Why does transformer secondary voltage not equal rated voltage?

[Clara Chung]

Homework Statement

Homework Equations

Please review the question and answer of question 34.

The Attempt at a Solution

1.Why does the answer take the rated power as the power output?
2.I tried to find the current like this:
R=200^2 / 100 = 400 ohm
I at Q = 400/[2(400)] = 0.5 A
I at P = 0.5/2 = 0.25 A
3. As the approach as shown as (2) is wrong. Either V is not equal to IR in the circuit Q or the secondary voltage is not 400/2 = 200V.
4. As question 36 used a similar method as (2), V should be equal IR. So the only possibility is Vq not equal to Vp [Nq/Np].
5. However as the change of magnetic flux is the same everywhere in the iron core. By V=-NdΦ/dt.
Vp=- Np dΦ/dt...(a)
Vq=- Nq dΦ/dt...(b)
Vr=- Nr dΦ/dt...(c)
by (b)/(a)
Vq = Vp [Nq/Np]
6. So please tell me what's wrong. Thank you VERY much.

 

[cnh1995]

1.Why does the answer take the rated power as the power output?

The power rating of the load is nothing but the power consumed when operating at rated voltage. The rated voltage is 200V and secondary voltage happens to be 200V (because of the turns ratio). Hence, the load consumes the rated power.

 

[Clara Chung]

The power rating of the load is nothing but the power consumed when operating at rated voltage. The rated voltage is 200V and secondary voltage happens to be 200V (because of the turns ratio). Hence, the load consumes the rated power.

Oh I see. As you have introduce magnetomotive force before, although Vq = Vp Nq/Np ,however Np Ip = Nq Iq + Nr Ir ,therefore 2Ip = Iq + Ir, so Ip = Iq Am I right?

 

[cnh1995]

so Ip = Iq Am I right?

Yes.

As the approach as shown as (2) is wrong. Either V is not equal to IR in the circuit Q or the secondary voltage is not 400/2 = 200V.

Could you elaborate? Why do you think the secondary voltage is not 200V?