Friday, October 22, 2010

Trial Selangor 2010 ~ paper 2 ( transformer )

Question 10 :( Selangor trial 2010)

Diagram 10.3 shows a simple step-up transformer.
Rajah 10.3 menunjukkan sebuah transformer injak naik yang ringkas.

i) Explain why step-up transformers are used in the transmission of electricity.
Terangkan mengapa transformer injak naik digunakan dalam penghantaran elektrik.

Notes :
A transformer changes the magnitude of an alternating current voltage. ( Direct current is not transformed, ad dc does not vary its magnetic fields)
A step-up transformer supplied an output voltage that higher that the input voltage.

* Penting transformer hanya meningkatkan voltan elektrik sahaja. Elakkan tulis arus elektrik ( Current).

(ii) You are required to modify the transformer in Diagram 10.3 so that it can be used as a laptop adapter with high efficiency and output of 20 V direct current.
Your suggestions and explanation should be based on the following aspects:

Anda dikehendaki mengubahsuai transformer pada Rajah 10.3 supaya ia boleh digunakan sebagai adapter komputer riba yang berkecekapan tinggi dengan mengeluarkan output arus terus 20 V.
Cadangan dan penjelasan anda haruslah berdasarkan aspek-aspek berikut:

a.
Type of the core used.
Jenis teras yang digunakan.

Note:
The alternating current flowing through the transformer continually magnetises and demagnetises the core. work has to be done to change the magnetic field in core which contributes to energy loss.
Use cores made from soft-iron can be easily easily magnetised and demagnagtised.

Materials and diameter of the wire used.
Bahan-bahan dan diameter dawai yang digunakan.

Note:
All wires will have resistance, heat is produced when current flow through them.r
The resistance will reduce as the wire is thicker. ( bigger diameter )

c.
Ratio of the number of tums in the primary coil to the secondary coil.
Nisbah bilangan lilitan gegelung primer kepada gegelung sekunder.

Note:
Ratio of the number of tums in the primary coil to the secondary coil is called turns ratio ( also known as voltage ratio)
* write the primary turns no. first.
Example => 1 to 20 is step-up transformer, whereas a 20 to 1 would be a step-down transformer.

d.
The arrangement of the primary coil and the secondary coil.
Susunan gegelung primer dan gegelung sekunder.

Note:
Electrical energy is lost when a fraction of the magnetic field produced by the primary coil does not link with the secondary coil.

e.
The number of diodes used.
Bilangan diod yang digunakan.

Note:
Two full-wave rectification methods.

Bridge rectifier--> four diode are used
and
center tap rectifier --> two diode are used

suggested answers:

i) The greater the transmission voltage, the smaller the current in the
power lines.
ii) Power loss during transmission due to resistance will be smaller.

a.
-
laminated soft iron core
~ easily magnetised or
demagnetised
~ to reduce the effect of eddy
current .

b.
-copper wire & large diameter
~
reduce heat loss due to
resistance

c.
Np : Ns
240 : 20
24 : 2
12 : 1
~ to reduce output voltage

d.
-Wind the secondary coil on
top of the primary coil
~ to reduce the leakage of
magnetic flux

e.
- 2 or 4 diodes
~to produce full-wave
rectification

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