Transformer Open Circuit and Short Circuit Test

Open Circuit Or No Load Test and Short Circuit Or Impedance Test On Transformer can carry out economically without loading a Transformer.

Open Circuit Test And Short Circuit Test on Transformer are performed to find the parameters of the equivalent circuit of the transformer and losses of the transformer. These two tests are conducted to determine the core loss, copper loss, and equivalent circuit parameters of a transformer.

The performance of a transformer can be calculated on the basis of a Transformer  Equivalent Circuit which contains four main parameters:

1. the equivalent resistance R₀₁ as referred to primary (or secondary R02); 
2. the equivalent leakage reactance X01 as referred to primary (or secondary X02);
3. the core-loss conductance G0 (or resistance R0); 
4. the magnetising susceptance B0 (or reactance X0). 

These constants or parameters can be easily determined by two tests:

1. open-circuit test and 
2. short-circuit test. 

These tests are very economical and convenient because they furnish the required information without actually loading the transformer. 

In fact, the testing of very large a.c. the machinery consists of running two tests similar to the open-circuit and short-circuit tests of a transformer.

Open Circuit Or No Load Test On Transformer

Open circuit test is performed in a transformer to find the resistance of core and shell France we apply rated voltage in open circuit test because to find maximum admittance or resistance of core and excitation branch.

The two components of no-load current can be given as,

I_μ = I₀sinΦ₀   and    I_w = I₀cosΦ₀.
cosΦ₀ (no load power factor) = W / (V₁I₀). ... (W = wattmeter reading)

From this, shunt parameters of an equivalent circuit of the transformer (X₀ and R₀) can be calculated as

X₀ = V₁/I_μ  and  R₀ = V₁/I_w.

(These values are referring to the LV side of the transformer.)
Hence, it is seen that the open-circuit test gives core losses of transformer and shunt parameters of the equivalent circuit.

Short Circuit Or Impedance Test On Transformer

we apply a short circuit test to find the series impedance of a transformer by adjusting primary voltage until in secondary winding current equal to rated current.

Sense input voltage is low current flowing through excitation branch is negligible and all voltage will drop in a transformer due to series element in a circuit.

The ammeter reading gives the primary equivalent of full load current (I_sc).

The voltage applied for full load current is very small as compared to rated voltage. Hence, core loss due to small applied voltage can be neglected. Thus, the wattmeter reading can be taken as copper loss in the transformer.

Therefore, W = I_sc²R_eq....... (where R_eq is the equivalent resistance of the transformer)
 Z_eq = V_sc/I_sc.

Therefore, the equivalent reactance of the transformer can be calculated from the formula  Z_eq² = R_eq² + X_eq².

These values are referredas  to the HV side of the transformer.
Hence, it is seen that the short circuit test gives copper losses of the transformer and approximate equivalent resistance and reactance of the transformer.

You may know the details about the electrical transformer from the following articles:

 

1. Working Principle of Transformer;
2. Transformer Construction;
3. Core-type Transformers;
4. Shell-type Transformers;
5. Elementary Theory of an Ideal Transformer;
6. E.M.F. Equation of Transformer;
7. Voltage Transformation Ratio;
8. Transformer with losses but no Magnetic Leakage;
9. Transformer on No-load;
10. Transformer on Load;
11. Transformer with Winding Resistance but no Magnetic Leakage;
12. Equivalent Resistance;
13. Magnetic Leakage;
14. Transformer with Resistance and Leakage Reactance;
15. Simplified Diagram;
16. Total Approximate Voltage Drop in Transformer;
17. Exact Voltage Drop;
18. Equivalent Circuit Transformer Tests;
19. Open-circuit or No-load Test;
20. Separation of Core Losses;
21. Short-Circuit or Impedance Test;
22. Why Transformer Rating in KVA?;
23. Regulation of a Transformer;
24. Percentage Resistance, Reactance, and Impedance;
25. Kapp Regulation Diagram;
26. Sumpner or Back-to-back-Test;
    
27. The efficiency of a Transformer;
28. Condition for Maximum Efficiency;
29. Variation of Efficiency with Power Factor;
30. All-day Efficiency;
31. Auto-transformer;
32. Conversion of 2-Winding Transformer into Auto-transformer;
33. Parallel Operation of Single-phase Transformers;
34. Questions and Answers on Transformers;
35. Three-phase Transformers;
36. Three-phase Transformer Connections;
37. Star/Star or Y/Y Connection;
38. Delta-Delta or ∆/∆ Connection;
39. Wye/Delta or Y/ Connection;
40. Delta/Wye or ∆/Y Connection;
41. Open-Delta or V-V Connection;
42. Power Supplied by V-V Bank;
43. Scott Connection or T-T Connection;
44. Three-phase to Two-Phase Conversion and vice-versa;
45. Parallel Operation of 3-phase Transformers;
46. Instrument Transformers;
47. Current Transformers;
48. Potential or Voltage Transformers.