Skip to main content

OC and SC tests, efficiency and regulation

Short answer

OC test on LV side with HV open gives iron loss Pi and parameters Ro, Xo. SC test on HV side with LV shorted gives full-load copper loss Pcu and R₀₁, X₀₁. Efficiency η = x S cos φ / (x S cos φ + Pi + x² Pcu). % regulation ≈ (I₂ R₀₂ cos φ ± I₂ X₀₂ sin φ)/V₂ × 100.

Discuss how to conduct open-circuit and short-circuit tests on a single phase transformer. From the test results how the efficiency and voltage regulation of the transformer is determined?

Open-circuit (OC) and short-circuit (SC) tests give the core-loss and copper-loss data without loading the transformer.

Open-circuit test (no-load test)

        W   A
 o──[W]─[A]──+───────+ o
 AC          V     (HV winding
 supply (LV side)    open)
  • The meters (wattmeter, ammeter, voltmeter) are connected on the low-voltage side with rated voltage V0 applied; the HV side is left open.
  • Readings: V0, I0, W0. Since I0 is only 2–5 % of rated, copper loss is negligible, so W0 = iron loss Pi.
cos φ0 = W0/(V0 I0)
Iw = I0 cos φ0 ,  Iμ = I0 sin φ0
Ro = V0/Iw ,  Xo = V0/Iμ

Short-circuit test

  • The HV side is supplied through a variable-voltage source (e.g. autotransformer); the LV side is short-circuited with an ammeter.
  • The voltage is raised slowly until the rated current flows. This voltage is only 5–10 % of the rated voltage, so flux and iron loss are very small. Readings: Vsc, Isc, Wsc.
  • Wsc = full-load copper loss Pcu.
Z01 = Vsc/Isc ,  R01 = Wsc/Isc² ,  X01 = √(Z01² − R01²)

Referring to the other side uses the factor K².

Efficiency

For a fraction x of full load at power factor cos φ:

η = x S cos φ / ( x S cos φ + Pi + x² Pcu ) × 100 %

Pi from the OC test, Pcu (at rated current) from the SC test. The load at which η is maximum is x = √(Pi/Pcu).

Regulation

Regulation = (E2 − V2)/E2 × 100 %. Using R02, X02 from the SC test and the rated secondary current I2:

% regulation ≈ (I2 R02 cos φ ± I2 X02 sin φ) / V2 × 100

"+" for lagging loads, "−" for leading loads. Maximum regulation occurs at power factor cos φ = R02/Z02.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Electrical Machine chapter 2 questions →