Q: What is load loss, what causes it, and how do we measure it?
A: Load loss (Pₖ) is the power dissipated in the windings due to load current and stray flux, proportional to I².
Components of load loss:
• I²R loss (~70–85% of Pₖ) - DC resistance Joule heating.
• Eddy current loss in windings (~10–20%) - skin + proximity effect.
• Stray loss in structural parts (~5–10%) - tank, clamps, core bolts.
Test procedure - Short-Circuit Impedance Test:
|
Step |
Action |
Notes |
|
1 |
Short-circuit secondary (LV side) |
Use heavy-duty shorting bars |
|
2 |
Apply reduced voltage to the primary |
Start from zero; increase to rated current |
|
3 |
Measure Vₖ, Iₖ = I_rated, Pₖ |
Vₖ is typically 5–15% of Uₙ |
|
4 |
Record Vₖ% = Vₖ / Uₙ × 100% |
Impedance voltage percentage |
|
5 |
Correct Pₖ to 75°C |
Pₖ(75) = Pₖ(T) × (235+75)/(235+T) |
Abnormal Pₖ indicates:
• > +15% of guaranteed → high-resistance connections, broken strands, wrong tap.
• Trending upward → developing loose connections, winding deformation (eddies).
• Vₖ% changed > ±10% → winding deformation (IEC limit).
Acceptance: Pₖ ≤ guaranteed + 15% at 75°C; Vₖ% within ±10% of guaranteed.