Getting the Buzz Out Electromagnetism for Beginners Chris Greacen Words 2143 Any time electricity flows in a wire, a magnetic field is produced. In turn, a changing magnetic field can make electricity flow in a wire. This sounds innocent enough, but these two physical laws are the keys to the strange world of electromagnetism. In renewable energy homes, microhydro and wind generators convert mechanical motion into electrical current by way of magnetic fields. Indeed, most of the utility electricity produced in the world is made this way. Transformers, DC to DC converters, and inverters use magnetic fields to convert power from one voltage to another, and from DC to ac. This link between electricity and magnetism is also the root of most obnoxious electrical noise problems in renewable energy homes. Notice a buzz on your stereo when your inverter is running? How about spurious lines on your television? Does your radio crackle and hiss when your charge controller starts regulating? Can you hear your inverter in your telephone? These are the sounds and sights of magnetic fields from your renewable energy equipment getting where they shouldn't. Knowing a little about how electricity and magnetism effect each other will give you a leg up in understanding how to use electromagnetism when you want to, and how to reduce electromagnetic "noise" in your home. Magnetism Basics Maxwell's four equations linking electricity and magnetism were codified in the 1860s. Let's take a non-mathematical look at two of them. 1) Ampere's Law: current flowing in a wire creates a magnetic field wrapping around the wire. To determine the direction of the magnetic field, look at your right hand. Point your thumb in the direction the current is flowing, and your fingers wrap around in the direction of the magnetic field. This is called the "right hand rule". See figure 1. The magnetic field diminishes in strength as you get farther from the wire. If the wire is a long straight single wire, its magnetic field decreases as 1/r where r is the distance from the wire. 2) Faraday's law: A changing magnetic field produces a voltage in a wire in the direction which opposes a change in current flow. Snakes! Imagine current is a stream of mice in a glass tube. They want to get to a hunk of Jarlsberg Swiss which Dr. Klge has placed on top of the tube. This cheese provides the incentive (or voltage) for the mice to climb through the tube. We might expect that the more cheese is put on top, the more mice would attempt to make the trip. Also, if the pipe was wider in diameter (less resistance), more mice (greater current) would flow. To keep the mice from piling up at the cheese, Dr. Klge removes them from the cheese after a small nibble. So far, so good, for the mice... The Boa Constrictor But there's a strange bit of magic at work. The travelling mice create, as a bizarre product of their collective unconscious, a boa constrictor (magnetic field) wrapped around the tube. The more mice travelling in the tube, the bigger the boa. Now remember, this is an imaginary boa. It doesn't eat any mice. But still, seeing this boa appear from thin air is disturbing, especially if you're a mouse. Understandably, some of these mice re-evaluate the relative merits of eating cheese at least right away. But some of the brave mice continue. Strangely enough, these mice don't mind the boa constrictor, as long as she doesn't grow. If the stream of mice is constant, the boa doesn't grow, and the mice decide it is safe after all. Of course, the more mice running through the tube, the bigger the boa grows... There's a balance reached in the steadily growing stream of mice that run through the tube, keeping the boa growing slowly enough that the fear of the growing boa (and the crowding in the tube) is outweighed by their desire for cheese. At some point a limit is reached caused by the crowding of the tube and how fast the mice can pile out of their cage, and how fast Dr. Klge can put them back into the cage. In an electrical circuit, this limit is determined by the total resistance of the circuit. The Cheese Is Gone Once the Boa is nice and fat, Dr. Klge back his hunk of gnawed Jarlesberg Swiss cheese, but he stays at the tube to watch what happens. For an instant the mice no longer have an incentive to go through the pipe, and as less mice travel through the pipe, the boa starts to shrink. The shrinking boa has the opposite effect that the growing boa did. All the mice want to catch a glimpse of the great snake before she disappears. Even without the cheese, mice are enticed to journey through the tube.The faster the boa shrinks, the more frantic the mice are to see the snake before she's gone. But again, the more mice, the slower the boa shrinks, and the less incentive the mice have to travel in the tube. Again a balance is reached in the steadily decreasing stream of mice. Eventually the stream slowly dies to zero, and the snake disappears. Ok, so it's a strange way to imagine electricity and magnetism. If you come up with a better way which doesn't involve large imaginary snakes and small rodents, let me know. It does have one thing going for it: the symbol for magnetic field is B. Think Boa. To recapitulate: apply cheese (an external voltage like a battery) mice start up the tube (current starts) the bigger the stream of mice, the bigger the boa (magnetic field wrapping around wire is proportional to current in the wire) The faster the boa grows, the less the mice want to make the journey (a growing magnetic field induces a proportional negative voltage) Remove the cheese (remove the external voltage) Shrinking boa entices sight-seeing mice (a shrinking magnetic field induces a proportional positive voltage). The snake adds "inertia" to the stream of mice. It makes the stream of mice harder to get started, and harder to stop. In a circuit, this inertia is called induction, and a circuit piece designed to cause induction is called an inductor. If it causes lots of induction, it is said to have high inductance. How to Increase induction A straight piece of wire has very little inductance. To increase inductance, make the wire into a coil. Two