Topic 5 DC Source of Electricity

7/27/2019 Topic 5 DC Source of Electricity

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UNIVERSITI KUALA LUMPUR

Malaysian Institute of Aviation Technology

Revision :

Original Date : 15/01/07Page No :1

5. DC Sources of Electricity A battery is a device that stores chemical

energy and makes it available in an electrical

form. It composed of two or more cell that convert

chemical energy into electrical energy.

+ -+ -+ -+ -

Battery 12VSingle Cell


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5. DC Sources of Electricity (contd) The cell itself consists of two different conducting

materials, called electrodes, which are immersed

in an electrolyte.

A simple cell consist of; two electrodes (terminal)

positive and negative.

An electrolyte

a liquid which is

a conductor, either

acid or alkaline.

Electrode

Electrolyte

+ -

Loads

Electrode

Electrolyte

+ -

Loads


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5. DC Sources of Electricity (contd)

The chemical reaction between the

electrodes and the electrolyte results in a

separation of electric charges, in the form

of ions and free electrons.

Then the two electrodes have a difference

of potential that provides voltage outputfrom the cell.


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The function of a battery is to provide a source

of steady DC voltage of fixed polarity.

The purpose of battery on the aircraft is for

starting small engine and for emergency use.

to supply power only to the essential load e.g. radio

communication in order for the pilot to communicate withthe ground tower, gyros (e.g. HSI, AH), escape light, etc.


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5. DC Sources of Electricity (contd) The cells of a battery are classified as

- Primary Cell.

- Secondary Cell.


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5. DC Sources of Electricity (contd) Primary cells (battery).

- To provide small / low power output e.g. the dry cell also known as

Leclanche Cell.

- They are called dry cell because the electrolyte is a paste rather than aliquid.

- They are not rechargeable. The cell will not function once their

chemicals are used up and the cells have to be thrown away.

- Examples are zinc-carbon cell, alkaline manganese cell and silver

oxide cell.


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Zinc-Carbon cells

- The construction of Zinc-Carbon cell consist of;

+ ve plate (anode) used carbon rod.

ve plate (cathode) used zinc casing.

Electrolyte is ammonium chloride withmanganese dioxide in a paste form.


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- The output of the dry cell is 1.5 V per cell regardless of their size.

- The size will determine the amount of current it will supply.

A manganese dioxide use as a depolarizer to ensure a chemical

reaction involving the constituents of both terminals; this reactioncauses a current to flow through a conductor that connects the

positive and negative terminals.


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5. DC Sources of Electricity (contd) Alkaline Manganese cells

- The purpose is to provide longer life

compare Zinc-Carbon cell.

- The construction of Alkaline Manganese

cell consist of;

+ ve plate used carbon rod

ve plate used manganese dioxide lined steel casing.

Electrolyte is potassium hydroxide and has a lower resistance than

ammonium chloride thus provide more load current.


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5. DC Sources of Electricity (contd) Mercury cells

- These cells have nearly constant voltage at low discharge currents

making them ideal for hearing aids, calculators, photographic

cameras and electronic watches.

- It has a high capacity in small size and very long shelf life, up to 10

years.

- Due to the content of mercury, and the resulting environmental

concerns, the sale of mercury batteries is banned in many countries.


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- The construction of Mercury cell consist of;

+ ve plate used

mercury oxide.

ve plate used zinc

casing.

Electrolyte is potassium

hydroxide.


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Lead Acid Cells

The purpose is to provide power supply to aircraft during emergency,

starting, etc (if the usages for aircraft).


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The separator prevents +ve and ve plate from touching but allow

sulphuric acid to pass through (porous material).

6 or 12 cells, each cell rated 2.1 volts.


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2 volt x 6 cells are connected in series will give 12 volts


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5. DC Sources of Electricity (contd) Indication of fully charged condition.

Terminal voltage - 2.7 V/cell on charged

- 2.1 V/cell off charged- 1.8 V/cell discharged

or terminal voltage between 30.0 to 32.4 V (on charge), if

terminal voltage below 28.5 V, it should be withdrawn fromservice.


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Voltage Test


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Specific gravity (S.G.) of electrolyte

- 1.275 and 1.300 at 80F or 27C reference).

- use hydrometer to check the S.G.

Gassing freely.


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Capacity

- ability to produce a given amount of current for specific time e.g. 1A

for 1 Hour which is known as 1 Ampere/Hour (A/H) .- more than 80% efficiency fit for aircraft use

Efficiency (%) = Output capacity X 100

Rated capacity


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Factor affecting capacity of a battery;

number of plate / cell

plate area

strength of electrolyte

temperature


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Note : Charging current is 4 A for

10 Hrs


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Note :

Total battery voltage must

not exceed charger voltage

Charging current is 2A for 10

Hrs


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Constant voltage charging

-when battery connected in an aircraft, car, or aother vehicles.

- generating voltage system slightly higher than

battery voltage.

- the amount of current flow into a battery being

charge is determined by the state charge of

battery.- no need monitoring device


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Note :

When the battery is slightly weak, the charging current will increase. The

charging current decrease as the battery towards fully charged condition. Thus

no need monitoring device.


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Capacity Test

- carried out after initial charge.

- 3 monthly.

- at any time if battery capacity in doubt.

- acceptable capacity for use on aircraft is 80% and above.


