Understand Transformer - Basic

photo take from wikipedia.

Transformer

Transformer is an electric device used to transfer electric energy from one circuit to another  through inductively coupled conductors. The transformer basic application is step up / step down AC Voltage. Normally, the power transformer is design at operation frequency 50Hz until 60Hz.

3 basic parameter you will found on transformer. There are primary voltage, secondary voltage and VA rating. The primary voltage and secondary voltage is indicate the transformer is step up / step down transformer. 

We may found some different size transformer but have same voltage rating. what difference between those transformer? Please take note its VA parameter is difference. What is VA parameter?

The VA is maximum power of transformer. because of  transformer is an inductive device. When AC signal meet with inductance and capacitance may result in periodic reversals of the direction of energy flow.

The power will transform into real power (measure in watt) and reactive power (measure in VAr [Volt-Amp-Reactive]). As what we know, the real power and reactive power may change depend transformer's load type. In inductive circuit, the capacitor may help on improve the real power.

Note The real power is energy will drop on load and reactive power is energy will returns to the source in each cycle (AC).

The VA is total "apparent power" supplied. This is to prevent over loading a transformer. Relationship between VA, Watts and VAr:-

VA² = W² + VAr²

Example:-

The transformer rating as below:-

Primary voltage = 230V

Secondary voltage = 15V

VA rating = 3A/50VA

1. Input voltage at primary is 240Vac, how much output voltage and max current? 

    a. Secondary voltage = (Vin/Vpri) x Vsec

                                     = (240V/230V) x 15V

                                     = 15.65V

    b. Secondary current = VA / Vout

        (max)                    = 50VA / 15.65V

                                     = 3.19A 

                                     ≈ 3A (limit by current rating)

2. How much input current, when maximum output current?

    a. Primary current = VA / Vin

                                = 50VA / 240V

                                ≈ 0.208A

3. If input voltage at secondary voltage is 12Vac, how much output voltage?

    a. Primary voltage = (Vin/Vsec) x Vpri

                                = (12V/15V) x 230V

                                = 184V