things happen. First, current in each turn of the wire creates a magnetic field wrapping around it. Inside the coil, these magnetic fields add to make a strong magnetic field in the center (see figure XX). Therefore, in a long tightly wound coil, the magnetic field inside is proportional to N, the number of turns. Second, when the magnetic field grows or shrinks, it crosses over each wire turn. So the voltage induced in the wire from a changing magnetic field is also proportional to N. These two go together to make up inductance. For a tightly wound inductor, the inductance (the "inertia" of the current) is proportional to N2. The mice would see the snake growing N times as fast. But even if it weren't they would be N times as reluctant to go to the cheese because they'd have to pass through the boa's coils N times. To make even more inductance, stick a hunk of iron inside the coil. Magnetic fields like to travel through iron much more than they like to travel through air. Iron inside the core can increase inductance by thousands of times. Dr. Klge's Plumbing Inductor Dr. Klge's ducks are back. The good doctor has found, to his annoyance, that they derive great joy from jumping up and down on his garden hose when he goes out to water his sunflowers. Dr. Klge, as always, has found a technical solution: a plumbing inductor to smooth out the flow of water. When he turns on the faucet, current flows slowly because it has to get the turbine and the flywheel spinning. When ducks jump on the hose, the inertia of the flywheel keeps the pumping water, maintaining an even flow as the flywheel slowly looses momentum. When ducks jump off the hose, the flywheel gains speed. An electrical inductor is useful in a similar problem. The output of modified sine wave inverters have sharp, abrupt changes in voltage and current. This is like the ducks jumping up and down on the hose. If your radio is plugged into your inverter, and it buzzes, try adding some inductance to the cord which supplies the radio. An easy way to do this is to wrap the cord (or an extension cord) around a piece of iron pipe. Broadcasted Magnetic Fields The magnetic field from a long straight wire (like a single power line) is most intense right next to the wire. Away from the wire, the field decreases in strength as 1/r. Go twice as far away, and the field is half as strong. If the source of a magnetic field is a single object (like an inverter or a transformer) then the magnetic field drops off faster than 1/r. Faraday's law said that changing magnetic fields will induce voltages in a wire. It didn't say anything about where these changing magnetic fields have to come from. In the snake and mice example, the current in the wire made a changing magnetic field which affected current in the same wire. But also, changing current in one wire will induce voltages in a neighboring wire. This is the principle behind radio. The radio station makes big changes in the current in their transmitting antenna, making big changing electric and magnetic fields. This forms an electromagnetic wave travelling outward in all directions, with power sloshing back and forth between an electric field and a magnetic field (see your favorite physics book for more on this). Miles away, the antenna on your radio is the "neighboring wire" with induced voltages from these changing magnetic fields. Your radio changes these voltages into signals you can hear. Unfortunately, inverters and linear current boosters and some charge controllers and generators also work like radio transmitters. These devices can make big pulses of current in your wiring. Then your electrical wiring acts like an antenna, broadcasting noise. The effect is especially strong in low voltage wiring because of big current pulses, and magnetic fields are proportional to current (Ampere's law). The result is a buzz or crackle or hum in anything which works as radio receiver (your radio, TV, or radio telephone). Reducing Electromagnetic Noise Since field strength drops off the farther you are from the source, the first solution to noise problems is to banish the offending electromagnetic banshee to the hinterlands. Put your inverter out in a "power shed" away from your house. Avoid running your stereo speaker wires and telephone wire parallel to or next to power wiring. Also, establishing a good ground (with a ground rod) for radio, etc. will help reduce noise. For more reduction, twist long positive and negative (for DC) or hot and common (for ac) wires into a twisted pair. Since equal currents travel in opposite directions in the two wires, they create magnetic fields which oppose and cancel each other. The hundreds of feet of large gauge wiring to our PV arrays form potentially a huge antenna. When our charge controller begins regulating, the full current from the pvs is quickly turned on and off. Changing currents means changing magnetic fields, which means a broadcasting antenna. To reduce the broadcasted energy, we twisted together the hundred of feet of 0 gauge copper wires (Grunt!) going to each array. This is done with five good humored people one at the middle of the two wires, and one at each end, in a May-pole like dance with the wire ends. Twisting small signal wires will reduce the noise they pick up. The next step is shielding. If wires are encased in a grounded conducting sheath (like metal conduit), then the electric field in an electromagnetic wave is grounded out, and therefore much less electromagnetic noise escapes. A conducting shield with high "magnetic permeability" can provide some isolation from the magnetic noise. For more on magnetic fields see: Health Effects: "ElectroMagnetic Fields and Home Power Systems": HP 23, p 24; "Reducing AC Magnetic Fields", HP 24, p 62. "Some Environmental Hazards of Lighting Systems", HP 30, p 32. "Yer Basic Alternator", Home Power #20, page 10; "How Electric Motors Work": HP 34, p 38 "How an Inverter Works": HP 23, p 53; "What's an Inverter", HP #32, p22; Linear Current Boosters: HP29, p 53; HP 12, p 19; HP 17 p 39; HP 25, p 16; HP 28, p 34. Access Author: Chris Greacen, Home Power Magazine, POB 520 Ashland, OR 97520 916-475-3179