Research team developing Tesla coil designs
Mention a Tesla coil (TC) to many people, and they might guess it has something to do with the pioneering electric car named after Nikola Tesla, a contemporary and rival of Thomas Edison. They would be correct, in part. Tesla’s development of ac generation and transmission technology has become the global standard for electrical power transmission, without which there would likely be no electric cars. But the cars themselves do not use TCs, nor does much of anything else, other than some high school labs and science museums. The coils were Tesla’s short-lived effort to create an open, wireless technology that would transmit power around the globe without cables.
Now, a century after the implementation of ac, researchers at The Geek Group National Science Institute, a science, technology, engineering, and math (STEM) collective in Grand Rapids, Mich., are revisiting Tesla’s vision and seeking to discover new uses. With the help of linear motion technology, they have set out on an ambitious program of research and experimentation that would have been impossible in Tesla’s day. Thomson Industries is donating a high-precision ball screw assembly that will help The Geek Group’s high-energy engineering team wind the thousands of coils they will need for their experiments.
The premise of a TC is basic physics. Current through a wire generates a magnetic field, a changing magnetic field around a wire creates current, capacitors and inductors store energy to form a resonant circuit—an electrical pendulum.
In a TC, a power supply charges a capacitor bank to a modest voltage until a switch connects it to a low turn count primary coil at the base. This causes a large ac "ringing" or oscillation to flow through this coil, which causes a large magnetic field to envelop a much larger, high turn count secondary coil.
This causes current to start flowing in the secondary coil, charging a capacitor formed between the terminal on top, called a "top load," and earth ground. This resonant circuit continues oscillating and transferring power between the coils until either the switch is opened or the ringing completely decays. For a sufficient time after the pulse, a properly-timed additional pulse can cause the oscillation to increase in amplitude until major energy losses start occurring in the form of sparks, arc, and corona discharges.
In a normal, well-understood power distribution transformer, the wires are quite close and strongly coupled such that huge currents can flow between them under the wrong conditions. In a TC, this close coupling is actually problematic because it forces a strong reliance on the turn ratio between the primary and secondary coils. This strong coupling also allows power stored in the secondary coil to potentially leak back to the primary coil. By greatly reducing the ability of these two coils to interact, a TC can cause the oscillation to overshoot the voltage levels that a traditional transformer calculation would indicate.
In a TC, the ratio may be in the order of 1:20 to 1:100, and traditional calculations would suggest output voltages in the order of hundreds of kilovolts to a million volts. But because of the loose coupling, the voltages are allowed to overshoot, realizing outputs from tens to hundreds of megavolts.
A similar effect can be observed on a swing set. A parent’s short, well-timed, 24-inch push is able to start and increase the swinging until a gleeful child is hurtling through an arc that is dozens of feet in length. Such movement would not be possible if the parent’s grasp was retained on the swing.
Many of Tesla’s original goals for his eponymous coils have been superseded by cheaper, smaller, and more efficient inventions. Today, TCs have been relegated largely to curiosity and science experiments. The Geek Group researchers, however, believe that TCs have tremendous potential that can be achieved by appropriately tuning the wire-wrapping strategy. Chris Boden, CEO of The Geek Group, believes that by adjusting the wrapping, he can tune the coil to achieve higher states of resonance.
"We might experiment with a logarithmic-tapered winding, working out the physics and the math to determine the algorithms that would achieve the ideal resonance," said Boden. "We might experiment with the spacing of the windings of the secondary coil, maybe winding the turns next to each other at the start and spacing them to 0.005 in. after the first inch, and increasing them gradually until you get to the end of the coil where they may end up 8 to 10 in. apart, because the flux density [electromagnetic force] is so low. Or maybe we do it all in reverse because the low flux density might actually warrant closer windings at the top, and so on. We won’t know until we start working with it."
The secret is in the windings
Most TCs in use today are created by manually wrapping copper wire around a PVC pipe that provides the air gap. Manual winding is possible and encouraged for the smallest classroom applications, which can involve a secondary coil that is 1 or 2 ft. by 4 in. But in winding thousands of secondary coils, some at lengths of up to 8 ft., manual winding not only would be prohibitively painstaking, it also would fail to provide the precision that would be necessary for perfect resonance.
Part of the winding process involves coating the wire with epoxy or polyurethane, which seals and insulates the wrapping. Even a tiny amount of moisture can interfere with the experiment. Coating is most effective when done during the winding. Because it must dry and cure in real time, the pipe must keep turning constantly for up to a week. During that time, the windings must be positioned straight, with no gaps, except as defined by the experiment’s protocol. Depending on what is being tested, an experiment could require up to 20 exact duplicates. Such consistency is relatively easy to attain when the windings are close to each other, as in most TCs today. However, in experiments that involve varying the spacing, as The Geek Group wants to do, precision is critical. To achieve this, The Geek Group researchers are building a winding machine, which is where advanced linear motion comes in (see Figure 1).
Automating the winding process
The Geek Group’s winding machine can provide the slow and steady motion needed to keep the coil turning steadily for days at a time. Key to the precision will be an 8-foot precision ball screw that converts the rotary motion of a servomotor into the linear motion necessary to guide a wire feed apparatus across the coil (see Figure 2). Finding the technology that matched up to the high standards held by The Geek Group was a challenge taken on by the Institute’s Internet Relay Chat (IRC) team.
"We set our IRC team on the task of finding the best linear motion technology in the industry," said Boden. "The IRC is a 24/7 nonstop think tank, composed of a couple hundred experts from many science and technology disciplines. They analyzed about a dozen different products and concluded that the Thomson precision ball screw could do exactly what we needed and exactly how we wanted to do it."
A customer support engineer guided The Geek Group team in selecting the exact configuration that would meet its needs. The product was a quick-install ball screw assembly, so named because most of the assembly and configuration is done before shipping, which avoids precision problems that might result from assembling components on-site. The final configuration consisted of an 8-foot ball screw just under 1 in. in diameter (see Figure 3).
Flanged bearing mounts support the screw on both ends of the support rack, and the drive nut translates as a servomotor turns the drive, which is flange-mounted to the bottom of a steel plate connected to the wire feed assembly. While one servomotor keeps the pipe turning at a constant rate from 1 to 200 rpm, a second servo keeps the nut moving across the threaded drive, and with it, the feed apparatus gliding slowly over the coil at a speed of less than 1 in. per minute.
Ball screw drives convert rotary motion into linear motion, or vice versa, and consist of a ball screw and a ball nut. These are packaged as an assembly with recirculating ball bearings. The interface between the ball screw and the nut is made by ball bearings that roll in matching forms. With rolling elements, the ball-screw drive has a low-friction coefficient and typically is greater than 90% efficient. The forces transmitted are distributed over a large number of ball bearings, giving a low relative load per ball.
The winding machine will enable experimentation to test the impact of numerous winding strategies on a magnetic field across the secondary coil (see Figure 4).
Coil production began in May 2017, and The Geek Group has plans for larger coils and experiments in the future (see Figure 5). Will they end up realizing Tesla’s vision? Will they discover new capabilities of the TC? If nothing else, we can be sure that The Geek Group’s high-energy team, the 75,000 Geek Group online subscribers, the more than 15 million viewers of its online videos, and possibly the entire scientific engineering community will wind up knowing a lot more about a powerful, yet all but abandoned, technology.
Jeff Johnson is the global product line manager at Thomson Industries Inc. in Wood Dale, Ill. He is responsible for lead screws, ball screws, and screw jacks.
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