Losses in a Transformer


Losses in a Transformer

Transformer losses are broadly classified as no-load (core or Iron losses) and load losses (copper losses). No-load losses occur when the transformer is energized with its rated voltage at the primary winding, but the secondary winding is open circuited so that no through current or load current flows. In this case, full flux is present in the core, and only the necessary exciting current flows in the primary and secondary windings. The losses are predominately core losses due to hysteresis and eddy currents produced by the time-varying flux in the core steel. Copper losses occur when the output is connected to a load so that current flows through the transformer from input to output terminals. So, from the above explanations transformer losses are :

  1. Core losses
  2. Copper losses
  3. Interlaminer losses
  4. Stray losses
  5. Tieplate losses
  1. Core losses

These consist of hysteresis and eddy current losses and occur in the transformer core due to the alternating flux. These can be determined by open-circuit test.

Hysteresis loss, = kh f B1.6 watts/m3

Eddy current loss, = ke f2 B2 t2 watts/m3

Both hysteresis and eddy current losses depend upon :

  • maximum flux density Bm in the core
  • supply frequency f.

Since transformers are connected to constant-frequency, constant voltage supply, both f and Bm are constant. Hence, core or iron losses are practically the same at all loads.

Iron or Core losses, Pi = Hysteresis loss + Eddy current loss = Constant losses

The hysteresis loss can be minimized by using steel of high silicon content whereas eddy current loss can be reduced by using core of thin laminations.

  1. Copper Losses

These losses occur in both the primary and secondary windings due to their ohmic resistance. These can be determined by short-circuit test.

  1. Interlaminar losses

The core laminations are coated with a glass-like insulating material. This is usually very thin to keep the space factor reasonably high (>96%). Like any other material, the coating is not a perfect insulator. Thus, eddy currents driven by the bulk flux in the core can flow across the stacked laminations that comprise the core, that is, normal to their surfaces. Of course, the eddy current paths are completed within the laminations where the resistance is much lower. The coating must be a good insulator to keep these losses low relative to the normal interlaminar losses. The insulating value of the coating is determined by measuring the resistance across a stack of laminations.

The interlaminar loss should be compared with the normal loss at the same peak induction. For typical values of the parameters, this loss is generally much smaller than the normal loss and can be ignored. However, a high enough interlaminar resistance must be maintained to achieve these low losses.

  1. Stray losses

Stray losses are losses caused by stray or leakage flux.

  1. Tieplate Losses

The tieplate (also called the flitch plate) is located just outside the core in the space between the core and the innermost winding.

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