Transformers - Definition & Terms

Primary winding:        The winding where incoming power supply is connected. Usually this refers to High Voltage side in distribution transformers

Secondary winding:    The winding where the principal load is connected. Usually this refers to Low Voltage side in Distribution transformers.

No load loss:               The losses taking place in a transformer when only primary winding is energized and all secondary windings are open. They represent constant losses in a transformer.

Dielectric loss:            The losses taking place in a stressed dielectric medium (insulation) subjected to stress reversals.

Iron losses:                 The losses taking place in the magnetic core. There are two types; hysterisis losses and eddy current losses.

Hysteresis losses:      This loss depends upon the area of the hysteresis loop, which is depending upon the maximum flux density, the type of material and frequency. It is independent of the waveform

Eddy current losses in core:

                                    This is loss due to circulating currents induced by voltage in the thickness of core laminations. It depends upon thickness of lamination, path resistance which is depended upon the type of material, R.M.S. flux density i.e. waveform and square of frequency

Eddy losses in a conductor:

For a thick conductor, the induced voltage within the conductor cross section due to self linkage and due to current in other conductor varies. The difference in induced voltage in the local path in the thickness of the conductor causes extra eddy current loss. This loss varies with square of current and square of frequency.

Stray losses:               All current dependant losses in a winding other than the basic I2R losses. Stray losses include eddy loss in the conductor, eddy losses in structural paths in close

proximity to outgoing conductor and the eddy loss in general in the structural parts. In dry type transformers, the last two mentioned types of stray losses are absent.

Form factor:                It is the ratio of the r.m.s. value of a waveform to the average value over one half cycle. For a sine wave the value of form factor is 1.11. For distorted waves with higher peak values, the form factor is higher.

Harmonics:                 Frequencies other than the main fundamental frequency of current or voltage which are present in a distorted wave as multiples of base fundamental frequency.

  

Transformer Polarity:

This refers to the relative direction of the induced voltages between the high voltage terminals and the low voltage terminals. During the AC half cycle when the applied voltage (or current in the case of a current transformer) is from H1 to H2 the secondary induced voltage direction will be from X1 to X2. In practice, Polarity refers to the way the leads are brought out of the transformer.

Burden:                       The load on an instrument transformer is referred to as a “burden”.

Short circuit impedance & Impedance voltage:

The impedance voltage of a transformer is the voltage required to circulate rated current through one of the two specified windings; when the other winding is short circuited with the winding connected as for rated operation. The short circuit impedance is the ratio of voltage and current under above conditions. The resistive component of short circuit impedance, gives a parameter for estimating load losses. These losses include eddy current losses in the conductors and structure as a small portion. Their contribution is materially enhanced due to harmonic currents in load. Exact determination by test is difficult and simplified test at low current suffers from the disadvantage of a high multiplying factor; but it is expected to give representative values.