Glossary · Making and operating
AC loss turns changing current and magnetic field into heat
Zero resistance to steady current does not eliminate loss under alternating current or changing magnetic fields. AC loss describes that dissipation in superconducting conductors and their surrounding materials. It sets part of the refrigeration load in cables and electrical machines. [1]
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- DefinitionAC loss
- Energy dissipated by a superconducting conductor or assembly under changing current or magnetic field. Contributions include superconducting hysteresis, coupling currents, and eddy currents in normal metals, with their relative importance depending on geometry and operating conditions.
Hysteresis loss comes from irreversible flux response
Changing current or field moves magnetic flux through a type II superconductor. Pinning makes that response depend on its previous state. Energy is dissipated over a cycle because the changing flux distribution does not follow a perfectly reversible path. This superconducting hysteresis loss can occur while the conductor remains below its nominal critical current. [1]
The current amplitude, field amplitude, field orientation, and superconducting geometry influence the loss. A tape in a changing perpendicular field can behave differently from the same tape in a parallel field. Dividing a conductor into narrower filaments can reduce hysteresis, but the resulting assembly must also control electrical coupling between them. [1]
Coupling currents link superconducting elements
Changing magnetic flux can drive circulating currents through loops that connect superconducting filaments, strands, or tapes. Where those loops include resistive paths through normal metal or contacts, the currents dissipate energy. This is coupling loss. Its magnitude depends on loop geometry, transverse resistance, and the timescale of the changing excitation. [1]
Twisting or transposing conductors changes the flux linked by these loops. Electrical isolation can also reduce coupling, although an engineered conductor still needs appropriate stabilization and current transfer. Loss reduction therefore involves the complete conductor architecture. Narrow superconducting filaments alone do not guarantee low loss if large coupled loops remain available. [1]
Normal metals contribute eddy-current heating
Copper stabilizers, metal substrates, and nearby conductive structures can support ordinary eddy currents when magnetic fields vary. These currents dissipate energy through their finite electrical resistance. Magnetic materials can add their own hysteresis contribution. The total loss of a layered superconducting tape or cable therefore includes components outside the superconducting layer. [1]
A loss measurement must identify what it includes. Energy per cycle and average heating power are different quantities, and their relationship depends on excitation frequency. Electrical measurements and calorimetric measurements require appropriate boundaries and background corrections. A reported superconducting-layer loss should not be substituted for the thermal load of the entire cold assembly. [1]
Cables and machines need a dynamic heat budget
An alternating-current cable carries changing transport current and experiences its changing self-field. Motors and generators can expose superconducting windings to field ripple even when their intended excitation is steady. Fault-current limiters also need a distinction between normal-service AC loss and the much larger transient heating associated with a fault and recovery. [1]
Engineers must compare losses at the intended waveform, temperature, current, and field orientation, then include them in the refrigeration budget. Critical current is necessary information, but it does not establish low AC loss. The relevant design balances conductor geometry, current margin, stabilization, and heat removal for the complete operating cycle. [1]
Sources
- Computation of losses in HTS under the action of varying magnetic fields and currents: University of Bath recordIEEE Transactions on Applied Superconductivity, 2014
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