A transformer is an electrical device that changes (increases or decreases) the voltage of an alternating current (AC) supply without changing its frequency. The transformer ratio, also called the turns ratio or transformation ratio, is the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. This ratio decides whether the transformer increases the voltage (step-up) or decreases it (step-down), and it is equal to the ratio of the secondary voltage to the primary voltage.
The transformer ratio (turns ratio) is K = Ns/Np = Vs/Vp = Ip/Is.
Voltage is directly proportional to the number of turns.
Current is inversely proportional to the number of turns.
Step-up transformer: Ns > Np, K > 1, increases voltage.
Step-down transformer: Ns < Np, K < 1, decreases voltage.
A transformer works on the principle of mutual induction and only on AC.
In an ideal transformer, input power equals output power (Vp Ip = Vs Is).
The transformer (turns) ratio is given by:
K = Ns / Np = Vs / Vp = Ip / Is
Where: • K = transformation ratio (turns ratio) • Np = number of turns in the primary coil • Ns = number of turns in the secondary coil • Vp = voltage across the primary coil • Vs = voltage across the secondary coil • Ip = current in the primary coil • Is = current in the secondary coil
Note: The voltage is directly proportional to the number of turns (Vs/Vp = Ns/Np), but the current is inversely proportional (Ip/Is = Ns/Np). This is because, in an ideal transformer, the input power equals the output power: Vp × Ip = Vs × Is.
Depending on the turns ratio, a transformer is of two types:
Step-up transformer: • The secondary has more turns than the primary (Ns > Np), so K > 1. • It increases the voltage (Vs > Vp). • The current decreases in the secondary. • Used to raise voltage for long-distance transmission of electricity.
Step-down transformer: • The secondary has fewer turns than the primary (Ns < Np), so K < 1. • It decreases the voltage (Vs < Vp). • The current increases in the secondary. • Used to lower the voltage for homes and appliances.
Example: A transformer has 100 turns in the primary and 500 turns in the secondary, with a primary voltage of 220 V. • Turns ratio K = Ns/Np = 500/100 = 5 • Secondary voltage Vs = K × Vp = 5 × 220 = 1100 V Since Vs > Vp, this is a step-up transformer.
Key points: • A transformer works only on AC, not DC. • It works on the principle of mutual induction. • In an ideal transformer, power is conserved (Vp Ip = Vs Is). • The frequency does not change.
The transformer ratio, also called the turns ratio or transformation ratio, is the ratio of the number of turns in the secondary coil to the number of turns in the primary coil. It is given by K = Ns/Np = Vs/Vp = Ip/Is, and it determines whether the transformer is step-up or step-down.
The turns ratio is K = Ns/Np = Vs/Vp = Ip/Is, where Np and Ns are the primary and secondary turns, Vp and Vs are the primary and secondary voltages, and Ip and Is are the primary and secondary currents. The voltage is proportional to the turns, while the current is inversely proportional.
In a step-up transformer the secondary has more turns than the primary (K > 1), so it increases the voltage and decreases the current — used in power transmission. In a step-down transformer the secondary has fewer turns (K < 1), so it decreases the voltage and increases the current — used to supply lower voltage to homes and appliances.
In an ideal transformer, no power is lost, so the input power equals the output power: Vp × Ip = Vs × Is. Since the voltage increases with the number of turns, the current must decrease in the same proportion to keep the power constant. That is why the current is inversely proportional to the number of turns.
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