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Among the Novice Hams - Inductors | |
Pliers of the amateur radio hobby have since the beginning put forth a lot of effort training fledgling entrants in the realm of electronics and communications. Up until the latter part of the last century, there were a number of magazines - Popular Electronics among them - that would regularly print articles covering the basics of electronics. Other than the ARRL's QST magazine, it seems maybe Nuts and Volts is the only monthly still in print that you can go to for such information. I suppose it was inevitable with the emergence and now domination of the Internet as a source for most knowledge. The Among the Novice Hams column in Popular Electronics often included short primers on subjects like the basics of capacitors and inductors. Here is one on inductors from March 1958. The basics still apply. Among the Novice Hams By Herb S. Brier, 9EGQ
In the January column, as part of our discussion of the basic electronic theory on which the General / Conditional / Technician license examination is based, we talked about capacitance and capacitors. This month, we'll cover inductance and inductors, which are also referred to as coils, chokes, and reactors. First let's learn a bit about current and magnetism before taking up inductance itself. Current and Magnetism. Imagine that a source of direct current, such as a battery, is connected across the ends of a length of wire or other conductor. An electric current, which consists of electrons in motion, will flow through the wire. If we bring a magnetic compass near the wire, the compass needle will be deflected from its normal position. The greater the current flowing through the wire, the more the needle will be deflected. If we reverse the battery terminals, it will be deflected in the opposite direction. We have shown that electrons in motion (electric current) in a conductor generate a rotating magnetic field around the conductor. We have also found that the direction in which the electrons are moving determines the direction of rotation of the magnetic lines of force. According to the "left-hand rule," when a conductor carrying current is grasped in the left hand with the thumb pointing in the direction in which the current is flowing (towards the positive terminal), the fingers point in the direction of rotation of the magnetic field. If we substitute a sensitive microammeter for the battery across the ends of the conductor and rapidly move a powerful permanent magnet across the conductor, the meter pointer is momentarily deflected. The direction in which the magnet is moved determines the direction in which the meter pointer is deflected. The speed of the magnet determines how much the pointer is deflected. Thus, a magnetic field moving across a conductor induces (causes to flow) a current in the conductor. A current will also be induced in the conductor if the magnet is held still and the conductor is swept across its poles. The magnitude of the effect is small in a straight length of wire. If the wire is wound into a coil like thread on a spool, the effect is greatly increased. Then the magnetic lines of force around the wire act upon each turn and on adjacent turns as well. Figure 1 illustrates this action two-dimensionally. If we insert a soft iron core inside the coil, even more current flow takes place, because the magnetic lines will travel through the iron much easier than through air. Consequently, the iron core concentrates the magnetism around the turns of the coil.
Fig. 1. How magnetic lines of force around a conductor carrying current (A) are concentrated by winding the conductor into a coil (B); total magnetic flux path around the tightly wound coil is shown in (C). Self Inductance. Suppose we connect a coil containing thousands of turns of wire wound around an iron core, a source of direct current, a voltmeter, an ammeter, and a switch, as shown in Fig. 2. When the switch is closed, the voltmeter immediately indicates the full battery voltage across the coil terminals. But the ammeter pointer moves slowly up to a position determined by the resistance of the wire in the coil and the applied voltage. When the switch is opened, however, the ammeter pointer immediately drops back to its zero position, but the voltmeter pointer flips up far beyond its previous position before it drops back to zero. There will also probably be quite a large spark across the opening switch contacts. What happened? When the switch is first closed and current starts to flow into the coil, a strong magnetic field starts to build up around the coil. This expanding magnetic field is moving; therefore, it builds up an electromotive force of its own in the coil. This induced electromotive force is exactly opposite to the applied electromotive force. Consequently, it opposes the flow of current into the coil-but it cannot cut off the current completely. If it did, there would be nothing to generate the magnetic field. So the current slowly increases to its steady value and supports a steady magnetic field around the coil, but the process does take time. When the switch is opened, the incoming current drops instantly to zero and kicks the props out from under the magnetic field, which is thus forced to collapse instantaneously. While it collapses, the energy it contains is instantly converted back into an electromotive force in the coil, which builds up in voltage until it is sufficient to arc across the open switch contacts. These effects are due to the inductance of the coil, which is measured in henrys. By definition, a change of one ampere per second in the amount of current flouring through an inductance of one henry generates an electromotive force of one volt in it. The technical name for a coil containing inductance is an inductor. In radio work, the terms millihenry (0.001 henry), abbreviated mh., and microhenry (0.000001 henry), abbreviated μh., are also used. Applying A.C. Figure 3 shows what happens to the current and voltage in an inductor if an a.c. generator is substituted for a d.c. generator. For simplicity, let us assume that the a.c. generator voltage is maximum (point A) when we close the switch. Immediately, this voltage tries to force current through the inductor. But zip! The resulting magnetic field immediately generates a counter voltage in the inductor, which sharply limits the amount of current that can flow into it. However, as time passes, the generator gradually forces more current into the inductor, even though the generator voltage is decreasing at the same time, until 1/4 cycle or 90° later (point B), the current reaches its maximum value, just as the generator voltage has decreased to zero. Immediately, the a.c. generator voltage starts increasing in the opposite (negative) direction and tries to force a current through the inductor in that direction. But, as soon as the current tries to reverse direction, the magnetic field generated by the current flowing in the original direction starts to collapse, and its energy is converted back into an electromotive force that tends to keep the current flowing in the original direction.
Fig. 2. Theoretical circuit used to illustrate the meaning of inductance as discussed in the text.
Fig. 3. Current and voltage relationships in an inductive circuit when alternating current is applied. At first, the electromotive force from the collapsing field is strong; so the current is high. As the cycle continues, however, this energy is used up, while the generator voltage is increasing. Thus, at the end of 1/2 cycle or 180° (point C), the current has decreased to zero, just as the generator voltage reaches its maximum negative value. At this point, current starts flowing into the inductor in the opposite direction, and the action of the current and voltage is like that of the previous half cycle. At the end of a complete cycle (point E), the current and voltage relations are exactly as they were when the switch was closed. These series of actions continue as long as alternating current is fed into the inductor. Inductive Reactance. Obviously, inductance opposes the flow of alternating current through it. This opposition is called inductive reactance and is measured in ohms. The formula for calculating it is: XL = 2 π FL; where π (pi) 3.14, F is the frequency in cycles per second, and L is the inductance in henrys. The formula is also correct if the frequency is expressed in kilocycles and the inductance in millihenrys, or the frequency in megacycles and the inductance in microhenrys. Don Jensen, KN6VXM, worked the 48 states and Europe with a home-brew 6146 transmitter running 50 watts. Now he uses a new Johnson Ranger transmitter. An example will show that there is nothing mysterious about the formula. Question: What is the inductive reactance of a 10-henry choke (inductor) at a frequency of 60 cps? Answer: XL = 2 X 3.14 X 60 X 10 = 3768 ohms. At 600 cycles, its reactance is 37,680 ohms. Inductive reactance is directly proportional to frequency and inductance. This is just the opposite of capacitive reactance, where the reactance is inversely proportional to frequency and capacitance. Another difference between inductive and capacitive reactance is that, in a purely capacitive circuit, the current leads the voltage by 90°, while in a purely inductive circuit the current lags the voltage by 90°.
Posted August 13, 2012 | |
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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ... All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged. My Hobby Website: AirplanesAndRockets.com
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