TRANSFORMER QUESTIONS - NEED YOUR ANSWERS

1.      Prove that in case of a single phase transformer emf per turn is constant. Also draw the phasor diagram at leading, lagging and unity power factor.

2.      Develop the equivalent circuit of transformer referred to (i) primary side (ii) secondary side.

3.      Explain why the rating of a transformer is specified in terms of kVA and why do we prefer to conduct Open Circuit Test on LV side and Short Circuit Test on HV side?. What is voltage regulation?

4.      A 50 kVA, 4400/220 V transformer has R₁ = 3.45 Ω, R₂ = 0.009 Ω. The values of reactances are X₁ = 5.2 Ω and X₂ = 0.015 Ω. Calculate for the transformer equivalent resistance and reactance (i) referred to the primary side (ii) referred to the secondary side.(iii) also calculate total copper losses first using individual resistances of the two windings and then using equivalent resistances as referred to each side. (Ans.:- (i) 7.05 Ω, 11.2 Ω (ii) 0.0176 Ω, 0.028 Ω, (iii) 910.4 W

5.      Following results were obtained on a 100 kVA, 11000/220 V single phase transformer:

(i)         O.C. Test (LV side)                220 V, 45 A, 2000 W                                                

(ii)        S.C. Test (HV side)                500 V, 9.09 A, 3000 W                                 

Determine equivalent circuit parameters of the transformer referred to low voltage side.

(Ans.:- R_LV = 0.0145 Ω, X_LV = 0.0165 Ω, Z_LV = 0.175 + j0.45 = 0.483 & 68.80⁰)

6.       The following results were obtained on a 50 KVA, 2400/120 V single phase transformer                   

Short circuit test (HV side)                                         :           810 W, 20.83 A, 92 V          

Open circuit test (LV side)                                         :           396W, 9.365 A, 120 V

Calculate (i) the equivalent circuit parameters, (ii) efficiency at full load, 0.8 power factor lagging. (ii) Efficiency at half of the full load, 0.82 power factor lagging. (Ans.:- Ro=24.2, Xo=4.992, Z02=55, R02=36.3, X02=41.31, 97.07%)

7.      A 100 kVA transformer has 400 turns on the primary and 80 turns on the secondary. The primary and secondary resistances are 0.3 Ω and 0.1 Ω respectively and the corresponding reactances are 1.1 Ω and 0.035 Ω respectively. The supply voltage is 2200 V. Calculate the voltage regulation and secondary terminal voltage for full – load having a p.f. of (i) 0.8 lagging (ii) 0.8 leading. (Ans.:- (i) 3.36%, 425.2V (ii) -1.54%, 446.8V)