**Transformer**

- There are mainly two kinds of losses in a transformer, namely (i) core loss and (ii) copper loss

**Core loss**

- Core loss consists of two components, hysteresis loss and eddy current loss.
- Hysteresis loss depends upon both the applied voltage and frequency.
- Eddy current loss is proportional to the square of the applied voltage and independent of frequency f.
- For any voltage (v α B
_{m }f), if f decreases B_{m}increases correspondingly and if f increases B_{m }decreases correspondingly. Hence the eddy current loss is independent of frequency f.

**Copper loss**

- When a transformer is loaded, current flows, this results copper loss occurs in both the primary and secondary winding resistances.
- Copper loss should be calculated at 75
^{0}

**Dielectric loss **

- This loss occurs in the insulating materials (such as, in the
**transformer oil**and the**solid insulation**of h.v. Transformers).

**Open circuit and short-circuit Tests**

- These two tests on a transformer, help to determine (i) the parameter of the equivalent circuit (ii) the voltage regulation and (iii) efficiency.

**Open circuit test**

- This test is carried out to determine (i) core loss at rated voltage and frequency (ii) shunt branch parameters and (iii) turn ratio of the transformer.
- The alternating source is connected to the low voltage side of the transformer.
- The high voltage side is left open circuited.
- 2% to 6% of rated current is required in this test.

**Short circuit test**

- This test is carried out to determine (i) copper loss at rated current and frequency and (ii) the equivalent resistance and equivalent leakage reactance.
- The high voltage side of the transformer is connected to alternating source.
- The low voltage side of the transformer is short-circuited.
- A primary voltage of 2 to 12% of its rated value is sufficient to circulate rated currents in both primary and secondary windings.