PPE Capacitors etc

4) Time constant-The time that is required for the current

to rise (or fall) 63.2% of the maximum change possible.

T=RC.

e) Capacitive reactance

(1) Definition-The opposition to current flow provided by a

capacitor.

(2) Formula- Xc = 1/ 2πfC

(3) E and I's relationship-The current leads the voltage by

900; ELI the ICE man.

D. Transformers

1. Definition- A device that couples electric energy from one circuit

to another via a moving electromagnetic field.

2. Theory of operation

a) Mutual inductance-The transformer allows for two circuits to

be electrically separate, yet power can still be transferred from

one circuit to another by the mutual coupling of two (or more)

coils. As a magnetic field builds and falls its lines of flux cut

across the other coil, inducing a voltage across that coil.

b) The primary winding-The coil that is connected to the AC

power supply.

c) The secondary winding-The coil that is connected to the

load; power is being induced into this winding.

(3) Voltage and Current relationship- Ep IP = Es Is

e) Power Transformation Power into an ideal transformer is

equal to the power out of it. There can be no power gain in a

transformer.

3. Uses

a) Step down-Takes a high input voltage and brings it down to

a lower voltage; Np > Ns.

b) Step Up-Takes a low input voltage and brings is up to a

higher voltage; Np < Ns.

c) Isolation-Used to isolate electrical equipment. There is no

electrical connection between the primary and secondary. Np

= Ns.

d) Impedance matching-An electrical circuit will transfer the

most power into a circuit that has the same impedance as the

supply. It is therefore sometimes necessary to match the

impedance of a supply to a load. Zp/ZS = (Np/NS)2

4. Power Losses in Transformers

a) Coil resistance

(1) The problem-Any conductor has a certain amount of

resistance. This resistance causes power to be lost as

heat in a transformer.

(2) The solution-Use the best conductor possible in a

transformer; generally this will be copper.

b) Flux leakage

(1) The problem-Some of the flux goes through the air

instead of through the core. This flux does not cut

across the secondary coil.

(2) The solution-Use a core material with the highest

permeability possible and surround the core as much as

possible.

c) Hysteresis loss

(1) The problem-As the magnetizing force from the

primary reverses some magnetism is retained in the

core and thus some of the reverse magnetizing force

must be used to reverse the retained magnetism. The

product of this is heat.

(2) The solution-The core material should have a high

permeability and a low retentivity. This will result in a

material with a narrow hysteresis loop.

d) Eddy currents

(1) The problem-As the flux changes within the core of

the transformer, current is induced into the core of the

transformer itself. The result of this is heat.

(2) The solution-The transformer cores are made of

laminated material. Each sheet is insulated from the

other sheets with a lacquer coating. This prevents

currents from flowing in the core of the transformer.

5. Phasing

a) The phase relationship-If two coils were wrapped around the

same core in the same direction the polarity of the secondary

would be exactly opposite that of the primary.

b) The dot notation-In a diagram the ends of the transformer

are marked with a dot to show which ends will have the same

instantaneous polarity.

II. Ignition Systems

A. Introduction

1. What is an ignition system?

a) Starting device-Every type of engine needs a system to first

get the fuel burning as the engine starts.

b) Igniting device-Some engines, such as gas reciprocating

engines need a continuous source of ignition to light the fuel

air mixture for each combustion cycle. Others, such as

turbine and diesel engines do not need the ignition source

once the engine has started.

2. What is required of an ignition system (in a reciprocating aircraft

engine)

a) Reliability-The ignition system must be able to ignite the fuel

air mixture for every cylinder, on every cycle during the entire

time that the engine is operating.

b) Timing-During operation the fuel air mixture must be ignited

at a specific time; about 250 before top dead center of

compression.

Engine speed 2200rpm

Revolutions sec 36.667r/s

Time per Revolution .0273sec/r

Time per degree75.8µsec/degree

c) Starting-For starting an engine we need a reliable means of

igniting the fuel air mixture and this must still take place at a

specific point in time, this time however must be close to TDC

of compression stroke. This prevents the engine from being

kicked back (rotating in reverse) as it starts.

3. Parts of the ignition system

a) Spark creators

(1) Battery ignition-- needs an external power source

such as a battery. Both mechanical and solid state

types are available.

