- 2073 Bhadra · 8 marks
Why Cu loss/iron loss neglected in tests
Short answer
In OC test, secondary open, primary carries no-load current (2–5% rated), so copper loss I₀² R is <0.1% of full-load value, negligible compared to iron loss. In SC test, applied voltage is 5–10% rated, flux and iron loss are ~1% rated, negligible; full-load current makes copper loss dominant.
Copper loss is assumed to be negligible in no load test and iron loss is assumed negligible in short circuit test. Explain why it is so.
Both losses are neglected because of how the test conditions make one of them very small compared with the other.
Copper loss in the no-load (OC) test
- In the OC test, rated voltage is applied to the primary with the secondary open. The secondary carries no current, so there is no secondary copper loss.
- The primary takes only the no-load current I0, which is about 2–5 % of the full-load current.
- Copper loss varies as the square of current: Pcu(no-load) = I0² R1 ≈ (0.03)² × Pcu(full load) ≈ 0.001 × full-load copper loss, which is less than 0.1 %.
- The iron loss at rated voltage is much larger than this small copper loss. So the wattmeter reading W0 is taken as entirely the iron loss.
Iron loss in the short-circuit (SC) test
- Core losses depend on the flux, which is proportional to the applied voltage: Pi ∝ Φm² ∝ V².
- In the SC test, the secondary is short-circuited, and the voltage applied to the primary only needs to circulate the rated current through the series impedance of the windings. This is only 5–10 % of the rated voltage.
- At about 5–10 % voltage, flux is about 5–10 % of normal, and the iron loss falls to about (0.1)² = 1 % or less of its rated value, which is very small.
- Also, almost the whole supply is used to drive the full-load current through the windings, so the magnetising branch carries negligible current.
- The wattmeter reading Wsc therefore equals the full-load copper loss.
| Test | Voltage | Current | Dominant loss | Neglected |
|---|---|---|---|---|
| OC | Rated | 2–5 % of rated | Iron | Copper |
| SC | 5–10 % of rated | Rated | Copper | Iron |
✓ Answer checked by Claude Opus 5.5 (high effort)
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