In a single circuit, consisting of a straight wire and a parallel return wire there is little or no self-induction. When a circuit containing a primary induction coil and a battery is closed there is no spark because at the instant of closing the circuit the current is at rest and on account of self-induction the current cannot at once rise to its full value.
Mutual Induction.—This is a particular case of electromagnetic induction in which the magnetic field producing an electromotive force in a circuit is due to the current in a neighboring circuit.
The effect of mutual induction may be explained with the aid of fig. 135. If, as illustrated, a circuit including a battery and a switch, be placed near another circuit, formed by connecting the two terminals of a galvanometer by a wire, it will be found that whenever the first circuit, 1, is closed by the switch, allowing a current to pass in a given direction, a momentary current will be induced in the second circuit, 2, as shown by the galvanometer. A similar result will follow on the opening of the battery circuit, the difference being that the momentary induced current occurring at closure moves in a direction opposite to that in the battery circuit, while the momentary current at opening moves in the same direction.
Currents, besides being induced in circuit 2 at make or break of circuit 1, are also induced when the current in 1 is fluctuating in intensity.
The most marked results are observed when the make or break is sudden, the action being strongest at the break of the current in 1.
The inductive effect of the current in the arrangement shown in fig. 135 is very weak.
Ques. What name is given to circuit 1?
Ans. The primary circuit.
Ques. What name is given to circuit 2?
Ans. The secondary circuit.
Ques. What names are given respectively to the currents in circuits 1 and 2?
Ans. The primary and secondary or induced currents.
Primary Induction Coils.—These represent the simplest form of coil, and are used chiefly in low tension ignition to intensify the spark when a battery forms the current source.
A primary coil consists of a long iron core wound with a considerable length of low resistance insulated copper wire, the length of the core and the number of turns of the insulated wire winding determining the efficiency. The effect of the iron core is to increase the self-induction.
The spark produced, as previously explained, is due to self-induction, and it should be remembered that in the operation of the coil, the spark occurs at the instant of breaking the circuit, not at the instant of making.
Secondary Induction Coils.—The arrangement shown in fig. 137, may be considered as a very simple or rudimentary form of secondary induction coil. In the actual coil, the primary and secondary circuits (corresponding to 1 and 2 in fig. 135) are made up of coils of insulated wire, as shown in fig. 143, the primary coil P, being wound over a core C and the secondary coil S being wound over the primary.
The one property of such an arrangement that makes it of great value for most purposes is that the voltage of the induced currents may be increased or diminished to any extent depending on the relation between the number of turns in the primary and secondary winding.
This relation may be expressed in the following rule:
The voltage of the secondary current is (approximately) to the voltage of the primary current as the number of turns of the secondary winding is to the number of turns of the primary winding.
For instance, if the voltage of the primary current be 5 volts, the primary winding have 10 turns and the secondary 100 turns, then
The watts in each circuit are approximately the same; hence: if, for instance, the current strength in the primary circuit be 5 amperes, the watts in primary circuit are 5 × 5 = 25. Accordingly, for the secondary circuit the current strength is:
From this, it is seen that where the voltage is raised in the secondary circuit, the current flow is small as compared to that in the primary circuit; therefore, heavy wire is used in the primary winding and fine wire in the secondary, as indicated in figs. 137 and 143.
For most purposes a very much higher secondary voltage is required than in the example just given.
Secondary induction coils may be divided into three general classes:
- 1. Plain coils;
- 2. Vibrator coils;
- 3. Condenser coils.
The plain coil gives but one spark when the primary circuit is made and broken, while the vibrator coil gives a series of sparks following each other in rapid succession.
Plain Secondary Induction Coils.—Coils of this class are very simple and consist of:
- 1. Core;
- 2. Primary winding;
- 3. Secondary winding.
The construction of a plain coil, such as would be suitable for ignition service, is about as follows:
The core is made of soft annealed iron wires (No. 20 B and S gauge) from one-half to three-quarters of an inch in diameter and about six inches long. Over this core is slipped a spool of insulating material (hard rubber or composition), on which is wound first the primary winding of the coil, which consists of several layers of about No. 18 B and S gauge silk insulated magnet wire.
