Transformer

Thread Starter

lemon

Joined Jan 28, 2010
125
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 :)
 

mik3

Joined Feb 4, 2008
4,843
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)
 

hgmjr

Joined Jan 28, 2005
9,027
Yeah, I was thinking potential difference but I disdain having to guess what the initials mean when there are multiple possibilities.

hgmjr
 

Thread Starter

lemon

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

Thread Starter

lemon

Joined Jan 28, 2010
125
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:confused:
 

mik3

Joined Feb 4, 2008
4,843
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:confused:
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
 

Thread Starter

lemon

Joined Jan 28, 2010
125
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.:confused:
 

Thread Starter

lemon

Joined Jan 28, 2010
125
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
 

mik3

Joined Feb 4, 2008
4,843
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
You are right, I got confused. :rolleyes:

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

Thread Starter

lemon

Joined Jan 28, 2010
125
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? :D
 

mik3

Joined Feb 4, 2008
4,843
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? :D
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.
 

Thread Starter

lemon

Joined Jan 28, 2010
125
Wow! I can't tell you how good that feels. You've been a rock with all my questions today mik3. Thank you so much.
Regards
 
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