Motors Part 11
[Ill.u.s.tration: _Fig. 46. Making the Circuit._]
Plurality of Loops.--The dynamo may have a plurality of loops, which are called _coils_, and there may be a single magnet or any number of magnets. Instead of driving these coils past the face of the magnet, or magnets, the latter may be driven past the coils. In fact with most of the alternating current machines the fields are the rotating parts and the armatures, or the coils, are fixed.
The voltage is increased if the coils have a large number of turns on the armature, and also if the armature, or the turning part, is speeded up. Voltage will also be higher if larger or more powerful magnets are used in the magnetos.
The Electro-Magnet.--The permanent magnet, such as is used in the magneto, is distinguished by the fact that it contains a permanent charge of magnetism, but this is not an _electro-magnet_. This is a magnet made of soft iron, so it will be readily demagnetized. While not shown in the diagrams, an iron core may be placed within the loop or coil, and this is done in all dynamos, because the iron core acts as a carrier of the magnetism, concentrating it at the center, because it is a much better conductor than air.
[Ill.u.s.tration: _Fig. 47. The Dynamo._]
[Ill.u.s.tration: _Fig. 48. The Magneto._]
The Dynamo Form.--Consult the diagram, Fig. 47. The iron heads A represent the bar in the previous diagrams, and B the wire around the bar. C is the armature, which in this case represents a number of loops, or coils, and D is the commutator, which is used in the direct current machine to correct the alternations referred to in the previous diagrams, so as to send the current in one direction only, the commutator brushes E being used to carry off the current for use.
The Magneto Form.--The metal loop F, in Fig. 48, being a permanent magnet, the armature, G, formed of a plurality of loops, has no field wires to connect with it, as in the case of the dynamo.
Advantage of the Magneto.--The magneto has a p.r.o.nounced advantage over the dynamo, as a source of power for ignition purposes, in the particular that the strength of the magnetic field is constant. In a dynamo this varies with the output, because when used on an automobile where the speed is irregular, the voltage will vary. The voltage of the magneto is a constant one, and is thus better adapted to meet the needs of ignition.
Induction Coil.--The induction coil is a device which is designed to produce a very high voltage from a low tension, so that a current from it will leap across a gap and make a hot spark.
We stated in a previous section that a current leaps across from one conductor to another, so that electricity can be transferred from a wire to another not touching it, by means of induction.
Look at Fig. 49, which represents two wires side by side. The current is flowing over one wire A, and by bringing wire B close to A, but not touching it, a current will be induced to leap across the gap and the wire B will be charged. If the ends of the wire B are brought together, so as to form a circuit, and a current detector is placed in the circuit it will be found that a current is actually flowing through it, but it is now moving in a direction opposite to the current flowing through A.
[Ill.u.s.tration: _Fig. 49. Current by Induction._]
Changing the Current.--But we have still another thing to learn. If the two wires are not of the same thickness it would not prevent the current from leaping across, but another astonishing thing would result.
First, we shall use a wire B double the thickness of wire A. If now, we had an instrument to test the voltage and the amperage, it would be found that the voltage in B is less than that in A, and also that the amperage is greater.
Second, if the conditions are reversed, and the wire A is thicker than B, the latter will have an increase of voltage, but a lower ampere flow than in A.
Now this latter condition is just what is necessary to give a high tension. Voltage is necessary to make a current leap across a gap. By this simple ill.u.s.tration we have made an induction coil which may be used for making a high tension jump spark.
Construction of a Coil.--Two wires side by side do not have the appearance of a coil, and even though such an arrangement might make a high tension current, it would be difficult to apply. To put the device in such a shape that it can be utilized, a spool is made, as shown in Fig. 50.
[Ill.u.s.tration: Fig. 50. Induction Coil.]
This spool A has a number of layers of thick, insulated wire B first wound around it, the layers being well insulated from each other, and the opposite ends brought out at one end or at the opposite ends, as shown at C, D. On this is a layer of finer wire, also insulated, this wire E having its terminals also brought out at the ends of the spool, and after the whole is thus wound, the outside of the coil is covered with a moisture proof material.
The Primary Coil.--The winding of thick wire is called the _primary_ coil. The current from the battery or the electric generator is led to this inner coil.
[Ill.u.s.tration: _Fig. 51. Typical Induction Coil._]
The Secondary Coil.--The fine wire wrapping represents the secondary coil, which is raised to a high voltage, and this actuates the sparking mechanism.
In the art it is customary to ill.u.s.trate the various contrivances by certain conventional forms. Fig. 51 shows the manner of designating an induction coil in a diagram, in which the heavy zig-zag line indicates the primary, and the lighter zig-zag lines the secondary coil.
