section 17.3 transformer

8/14/2019 Section 17.3 Transformer

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Manhattan Press (H.K.) Ltd. 1

Section 17.3Section 17.3

TransformerTransformer

Section 17.3Section 17.3

TransformerTransformer

Voltage ratio and turns ratioVoltage ratio and turns ratio

Efficiency of transformerEfficiency of transformer


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change magnitude of

a.c. voltage

17.3 Transformer (SB p. 162)

Transformer

Go to

Discussion 2Discussion 2

soft-iron core woundwith two coils

transformer


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Transformer

low voltaged.c. power

supply centre-zerogalvanometer

switch

coilA coil B

transformer


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(1) at the instant

switch is closed


Transformer

magnetized


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(2) switch remains closed


Transformer

field no changeno induced current


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(3) at the instant

switch is open


Transformer

oppose thechange

Iflows inopposite direction


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Transformer

change current in a coil

induce current inanother coil

mutual induction

primary coil secondary coil

to an energy

source

soft-iron core

output

CAL

Workshop 2

Transformer
http://cal/E-SimPhy_408.exe


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Transformer

light

bulb

low voltage a.c.

power supply

transformer

secondary

coil

light

bulb

primary

coil

a.c. sets upalternating

magnetic field in

core

a.c. induced

glows

continuously


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Expt. 17D

Transformer

to a.c. powersupply

CRO

Voltage ratio and turns ratio


secondary coilprimary coil
http://movie/Expt%2017D/17D.avi


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s

p

s

p

N

N

V

V=

Vp

Np

VsNs



Thinking 2Thinking 2

voltage ratio turns ratio

primarycoil

secondarycoil


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input

voltage

(Vp)

output

voltage

(Vs)

soft-iron core

NsNp

circuit symbol

(Ns>Np)



step-up transformer

(Vs>Vp)


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input

voltageoutput

voltage

soft-iron core

Ns

Np

circuit symbol

(Ns


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Efficiencyof a transformer

Efficiency of transformer


Go to

Discussion 3Discussion 3


%100powerelectricalInput

powerelectricalOutput=

%100pp

ss=

IV

IVe


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inputpower



outputpower

ideal

transformer

(e = 100%)

ppss IVIV =

s

p

p

s

s

p

N

N

I

I

V

V

==

Forideal transformer


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transformer

input

power

output

power

input

poweroutput

power

power

loss

transformer

ideal

(e= 100%)

practical

(e< 100%)





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Methods to improve efficiency of transformer



input

voltageoutput

voltage

soft-iron core

secondary

coil

primary

coil

(1) resistance

coils heat up

useconduc

tingwiresof

smallerresis

tance


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input

voltageoutput

voltage

soft-iron core

secondary

coil

primary

coil



reduceedd

ycurrentby

laminationof

core

(2) eddycurrent


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(3) continuousmagnetization&

demagnetization

heats up coils

usesoft-ironcore


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Example 4:Example 4:In a TV set, a 220 V a.c. is stepped up to 22 000 V by a

transformer.(a) What the turns ratio is needed for the transformer?

(b) If the input power is 100 W and the efficiency of the

transformer is 95%, what is the output current?

Solution

(b) Output power = Input power Efficiency

= 100 95%

= 95 W

ByP= VI95 = 22 000 I

I= 0.004 3 A

The current is small because

the voltage is high.


(a) By

1001

22000

220

s

ps

p

s

p

s

p

=

=

=

N

N

N

N

N

N

V

V


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Class Practice 3:Class Practice 3:A transformer is used to step down a 220 V mains supply to

run an electrical appliance of 110 V, 550 W.(a) If the primary coil has 1 000 turns, what is the number of

turns of the secondary coil?

By

________ = ________

Ns = ________

Ans

wer

500

110220


s

p

s

p

N

N

V

V

=

s

0001

N


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Class Practice 3: (Cont)Class Practice 3: (Cont)(b) What is the current drawn by the appliance?

Current = __________________________

(c) It is found that the current drawn from the mains is 2.8 A.

What is the efficiency of the transformer?

Output power = _______________

Input power = _______________

Efficiency = _______________

Ans

wer

Ans

wer

Ans

wer

Ans

wer

550 W

VI= 220 2.8 = 616 W

A5110

550==

V

P


%89%100616

550=


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To section 17.4
http://e-ch17_04.ppt/http://e-ch17_04.ppt/


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Discussion 2:Discussion 2:

One day, Louis found that the 6 V battery of his

shaver was running out. Luckily, he got an

adaptor. Then the shaver was connected to a

socket through the adaptor. Louse realized that

the electricity from the socket was an a.c. outputof 220 V and wondered why the shaver did not

overload. Do you know why? Answer

The adaptor is a transformer, which changesthe voltage from 220 V a.c. to 6 V a.c. and then 6 V d.c.

Therefore the shaver can work properly.Return to

TextText



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Two resistors Pand Q are connected to an idealtransformer. What are the changes in the

readings of the ammeter and the voltmeter when

the switch Sis open?

Ans

wer



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Thinking 2 (Cont)Thinking 2 (Cont)

Return to

TextText

1. When Sis open, the current only passes throughP. The output power is decreased. Since the input

power is directly proportional to the output

power, it is decreased too. ByP=IpV

p, the

current in the primary coil is decreased. Therefore,

the reading of the ammeter decreases.


2. The reading of the voltmeter remains unchanged

because by Vp / Vs =Np /Ns, the secondary voltage

depends on the primary voltage and the turns ratio

of the transformer only.


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Discussion 3:Discussion 3:

After Sally has used an adaptor to charge a

mobile phone, the adaptor becomes very hot.

Why? What is the danger if the adaptor becomes

too hot? What should you do to avoid it?Ans

wer


The heat produced is due to the energy lost during the

process when the input electrical energy is converted

to the output electrical energy. If the adaptor becomes

too hot, it causes fire. Therefore, we should not usean adaptor to charge an electrical device for too long.

Return to

TextText


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Recharger of electrical toothbrushHave you used an electrical toothbrush as

shown? Somehow, the plastic recharger can

charge the toothbrush without a metal electricalcontact. How can the recharger charge the

toothbrush?Ans

wer


recharger

electrical

toothbrush


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Thinking 3 (Cont)Thinking 3 (Cont)

Return to

TextText

When the toothbrush is placed on the recharger, thecoil inside the toothbrush rests inside another coil in

the recharger. An alternating current passes through

the coil of the recharger to produce a variable

magnetic field. Therefore, a current is induced in the

coil of the toothbrush to charge the toothbrush.



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Can an efficient transformer step up energy?Ans

wer


input

voltage(Vp)

output

voltage(Vs)

soft-iron core

secondar

y coilprimarycoil

No. Transformer can only step up

voltage. In all cases, energy is

conserved.

Return to

TextText


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Explain why thicker wire is used to make coils intransformers. Ans

wer


VpV

s

soft-iron core

NsNp

Since the resistance of a wire of greater

cross-sectional area is smaller. It can reduce theheat loss by the coil.

Return to

TextText

section 17.3 transformer

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