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Revision :Original Date : 15/01/07

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stored in clean, dry, well ventilated area. always stored in fully charged condition this is to

prevent sulphation take place.

give freshening charge every 2 to 4 weeks.

Storage


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5. DC Sources of Electricity (contd) Nickel Cadmium Cells

The purpose is to provide high current or capacity output within ashort period main used for turbine engine starting.

An improvement of the Lead Acid Battery.

The construction of NiCad Cell consist of;

- + ve plate used powdered nickel or sintered / nickel hydroxide.

- ve plate used cadmium hydroxide.

- The separator is using nylon and cellophane to keep plate apart



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Nickel Cadmium Cell


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- The electrolyte is using potassium hydroxide (30%) and

distilled water (70%).

S.G. between 1.24 to 1.30 at roomtemperature.

No S.G. change during discharging or

recharging and this electrolyte act as a

conductor.

- The casing is made of polystyrene or nylon cell case and

sealed.



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- For each cell the voltage is 1.55 to

1.80 V for open circuit voltage.

-For 9 to 10 cells are used for 12 Vaircraft system.

- For 19 to 20 cells are used for 24 V

aircraft system


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Adjust the electrolyte level

immediately after fully charge

condition with distilled water

Remove vent plug using vent

wrench to top up distilled water


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Internal short circuit between the cells are indicated that the battery

wont hold charge. Check for leakage test. Check all connection any sign of corrosion or burning. The

connectors must be torque tight.

Adjust the electrolyte level immediately after fully charge condition

with distilled water only.


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UN V S U U U



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5. DC Sources of Electricity (contd)Leakage Test

- check between terminal and steel casing using ammeter.

- leakage current must not more than 100mA, if more clean the steel casing.


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- If cell unbalance still occurs, discharge cell voltage to

approximately 0.2 V/cell, then use shorting strap across each cell

for 3 to 8 hours to completely discharged them. This process

known as Deep Cycling (equalization of cells).

- Recharge with constant current charger. Each cell should have a

voltage of 1.55 to 1.80 V at temperature between 21C (70F) and

26C (80F).


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Neutralizing agent or cleaning agent for

alkaline spillage is dilute boric acid,

vinegar, lemon juice.



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Specific Gravity

Specific gravity is the ratio of the weight of a certain amount of a

given substance compared to the weight of the same amount of

pure water.

The specific gravity of pure water is 1.0.

Any substance that floats has a specific gravity less than 1.0. Any substance that sinks has a specific gravity greater than 1.0.



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To measure a battery's specific gravity, use aninstrument called a HYDROMETER. Is a glass

syringe with a float inside it.

The float is a hollow glass tube sealed at both

ends and weighted at the bottom end, with a scale

calibrated in specific gravity marked on its side.



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To test an electrolyte:

-Draw the electrolyte it into the hydrometer using the suctionbulb.

- Draw enough electrolytes into the hydrometer to make the

float rise.

- DO NOT draw in so many electrolytes that the float rises into

the suction bulb


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The float will rise to a point determined by the specific gravity of

the electrolyte.

- If the electrolyte contains a large amount of active ingredient,

its specific gravity will be relatively high.

- The float will rise higher than it would if the electrolyte

contained only a small amount of active ingredient



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NOTE:Hydrometers should be flushed with fresh water after each

use to prevent inaccurate readings. Storage battery

hydrometers must not be used for any other purpose.

To read the hydrometer, hold it in a vertical position and read the

scale at the point that surface of the electrolyte touches the float.

Refer to the manufacturer's technical manual to determinewhether or not the battery's specific gravity is within

specifications



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When cells connected in series, there is an

increase in the voltage and current (ampere)

remains constant.

+ - + - + -

1.5V

0.5A

1.5V

0.5A

1.5V

0.5A

+ -+ - + -+ - + -+ -

1.5V

0.5A

1.5V

0.5A

1.5V

0.5A

Voltage = 1.5 V x 3= 4.5 V

Current = 0.5 A


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Internal Resistance (IR).

The resistance present inside a battery while connected to aload.

Restricts the movement of current inside of any power source,

including battery.

As a cell is discharged, its internal resistance increases.

Therefore, its output voltage decreases for a given value of load

current.



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The voltage drop across resistor in series circuit will slightly

less than the power source, hence due to IR or of the power

source.

In some applications, the changes in cell terminal voltage are

so small that they make no practical difference.



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Thermocouple

Thermoelectric temperature sensor which consists of two

dissimilar metallic wires, coupled at the probe tip (measurement

junction) and extended to the reference (known temperature)junction.

The temperature difference between the probe tip and the

reference junction is detected by measuring the change in voltage

(electromotive force, EMF) at the reference junction.


M l i I i f A i i T h l


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The absolute temperature reading can then be obtained by

combining the information of the known reference temperature

and the difference of temperature between probe tip and the

reference.

HOT JUNCTION

COLD JUNCTION


M l i I tit t f A i ti T h l


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Metal pairs most commonly used are Chromel / Alumel and

Iron / Constantan.

This principal is used for aircraft Cylinder Head Temperature

(CHT) and Exhaust Gas Temperature (EGT) Instrumentation.


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Mala sian Instit te of A iation Technolog


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DC SOURCES OF ELECTRICITY

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Topic 5 DC Source of Electricity