(2) Magneto--Self contained generator; Aircraft need 2

sources of ignition for these reasons:

(a) 14 CFR 33.37 states that a spark plug ignition

system requires dual ignition including two spark

plugs per cylinder, two separate electric circuits

with separate electric sources.

(b) Safety: engine can run (although roughly) on 1

magneto, this will allow some factor of

redundancy.

thus need two sources of ignition to achieve a

complete and even fuel burn.

b) Sample Starting Mechanisms (Aircraft)

(1) Shower of sparks

(2) Induction vibrator

(3) Impulse coupling

c) Transportation of the spark to cylinder-Ignition harness

d) Spark delivery-Spark plugs

B. Battery Ignition systems

1. Parts required

a) Power source-Usually a battery charged by a generator

system; it is required, if there is no power there is no ignition.

b) Ignition coil-A transformer with a step up ratio of about 1:100

c) Cam and Breaker-Creates pulsating DC through the primary

coil of the transformer. This allows the spark to be created at

a given instance in time.

d) Capacitor-Placed in parallel with the points

e) Distributor-Contains an electrode for each cylinder and a

distributor finger which selects the proper electrode.

f) Spark Plug-A high voltage across the secondary coil will

cause current to flow through the distributor, ignition lead, and

across the gap between the spark plug electrodes. The arc

created when the secondary current flows will ignite the fuel-

air mixture in the cylinder.

2. The theory

a) The primary circuit

(1) Primary coil-Current flows in the primary coil when

the breaker points are closed thus producing a magnetic

field in the core of the transformer.

(2) Breaker points-When the points open the magnetic

field collapses due to the extremely fast change in

current in the primary circuit.

(3) Capacitor-According to Lenz's law current wants to

keep flowing through the primary circuit which would

cause an arc at the points if the capacitor was not

installed in the circuit. The capacitor performs two

functions for us:

(a) It allows current to flow into it while the points

begin opening thus preventing the points from

being damaged by arcing.

(b) Because it prevents arcing across the points it

allows the magnetic field to collapse more quickly

once the points open.

b) The secondary circuit

(1) The secondary coil-As the magnetic field created by

current flow through the primary circuit collapses it cuts

across the secondary coil creating a voltage of around

20,000 to 25,000 volts.

(2) The distributor-Selects the proper cylinder to fire by

making the easiest path for secondary circuit current

the spark plug gap in the particular cylinder that is to be

fired.

(3) The ignition harness and spark plug-The path that

allows the high tension current to flow to the gap at the

end of the spark plug.

C. Magneto ignition systems

1. The parts required

a) Magnet and Magnetic circuit-Permanent magnet, which is

usually made of some alloy such as ALINCO (Aluminum, iron,

nickel, and cobalt), is rotated between field poles which are

made of laminated, soft iron.

b) Transformer-Is built around part of the magnetic circuit. The

primary contains about 250 windings of wire and the

secondary contains about 20000. The windings are usually

covered with a hard plastic or rubber. Generally there are

three electrical connections to the transformer, primary,

secondary, and the ground.

c) Cam and breaker points-Cam spins with the rotor shaft and

pushes the breaker points open at the optimal time. Breaker

points are usually made of some heat resistant metal such as

tungsten. Breaker points open the primary circuit.

d) Capacitor-Placed in parallel with the points. The plates are

made of an aluminum foil with a paper dielectric. Generally

they have a capacitance between .35µF and .4µF.

e) Distributor-Composed of a material with very good

insulating qualities. Examples of these materials are bakelite

and a thermal setting plastic.

2. The theory

a) The magnetic circuit-Without the primary attached: The Flux

flow alternates with the position of the magnet. The flux flow

is greatest when the magnet is directly in line with the pole

shoes; this is called Full register position. The flux flow is

least when the magnet is perpendicular with the field poles;

this is called Neutral.

b) The primary circuit-If the primary circuit is complete then

there will be current induced in the primary circuit due to a

changing magnetic field. The current following through the

primary causes the flux flow graph to shift to the right. This

takes place because the current flowing through the primary

circuit creates its own magnetic field that is summed with the

magnetic field from the magnet.

c) The result of the two combined; E-gap-To get the maximum

energy out of the secondary coil we want to open the primary

points at the instant that will provide us with the greatest

change in over all flux in the magnetic circuit. This point is

called E-gap and it is usually around 100 after the magnet

passes through Neutral

d) The secondary circuit-When the points are opened up at E-

gap the current flowing through the primary circuit ceases and

thus the magnetic field created by the primary current

collapses. As it collapses it cuts across the secondary

windings creating a very high voltage potential across the

secondary.

