PART 1: THEORY OF FOUR POLE MAGNETO

and there are many different types of Magneto's in use today on our modern aircraft engines many different types and models yet they all have the same job to do each magneto is designed to supply the electrical current necessary to fire the cylinders of a certain type of aircraft engine the operation of an aircraft magneto depends largely upon just two things magnets and wire to understand the part each plays in the magneto we must first know how magnets and wire act under certain conditions first let's consider the steel magnate the steel magnate is called a permanent magnet because it always attracts or magnetizes a piece of iron every magnet has two poles one is called the North Pole and the other is called the South Pole either pole attract iron which is a magnetizable metal when the poles which are alike get near each other like this they shove away or repel each other if the polls are not alike they attract or draw toward each other the forces causing this action cannot be seen but their effect can be seen by sprinkling iron filings over a paper near the magnet the magnets force acts in lines lines of force these are also called lines of flux or flux lines we speak of all these lines of flux as a field a magnetic field of flux since this field is useful in operating a magneto let's try it out and see what it can do here is an ordinary soft iron nail it is not a magnet but when it touches either Pole of the magnet it stays there the lines of flux pass through the nail because the soft iron makes an easier path to follow than the air this concentrates the lines in the nail which now acts like a magnet so we call it a temporary magnet when the nail moves away from the magnet however it soon loses its magnetism and is just an ordinary nail again so it is called a temporary magnet as long as the permanent magnets lines of flux pass through it the end of the nail next to the permanent magnet has the opposite Pole otherwise it would be shoved away the attraction between these poles also shows that they are unlike the lines of flux always follow this route out of the North Pole into the South Pole through the magnet and around again each line makes a closed loop these facts are all we need to keep in mind about magnets now for the wire by using a permanent magnets field and a length of copper wire a current can be produced connecting this wire to a meter shows that no current is flowing through the loop but when the closed loop of wire passes through the magnets field a current flows we are by this simple movement actually changing mechanical energy to electrical energy when a current is set up in this way it is called an induced current the voltage and hence the current has been induced or built up in the wire as long as a closed loop of wire keeps passing through the lines of flux current flows in the wire if the wire is held still and the field moves like this a current is again set up or induced in the wire but as soon as there is no movement either by the wire or by the field the current stops flowing to simplify this suppose that the magnet has only three lines of flux to get a unit of measurement let's say that every time one of these flux lines passes through one turn of wire one flux linkage is made then three lines of flux passing through or linking through one turn of wire makes three flux linkages the meter shows the current flow now suppose we call the wire like this and use a soft iron core to concentrate the flux lines through the coil when three lines of flux linked through five turns of wire 15 flux linkages are made and the current increases we can further increase this current by changing just one thing the rate at which these lines of flux are cut moving the same field three lines of flux through the same coil with five turns of wire faster gives more current only 15 flux linkages are made but they are made faster so the greater the rate of change in flux linkages the greater the induced current there are other ways to increase the rate of change in flux linkages when a greater induced current is needed for example strength of the field can be increased by using a stronger magnet the rate of change in flux linkages is now greater and the current increases the amount of current can also be increased by using a coil with more turns of wire because the number of flux linkages will be increased and of course increasing the speed of the change by moving the magnet faster also does the trick remember the amount of induced current depends upon the rate of change of flux linkages now let's see how a wire acts when it carries the current here we have a source of continuous direct current suppose this steady current is sent through the wire a magnetic field will be set up around the wire this field is also made up of closed lines of flux when the wire is cut the current stops and the field around the wire disappears with this in mind let's replace the continuous flowing current of the battery with an induced current as current is induced in a wire by a change in flux linkages a magnetic field is set up around the wire let's watch the unusual action of this field around the wire as the current changes as the linkages increase current starts in the wire the current in the wire sets up a magnetic field around it and this magnetic field like a giant hand acts against or opposes the change in flux linkages the wires magnetic field opposes the very change which is inducing the current in the first place not only that but as soon as the flux linkages start to decrease the magnetic field around the wire again acts like a giant hand to oppose the decrease in other words the field now tries to keep the flux linkages from dying out or decreasing this action was first