A transformer with turns ratio Np:Ns = 100:50. If the primary is supplied with 200 V, what is the secondary voltage? If the rated secondary current is 4 A, what is the primary current (ideal)?

• A. Vs = 200 V; Ip = 1 A
• B. Vs = 100 V; Ip = 1 A
• C. Vs = 50 V; Ip = 4 A
• D. Vs = 100 V; Ip = 2 A

Answer: (D)

The main idea is how an ideal transformer links voltages and currents through the turns ratio. For an ideal transformer, Vp/Vs = Np/Ns and Ip/Is = Ns/Np, which also means power is conserved: Vp × Ip = Vs × Is.

Here the turns ratio is Np:Ns = 100:50, which simplifies to 2:1. So the secondary voltage is scaled by the turns ratio: Vs = Vp × (Ns/Np) = 200 V × (50/100) = 100 V.

With the secondary current specified as 4 A and using the current relation Ip/Is = Ns/Np, you get Ip = Is × (Ns/Np) = 4 A × (50/100) = 2 A. This also matches power balance: 200 V × 2 A = 100 V × 4 A = 400 W.

Thus the secondary voltage is 100 V and the primary current is 2 A.