3. The nature of high voltage

a) Persistent-The current will find a path to complete the

circuit. Even air will act as a conductor.

b) Prefers the path of least resistance

(1) Air as a conductor-Air is a conductor, particularly

when it has become ionized by the first bit of current

flowing though it. As air pressure decreases the

insulating qualities of the air also decreases. This can

cause problems for reciprocating engine aircraft that fly

at relatively high altitudes.

(2) Moisture-Is conductive when mixed with

contaminants such as oil or carbon dust.

(3) Carbon tracking-If a current arcs across the surface of

a distributor where oil or carbon contaminants are

present then the spark will evaporate any moisture and

burn the contaminants in its path; this leaves a

conductive (relative to the air around it) trace. This path

may become the path of least resistance for the high

potential current rather than its proper path.

c) Insulation deterioration-A high voltage repeatedly applied

across an insulator may tend to cause the insulator to break

down after time.

4. Magneto Flavors

a) Transformer placement

(1) High tension-The transformer is placed within the

magneto. This was the first type of magneto to be made

and it is the most popular at the present time.

(2) Low tension-Was designed around WWII when there

was a big push to have piston powered aircraft fly

higher and higher. Only the primary circuit was put

within the magneto. Each spark plug had its own

transformer and these were placed as close as practical

to the spark plug.

b) Magnets

(1) Numbers-Came with 2, 4, 6, or 8 poles.

(2) Rotating or Not-Some magnetos had two stationary

magnets and a piece of soft iron that was rotating within

the magnetic circuit. This is called a polar inductor

magneto.

c) Cams

(2) Compensated-Used in high powered radial engines to

compensate for the master rod tipping from side to side.

The lobes are ground a few degrees off to one direction

or the other to insure that the spark takes place at the

optimal time during the combustion cycle.

d) Distributor placement

(1) Integral to the magneto-Most magnetos on small

reciprocating engines.

(2) As a separate unit-Some large radial engines place the

distributors separately from the magneto, i.e. R2800.

e) Dual magnetos

(1) Parts in common

(a) Rotating magnet (driven by engine)

(b) Housing

(2) Independent parts-All other parts are separate.

f) Mounting

(1) Clamp mounting-The magneto has a lip which is held

to the engine with two clamps. A circular gasket is

used. This type of timing allows for more of a timing

range (i.e. Slick 4200).

(2) Flange mounting-The magneto has two ears with

oblong slots through which the mounting studs pass

(i.e. Bendix S-20).

g) Drive type

(1) Gear driven-A gear is directly placed on the magneto

drive shaft. This gear meshes with a gear in the

engine's accessory housing.

(2) Ear and cushion-A drive accessory with a pair of ears

is directly placed on the magneto drive shaft. These

ears fit into a gear that is placed in the engine's

accessory housing. There are two rubber cushions that

fit into the gear which protect the magneto from

vibrations. Extra caution must be used when removing

and installing magnetos with this type of drive in that it

is easy to drop the rubber cushions into the accessory

housing. A thick, black grease affectionately called

"gorilla snot" can be used to help keep the rubber

cushions from falling out of their gear.

(3) Impulse coupling-Has the same type of drive

mechanism as the "Ear and cushion" only instead of

just using a drive accessory with ears, the magneto has

ears.

h) High altitude flying (such as with turbo prop aircraft)

(1) The problem-As the air becomes thinner it provides

less of an insulator, hence where we are relying on the

insulating property of air, such as in the distributor,

there can be a problem when operations take place at

high altitudes.

(2) The solutions

(a) Low tension system-By keeping all the high

voltages out of the magneto and as close to the

spark plug as possible the possibilities of stray

sparks is minimized.

(b) Distributor design-By creating a magneto with a

larger distributor the insulating properties of the air can be maximized. Such as is used in the Bendix S-1200 magnetos