Electromagnetic Can Crusher Victoria Meadows and Matthew Kundrock Advisor: Dr. Gore Introduction Our Capstone Project was to build an Electromagnetic Can Crusher, a device that will crush an aluminum can placed within a conductive coil by releasing a large amount of current through the coil in a short time frame. Our device was built to test the theory behind crushing cans electromagnetically and was built to mirror other examples we found during our research. Those devices used similar setups to crush cans with varying degrees of success. In the device’s design, a transformer was used to ramp up the voltage that would be stored in our capacitor bank made of three capacitors in parallel. A spark gap wired in parallel with the capacitors was used as a switch to transfer a large amount of current into a copper coil. The current running throughout the coil created a magnetic field within the can. As a result an induced current was created within the can and the reaction between the magnetic field and the eddy current caused the can to be deformed. Theory An electromagnetic can crusher makes use of faraday’s law. This law, as shown in Equations 1 and 2, states that any change in magnetic field (
) over time within a coil will cause an electromotive force. The amount of voltage is controlled by N or the number of turns in the coil. Equation 1 Equation 2 When the capacitor’s discharge voltage reaches the breakdown voltage of air the spark gap will fire. The quick burst of current through the spark gap will create a current in the coil that increases exponentially for a fraction of a second. Though the circuit is only completed for a very short time, the current ramps up to several thousand amps. This massive rate of increase will produce a strong magnetic field within the coil. The fast change in magnetic field field (
) over the change in time creates a force that induces another current within the can in the opposite direct of the coil’s current. The new current reacts with the magnetic field of the first to crush the can. Methods Figure A: Can Crusher Circuit The first major circuit component is the transformer (T1). Rated at 75 milliamps for the desired 300 volts, the transformer allowed the capacitors to charge to 300 volts. With this amount of energy stored, the capacitors should be able to crush a can nicely. The charge time for the capacitor bank was a lot longer than originally designed because the transformer could only deliver 75 milliamps of current. The transformer output is still AC so we had to use a bridge rectifier circuit (DB1) to change the current to DC. When capacitors are supplied an AC current the capacitors will charge a little bit and immediately discharge when the current alternates, preventing the storage of any significant amount of energy. With the current in DC form then the capacitors would be able to charge. The conversion from AC to DC is done by the full bridge rectifier. The current was wired to a resistor with a total resistance of 3.75 kΩ (R1). This resistor prevented the current of the charge circuit from going over 75 milliamps. From here the capacitors are charged in parallel (C1, C2, C3). These three capacitors rated at 15 millifarads each totaled to 45 millifarads. Each of these capacitors are rated for 450 volts and by definition were able to handle the desired 300 volts safely. The circuit then splits into three additional paths; one being our spark gap and coil (V2, L1), one being our discharge circuit (R2, S2), and the final being our voltmeter. These paths each fulfill one of the goals in the design, a way to record the voltage as the capacitors charged, a way to discharge the capacitors should the spark gap not fire, and the actual coil and spark gap required to crush a can. The spark gap was originally two steel L ­ brackets fastened to a piece of wood with wire leads attached to each end and two bolts coming to a point in the middle. Figure B: Original Spark Gap Design This simple design was used because it was easy to adjust the gap length as needed. Unfortunately this design was heavily flawed with the bolts being knocked out of alignment when the spark gap fired and the thread on the bolts fused together, making the gap impossible to adjust without replacing the bolts. The desired gap distance was also difficult to measure without more precise tools and the breakdown voltage of air, which determined the distance that was needed between the bolts, was changing daily. The breakdown voltage of air is heavily dependant the humidity of the air as well as what particles were floating around on a particular day. These factors are not easily accountable without some way to reliably measure the breakdown voltage of air. To counteract this, an improved hammer drop system was designed which connected the two wire leads by dropping a hinged piece of wood with a metal bar across the length aligned with the leads. Figure C: Improved Spark Gap Design Even though the metal plate melted to the wire terminals, this new design proved fairly effective and it was much easier to reset after a successful trial than the original design. A voltmeter was connected in parallel with the capacitors to track the voltage during the charge and discharge cycles. This was added not only as a safety feature, but a component that was used to measure the voltage at certain intervals during the charge cycle. This information was used as our experimental charge time, discussed later in this report. A discharge circuit provided a safe way of discharging the capacitors in case the spark gap did not fire as planned. A resistor was placed in parallel with the capacitor bank and was wired in series with another switch so the capacitors would discharge when needed. The resistor has a resistance of 3.9 kΩ which limited the current and speed of which the capacitors were being discharged when the discharge switch was flipped. A copper coil with three turns was formed in series with the spark gap. This coil will create the magnetic field needed to crush the can when the capacitors discharge. A three turn coil was used because it has a low inductance value while still being able to generate a strong magnetic field. Having a high inductance is not desirable because the current will be slowed through the coil and the magnetic field will not ramp up quickly enough to generate a strong eddy current in our can. AWG 2 copper wire was used in the coil and for the connections between the capacitors. At 6.544 millimeters in diameter, this wire is high enough gauge to be able to withstand the current we would be sending through it as well as being easy to purchase and physically stable enough to form the coil without any major support structures. Data The charge cycle for our capacitors produced a logarithmic growth in voltage over the time frame we measured as seen in Figure B. The voltage was recorded every 10 seconds and plotted using Microsoft Excel. A line of best fit was included to The equation for the line of best fit is: The graph in Figure D plots our experimental and theoretical charge times. These curves are very similar and both represent a logarithmic charge of the capacitors. The theoretical charge time was calculated by substituting the resistance (3750 Ω), total capacitance (0.045 Farads), time (seconds), and the initial voltage (0 Volts) into the following equation: Figure D: Capacitor Charge Cycle Figure E shows the theoretical discharge cycle. It was calculated using Equation 5 by substituting the resistance (3900 Ω), total capacitance (.045 Farads), time (seconds), and the final voltage (0 Volts) into the following equation: In order to calculate what size resistor was required to discharge the capacitors at a safe rate, Equation 6 was used by substituting the maximum power the resistor could handle (25 Watts), the maximum voltage (300 Volts), and the resistance value of the resistor (3900 Ω). If the energy that was being dissipated per second was less than the power rating of the resistor then the resistor should withstand the current and be able to discharge the capacitors safely. Figure E: Capacitor Discharge Cycle Discussion and Conclusions After testing the Can Crusher several times, noticeable dents appeared around the circumference of the cans as well as additional vertical pinches left over from the magnetic field. Though the crusher did not provide the anticipated results, it performed fairly well considering the budget constraints that limited it. The crusher’s biggest limiting factor was the voltage rating of the capacitors we used. Three hundred volts was not enough to crush the can as much as desired, and higher rated capacitors can go upwards of $400 each which was way too expensive for our planned budget. Another limiting factor was our transformer’s maximum current rating. The transformer was rated at 75 milliamps. With this current rating, the amount of time to charge our capacitors took 8 minutes. With larger capacitors, the charge time would increase significantly unless we had a stronger transformer to compensate. The final problem we ran into was our spark gap. Our original design was poor, unreliable, and unsafe. The second design of a hammer drop spark gap proved to be more efficient, but a few improvements could have been made. A more stable hammer drop spark gap with a more precise dropping mechanism would be preferred as compared to the string method used. Improvements could also have been made to the quality of materials used as many of the parts, in the spark gap especially, tended to weld together when the capacitors discharged. Bits of metal even flew off the terminals of the original spark gap and burned bits of the wooden containment box which could have started a fire if we were not careful. Using a metal that has better conductivity. Appendices: Digi­Key Schemit Software Schemit software was used to create a schematic of our circuit.