After the primary winding has been wound over the insulated core, and the ends have been properly brought out through the heads of the spool to be connected to binding posts thereon, a layer of insulating material is applied over the primary wire, and the secondary winding is then wound on.
The wire for the secondary winding consists of about No. 36 B and S gauge silk covered magnet wire, the amount used varying considerably, depending on the desired voltage of the secondary current.
When all the wire has been wound on, the ends are brought out to the binding posts, the coil is soaked in shellac dissolved in alcohol and baked, or in melted paraffin or a paraffin compound, and allowed to cool. It is then placed in either a cylindrical hard rubber shell or in a hard wood box.
The proportions of such coils vary greatly; for motor cycle use they are made long and of small diameter (10×21⁄2 inches for instance), while for some other purposes short and thick coils are found more convenient.
Ques. How may the coil just described be connected for demonstrating purposes?
Ans. Connect the ends of the secondary winding to fixed insulators and bend the ends so they are about 1⁄8 inch or less apart. Connect one end of the primary winding to a battery and brush the other end of the primary winding against the other terminal of the battery as indicated in fig. 137.
Ques. What happens when the primary circuit is made?
Ans. An electric pressure is induced in the secondary circuit, but of not enough intensity to cause a spark to jump across the air gap.
Ques. What happens when the primary circuit is suddenly broken?
Ans. A spark is produced both at the point of break in the primary circuit and at the air gap in the secondary circuit.
Ques. Why is a spark produced at the air gap at break and not at make of the primary current?
Ans. Because when the current is flowing it cannot be stopped instantly on account of self-induction, that is, it acts as though it possessed weight.
If the reader has charge of a gas engine with a make and break ignition system, he will often avoid vexatious delays in locating ignition troubles, if he remember that one of the most important conditions for obtaining a good spark is that the break take place with great rapidity. This, of course, involves that the ignition spring be adjusted to the proper tension.
Secondary Induction Coils with Vibrator and Condenser.—A plain secondary coil, such as just described, will only give feeble sparks for its size for the following reasons: The inductive effect of the primary winding in the secondary depends as previously explained on the rate at which the current in the primary winding decreases or dies out.
If a strong inductive effect is to be produced in the secondary, the current in the primary must stop suddenly. This is prevented by self-induction in the primary winding, which opposes any change in the current strength. The direct result is that, as the primary circuit is broken, a spark appears at the break, which means that the current continues to flow after the break has occurred, dying down comparatively slowly, hence, the inductive effect on the secondary winding is small.
The spark at the break in the primary circuit is even larger than that in the secondary circuit, and as this primary spark serves no useful purpose, but, on the contrary, quickly burns away the contact points, such an arrangement is obviously defective.
The vibrator-condenser coil is designed to overcome this trouble and also to give a series of sparks following in rapid succession instead of one.
It should be noted that a series of sparks following each other with considerable rapidity may be obtained with a plain coil by placing a mechanical vibrator in the primary circuit, as used on some motor cycle ignition circuits.
The object of the vibrator, of a vibrator-condenser coil, is to rapidly make and break the primary circuit during the time the primary circuit is closed externally. It consists of a flat steel spring secured at one end, with the other free to vibrate. At a point about midway between its ends, contact is made with the point of an adjusting screw, from which it springs away and returns in vibrating. The points of contact of blade and screw are tipped with platinum. One wire of the primary circuit is connected to the blade and the other to the screw, hence, the circuit is made when the blade is in contact with the screw and broken when it springs away.
A condenser is used to absorb the self-induced current of the primary winding and thus prevent it opposing the rapid fall of the primary current.
Every conductor of electricity forms a condenser and its capacity for absorbing a charge depends upon the extent of its surface. Hence, a condenser is constructed of conductive material so arranged as to present the greatest surface for a given amount of material.
The usual form of condenser for induction coils as shown in figs. 141 and 142 is composed of a number of layers of tin foil separated by paraffin paper, each alternate layer being connected at the ends.
