Q: What exactly is no-load loss, and what does the test procedure look like?
A: No-load loss (P₀), also called iron loss or core loss, is the power consumed when the transformer is energized at rated voltage and frequency with the secondary open-circuited.
Components of no-load loss:
• Hysteresis loss (~60–70% of P₀) - energy to reverse magnetic domains (∝ f × B¹·⁶).
• Eddy current loss (~25–35% of P₀) - circulating currents in laminations (∝ f² × B² × t²).
• Dielectric loss (~1–5%) + I²R in primary winding (negligible).
Test procedure (IEC 60076-1 / IEEE C57.12.90):
|
Step |
Action |
Notes |
|
1 |
Open-circuit secondary (LV side) |
Ensure safety - secondary may reach dangerous voltage |
|
2 |
Apply rated Uₙ at rated f to the primary (HV side) |
Waveform THD < 5% |
|
3 |
Measure V, I₀, P₀ |
Use class 0.2 power analyzer |
|
4 |
Record I₀ as % of rated current |
Typically 0.5–5% |
|
5 |
Correct to 20°C reference |
Correction is minor for P₀ |
Typical P₀ values:
• Distribution (100 kVA–2.5 MVA): 0.3–0.8% of Pₙ
• Medium power (5–50 MVA): 0.15–0.4%
• Large power (100–500 MVA): 0.08–0.2%
• Amorphous core: ~70% lower than grain-oriented steel
High P₀ indicates: core degradation, interlaminar shorts, poor joints, and over-excitation.
Acceptance: measured P₀ ≤ guaranteed + 15% (IEC 60076-1).