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1.

A step-down transformer has a turns ratio less than 1.

Answer» Ns/Np is known as the turns ratio. In step down transformer the number of turns in secondary winding (Ns) will be less than the number of turns in primary (Np). Therefore, the ratio will be less than 1 for step down transformer. Ex: Np = 10, Ns = 5; Turns ratio = 5/10 = 0.5.
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2.

To step 110 V ac down to 20 V ac, the turns ratio must be

Answer» Vp/Vs = Np/Ns Vs = (Ns/Np)Vp We required turns ratio i.e., (Ns/Np) = Vs/Vp = 20/110 = 0.181818182 [Note: p - primary; s - secondary] For step down transfer, the turns ratio will be less than 1. For simulation: Step-Down Transformer Circuit Simulator.
Discussion
3.

The primary winding of a transformer has 110 V ac across it. What is the secondary voltage if the turns ratio is 8?

Answer» V2/V1 = N2/N1 Turns ratio = N2/N1 = 8. V2/110 = 8 V2 = 8 x 110 V2 = 880v.
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4.

How many primary volts must be applied to a transformer with a turns ratio of 0.1 to obtain a secondary voltage of 9 V?

Answer» We know that, V2/V1 = N2/N1. Given V2 = 9v, N2/N1 = 0.1 Therefore, 9v/V1 = 0.1 V1 = 9v/0.1 = 90v. For more detail: Step Down Transformer - Circuit with Simulation.
Discussion