TRANSFORMER

A 5kVA single-phase transformer has

a turns ratio of 10:1 and is fed from a 2.5kV supply.

Neglecting losses, determine

(a) The full-load secondary current,

 (b) The minimum load resistance which can be connected to the secondary winding to give full load kVA,

 (c) The primary current at full load kVA.

answer

(a) N1/N2=10/1 and V1=2.5kV=2500V.

Since

    N1/N=V1/V2 secondary voltage  

        V2 = V1(N2/N1)= 2500(1/10)=250v

The transformer rating in volt-amperes=V2 i2 (at full load)

    i.e. 5000=250I2

Hence full-load secondary current,

   i2 = (5000/250) = 20A.

(b) Minimum value of load resistance,

            RL =(V2/V1)=(250/20)=12.5Ω

(c) (N1/N2)=(i2/i1)      from which primary current

              i1 = i2(N1/N2)= 20(1/10)=2A