| 1. |
- Frost, Anna E., 1988-, et al.
(författare)
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Do Lab Introductions Matter for Electrical Engineering Students?
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- In order to improve the experience and results of the laboratory work for first-year electrical engineering students, a lab introduction was introduced in a basic electrical engineering course. An increase in students who took the laboratory as well as the theoretical exam was observed when there was a lab introduction in 2018, compared to 2017. Marginally more students passed the laboratory exam, while significantly more students failed the theoretical exam. The students who participated in the lab introduction answered in a survey that they thought the course should have a lab introduction. Before implementing the lab introduction, some students who took the course in 2017, answered in a survey that they were positive to introducing a lab introduction and what they thought it should contain. A major part of the new lab introduction was based on their answers. It contained presenting the lab rules, but was also supposed to make the student reflect upon how they work with preparatory tasks and the labs. Compared to other studies on engineering students, this one looks at the perception of students who just started at the university.
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| 2. |
- Frost, Anna E., 1988-, et al.
(författare)
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Do pre-recorded lessons in combination with formative assessments fulfil the learning necessity of engineering students?
- 2022
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Ingår i: Bidrag från 8:e Utvecklingskonferensen för Sveriges ingenjörsutbildningar. - : Karlstads universitet. - 9789178672813 ; , s. 21-25
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- As an effort to increase digitization of education during the ongoing pandemic in Spring 2021, the course ”Electromagnetism II with electric circuits” at Uppsala University has been conducted in a blended form with both pre-recorded sessions and live online sessions. About half of the lectures were live sessions, and the rest were pre-recorded. Among the lessons, more than 75 % were pre-recorded. When the teaching is pre-recorded, the teachers do not get reactions directly from the students, either verbally or from facial expressions. Therefore,self-correcting quizzes were implemented for every lecture, and short formative evaluations were implemented for every lesson. The main objective was to increase the number of students participating in the lessons, which hopefully would lead to more students passing the course. More students watched the pre-recorded lessons than attended the lessons on campus previous years on the course. More students passed the exam on the course compared to previous years, but it cannot be excluded that this is partially or completely due to the exam being different due to the pandemic. The short formative evaluations had at least seven respondents during the first four lessons, but from lesson five and onward, the number of respondents was two or fewer. The quizzes which most students attempted were right after or right before the pre-exams (”duggor” in Swedish), but otherwise relatively few students answered the quizzes. When asked in the course evaluation, it varied a lot whether the students liked, disliked, or were neutral to have pre-recorded lessons, with slightly more students saying they preferred live sessions.
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| 5. |
- Frost, Anna E., 1988-
(författare)
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In the Air Gap of Linear Generators for Wave Power
- 2021
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Wave power conversion is one type of renewable electricity generation. Within wave power, there are many different concepts, whereof some utilizes linear generators for converting the energy in the ocean waves into electricity. A linear generator consists of a translator, which is moving and have magnets of alternating polarity, and a stator, which have conductors sur-rounded by laminated steel. Between the translator and stator is an air gap, which is only a few millimeters wide. All linear generators for wave power, to the author’s knowledge, are permanent-magnet synchronous generators. This thesis looks into the forces and power flow in the air gap of linear generators for wave power, with the purpose of improving their future performance. The studies have focused on permanent magnet synchronous generators for wave power, but several of the results should also be applicable for other applications of linear elec-trical machines.Depending on the design of the linear generators, the translator can move so long that it only partially overlap the stator. This is common among several different wave power concepts with linear generators. When the stator is only partially overlapped by the stator it is denoted as partial stator overlap. It is studied how partial stator overlap affects the generated electric-ity, the absorbed energy, and the tangential and normal force in the air gap. The generated electricity and absorbed energy of a linear generator are quadratically dependent on the partial stator-translator overlap is shown through Faraday’s law and simulations. Experimental data showed that the absorbed energy is both linearly and quadratic depending on partial stator over-lap, where the linear dependence is at least partially due to frictional losses. Simulated results confirm that voltage is linearly dependent on partial stator overlap, which means quadratic de-pendence between generated electric and partial stator overlap. The simulated forces showed a linear dependence.Decades ago, the Poynting vector was used to derive an expression for the power flow in the air gap of rotating electrical machines. In this thesis the equivalent expressions for both flat and tubular linear electrical machines were derived. The analytical results were also compared with results from simulations. Both the analytical expressions and simulations showed that tubular and flat linear electrical machines have slightly different behavior.
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| 6. |
- Frost, Anna E., 1988-, et al.
(författare)
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Power flow in the air gap of linear electrical machines by utilization of the Poynting vector : Part 1 - Analytical expressions
- 2022
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Ingår i: The Journal of Engineering. - : Institution of Engineering and Technology (IET). - 2051-3305. ; 2022:4, s. 377-388
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Tidskriftsartikel (refereegranskat)abstract
- Analytical solutions and estimations for the power flow in the air gap of linear electrical machines of different geometries are derived from Poynting's theorem. The different geometries considered are flat one-sided, multi-sided, and tubular linear electrical machines. The radial power flow for all considered geometries is dependent on the area of the air gap, the electric field, the magnetic field, and the load angle. The tangential power flow for both flat one-sided and tubular linear electrical machines is dependent of the area of the air gap, number of poles, the electric field, the magnetic field, and the load angle. The number of poles could be increased to decrease the tangential power flow in flat linear electrical machines. The expression for the tangential flow in tubular linear electrical machines is so complicated that it is difficult to draw conclusions from it.
