IntroductionCurrently, we all know Bluetooth and wireless internet and use them frequently in our day to day life and is regarded widely as a bedrock of modern communication. Numerous devices like Roku, Apple TV etc allow you to stream internet programming directly to your TV thanks to wireless internet. Many modern day security appliances work on Bluetooth and wireless internet. Now one wonders why charging of batteries etc cannot be done wirelessly. The answer is it can be done and you will be surprised that the concept is not at all new and it is based on the principle of Inductive Charging.
Figure 1 – Many modern day appliances work on the principle of wireless communication
Principle of Inductive Charging
Consider two coils separated by a distance (as shown in the figure below). One coil is the transmitter coil, which carries an AC current (to maximize the efficiency of power transfer the AC current is usually at high frequency). The receiver coil is not excited but you will notice that there is a current induced in the receiver coil due to the principle of induction. This current can be used to charge a battery (for example a cell phone battery). The transmitter coil is also called the Power Transmitter. This principle works when the coils are at a close proximity to each other but for long distance (far field) wireless power transfer we use antennas. So transmitting and receiving coils are replaced by transmitting and receiving antennas.
Figure 2 – Wireless charging based on the principle of Induction
What is of interest to engineers?Engineers who design near field wireless charging devices are interested in the design of the coil that they will use, the efficiency of the wireless power transfer, the electrical characteristics of the coils (self, mutual and leakage inductance, coupling coefficient) and the ohmic losses in the coil that results in heating of the coils. EMS for SOLIDWORKS is an electro-thermal-motion simulation package that will help engineers design and validate their induction charging designs. Far-field wireless power transfer antennas are analyzed using HFWorks product.
Model setup in SOLIDWORKSThe transmitter coil and receiver coil details are shown in the table below and the SOLIDWORKS assembly file is created based on the dimensions given below. The distance between the Transmitter and Receiver coil is about 3 mm.
|Coil type||Wire||Dia (out)||Dia (inner)||No of turns||No of layers|
|Transmitter coil||Planar circular coil||AWG 18||42.61 mm||20.81 mm||10||Single|
|Receiver coil||Planar spiral coil||AWG 15||19 mm||6 mm||5||Single|
Figure 3 – SOLIDWORKS model of the transmitter and receiver coil
Simulation objectivesUsing EMS, we are interested in finding out the following – Inductance of the coils, coupling coefficient between the coils, Ohmic losses in the coil, induced voltage in the receiving coil and finally the magnetic field generated by the coils. To compute the coupling coefficient as a function of the distance between the coils, a parametric study in EMS will automatically compute all of the above quantities for various positions of the coils.
InductanceThe table gives the self, mutual and leakage inductance as calculated by EMS.
|Self Inductance (mH)||Mutual Inductance (mH)||Leakage Inductance (mH)|
|Transmitting coil||3.38 mH||0.21 mH||2.65 mH|
|Receiving coil||0.27 mH||0.21 mH||0.212 mH|
Coupling coefficient and efficiency of the charging deviceThe coupling coefficient as calculated by EMS is 0.22 (close to 22%). It means that 22% of the flux generated by the transmitting coil will cut the receiving coil. The efficiency of the device is basically the ratio of the collected energy and input energy. For this configuration, the efficiency is around 73.5%. With an increased distance between the coils, the efficiency will drop. The efficiency also varies with the frequency of the input signal. Hence the EMS software along with SOLIDWORKS will help in predicting the correct distance and the correct amount of excitation in the transmitting coil to ensure maximum power transfer.
Power and Efficiency
|Input Power (mW)||Output Power (mW)||Induced Voltage (mV)|
|Transmitting coil||3.8 mW|
|Receiving coil||2.78 mW||82 mV|
3D plotsThe figures below show plots of Magnetic flux density and Current density. EMS can help you visualize results inside your geometry and get a good understanding of what is going on.
Figure 4 – Section plot of the magnetic flux density created with the power transmitter
Figure 5 – Magnetic flux density on the coils
Figure 6 – Current density plot on the receiver coil
Figure 7 - Section of the current density to see how the current is distributed inside the cross section