noted by a man named lens it is of great importance in the operation of a magneto the magnetic field a powder coil of wire carrying an induced current opposes the flux change which is producing the current if the flux linkages are increasing the field around the wire opposes the increase if the flux linkages are decreasing the field opposes the decrease this magnetic field around the coil of wire is against any change taking place even though the current in the wire wouldn't be there without a change in flux linkages now let's notice the direction of the induced current as the magnet moves away from the coil and the lines of flux through the core core decrease a current is induced in this direction now if we turn the magnet over reverse the position of the poles and increase the lines of flux through the coil core by moving the magnet up to it the current induced will have the same direction as before if we can do this quickly enough we will get a single big surge of current all in one direction so our problem is to decrease the flux through the coil reverse the poles and then increase the flux through the coil core in the other direction keeping this problem in mind along with the basic facts about magnets and wire let's build a device which will produce a high tension current to a soft iron core arms of soft iron are added these arms are called pull extensions now instead of turning the magnet over to reverse the poles we can put two magnets on a shaft so that they can be rotated notice that we now have four fields of flux around the 4-pole magnet to improve the path for the magnets field soft iron pole shoes are added now let's see what effect the turning of the magnet has on this soft iron core as the poles of the magnet moved toward the pole shoes the lines of flux increase through the core this flux made by the magnet as it turns is called the static flux at this point the greatest number of flux lines are passing through the core this is called the full register position of the magnet as the magnet moves on the lines of flux going through the core decreased 20 is called the neutral position of the magnet as the North Pole moves up to the next Pole shoe the lines of flux build up to a maximum in the opposite direction so now by simply turning the magnet we are able to decrease the flux through the core reverse the poles and then increase the flux through the core in the opposite direction so far we have seen the static flux as a few lines moving along their path as a matter of fact there are thousands of lines of flux and from this point on we will think of the flux as a stream moving along the magnetic circuit now let's use this static flux to induce a current in a coil of copper wire this is called the primary coil it is the first wire to get the current with the primary coil in place let's see what happens to the static flux as the magnet moves away from the full register position the amount of flux through the core decreases this change in flux starts a current in the primary coil the field around the primary coil like a giant hand tries to keep the static flux flowing this makes the primaries field work with the static flux of the magnet this combination of the primaries flux and the magnet static flux gives a resultant flux and the resultant flux continues to flow through the core even though the magnet has reached the neutral position in this way the primary current maintains the original field in the coil core but of course not quite up to its original strength however as the magnet starts to move towards the next pole shoe its lines of flux try to get through the coil core in the opposite direction but this is prevented by the magnetic field of the primary current right here is a chance to get a terrific rate of change in flux linkages because if we can break the primary circuit at this instant its current drops to zero without the primaries help the original field through the core disappears and the flux of the magnet has a clear path through the coil cord in the other direction in this split instant then the rate of change in flux is tremendous changing as it does from a high in this direction and up to a high in the other direction in order to break the primary of this instant a breaker or interrupter assembly is needed by grounding this end of the primary to the magneto and connecting the other end of the primary to the breaker and the breaker to the ground here the primary circuit can be easily made complete by closing the contact points or broken by opening the contact points this breaker is really an automatic switch it has a movable arm or spring assembly connected to the points the movable arm is operated by this cam on the magnet chair which opens the points when the rotating magnet reaches a certain number of degrees beyond the neutral position a distance called the egapp in practice the egapp is sometimes measured at this point by breaking the primary at this point the rate of change in flux will be greatest in order to get the necessary output the high rate of change in flux linkages can be further increased by adding a second coil this second coil or secondary made of many thousands of turns of fine wire is wound around the primary coil on the same core in this way the extremely rapid flux change will be linked into it the secondary coil completes its circuit like this this end goes to the distributor and the distributor directs the current on to the proper spark plug the spark plug grounds the current through the engine in this way the secondary can complete its circuit back through the primary winding to its beginning now there is a complete path for the secondary to travel let's see what happens as the magnet turns this time when the primary is suddenly broken the