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| 7. |
- Frost, Anna E., 1988-, et al.
(författare)
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Power flow in the air gap of linear electrical machines by utilization of the Poynting vector : Part 2 - Simulations
- 2022
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Ingår i: The Journal of Engineering. - : Institution of Engineering and Technology. - 2051-3305. ; 2022:9, s. 883-891
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Tidskriftsartikel (refereegranskat)abstract
- Different types of linear generators are simulated and their power flow in the air gap is investigated. The results are compared to the analytical expressions derived in Part 1. The simulations and the analytical expressions in Part 1 show the same general behavior, but the magnitudes are lower for the analytical expressions. One explanation for the difference in magnitude can be that the harmonics of the electric and magnetic fields contribute to the power flow, which is not accounted for in the analytical expressions. Due to results from Part 1, it is investigated if changing the number of poles can decrease the tangential power flow while the normal power flow stays the same. As was suspected, changing the number of poles affected several other factors, which lead to an increase in the normal power flow when increasing the number of poles, even though the electrical power was the same. The tangential power flow also decreased for three out of four generators. Thereby, increasing the number of poles with the same length of the machine, at the cost of reduced pole-pitch, should be done with precaution.
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| 8. |
- Frost, Anna E., 1988-
(författare)
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Power transfer in the air gap of linear generators for wave power
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An increased need for electric energy and share of renewable energy is important for a more sustainable future, to which wave power can contribute. Some concepts for wave power use linear generators, and the studies presented here focus on the power transfer and forces in the air gap of linear generators.Based on Poynting's theorem, analytical expressions were derived for how much power is transmitted in the air gap of linear electric machines. The equations were derived for flat linear generators as well as for cylindrical linear generators, and for both the radial and the longitudinal power flows. The radial power flow contributes to the electricity that can be taken from the generator, while the longitudinal contributes to unwanted effects such as vibration. The equations show that for flat linear electric machines, vibrations and other unwanted power transmissions can be reduced by increasing the number of poles in the machine, without decreasing the desired power transfer. Otherwise, the same factors that increase the desired power transfer also increase the unwanted.By studying the generator as a damping force and a lumped circuit, it was derived that the absorbed energy of a linear generator is quadratically dependent on the partial stator-translator overlap if the inductance of the generator is not affected by a partial overlap. Consequently, the forces in the air gap also depend quadratically on the relative overlap between the stator and the translator.Data were collected during two experiments in order to determine whether the quadratic dependence between partial stator-translator overlap and energy transfer could be seen experimentally. Linear regression analysis of the experimental data shows that the relationship is somewhere between linear and cubic, but did not clearly show a quadratic relationship. The inductance showed no dependence of the partial stator-translator overlap.
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| 9. |
- Frost, Anna E., 1988-, et al.
(författare)
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What influences the Generator Damping atPartial Stator Overlap in a Linear Generator for Wave Power?
- 2021
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Ingår i: Proceedings of the 14th European Wave and Tidal Energy Conference. ; , s. 2245-1-2245-11
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Konferensbidrag (refereegranskat)abstract
- Experimental data is fitted against partial overlap between the stator and the translator to different powers from one to four to see which relations can be found between electric power, copper losses, other losses, and the dampening force. The other losses consist of iron losses and frictional losses together. The results differ depending on whether the translator is moving up or down, especially for the electric power. Why that is would need further studies to explain. The electric power shows mainly a quadratic dependence on partial overlap, but a cubic dependence as well cannot be excluded, especially when the translator is moving down. The copper losses shows aquadratic dependence on partial stator overlap. The other losses show mostly a linear dependence on the partial stator overlap. The dampening force shows mainly a linear dependence on partial overlap, but a quadratic dependence as well cannot be excluded, but this is probably since the electric power and the copper losses affect the dampening force except when there is no load.
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| 10. |
- Kontos, Sofia, et al.
(författare)
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An Overview of MnAl Permanent Magnets with a Study on Their Potential in Electrical Machines
- 2020
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 13:21
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, hard magnetic materials for future use in electrical machines are discussed. Commercialized permanent magnets used today are presented and new magnets are reviewed shortly. Specifically, the magnetic MnAl compound is investigated as a potential material for future generator designs. Experimental results of synthesized MnAl, carbon-doped MnAl and calculated values for MnAl are compared regarding their energy products. The results show that the experimental energy products are far from the theoretically calculated values with ideal conditions due to microstructure-related reasons. The performance of MnAl in a future permanent magnet (PM) generator is investigated with COMSOL, assuming ideal conditions. Simplifications, such as using an ideal hysteresis loop based on measured and calculated saturation magnetization values were done for the COMSOL simulation. The results are compared to those for a ferrite magnet and an NdFeB magnet. For an ideal MnAl hysteresis loop, it would be possible to replace ferrite with MnAl, with a reduced weight compared to ferrite. In conclusion, future work for simulations with assumptions and results closer to reality is suggested.
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