[Ill.u.s.tration: _Fig. 52. Contact Maker._]
Contact Maker.--A simple little device used in the primary circuit of an induction coil, is known as a _contact maker_. This, as shown in Fig.
52, is merely a case A, through which is a shaft B that carries within the sh.e.l.l a cam C. A spring finger D has its free end normally bearing against the cam, and when the nose on the cam moves out the spring finger, the latter is moved outwardly so it contacts with a plug E in the side wall of the case, although it is insulated therefrom. This contact establishes a current through the plug, spring finger and case.
The diagram, Fig. 53, ill.u.s.trates the principles of construction and arrangement of a high tension jump spark ignition, in which the electrical source is a battery actuating an induction coil.
High Tension With Battery and Coil.--The battery A has one side connected up by wire B with one terminal of the primary C in the induction coil, and the other side of the battery has a wire D leading to the contact maker. A switch E is placed in the line of this wire.
[Ill.u.s.tration: _Fig. 53. Typical Circuiting, Jump Spark Ignition._]
The other terminal of the primary has a wire F leading to the insulated contact plug G of the contact maker. This completes the generating circuit. The cam H is on a shaft I, which travels one half the speed of the engine shaft.
One side of the secondary coil J has a wire K leading to the spark plug, while the other terminal of the secondary has a wire L which is grounded on the engine M.
When the nose of the cam pushes over the spring finger and closes the cam, the circuit through the finger flows through the primary coil and excites the secondary. When the cam again immediately breaks the circuit a high tension current is momentarily induced in the secondary, so that the current leaps the gap in the spark plug and makes the spark.
[Ill.u.s.tration: _Fig. 54. Metallic Core, Induction Coil._]
Metallic Core for Induction Coil.--In the previous description of the induction coil it was stated that the spool might be made of wood. These coils are also provided with metal cores, which can be used to make what is called a vibratory coil.
The Condenser.--A necessary addition to the circuiting provided by an induction coil, is a _condenser_. This is used in the primary circuit to absorb the self-induced current of the primary and thus cause it to oppose the rapid fall of the primary current.
The condenser is constructed of a number of tinfoil sheets, of suitable size, each sheet having a wing at one end, and these sheets are laid on top of each other, with the wings of the alternate sheets at opposite ends. Very thin sheets of waxed paper are placed between the tin foil sheets so that they are thus insulated from each other.
The wings at the ends are used to make connections for the conducting wires. The device is not designed to conduct electricity, but to act as a sort of absorbent, if it might so be termed. The large surface affords a means where more or less of the current moves from the conductor at one end to the conductor at the other end, and as it is designed to absorb a portion of the current in the line it is merely bridged across from one side of the circuit to the other.
[Ill.u.s.tration: _Fig. 55. Condenser._]
The diagram, Fig. 55, represents the conventional form of ill.u.s.trating it in sketching electrical devices.
Operation of a Vibrator Coil.--The ill.u.s.tration, Fig. 56, shows the manner in which a vibrator coil is constructed and operated. The coil comprises a metal core A, the primary winding B being connected at one terminal, by a wire C, with a post D, and the other terminal by a wire E with one side of a battery F. A switch G is in the line of this conductor.
[Ill.u.s.tration: _Fig. 56. Vibrator Coil and Connections._]
The post D holds the end of a vibrating spring H, which has a hammer H'
on its free end, which is adapted to contact with the end of the metal core A, but is normally held out of contact, so that it rests against the end of an adjusting screw I which pa.s.ses through a post J.
The post J is connected up with the battery by a wire K, and a wire L also runs from the wire K to the conductor C, through a condenser M.
The secondary coil N, has the outlet wires O, P, which run to the spark plug Q on the engine.
The operation is as follows: When the switch G closes the circuit, the battery thus thrown in the primary coil magnetizes the core A, and the hammer H' is attracted to the end of the core, thus breaking the circuit at the contact screw I. The result is that the core is immediately demagnetized, and the spring H draws the hammer back to be again attracted by the core which is again magnetized, so that the hammer on the vibrator arm H goes back and forth with great rapidity.
From the foregoing explanations it will be understood how the primary induces a high tension current in the secondary, and in order that the spark may occur at the right time, a _timer_ for closing and opening the primary circuit must be provided. By this means an induced high tension current is caused to flow at the time the spark is needed in the cycle of the engine operation.
_The Distributer._--The distributer is a timing device which controls both the primary and the secondary currents, and it also has reference to the revolving switch on the shaft of a magneto whereby the current is distributed to the various cylinders in regular order.
Fig. 57 shows a form of distributer which will ill.u.s.trate the construction. A is the shaft which is driven at one half the engine speed. It is usually run by suitable gearing direct from the shaft of the magneto.
[Ill.u.s.tration: _Fig. 57. The Distributer._]