The following project was originally my year end physics 12 project I did in 2014.  I had a lot of fun working on it and found some really cool things so I thought I would include it in the blog.  I apologize for the “dryness” of this post.  The majority of this post is pulled directly from my paper and focuses on things that I found rather than how I created the charger.  I would love to come back to this when I have more time and create a simplified and more interesting “how to” tutorial.


 

 

Electromagnetic induction (otherwise known as wireless charging) is when you transfer energy from one coil to a second coil of wire by fluctuating magnetic fields. This technology allows energy to be transferred without the need of a physical connection. Electromagnetic induction has been around for many years in appliances such as electric toothbrushes and peacemakers but no devices seem to utilize the technology for long-range wireless charging. I wanted to see if it was possible to charge devices from further distances.

 

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Questions I Had:

  • Can you transfer energy using electromagnetic induction effectively and safely through the air?
  • What factors affect the range of the power transfer? How far can I efficiently transfer power?
  • Does the power drop off exponentially the larger the range?
  • What happens when you get the coils to resonate at their resonating frequency?

Supplies

  • 2 x Mosfets
  • 2 x 10k ohm resistors
  • 2 x Ultrafast Diodes
  • 2 x Twelve-volt zener diodes
  • 2 x 18k ohm resistors
  • 2 x 12k ohm resistors
  • 1 x Ferrite toroid
  • 1 x Oscilloscope
  • A few tank capacitors

My Setup

induction inductino 3 inductino2

 

Design/How Induction Works

Primary coil circuit

The circuit takes in DC power and the mosfets (transistors) switch on and off at a high frequency causing the current in the primary coil to alternate (AC). The fluctuating current causes the magnitude of the magnetic field to change at a very fast rate. The change in magnitude creates and maintains magnetic flux. Faraday’s law of induction dictates that a changing magnetic flux will induce a voltage in a secondary coil. The secondary coil will not have an induced voltage if the magnetic flux does not change. If the primary coil were powered directly with DC the magnitude of the magnetic field would never change. This would result in no change of magnetic flux, which means there would be no induced voltage in the secondary coil.

Two mofets sitting on heat sinks

How to Increase Efficiency and Range

Optimizing electromagnetic induction is often referred to as an art form because of the complexity and precision involved. Things like coil radius, length, number of turns and frequency play a big role is increasing the range of the power transfer.

The size of the coil radius increases the area inside the coil (A). An increase area has a direct effect of the size of the magnetic flux (ΦB). A larger magnetic flux (ΦB) increases the induced EMF in the secondary coil. This will increase the power received in the secondary coil without drawing any more power. The efficiency and range increase.
The number of coils affects the output and input voltages of the primary and secondary coils. The magnetic field formula dictates that increasing the number of turns in the primary coil increases the “coils/length” ratio (N/L), which increases the magnitude of the magnetic field (B). The larger the B value, the larger the magnetic flux and the larger the EMF induced voltage in the secondary coil is. Increasing the number of turns in the secondary coil directly increase the voltage in the secondary coil. Since the overall power remains the same, increasing the voltage will decrease the current at an equal and opposite rate (P=V*C).
The number of coils affects the output and input voltages of the primary and secondary coils. The magnetic field formula dictates that increasing the number of turns in the primary coil increases the “coils/length” ratio (N/L), which increases the magnitude of the magnetic field (B). The larger the B value, the larger the magnetic flux and the larger the EMF induced voltage in the secondary coil is. Increasing the number of turns in the secondary coil directly increase the voltage in the secondary coil. Since the overall power remains the same, increasing the voltage will decrease the current at an equal and opposite rate (P=V*C).

 

Resonance

Resonant inductive coupling drastically increases the efficiency and range of the induction coils. Each tank circuit (coil and capacitor) has a frequency that it resonates at. Tuning the circuit to oscillate at the tank circuit’s resonating frequency causes the magnetic coil to couple and efficiently transfer a large percentage of its power, even if it is some distance away.

Non-resonating induction depends on a primary coil to “push” a magnetic field through a secondary coil. This requires the primary and secondary to be close together and typically requires a magnetic core to contain and direct the magnetic field through the secondary coil. Resonating induction works on the principle, making the coils “ring” or resonate with each other rather than “pushing” a magnetic field through a coil.

Induction

Resonating inductive coupling

Traditional Induction

 

 

 

 

 

 

 

Advantages of Resonating Induction
  • Charge from farther distances.
  • Coil size can be much smaller.
  • Magnetic field can be much smaller. This decreases the likelihood that the magnetic field would interfere with other electronics around it.
  • Increases efficiency. Decreases power loss
Disadvantages of Resonating Induction
  • Much more difficult to create and manufacture.

 

This graph show how the peak-to-peak voltage in the secondary coil drops off exponentially the farther it is from primary coil.

This graph show how the peak-to-peak voltage in the secondary coil drops off exponentially the farther it is from primary coil.

This graph shows how much smaller the peak-to-peak voltage is on a coil that is not tuned to resonate.  Note:  This the measured peak-to-peak voltage measured on the Oscilloscope not the actually voltage going through the circuit.

This graph shows how much smaller the peak-to-peak voltage is on a coil that is not tuned to resonate.
Note: This the measured peak-to-peak voltage measured on the Oscilloscope not the actual voltage going through the circuit.

 

Conclusion

This experiment proved that energy can be transferred efficiently and safely through the air over small distances, but power transfer efficiency drastically decreases over long distances. I learned that factors like coil radius, length, number of turns and frequency effect both efficiency and range of the power transfer. One of the biggest drawbacks, with electromagnetic induction, is the fact that the power drops off exponentially the further you are away from the primary coil. This means a lot of power is simply wasted when trying to achieve induction over distances greater than 2 to 3 cm. I learned that the trick, to long-range induction, is getting the coil circuits to resonate. Resonating induction drastically increases both the range and efficiency of power transfer. This means that you can charge over much farther distances. Resonating induction also means that coil size could be decreased, which would be more practical for consumer products such as cell phones.

 

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