# Transformer

Discussion in 'Homework Help' started by lemon, Feb 8, 2010.

1. ### lemon Thread Starter Member

Jan 28, 2010
125
2
A 36Ω resistive load is connected to the secondary of a transformer. The terminal p.d. at the secondary is 24v. If the primary is connected to a 480v a.c. supply, calculate:

a) the transformer turns ratio
b) the current and power drawn by the load
c) the current drawn from the supply

Relevant equations
1: V=IR
2:Vp/Vs=Np/Ns=ip/is
3: Np:Ns
P=IV

Solution:
a) Is=Vs/R = 24/36=0.667A
P=0.667Ax24V=16W

b) Ip=current drawn from the supply
Ip=Vp/R = 480V/36Ω=13.3A

Not sure if I'm getting this ok
Can somebody please check - thank you

2. ### mik3 Senior Member

Feb 4, 2008
4,846
67
No, you are not right about the primary's current.

Use that equatio:

Np/Ns=Is/Ip or

Ip=Ps/Vp, since Pp=Ps (ideally)

3. ### hgmjr Moderator

Jan 28, 2005
9,030
215
What do you mean by p.d.?

hgmjr

4. ### mik3 Senior Member

Feb 4, 2008
4,846
67
I guess potential drop even it is not a correct term for this case.

5. ### hgmjr Moderator

Jan 28, 2005
9,030
215
Yeah, I was thinking potential difference but I disdain having to guess what the initials mean when there are multiple possibilities.

hgmjr

6. ### mik3 Senior Member

Feb 4, 2008
4,846
67
Let's hope the OP will let a drop of lemon on p.d and make its taste better.

7. ### lemon Thread Starter Member

Jan 28, 2010
125
2
sorry. Didn't think that would cause confusion.
I meant potential difference.
Should I define things before I use abbreviations?

8. ### mik3 Senior Member

Feb 4, 2008
4,846
67
It should be better unless they are frequenctly used abbreviations.

9. ### lemon Thread Starter Member

Jan 28, 2010
125
2
Is=24v/36Ω=0.67A

Ip=Vp/Vs χ Is=0.67A χ 480v/24v = 13.3A

How can I calculate the number of turns if I don't know either number of turns on one or two?

Np/Ns=Is/Ip so Ns=Is/Ip x Np
but no Np

10. ### mik3 Senior Member

Feb 4, 2008
4,846
67
You do not need to know the actual number of turns but their ration which is Vp/Vs.

The input and output power is the same ideally.

Thus,

Pin=Pout

Vp*Ip=Vs*Is

therefore,

Ip=Vs*Is/Vp

11. ### lemon Thread Starter Member

Jan 28, 2010
125
2
Hi mik3:

So the transformer turns ratio is 20:1

for question b) I want the current and power drawn by the load. So this is Is and Ps.
Is =Vp*Ip/Vs
But I don't have the primary current and not sure how to get it from a circuit like this. I would attempt V=IR but there is no primary resistor.

12. ### mik3 Senior Member

Feb 4, 2008
4,846
67
Np/Ns=100/200=0.5

Vs=Vp*Ns/Np=6*200/100=12V

Is=Ps/Vs=24/12=2A

Pp=Ps=Vp*Ip

Thus, Ip=Pp/Vp=24/6=4A

13. ### lemon Thread Starter Member

Jan 28, 2010
125
2
Where did the values for the number of turns come from?
100 and 200?

14. ### lemon Thread Starter Member

Jan 28, 2010
125
2
I think you may have taken those values by mistake from one of the other questions you are currently helping me with mik3. My other question on transformers has 100 and 200 turns

15. ### mik3 Senior Member

Feb 4, 2008
4,846
67
You are right, I got confused.

The current drawn by the load is Is=Vs/R

16. ### lemon Thread Starter Member

Jan 28, 2010
125
2
So summary is:

a)Vp/Vs=Np/Ns=480v/24v=20:1

b) Is=Vs/R = 24v/36Ω=0.67A
P=IV, so 0.67A x 24v = 16.08W

c) Ip=Vs*Is/Vp = 24v*0.67A/480v=0.0335A

How's that?

17. ### mik3 Senior Member

Feb 4, 2008
4,846
67
The answer the is correct because the input power (primary) equals the output power (secondary) and thus Ip has to be less than Is because Vp is greater than Vs.

18. ### lemon Thread Starter Member

Jan 28, 2010
125
2
The answer the is correct because ???
Does this mean the answers to the above questions are correct?

Feb 4, 2008
4,846
67