# Calculation On Transformer

Discussion in 'Physics & Math' started by chikis, Mar 21, 2012.

1. ### chikisRegistered Senior Member

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328
Hello everybody, am confronted with this question:
A transformer with 5500 turns in its primary is used between a 240V a.c supply and a 120V kettle. Calculate the number of turns in the secondary.

The equation Es/Ep = Ns/Np can be applied in solving the problem.
Where Es = e.m.f of secondry. Ep = e.m.f of primary, Ns = number of turns in secondary coil, Np number of turns in primary coil

Using the formular:
Es/Ep = Ns/Np
Where Np = 5500turns
Ns = ? becuase am required to calculate for it.
For Es and Ep, I don't know the value to substitute in for Es and Ep respectively. Whether is the 240V or 120V. I don't know between 240V and 120V the one that is Ep or Es respectively.
I can't go further with the calculation since am unable to substitute in the values into the equation correctly. Can sombody help me here please?

3. ### AlphaNumericFully ionizedRegistered Senior Member

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6,697
Isn't the ratios of the EMFs just the ratios of the voltages, in ideal circumstances? Isn't the volt the unit of EMF?

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10,296
It's nothing but a simple ratio. The ratio of the two voltages equals the ratio of the turns in the windings. Since the ratio of the voltages - going from primary to secondary is 2:1 (which can also be expressed as 1/2) just do the same with the windings. Your secondary will have exactly half as many turns as the primary.

7. ### el esRegistered Senior Member

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272
Yes, it is a step-down transformer.

120/240=x/5500

8. ### rcscwcRegistered Senior Member

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721
The ratio of the two voltages equals the ratio of the turns in the windings. Sure.

But conservation of energy kicks in. If you reduce the output voltage, then ameragec SHALL increase. And some devices may not be able to tackle the increased currents. So a way has to found to decrease the currents too. Nothing special, battery emilinators have been around much longer than you realise. Your mobile charger does exactly that ie reduce voltage AND amperage.

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10,296
TOTALLY wrong!!! The available amperage increases, that's all. Seems your understanding of basic electricity is very poor. I suggest you hit the books again - and pay attention this time. :bugeye:

10. ### MikemccRegistered Senior Member

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13
It's a simple 2:1 ratio, so N[sub]s[/sub] = 2750. The secondary winding will need to have a current-carrying capacity capable of supplying the current drawn by the load. The primary must be capable of supplying half that current plus the current to account for the transformer losses.

11. ### chikisRegistered Senior Member

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328
I know is a simple ratio, is just that I can't tell between 240v and 120v, the one that is primary and secondary e.m.f respectively.

Who can give me a clue about that?

12. ### el esRegistered Senior Member

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272
Es=120

Ep=240

Ns=Unknown

Np=5500

e.m.f. is electromotive force in volts.

13. ### chikisRegistered Senior Member

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328

You have done well by defining those terms. Let me be more specific these time; how do you know that 240v is for primary and 120v is for secondary? Or suppose am faced with similar problem in future, how do I know the primary and the secondary e.m.f respectively.

14. ### el esRegistered Senior Member

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272
In the OP you stated a 240 v.a.c. supply. The supply voltage will be used across the primary winding. The problem states that you will want 120 v.a.c. to be provided by the secondary winding.

15. ### chikisRegistered Senior Member

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What do mean by "OP"?

16. ### el esRegistered Senior Member

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272
Original Post, opening post, or post #1

17. ### chikisRegistered Senior Member

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328
Now I understand. You have actually made my day. Thank you.

Thank you for that explaination.

18. ### rcscwcRegistered Senior Member

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721
Battery eliminators and mobile chargets do waste a lot of elecricity. Look into the designs thetof.

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10,296
That has nothing at all to do with the post that you made which I replied to. And I'm pretty sure that most people are aware of the wasted power in those devices.

20. ### chikisRegistered Senior Member

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328
Here is the working:
E[sub]p[/sub]/E[sub]s[/sub]=N[sub]p[/sub]/N[sub]s[/sub]
=240/120=5500/N[sub]s[/sub]
:.
N[sub]s[/sub]=5500*120/240
=2750
My thanks goes to all who contributed immensly to this thread.