Transformer:5 Important Conditions For Best Efficiency


A transformer is a simple electrical device, which uses the property of mutual induction to transform an alternating voltage from one to another of greater or smaller value.

The first constant-potential one was invented in 1885, and since then, it has become a necessity as an essential device for the transmission, distribution, and utilization of alternating current (AC).

There are different types of transformers having different designs suitable for different electronic and electric power applications. Their sizes range from Radio Frequency application having a volume less than a cubic centimeter, to huge units weighing hundreds of tons used in power grids.

shell form DBZ design transformer at 1885
Shell form DBZ design transformer at 1885, Image Credit – Zátonyi Sándor, (ifj.), DBZ trafoCC BY-SA 3.0
transformers in an electric substation, Image Credit – Allalone89Melbourne Terminal Station, marked as public domain, more details on Wikimedia Commons

They are most widely used in transmission and distribution of energy over long distance by stepping up the voltage output from the transformer so that the current is reduced and subsequently, the resistive core loss is less significant, so signal can be transferred over the distances to the substation contiguous to the consumers where the voltage is again stepped down for further use.

Basic Structure and Working of Transformer

The basic structure of a transformer generally consists of two coils wound around a soft iron core, namely primary and secondary coils. The ac input voltage is applied to primary coil and the ac output voltage is observed in the secondary side. 

As we know that an induced emf or voltage is only generated when the magnetic field flux is changing relative to the coil or circuit, hence, mutual inductance between two coils is only possible with an alternating, i.e. changing/AC voltage, and not with direct, i.e. steady/DC voltage.

working of transformer and leakage flux
Working of transformer and leakage flux
Image Credit :My self, Transformer fluxCC BY-SA 3.0

The transformers are used to transmute voltage and current levels as per the ratio of input to output coil turns. The turns in the primary and secondary coil are Np and Ns, respectively. Let Φ be the flux linked through both primary and secondary coils. Then,

Induced emf across the primary coil,  image001 2 = image002

Induced emf across the secondary coil, image003 2 = image004 2

From these equations, we can relate that  image005 3

Where the symbols have the following meanings:


Power, P = IpVp = IsVs

Relating to the previous equations, image007 4

Thus we have Vs = (image008)Vand Is = image009 2 IP

For step up: Vs > Vp so Ns>Np and Is<Ip

For step down: Vs <Vp so Ns < Np and Is > Ip

Primary and Secondary coil in a transformer

Primary and secondary winding
Image Credit: anonymous, Transformer3d colCC BY-SA 3.0

The above relation is based on some assumptions, which are as follows:

  • The same flux links both primary and secondary without any flux leakage.
  • The secondary current is small.
  • Primary resistance and current are negligible.

Hence, transformer efficiency cannot be 100%. Although a well-designed one can have an efficiency of up to 95%. For having higher efficiency the main four reasons of energy loss in it should be kept in mind.

Cause of Transformer energy loss:

  • Flux leakage: There is always some flux leakage as its almost impossible for all the flux from primary to pass to the secondary without any leak.
  • Eddy currents: The varying magnetic flux will induce eddy currents in the iron core, which may causes heating and hence energy loss. These could be minimized by using a laminated iron core.
  • Resistance in the winding: Energy is lost in the form of heat dissipation through the wires but can be minimized by the use of comparatively thick wires.
  • Hysteresis: When the magnetization of the core is repeatedly reversed by an alternating magnetic field, it results in expenditure or loss of energy by the generation of heat inside the core. This can be reduced by using materials having lower magnetic hysteresis loss.

We will be studying about Eddy currents and Magnetic Hysteresis in details in the further sections.

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