Open Circuit and Short Circuit Test of Transformer: A Complete Guide for AE/JE Exams

In transformer testing, Open Circuit (OC) and Short Circuit (SC) tests are crucial methods to determine parameters such as core loss, copper loss, efficiency, and voltage regulation—without actually loading the transformer. These tests are frequently asked in AE/JE Electrical exams. This blog will guide you through these tests in detail with theory, procedure, calculations, advantages, and FAQs.

📌 Why Perform OC and SC Tests?

  • To determine transformer equivalent circuit parameters.
  • To calculate core (iron) and copper losses.
  • To find efficiency and voltage regulation without full load.
  • To avoid practical loading of large transformers which is inefficient.

🔍 Open Circuit Test (No Load Test)

Purpose:

To determine core (iron) loss and magnetizing parameters (Rc and Xm).

Test Setup:

  • Conducted on the low voltage (LV) side.
  • High Voltage (HV) side is left open.
  • Instruments used: Voltmeter (V), Ammeter (A), Wattmeter (W).

Circuit Diagram:

Procedure:

  1. Connect LV side to rated voltage through voltmeter, ammeter, and wattmeter.
  2. Keep HV side open (no load).
  3. Note the values of V0, I0, and W0.

Calculations:

  • Core Loss (Wcore): W0
  • No load current (I0): Ammeter reading
  • Power factor: cosφ = W0 / (V0 × I0)
  • Magnetizing Component: Im = I0 sinφ
  • Core Loss Component: Iw = I0 cosφ

⚡ Short Circuit Test

Purpose:

To determine copper loss and equivalent impedance parameters (Req, Xeq).

Test Setup:

  • Conducted on the high voltage (HV) side.
  • Low Voltage (LV) side is shorted.
  • Instruments used: Voltmeter, Ammeter, Wattmeter.

Circuit Diagram:

Procedure:

  1. Connect HV side to variable voltage supply through voltmeter, ammeter, and wattmeter.
  2. Short LV side terminals with thick wire.
  3. Slowly increase the voltage till rated current flows.
  4. Note Vsc, Isc, Wsc.

Calculations:

  • Copper Loss (Wcu): Wsc
  • Power factor: cosφ = Wsc / (Vsc × Isc)
  • Impedance (Zeq): = Vsc / Isc
  • Resistance (Req): = Wsc / Isc2
  • Reactance (Xeq): = √(Zeq2 - Req2)

📘 Equivalent Circuit Representation

From the above values, we can draw the approximate equivalent circuit of a transformer referred to either LV or HV side.

⚙️ Efficiency & Regulation Using OC and SC Tests

Efficiency Formula:

Efficiency (%) = (Output / (Output + Losses)) × 100
Losses = Wcore + Wcu

Voltage Regulation Formula:

Regulation (%) = [(I2Req cosφ ± I2Xeq sinφ) / V2] × 100

🎯 Advantages of OC and SC Tests

  • No need to load transformer fully.
  • Less power consumption during testing.
  • Provides accurate estimation of parameters.

📚 Example Numerical Questions (AE/JE Level)

Q1. A transformer gives the following results in OC Test: V=230V, I=0.8A, W=80W. Calculate core loss and power factor.

Core Loss = W = 80W
cosφ = 80 / (230 × 0.8) = 0.435 approx.

Q2. In SC test: V=15V, I=5A, W=60W. Find Req and Zeq.

Req = 60 / (52) = 2.4 Ω
Zeq = 15 / 5 = 3 Ω
Xeq = √(32 - 2.42) = √(9 - 5.76) = 1.6 Ω

❓ FAQs – Open & Short Circuit Test

Q1: Why is the OC test performed on LV side?

Because the rated voltage can be easily applied with low power consumption and the current is measurable.

Q2: Why is the SC test performed on the HV side?

Because the HV side has higher impedance, making it safe to apply low voltage to circulate full-load current.

Q3: Are copper losses constant?

No, they vary with the square of load current. Core losses remain nearly constant.

Q4: Can these tests be done on any transformer?

Yes, they are universal tests for power and distribution transformers.

Q5: What happens if we apply full voltage during SC test?

It may cause heavy current and damage the transformer. Always apply voltage gradually until rated current flows.

📝 Conclusion

OC and SC tests provide valuable insight into transformer performance. They are essential for determining losses, efficiency, and voltage regulation, all while avoiding the need to fully load the transformer. For AE/JE aspirants, understanding the step-by-step procedure and calculations is crucial for both theory and numerical questions in the exam.

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