Back To Back Test Transformer


Main objective

  • The main objective of this test is to determine efficiency at any load, regulation and temperature rise of the transformer.
  •  If the high-capacity transformer is loaded for long time, temperature rise is known but it is very difficult to arrange suitable load for that transformer and also large amount of wastage of energy.



  • In back to back test, two transformers of same kVA rating must be necessary. 
  • The primaries of both transformers T1 and T2 are connected in parallel and secondary of both transformers are connected in phase opposition to each other. 
  • With switch S open, when the primaries of the both transformers are energized, wattmeter W1 indicates total iron loss for the two transformers.

       Iron loss per transformer = Wi = W1 / 2

  • When switch S is closed, wattmeter W2 and ammeter A2 do not indicate any deflection because VGH and VIJ are equal in magnitude but direction phase opposition to each other. 
  • The auto-transformer T3 injects voltage into secondary circuits till the ammeter A2 shows the full load current.
  •  In that time, wattmeter W2 indicates full load copper loss of both transformers.

     ∴ Copper loss per transformer = Wcu = W2 / 2

  • The reading of W1 does not change because the current flow through path BAFCDEB that does not include wattmeter W1
  • Obviously, the power taken from supply is twice the total losses of both transformers.

Efficiency = kVA Cos Φ / { kVA Cos Φ + Wi + Wcu } × 100%


  • kVA Cos Φ = Output power
  • Wi = Iron losses per transformer
  • Wcu = Copper loss per transformer
Temperature Rise

  • To measure temperature rise, two transformers are kept at rated load for several hours till the maximum temperature is attained. 
  • This test is also known as heat run test. The main disadvantage of this test is that two transformers of same rating must be necessary.

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