Technology topic

Hollow core vs strip wound coils

Hollow core conductor has been a traditional choice of construction for high-power electromagnets of all types. However, for many magnets, strip wound coils can offer significant advantages including cost savings

It’s not all about power density

Historically, hollow core conductor was considered to be a better material for coil construction because of its ability to transmit more power through its cross-sectional area (power density). By internally cooling the copper, power density of around 6-8 A/mm2 can be achieved in hollow core while strip wound coils are usually rated around 3-4 A/mm2 (although in some special cases, current densities of up to 10 A/mm2 can be achieved). However, hollow core conductor winding is difficult with complicated and expensive manifolds and watertubing circuits required to minimize pressure drop in each pancake, and to achieve electrical isolation between the water-supply and electric power supply circuits. Strip coils on the other hand, can have a very simple water circuit and very simple electrical connections that are inexpensive and inherently isolated from each other.

Another assumption is that because of the difference in power density, a much larger strip coil is needed to produce the same field as a hollow core. However, hollow core conductor is less dimensionally accurate, more difficult to wind precisely to shape and requires thick (around 0.50 mm or .020”) fiberglass insulation. Strip coil can be very accurately wound and is normally insulated with thin, 0.075 mm (0.003”) Mylar. For many applications, a strip coil can replace a hollow core coil within the same dimensional envelope.

Strip coils aren’t just round

Thin strip can be wound to a tighter radius than hollow core, and complex 2D shapes including convex curves can be achieved with the use of special formers. With precisionmachined cooling plates helping to stabilize the coils, accuracy of the shape is ensured. Where high precision is required, molds can be constructed to keep the coil within a strict design envelope.

Buckley Systems currently manufactures strip coils from 260 g (9 oz) to 940 kg (2072 lbs). Larger strip coils can be made but the advantages of strip over hollow core generally reduce beyond this size. For very small coils, insulated wire, despite its lower power density is often the best option.

Cost savings

Per kilogram, copper strip is considerably cheaper than hollow core conductor. Hollow core conductor also needs to be wrapped with fiber-glass insulation before it is wound, adding another process and expense to the operation. Depending on the coil, savings of up to 15% can be achieved with no loss of performance by using a strip wound coil instead of a hollow core coil. Strip wound coils have proven to be just as reliable than their hollow core coil equivalents with some Buckley Systems manufactured strip coils still in use after over 30 years’ service.

Accuracy

Copper strip is more dimensionally accurate than hollow core. When hollow core conductor is bent, the outside of the radius is stretched while the inside is compressed. As well as varying the cross section, the copper can work harden which in turn makes the assembly process difficult. The internal cooling of hollow core conductor can also be restricted when bent, increasing pressure drop. Strip coil performs much closer to calculated resistance measurements than hollow core.

Conclusions

For many 2D coils, copper strip is a cost effective, more efficient alternative to hollow core conductor for new magnets. Hollow core coils can often be swapped out for strip coils with little or no modification to existing magnets. However, for 3D saddle coils and very large magnets, hollow conductor is still the best option. Talk to the design team at Buckley Systems who will be more than happy to advise you on the best construction type for your next project.

This piece was reprinted by D-Pace, Inc. from the Buckley Systems Fall 2017 technical bulletin with permission from Buckley Systems Ltd. It may not be reproduced in any form without permission or license from the source.