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June 26, 2012

Why we are use 11KV / 22KV / 33KV / 66KV / 110KV / 230KV / 440KV this type of ratio. Why can’t we use other voltage ratio like 54KV / 99KV etc?

When an alternator generates voltage, we always use a multiple of 1.11 because for a pure sine wave the FORM FACTOR is the  ratio of rms value of voltage or current with the avg. value of voltage or current and for pure sine wave rms value of current is Imax/root '2' and avg. value is 2Imax/pie and which comes out to be 1.1;

We can't have a combination of other then a multiple of 1.11*.
So we can see all the voltages are made inevitably multiple of this value (1.1, which is the form factor of ac wave).

Also it provides us the best economic construction of step up and step down transformers.

* In the case of a Square Wave ie. a digital wave, the RMS and the average value are equal; therefore, the form factor is 1.

June 13, 2012

Under what condition is d.c. supply applied safely to the primary of a transformer?

When the primary winding of a transformer is to be connected to a d.c. supply, a high resistance is connected in series with the primary. this series resistance limits the primary current to a safe d.c. value and thus prevents the primary from burning out.

What will happen if the primary of a Transformer is connected to d.c. supply?

If the primary of a Transformer is connected to a d.c. supply, the primary will draw a steady current and hence produce constant flux. Consequently, no back e.m.f. will be produced. The primary winding will draw excessive current due to low resistance of primary. The result is that the primary will overheat and burn out or the fuses will blow. Care must be taken not to connect the primary of a Transformer across the d.c. supply. 

Source: Books of V K mehta. 

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