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Total Voltage Drop in a Transformer

(i) Approximate voltage drop. When there is no-load on the transformer, then,

V1 = E1 (approximately).

and E2 = KE1 = KV1

Also    E2 = 0V2, where 0V2 is secondary terminal voltage on no-loud

E2 = 0V2 =KV1

V2 = secondary voltage on load.

Refer Fig. 32. The procedure of finding the approximate voltage drop of the transformer as referred to secondary is given below:

  • Taking O as center, radius OC draw an arc cutting OA produced at E.

The total power voltage drop I1Z02 = AC = AE which is approximately equal to AG.

  • From B draw BF perpendicular on OA produced. Draw CG perpendicular to OE and draw BD parallel to OE.

Approximate voltage drop = AG = AF + FG = AF + BD            [FG=BD]

= I2R02 cos ɸ + I0X02 sin ɸ

This is the value of approximate voltage drop for a lagging power factor.

Figs. 33 and 34 refer to unity and leading power factor respectively.

  • The approximate voltage drop for a leading power factor becomes:

(I2R02 cos ɸ – I2X02 sin ɸ)

  • The approximate voltage drop for a transformer in general is given by :

(I2R02 cos ɸ ± I1X02 sin ɸ)                                               … (12)

  • The voltage drop as referred to primary is given by:

(I1R01 cos ɸ ± I1X02 sin ɸ

  • Percentage voltage drop in secondary