WAZIPOINT Engineering Science & Technology: Transformer: Percentage Resistance, Reactance, and Impedance

Thursday, February 2, 2023

Transformer: Percentage Resistance, Reactance, and Impedance

The quantities Percentage Resistance, Reactance, and Impedance of a transformer are usually measured by the voltage drop at full-load current expressed as a percentage of the normal voltage of the winding on which calculations are made.

(i) Percentage resistance at full-load

The percentage resistance and reactance of a transformer are 2% and 4% respectively. The approximate regulation on full load at 0.8 pf lag

Percentage resistance at full-load

(ii) Percentage reactance at full-load

Percentage reactance at full-load

The reactance of the transformer is nothing but leakage reactance of the transformer. This phenomenon in transformers is known as Magnetic leakage. Voltage drops in the windings occur the due to impedance of the transformer. Impedance is the combination of resistance and leakage reactance of the transformer.

(iii) Percentage impedance at full-load

The percentage impedance of a transformer is the volt drop on full load due to the winding resistance and leakage reactance expressed as a percentage of the rated voltage. It is also the percentage of the normal terminal voltage required to circulate full-load current under short circuit conditions.

Percentage impedance at full-load

It should be noted from above that the reactance and resistances in ohm can be obtained thus :

Percentage Resistance, Reactance, and Impedance

It may be noted that percentage resistance, reactance, and impedance have the same value whether referred to as primary or secondary.

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.

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