In English

Conductive material within Temperature Management

Johanna Ternström
Göteborg : Chalmers tekniska högskola, 2014. 22 s.
[Examensarbete på avancerad nivå]

A contactor is an electronically controlled switch used for switching a power circuit. The principles have been the same for a hundred years, i.e. there is a contact system that does the job. The contactor is operated by a control voltage that causes it to open or close. Common applications include starting and stopping of short-circuited motors, purely resistive loads, and bypass applications. These are all usually AC applications. The recent emergence of solar energy applications has increased the demand for products that can also handle DC. The difficulty with DC is to break the current. Unlike a direct current, an alternating current will have a zero-crossing point at every half cycle. During this period of low current, the arc can be extinguished with relative ease by preventing re-ignition of the arc. For DC however, the current remains constant over time and the breaker needs to attenuate the ow of current. When the contacts are separated an electric arc occurs between them which has to be extinguished in some way for the current to be broken. This makes DC breaking extra difficult and special features need to be implemented in order to extinguish the arc and break the current ow. High voltage electric arcs causes significant damage to the electrodes and shorten the lifetime of the contactor. Therefore, minimizing the arc duration time is an important engineering challenge. This thesis is a pre-study of arc extinguishing techniques for a DC contactor. The goal is to be able to break 1500VDC using 4-poles connected in series. This thesis projet is divided into two parts. In the first part of the project, permanent magnets were used to suppress the arc and the simulations were performed in ANSYS Maxwel This report deals with the choice of material for a thermal management product at Mölnlycke Health Care. The aim with the product is to prevent hypothermia during surgery. The goal with this project was to investigate which material parameters that was of interest. The material parameters that were found most interesting were the thermal conductivity, the thermal diffusivity and the ability to keep a stable temperature over time. A model was built in Comsol to study the impact of different materials on the temperature on and in human skin. The simulations showed that water based materials could not spread the heat outside of the heater. In an environment that was not thermally and water isolated, the use of a water based material even led to a decrease in skin temperature. Measurements showed that different types of heaters had different requirement on the interface material. Some different commercial materials were tested and Hollywog, Medema and T ex were found to be most suitable on an iron heater and Hollywog, Medema and Heat sink were found to be most suitable on a an electrical heater. A hydrogel containing 67% water could also be used on the electrical type of heater if the water was not allowed to evaporate. Different types of conductive particles could be used to increase the thermal conductivity and thermal diffusivity in the material. Of the evaluated conductive particles, Carbon Black and Potassium Chloride were most suitable.l. The method considered all possible wirings of the contactor and compared the results to an existing 3-pole DC contactor. The proposed optimal solution uses steel plates surrounding the magnet and contact system to increase the magnetic ux density. Results show an increased efficiency of the magnets used by 164.8%. In the second part of the projects, a DC arc model was implemented in the computer program ANSYS Simplorer using theory found in literature. The model was extended to a model of 4-pole 1500VDC contactor. The model successfully simulates the currentvoltage characteristics of the contactor during the breaking process and predicts critical currents.

Publikationen registrerades 2015-01-02. Den ändrades senast 2015-01-14

CPL ID: 209405

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