WAZIPOINT Engineering Science & Technology: Transformer Back-to-Back or Sumpner Test

Friday, July 15, 2022

Transformer Back-to-Back or Sumpner Test

Sumner's Test or Back-to-Back Test of a Transformer

A full-load test of the large transformer is needed to determine the maximum temperature rise of a transformer. This kind of Transformer test is called a back-to-back test. The back-to-back test is also known as Sumpner's test or regenerative test of transformers.

Sumner's Test Circuit Diagram of Transformer

The circuit diagram or the connection diagram to carry out the transformer Sumpner's test also known as the transformer back-to-back test is shown below. Find herewith the following connection diagram where two transformers are connected in a parallel way to supply the AC voltage V1 and the low voltage supply V2
Watt meters  W1 and W2, Volt meter V, and Amp meter A are connected to monitor the result as below:

Transformer Back-to-back or Sumpner Test

How to perform the Back-to-back or Sumpner Test of a Transformer?

Transformer Back-to-back or Sumpner Test provides data for finding the transformer regulation, transformer efficiency, and transformer heating under load conditions.

 The condition of Sumpner or Back to Back test of a transformer is only employed when two similar transformers are available and connected in parallel.

One transformer is loaded on the other and both are connected to the supply. The power taken from the supply is necessary for supplying the losses of both transformers and the negligibly small loss in the control circuit. 

As shown in the figure, the primaries of the two transformers are connected in parallel across the same a.c. supply. With switch S open, the wattmeter W1 reads the core loss for the two transformers.

T1 and T2 are identical transformers and the secondaries of them are connected in voltage opposition.

EMF opposite each other,  EEF and EGH 

 So, both the EMFs cancel each other, as transformers are identical.

The current drawn from V1 is 2I0

where I0 is equal to the no-load current of each transformer. 

Thus input power measured by wattmeter W1 is equal to iron losses of both transformers.
i.e. iron loss per transformer Pi = W1/2.

 copper loss per transformer PCu = W2/2.

Sumpner Test Calculation procedure

  • Ammeter A1 = No-load current = 2I0
  • Voltmeter V1 = Applied rated input voltage (Primary voltage)
  • Wattmeter W1 = Core losses (iron losses) of both transformers = 2Pi
  • Ammeter A2 = Full load secondary current of both transformers = 2I2
  • Voltmeter V2 = Total voltage across the series connection of both secondary windings
  • Wattmeter W2 = Full load copper loss of both transformers = 2Pcu

Calculation of Sumpner or Back-to-back-Test

Why is Sumpner's test used?

Sumpner's test is used for large transformers, A full-load test on the large transformer is to be conducted to determine the maximum temperature rise of the transformer which is called a back-to-back test.

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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