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Table of Contents. • U.S. Government Printing Office; 1945 - 618779
Electromagnetism - What It Is
Now take a look at still another type of magnet. It is LIKE a natural or artificial magnet in its attraction but UNLIKE in its control. Its attraction is tremendous-it can hold tons of iron. But because this magnet is powered by an electric current, the magnetism can be turned on and off with the flick of a switch. Electrically-powered magnets are called ELECTROMAGNETS.
Electromagnets come in all sizes and shapes - and do all kinds of jobs. See the lifting magnet in figure 89. All electromagnets use a coil of wire and a core of iron to produce their magnetism. The coil furnishes the magnetic flux and the iron concentrates it. To understand how it works, you should start with -
The Magnetic Field Around a Conductor
All conductors carrying current are surrounded by a field-of flux. As in the case of artificial magnets, iron filings will make this field visible. Connect a wire to a battery and, as in figure 90, dip the wire in iron filings. The filings are attracted and held to the wire. This is proof of a magnetic field. Now open the circuit-the filings drop off. This is proof that THE FIELD EXISTS ONLY WHEN CURRENT IS FLOWING.
Figure 89. - Lifting electromagnet.
Figure 89. - Lifting electromagnet.
Figure 90. - Magnetism produced by current.
Figure 91. - Magnetic field around a conductor.
Figure 92. - Direction of the field around a conductor.
Figure 93. - The coil hand rule.
Figure 94. - Dot-cross method of indicating current directions.
Figure 95. - Flux directions - cross-sections.
Figure 97. - Magnetic field of a coil.
Figure 98. - Hand rule for coils.
Figure 99. - Equal ampere-turns.
Now run the conductor through a piece of cardboard as in figure 91. Connect the wire to a battery and sprinkle iron filings on the cardboard. The filings outline the exact shape of the field. Two characteristics stand out; the field is circular around the conductor, and, no lines cross. If you moved the cardboard to other parts of the wire, you'd find that THE FIELD SURROUNDS THE WIRE FOR ITS ENTIRE LENGTH.
The magnetic field around a conductor is like the apprentice electrician - going around in circles. BUT - magnetic circles are always in the same direction. Place compasses around the conductor IRON FILINGS as in figure 92. All the compasses point in a clockwise direction. This shows that the lines of force are clockwise.
Leave the compasses in place and reverse the current direction (switch battery connections). All the compasses reverse - now pointing in a counterclockwise direction. THE DIRECTION OF CURRENT DETERMINES THE FLUX DIRECTION.
The Coil Hand Rule
Magnetic fields around conductors are subject to frequent reversal by reversing current. And there is an easy and foolproof rule which connects the field direction and the current direction.
The wire hand rule is illustrated in figure 93. It says -
GRASP THE WIRE IN YOUR LEFT HAND SO THAT THE THUMB POINTS IN THE DIRECTION OF CURRENT FLOW. YOUR FINGERS WILL THEN POINT IN THE DIRECTION OF THE FLUX FIELD.
GRASP THE WIRE WITH YOUR FINGERS IN THE DIRECTION OF THE FLUX FIELD. THEN YOUR THUMB WILL POINT IN THE DIRECTION OF CURRENT FLOW.
This rule is used to tell flux direction if you know the current direction. Or, it will tell current direction if you know flux direction.
Imagine that you have determined flux direction with a compass. By using the wire hand rule you can tell which way the current is flowing-and consequently, you can tell whether the wire is connected to the positive or negative terminal of the source. Likewise, if you know which terminal the wire is connected to-you can use the wire hand rule to tell the direction of the flux field around the conductor.
Marking Current Direction
An arrow is usually used to mark current direction. This works fine on a long section of wire. But in diagrams where cross sections of wire are used, a tricky view of the arrow is employed. Compare the two drawings in A of figure 94. The top drawing shows an arrow coming out of the wire. If you cut this wire, making a cross-section, you'd see just the HEAD of the arrow coming out of the wire-bottom drawing. This is the label for current coming OUT of a cross-section. The current direction is reversed in figure 94-B. With this current direction, a cross-section of the wire shows the feathered tail of the arrow just disappearing down the wire. This is the label for current going IN a cross-section.
