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Impact of Surrounding Objects on Inductive Wireless Power Transfer

Odhiambo, Brenda LU (2019) EITM02 20191
Department of Electrical and Information Technology
Abstract
Many new models of smartphones and small electronic devices are using their wireless charging feature as a key selling point. This is due to the convenience of cable-free charging as well as not requiring multiple power adapters for charging different devices. However, with the increasing use of devices having wireless charging capability, safety and efficiency issues arise. Safety concerns are critical for manufacturers of these devices. In inductive wireless charging systems, knowledge on the impact of surrounding objects on the system is imperative in designing safer and more efficient systems. This thesis gives a quantitative analysis of the influence of foreign objects to an inductively coupled Wireless Power Transfer (WPT) system.... (More)
Many new models of smartphones and small electronic devices are using their wireless charging feature as a key selling point. This is due to the convenience of cable-free charging as well as not requiring multiple power adapters for charging different devices. However, with the increasing use of devices having wireless charging capability, safety and efficiency issues arise. Safety concerns are critical for manufacturers of these devices. In inductive wireless charging systems, knowledge on the impact of surrounding objects on the system is imperative in designing safer and more efficient systems. This thesis gives a quantitative analysis of the influence of foreign objects to an inductively coupled Wireless Power Transfer (WPT) system. This is done by measuring carefully chosen square metal plates of different dimensions and material properties, and quantifying their impact on the coil inductance, equivalent coil resistance and quality factor of the transmitter coil. Low-frequency electromagnetic simulations are performed in Ansys Maxwell to verify the experimental measurements. The results obtained by experiments and simulations are consistent with each other. These results are analyzed, and conclusions drawn pertaining to how the materials impact the system.

In particular, the tested metal plates increase the equivalent coil resistance as compared to that of the empty coil, due to the induced eddy currents on the plates. Simultaneously, eddy currents tend to decrease the coil inductance as compared to that of the empty coil. However, at frequencies lower than 100 kHz, the presence of ferromagnetic metal plates can actually increase the coil inductance, as these plates are strongly magnetized by the magnetic field generated by the coil currents. In general, the quality factor is decreased in the presence of the test metals. Further, the thickness of the metals influences the power loss due to eddy currents only if the metal thickness is below the skin depth of the metal. In addition, the square metal plates with side lengths equal to or greater than the coil diameter have similar impact on the coil parameters, indicating highly localized magnetic fields around the coil. The metal plates with side lengths smaller than the coil diameter have more unpredictable impacts on coil parameters. Investigations into the impact of foreign objects on both the transmitter and receiver coils are proposed for future work. (Less)
Popular Abstract
The global wireless charging market is expected to hit USD$71 billion by 2025. With the onset of Internet of Things (IoT) where many hundreds of millions more devices will be connected to the internet, opportunities for wireless charging applications will be further enhanced. Wireless charging of smartphones and wearable electronics is just one of many areas where modern day wireless charging is applied. In general, wireless charging solutions have been applied in consumer electronics, automotive industry and health care. Efforts are also made to promote the same technology for wirelessly powered kitchen appliances.

When wirelessly charging or powering a device, it is desirable to have maximum power transfer efficiency and guaranteed... (More)
The global wireless charging market is expected to hit USD$71 billion by 2025. With the onset of Internet of Things (IoT) where many hundreds of millions more devices will be connected to the internet, opportunities for wireless charging applications will be further enhanced. Wireless charging of smartphones and wearable electronics is just one of many areas where modern day wireless charging is applied. In general, wireless charging solutions have been applied in consumer electronics, automotive industry and health care. Efforts are also made to promote the same technology for wirelessly powered kitchen appliances.

When wirelessly charging or powering a device, it is desirable to have maximum power transfer efficiency and guaranteed safety of the user. Reduction in the power transferred is mostly caused by surrounding objects that interfere with the wireless charging system. The most obvious safety risk is overheating of the device, which can cause fires. Moreover, even if there is no safety risk, inefficient power transfer results in longer charging times. Therefore, it is important to understand how surrounding objects affect the wireless charging system, so that guidelines can be made to improve system design and appropriate measures taken when foreign objects disturb the system unexpectedly. Examples of foreign objects include coins, keys and safety pins, everyday metallic objects that may be placed near wireless charging pads as wireless devices are being charged.

Currently, the Qi Specification is the dominant wireless charging standard, and it includes safety mechanisms for identifying foreign objects and shutting down the charging process when they are detected. However, given the relatively recent adoption of the wireless charging technology, very little study has been performed on the behavior of wireless charging system when it is subjected to foreign objects. To address this knowledge gap, this thesis investigates the impact of surrounding objects on the metal coil used in the charging pad. Specifically, the surrounding objects are metal plates of different sizes and material properties, and the impact is quantified in terms of several coil parameters, including resistance and inductance. The metals chosen in this study are commonly used ones, including copper, aluminum and stainless steel. The results from the thesis yield practical insights and rule-of-thumbs that can be used for wireless charging system design. For example, metals with ferromagnetic properties tend to have less impact on coil inductance. The knowledge gained can also be used to improve the detection of foreign objects, which will further increase the safety of wireless charging applications. (Less)
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author
Odhiambo, Brenda LU
supervisor
organization
course
EITM02 20191
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Wireless power transfer, inductive coupling, Qi Specification, effect of metals
report number
LU/LTH-EIT 2019-711
language
English
id
8989358
date added to LUP
2019-07-04 11:55:19
date last changed
2020-03-11 10:15:39
@misc{8989358,
  abstract     = {Many new models of smartphones and small electronic devices are using their wireless charging feature as a key selling point. This is due to the convenience of cable-free charging as well as not requiring multiple power adapters for charging different devices. However, with the increasing use of devices having wireless charging capability, safety and efficiency issues arise. Safety concerns are critical for manufacturers of these devices. In inductive wireless charging systems, knowledge on the impact of surrounding objects on the system is imperative in designing safer and more efficient systems. This thesis gives a quantitative analysis of the influence of foreign objects to an inductively coupled Wireless Power Transfer (WPT) system. This is done by measuring carefully chosen square metal plates of different dimensions and material properties, and quantifying their impact on the coil inductance, equivalent coil resistance and quality factor of the transmitter coil. Low-frequency electromagnetic simulations are performed in Ansys Maxwell to verify the experimental measurements. The results obtained by experiments and simulations are consistent with each other. These results are analyzed, and conclusions drawn pertaining to how the materials impact the system. 

In particular, the tested metal plates increase the equivalent coil resistance as compared to that of the empty coil, due to the induced eddy currents on the plates. Simultaneously, eddy currents tend to decrease the coil inductance as compared to that of the empty coil. However, at frequencies lower than 100 kHz, the presence of ferromagnetic metal plates can actually increase the coil inductance, as these plates are strongly magnetized by the magnetic field generated by the coil currents. In general, the quality factor is decreased in the presence of the test metals. Further, the thickness of the metals influences the power loss due to eddy currents only if the metal thickness is below the skin depth of the metal. In addition, the square metal plates with side lengths equal to or greater than the coil diameter have similar impact on the coil parameters, indicating highly localized magnetic fields around the coil. The metal plates with side lengths smaller than the coil diameter have more unpredictable impacts on coil parameters. Investigations into the impact of foreign objects on both the transmitter and receiver coils are proposed for future work.},
  author       = {Odhiambo, Brenda},
  keyword      = {Wireless power transfer,inductive coupling,Qi Specification,effect of metals},
  language     = {eng},
  note         = {Student Paper},
  title        = {Impact of Surrounding Objects on Inductive Wireless Power Transfer},
  year         = {2019},
}