11.2 - Transformers

A transformer is a device which can be used to transfer electrical energy from one ac circuit to another at a different voltage. The basic structure of a transformer is of two coils (solenoids) joined by an iron core. 

The two coils are designated the Primary (P) and Secondary (S) coils. The ratio of the two coils determines the output voltage. Transformers which increase the output voltage are called step up transformers. Step up transformers have a NP:NS  ratio greater than 1:1.  While those which decrease the output voltage are called step down transformers, these transformers have a NP:NS ratio of less than 1:1.

The solenoids are defined by the number of coils of each (N).

An AC power sources (V_P) is connected to the Primary Coil (N_P). This alternating voltage causes a change in the magnetic field in the Primary Solenoid.  

A plug-in transformer, like that below, supplies 9.00 V to a video game system. (a) How many turns are in its secondary coil, if its input voltage is 240 V and the primary coil has 800 turns? (b) What is its input current when its output is 1.30 A? (a) N= 30 turns (b) 0.049 A = 49 mA

A large power plant generates electricity at 12.0 kV. Its old transformer once converted the voltage to 335 kV. The secondary of this transformer is being replaced so that its output can be 750 kV for more efficient cross-country transmission on upgraded transmission lines. (a) What is the ratio of turns in the new secondary compared with the old secondary? (b) What is the ratio of new current output to old output (at 335 kV) for the same power? (a) N_new:N_old = 2.2:1 (b) I_new:I_old = 0.45:1