Years ago, Benjamin Franklin jumped to the conclusion that the direction of an electrical current is from POSITIVE to NEGATIVE. Modern experiments have shown the real movement to be that of ELECTRONS - from NEGATIVE to POSITIVE. Nevertheless, Franklin's theory is still used in many electrical textbooks and in some Navy manuals. If you run across the old theory, DON'T let it confuse you. In those cases where you find that current is traced from positive to negative, simply use the OPPOSITE HAND from the one used in this book. Your answers will then be CORRECT. And throughout this book all explanations are based on the present-day idea-that electron flow is from NEGATIVE to POSITIVE.
Figure 95 shows cross-sections of two wires. BOTH flux direction AND current direction are labeled. Use the wire hand rule to check these labels. Your thumb should point down into the page for the right-hand drawing. And it should point up out of the page for the left-hand drawing.
Flux around a conductor consists of closed circular lines. These lines start as a dot in the center of the wire. As current commences to flow the circles expand from this dot. It's like the ripples made by a stone dropped in calm water. The larger the stone, the more and the larger the ripples. The more the current, the more the lines of force, and the larger the field. Flux is said to "blossom out" from the heart of a conductor. Hence, the strongest part of the field is close to the conductor and the weakest part is farthest away. This is logical-the farthest flux has been weakened by traveling through air, which has a high reluctance.
Fields Produced by Coils
Figure 96. - Magnetic polarity of a loop.
A single conductor produces a field - but no poles. And poles are important because machines make use of these points of flux concentration. To produce poles, bend the straight conductor of figure 95 into a loop. Now it looks like figure 96. Use the wire hand rule at a number of points on this loop.
You will find that the flux blends together in the center of the loop. This produces a north pole on one side of the loop and a south pole on the other side.
If a number of loops of wire are combined, as in figure 97, you have a HELIX COIL. Again the flux blends together in the center of the coil. You'd expect this coil to produce much stronger poles than those of a single loop. IT DOES. Again, check flux direction at a number of points on this helix coil. Notice that coiling the wire forces most of the flux to CONCENTRATE at the ends of the coil. There would be the same total flux if the wire were straightened out - BUT it would not be concentrated.
You can use the wire hand rule you already know for determining coil polarity. Or you can use another hand rule FOR COILS. This second coil hand rule states -
GRASP A COIL IN THE LEFT HAND SO THAT THE FINGERS POINT IN THE DIRECTION OF CURRENT FLOW. THEN THE THUMB POINTS TO THE NORTH POLE END OF THE COIL.
Figure 98 shows the difference in polarity for both current directions.
If a very strong magnetic coil is wanted, more turns of wire are built up in LAYERS. This produces a SOLENOID COIL. Now you have three types of coils. The single loop which is magnetically weak. The helix coil which is moderately strong, and the solenoid coil which is very strong. Notice that the magnetic strength of a coil depends on the number of turns of wire. For example, say that each turn produces 1,000,000 lines of force. Then a one-turn coil would produce poles having 1,000,000 flux lines. A ten-turn helix would produce poles having 10,000,000 flux lines. And a 150-turn solenoid would produce poles having 150,000,000 flux lines.
The idea that the flux increases in exact proportion to the number of turns of wire is used for all practical purposes, but, it is not quite correct. Some lines of force are lost in any coil because of the high reluctance air gap. Therefore, the total strength of the many-turn coils is a little less than the calculated strength.
Now suppose you took one of the helix coils - say the 10-turn helix - and doubled the current through the wire. Since the turns are in series, the current would double in each turn. Twice as much current produces twice as much flux. Now the 10-turn coil would have poles of 20,000,000 lines per pole.
Figure 99 shows two coils of EQUAL flux strength. A has 10 turns and 5 amperes; B has 20 turns and 2½ amperes. A has twice as much CURRENT but B has twice as many TURNS.