Fig. 143 is a diagram of a vibrator coil. CC represents the core composed of soft iron wires. PP is the primary winding and SS the secondary. There is no connection between these windings and they are carefully insulated. Y is the vibrator or trembler and D the center about which it vibrates. W is a switch used for opening and closing the primary circuit; B, a battery of five cells. The point of adjusting screw A rests against a platinum point R soldered upon the vibrator.
If the switch W be closed, the electric current generated by the battery B will flow through the primary winding. This will cause the core CC to become magnetized, and the vibrator Y will at once be drawn toward it. This will break the connection at R. The core, being made of soft iron, immediately upon the interruption of the current, will again lose its magnetism, and the vibrator will return to its original position. This again closes the circuit, after which the operation of opening and closing it is repeated with great rapidity so long as the switch W remains closed.
The cycle of actions may be briefly stated as follows:
- 1. A primary current flows and magnetizes the core;
- 2. The magnetized core attracts the vibrator which breaks the primary circuit;
- 3. The core loses its magnetism and the vibrator springs back to its original position;
- 4. The vibrator, by returning to its original position, closes the primary circuit and the cycle begins again.
Magnetic Vibrators.—Many types of vibrator are used on induction coils, the most important requirement being that the break occur with great rapidity. In order to render the break as sudden as possible, different expedients have been resorted to, all tending to make the mechanism more complicated, yet having sufficient merit in some cases to warrant their adoption.
In the plain vibrator, the circuit is broken at the instant the spring begins to move, hence, the operation must be comparatively slow.
In order to render the break more abrupt some vibrators have two moving parts, one of which is attracted by the magnetic core of the coil and moved a certain distance before the break is effected. A vibrator of this type is shown in fig. 146 and described under the illustration.
Vibrator Adjustment.—When a vibrator coil is used, the quality of the spark depends largely upon the proper adjustment of the vibrator. The following general instructions for adjusting a plain vibrator should be carefully noted:
- 1. Remove entirely the contact adjusting screw.
- 2. See that the surfaces of the contact points are flat, clean and bright.
- 3. Adjust the vibrator spring so that the hammer or piece of iron on the end of the vibrator spring stands normally about one-sixteenth of an inch from the end of the coil.
- 4. Adjust the contact screw until it just touches the platinum contact on the vibrator spring—be sure that it touches, but very lightly. Now start the engine; if it miss at all, tighten up, or screw in the contact screw a trifle further—just a trifle at a time, until the engine will run without missing explosions.
| Length of spark | 3⁄8 inch | 1⁄2 inch | 1 inch | 2 inches |
| Size of bobbin ends | 21⁄8 × 11⁄4 | 21⁄2 × 5⁄16 | 3 × 3⁄8 | 4 × 23⁄4 × 3⁄8 |
| Length of bobbin | 4 | 51⁄2 | 61⁄2 | 61⁄2 |
| Length and diameter of core | 41⁄4 × 7⁄16 | 6 × 5⁄8 | 61⁄2 × 3⁄4 | |
| Size of base | 71⁄4 × 31⁄4 × 11⁄2 | 9 × 5 × 2 | 141⁄2 × 6 × 13⁄4 | 12 × 71⁄2 × 31⁄4 |
| Size of tinfoil sheets | 4 × 2 | 51⁄2 × 31⁄4 | 6 × 4 | 6 × 6 |
| Number of tinfoil sheets | 36 | 40 | 40 | 60 |
| Size of paper sheets | 5 × 3 | 61⁄2 × 41⁄4 | 9 × 5 | |
| Primary coil | No. 18 | No. 18 | 2 layers No. 16, silk covered. | 2 layers 14 B. W. G., silk covered. |
| Secondary coil | 3⁄4 lb. No. 40 | 1 lb. No. 40 | 11⁄4 lbs. No. 38 | 21⁄2 lbs. No. 36 |
| Volts. | Distance. |
| (Inches.) | |
| 5000 | .225 |
| 10000 | .47 |
| 20000 | 1.00 |
| 30000 | 1.625 |
| 35000 | 2.00 |
| 45000 | 2.95 |
| 60000 | 4.65 |
| 70000 | 4.85 |
| 80000 | 7.1 |
| 100000 | 9.6 |
| 130000 | 12.95 |
| 150000 | 15.00 |
Points Relating to Ignition Coils.—1. Most ignition induction coils or “spark coils” as they are called, have terminals marked “battery,” “ground,” etc., and to short circuit the timer for the purpose of testing the vibrator, it is only necessary to bridge with a screw driver from the “battery” binding post to the “ground” binding post.