extremely rapid flux change links through the secondary coil and induces a strong current in it this current has enough force to jump across the spark plug gap notice the direction of the secondary current again in accordance with Lenz law as soon as current flows in the secondary a magnetic field is set up around the wire this field like a giant hand opposes the change in flux which is inducing the current soon after the secondary has completely discharged its current the points closed for the next primary build up this time however the flux has changed direction so the primary which it induces flows this way as the points open the flux urges this way through the core and the secondary flows in this direction this cycle occurs over and over as the flux changes direction the induced primary changes with it and the secondary changes Direction also so the secondary travels first one way and then the other to the spark plugs this setup works but a lot of current may be lost by arcing across the points the arcing results from this action the break of the primary circuit causes an extremely rapid flux change this change links the secondary as we know and gives a high-tension current but it also links the primary and tries to make a current flow in it as a result some current will jump the gap as the points are just separating this is because the breaker points open slowly compared to the speed of the flux change current jumping across the points would cause them to burn away in a short time to prevent this a condenser is connected across the points here to show the action of a condenser let's think of it as if it were a box with a rubber partition as current rushes in here for example this sudden shove on the partition causes the current to be forced out of the condenser but only for an instant because now the secondaries field opposes the flux change which made the primary current flow into the condenser in the first place all of this action occurs in the brief instant just after the contact points open with the breaker points and the condenser in action a high tension current is produced at regulated intervals the magneto is now ready for use next let's see how it fits into the aircraft ignition system here is the complete aircraft ignition system with its two Magneto's radio shielded harness spark plugs and switch this magneto is shown completely assembled the other is in skeleton form like our model and shows the electrical and magnetic circuits the condenser is connected across the breaker points these wires connect the primary grounding terminal on each Magneto with the switch when the switch is off this wire makes a direct path to ground for the primary circuit and when the breaker contact points open the primary current flows around through the switch instead of being interrupted so a high voltage in the secondary is impossible when the switch is off one end of the secondary is electrically grounded to the magneto and the other end runs to the distributor from the distributor finger the current jumps a small air gap to the electrodes of the distributor block and then goes to the spark plugs now that we have seen how the magneto works and its place in relation to the rest of the ignition system let's take a close look at an aircraft magneto as it is assembled there are several improvements over our model but the operation is just the same the rotating magnet looks like this and it fits into the magneto here the magnet is mounted on a shaft driven from the engine crankshaft in this way the engine powers the magneto the core pole extensions and pole shoes are made of laminated soft iron laminating or putting together thin sheets of soft iron with layers of special insulating compound between makes a unit which is more efficient than a solid piece of soft iron the laminated poll shoes and Paul extensions are cast into the housing here the primary condenser looks like this it is usually built of nickel foil plates carefully insulated from one another by mica and fits into the primary circuit where it is connected across the points like this this coil assembly in the real magneto looks like this the laminated core the primary winding with about 150 turns of heavy copper wire and the secondary winding with about 13,000 turns a very fine wire fits onto the pole extensions like this and the Baker assembly the spring and the breaker points fit here right next to the cam on the magnet shaft or compactness the distributor is made a part of the magneto assembly in single magneto units it is made of high-grade insulating material within SAT electrons there are of course just as many electrodes in the distributor block as there are cylinders in the engine the distributor finger is an insulated arm with a metal insert which carries the current from the secondary coil to the distributor block electrodes each electrode is connected to a spark plug like this and here the high tension current produces a powerful spark in the right cylinder at the right time as long as the engine run because to sum it all up as the engine turns the magneto shaft rotates the magnet the magnet see effect on the coils causes a high rate of change in flux linkages the current bus induced goes on its way to the distributor and is directed to the spark plugs in rapid-fire order rapid-fire order yes and faster this magneto can produce 25,000 sparks per minute over 2,000 in five seconds to get an idea of the speed with which the sparks occur let's flash two thousand dots on the screen in five seconds every dot represents a spark two thousand in five seconds let's look at that again 1 2 3 4 5 with a magneto supplying 25,000 accurately time sparks every minute the pilot knows he can depend on a constant source of electrical life in every engine