The strength of coils is measured in AMPERE-TURNS (NI - the N for the number of turns and the I for the amperage). The number of ampere-turns can be determined by multiplying the coil current in amperes by the number of turns of wire.
Strong coils can be made in two ways - either use a heavy current or put many turns on the coil. Here are two coils of equal strength: (1) has 1,000 turns and 0.1 amperes, (2) has 10 turns and 10 amperes. Both coils have 100 ampere-turns.
Cores - Flux Savers
Figure 100. - Applications of the coil hand rule.
Figure 101. - Answers.
Figure 102. Field poles of a motor.
Figure 103. - Cross-section of the lifting magnet.
Figure 104. - Electric door chime.
Figure 105. - Magnetic circuit breaker.
How can the air gap losses of a coil be reduced? You know that air is a high reluctance material, so simply substitute a low reluctance material for the air. Iron is the best material because of its high permeability. A bar of iron shoved down the center of a coil, makes it an IRON CORE helix or solenoid. Often, iron-core coils are made by winding the wire directly on an iron bar. The iron, because of its high permeability concentrates the flux within itself. Then the poles appear at the ends of the iron. Almost all commercial coils are iron-core solenoids.
Figure 100 has eight iron-core coil problems.
Problems (a), (b), (c), and (d) show terminal connections of the coils, but no polarity. How would you label the poles? Problems (e), (f), (g), and (h) shows polarity but no terminal connections. How would you connect the lead wires-to positive or negative? Figure 101 is the answer table. BELAY THE PEEKING until you've tried to get YOUR OWN answers!
Do you recall, back in figure 66, how an artificial magnet was made by a coil. This was an iron core helix. The iron core became the artificial magnet when removed from the coil. The magnetism held by the core was residual magnetism left from the magnetic field of the coil.
The field magnets of a motor are electromagnets - solenoid coils with iron cores. In figure 102 trace the path of the magnetic lines of force. Start at the N poles, the lines leaving these poles split - half going to the top S pole and half going to the bottom S pole. The flux travels through the S pole electromagnets and out their N pole ends. (Use the coil hand rule to locate the N poles). From the N pole ends of the top and bottom magnets, the flux travels through the iron of the frame and back to the south poles of the side magnets, and again out the N pole ends. Notice two things-the flux path is a complete circuit and the air gap is reduced to a minimum by using the iron frame as part of the magnetic circuit.
Figure 103 shows a cross-section of the same electromagnet pictured in figure 89 at the beginning of this chapter. Can you understand its construction now? A double-sized N pole is set up by the coil, and one-half of the flux from this N pole enters each of the S poles. When the magnet is unloaded, the flux travels in air. But when the magnet is loaded the flux travels through the scrap iron - holding the iron to the magnet. An ARMATURE is a piece of iron used to complete a magnetic circuit. The scrap iron acts as an armature in this electromagnet.
The Sucking Coil
Have you ever wondered how an apartment house door is opened by pushing a button in one of the apartments? How about door chimes? Do you know how they work? Do you understand the action of automatic switches? All these and many other devices use an electromagnet and a movable core.
When a solenoid coil is energized, it sets up a strong field. Any iron near this field has a strong pole induced. This pole is always opposite to the closest pole of the coil - setting up a strong attraction between the iron and the coil. If the coil is just started into one end of the solenoid, the magnetism will jerk it all the way into the coil. Doors are un-locked by making a part of the bolt the core of a solenoid. When the coil is energized, it sucks in the core (bolt) and the door is unlocked.
In a door chime, the hammer which hits the chime is attached to the core of a solenoid. The core is below the solenoid as in figure 104. When the solenoid is energized, the core is jerked upward carrying the hammer with it.
The circuit breaker - an automatic switch used for opening overloaded circuits - is shown in figure 105. This device is connected in series with the line. Normally, the contacts are closed but if the current rises over its safe rating, it makes the magnet strong enough to pull its armature against the core. This OPENS the contacts which had been completing the circuit. The circuit-breaker serves the same purpose as a fuse - protecting circuits from overload. It is better than a fuse because nothing burns out - the circuit breaker can be reset and used over and over again.
Chapter 12 Quiz