2. In adjusting the vibrator of an ignition coil, the latter should not require over one-half ampere of current.
3. A half turn of the adjusting screw on a coil will often increase the strength of the current four or five times the original amount, hence, the necessity of carefully adjusting the vibrator. When the adjustment is not properly made it causes, 1, short life of the battery, 2, burned contact points, and 3, poor running of the engine.
4. In adjusting a multi-unit coil, if any misfiring be noticed, hold down one vibrator after another until the faulty one is located, then screw in its contact screw very slightly.
5. The number of cells in the circuit should be proportioned to the design of the coil.
If the coil be described by the maker as a 4 volt coil, it should be worked by two cells of a storage battery or four dry cells. The voltage of the latter will be somewhat higher, but since their internal resistance is also greater, the current delivery will be about the same. Most coils are made to operate on from 4 to 6 volts.
6. It is a mistake to use a higher voltage than that for which the coil is designed, because it does not improve the spark and the contact points of the vibrator will be burned more rapidly, moreover, the life of the battery will be shortened.
CHAPTER XII
THE DYNAMO
The dynamo is a machine which converts mechanical energy into electrical energy by electromagnetic induction.
The word dynamo is used to designate a machine which produces direct current as distinguished from the alternator or machine generating an alternating current. In a broader sense, the word generator is used to denote any machine generating electric current by electromagnetic induction; the term therefore includes both dynamos and alternators.
Operation of a Dynamo.—A dynamo does not create electricity, but generates or produces an induced electromotive force which causes a current of electricity to flow through a circuit of conductors in much the same way as a force pump causes a current of water to flow in pipes. The electromotive force generated in the dynamo causes the current of electricity to pass from a lower to a higher potential in the machine, and from the higher back to the lower potential in the external circuit; that is, the dynamo generates electrical pressure which overcomes the resistance or opposition to the current flow in the circuit. The pump produces a mechanical pressure which, for instance, may be used to force water into an elevated reservoir against the back pressure due to its weight.
The point to be emphasized is that the dynamo does not create electricity (nor the pump water) but sets into motion something already existing by generating sufficient pressure to overcome the opposition to its movement.
Essential Parts of a Dynamo.—The dynamo in its simplest form consists of two principal parts:
- 1. The field magnet;
- 2. The armature.
Ques. What is the object of the field magnet?
Ans. To provide a field of magnetic lines or lines of force to be cut by the armature inductors as they revolve in the field.
Ques. What is an armature?
Ans. A collection of inductors mounted on a shaft and arranged to rotate in a magnetic field with provision for collecting the currents induced in the inductors.
A simple loop or turn or wire may be considered as the simplest form of armature.
Ques. How do armatures and field magnets differ in dynamos and alternators?
Ans. A characteristic feature is that in the dynamo the field magnet is the stationary part and the armature the rotating part, while in the alternator the reverse conditions usually obtain.
Ques. With respect to this feature, what names are sometimes given to the armature and field magnet?
Ans. The stator and the rotor depending on which moves.
Ques. What is the real distinction between an armature and a field magnet?
Ans. The name field magnet is properly given to that part which, whether stationary or revolving, maintains its magnetism steady during operation; the name armature is properly given to that part which, whether revolving or fixed, has its magnetism changed in a regularly repeated fashion when the machine is in motion.
Construction of Dynamos.—In the make up of a dynamo, as actually constructed, there are five principal parts, as follows:
- 1. Bed plate;
- 2. Field magnets;
- 3. Armature;
- 4. Commutator;
- 5. Brushes.
CHAPTER XIII
THE DYNAMO: BASIC PRINCIPLES
A dynamo is a machine for converting mechanical energy into electrical energy, by means of electromagnetic induction, the amount of electric energy thus obtained depending upon the mechanical energy originally supplied.
