (one-sided, two-sided or external), the added resistance and temperature rise at rated current, the field-to-current phase lag, and short-circuit type-test reports for the interrupter fitted with that coil.

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