Transformers are what allow us to transport electricity over long distances and

Question

Transformers are what allow us to transport electricity over long distances and use it in our homes/work/schools/etc… Although what we use from the wall has an RMS voltage of 120 V, electricity coming from nearby power substations is at an RMS voltage of 15,000 V! Transformers allow us to safely “step-down” the voltage to a useable value.

(a) When going from 15,000 V to 120 V, how many windings does the primary coil have if the secondary coil has 100 windings?

(b) An important part of how transformers work is that no power is lost in an idea transformer, so Pin equals Pout. If the output current for the above transformer is IRMS = 250 A, what is the current in the 15,000 V line?

Explanation / Answer

from the tranformer ratio formula

Vp/Vs = Np/Ns

15000V/ 120 V = NP/100

NP = 12500 turns

(B)

Pin = P out

Vrms I rms = V rms Irms

15000 I rms = 120 ( 250)

I rms = 2 A

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