The word dynamo is properly applied to a machine which generates[14] direct current, as distinguished from the alternator, which generates alternating current.
Ques. Define a dynamo with respect to its principle of operation.
Ans. A dynamo is a machine for filling and emptying conducting loops with magnetic flux, and utilizing the electromotive force thus induced in them for the production of current in the external circuit.
The fitness of this definition is apparent, having in mind the principles of electromagnetic induction.
Ques. What are the three essential parts of a dynamo?
Ans. The field magnet, armature, and commutator.
Ques. What is the object of the field magnet?
Ans. To provide a magnetic field, through which the conducting loops arranged on a central hub and forming the armature are carried, or the flux carried through them, so that they are successively filled and emptied of magnetic lines.
Ques. What is a commutator?
Ans. A device for causing the alternating currents generated in the armature to flow in the same direction in the external circuit.
Ques. Upon what does the voltage depend?
Ans. Upon the rate at which each conducting loop is filled and emptied of lines of force and the number of such loops with their grouping or connection.
Ques. How is the operation of a dynamo best explained?
Ans. By considering first the action of the simplest form of current generator, or elementary alternator.
Ques. Describe an elementary alternator.
Ans. It consists, as shown in fig. 165, of a single rectangular loop of wire A B C D, one end being attached to a ring F and the other to the shaft G, and arranged so as to revolve around the axis X X′, which is located midway between the two poles of the magnet. Two metallic strips or brushes M and S connected with the external circuit, bear on the ring F and shaft G, respectively, in order to “collect” the current generated in the armature when the machine is in operation. The long, straight, horizontal arrows joining the two poles of the magnet, represent the lines of force which make up the magnetic field between the poles. The field is here assumed to be uniform, as indicated by the equal spacing of the arrows.
Ques. What happens when the loop is rotated?
Ans. According to the law of electromagnetic induction, when the loop is rotated around its horizontal axis in the direction indicated by the curved arrow, an electromotive force will be induced in the loop, the magnitude of which depends on the rate of change of the number of lines of force threading through, or embraced by the loop.
That is, if the number of lines embraced by the loop be increased from, say, 0 to 1000, or decreased from 1000 to 0, in one second, the electromotive force generated will be two times as great as if the increase or decrease were only 500 lines per second.
Ques. Upon what does the direction of the induced current depend?
Ans. Upon the direction of the lines of force and direction of rotation of the loop.
Ques. How is Fleming’s rule applied to determine the direction of current?
Ans. In applying this rule, the horizontal portion of the loop, such as A B or C D (fig. 165), is to be considered as moving up or down; that is, the component of its motion at right angles to the lines of force is taken as the direction of motion. When the loop is in the position A B C D, such that its plane is vertical or perpendicular to the lines of force, the maximum number of magnetic lines thread through it, but when it is in a horizontal position, A′ B′ C′ D′, so that its plane is parallel to the lines of force, no lines pass through the loop. During the rotation from position A B C D to A′ B′ C′ D′, the number of lines passing through the loop is reduced from the maximum to zero, the reduction taking place with increasing rapidity as the loop approaches the horizontal position, the electromotive force thus induced increasing in like proportion. Continuing the rotation from the horizontal position A′ B′ C′ D′ to the inverted vertical position A B C D (fig. 166), the number of lines passing through the loop is increased from zero to the maximum, the increase taking place with decreasing rapidity as the loop approaches the inverted vertical position, the electromotive force thus induced decreasing in like proportion.
Ques. How does the current flow during the first half of the revolution of the loop?
Ans. It flows in the direction A B C D (fig. 165), as is easily ascertained by aid of Fleming’s rule.
Ques. What is the path of the current to the external circuit?
Ans. It flows out through brush M (fig. 165) and returns through brush S, thus making M positive and S negative.
Ques. What occurs during the second half of the revolution?
Ans. The wire A B (fig. 166), which before was moving in a downward direction, moves in an upward direction; hence, the current is reversed and flows around the loop in the direction A D C B (fig. 166), going out through brush S and returning through brush M. This makes M negative and S positive.