  High-temperature superconducting (HTS) Flux pumps are promising devices to
maintain steady current mode in HTS magnets or to energize rotor windings in
motors and generators in a contactless way. Among different types of flux
pumps, the dynamo-type flux pump has been very common due to its simple
structure and ease of maintenance. However, understanding the principle of
dynamo-type flux pump has been challenging despite the recent progress. For
addressing this challenge, the numerical modeling has proved to be an
appropriate tool. Modeling is usually fast, precise, cost-efficient and enables
to examine some details that is hardly possible via measurements. For this
purpose, an efficient numerical model based on Minimum Electromagnetic Entropy
Production (MEMEP) method has been used to study the performance of the flux
pump in open-circuit mode. This model is the fastest and most efficient model
of a flux pump with good agreement with experiments. In addition to the main
behavior of the flux pump, the influence of airgap on open-circuit DC voltage
of the flux pump has been investigated. This study revealed an important
property in HTS flux pumps that with increasing airgap, pumping voltage in
superconducting tape does not cease, but only reaches to insignificantly low
values, which is not measurable via experiments. Furthermore, the modeling
results of open-circuit DC voltage were compared to experimental ones obtained
from the article published in 2016 by Bumby \textit{et al.}
\cite{bumby2016anomalous} and the results showed good agreement.

Applied Physics;Superconductivity