Transformers Questions on “Voltage Regulation – 2”.
1. Voltage regulation of transformer is given by _____________
a) E2-V2/V2
b) E2-V2/E2
c) V2-E2/E2
d) V2-E2/V2
Answer: b
Clarification: Voltage regulation is defined as change in the voltage or rise in voltage when transformer is load is thrown off. Thus, it is the difference of the no load voltage with the full load voltage divide by full load voltage to get % increase.
2. On which load power factor zero voltage regulation will be achieved?
a) 0
b) 1
c) Leading
d) Lagging
Answer: c
Clarification: At leading power factor the voltage regulation can be negative or zero. This can be found from this equation % regulation = εxcosθ – εrsinθ. Bu substituting the appropriate value of angle one can check this mathematically.
3. A transformer has resistance and reactance in per unit as 0.01 and 0.04 pu respectively. What will be its voltage regulation for 0.8 power factor lagging and leading?
a) 3.2% and 1.6%
b) 3.2% and -1.6%
c) 1.6% and -3.2%
d) Can’t be defined
Answer: b
Clarification: Voltage regulation for lagging power factor = (R cosθ + X sinθ) × 100, Voltage regulation for 0.8 lagging power factor = (0.01×0.8+0.04×0.6) × 100 = 3.2%. Voltage regulation for leading power factor = (R cosθ – X sinθ) × 100, Voltage regulation for 0.8 leading power factor = (0.01×0.8-0.0 4×0.6) × 100 = -1.6%.
4. At which power factor one will get maximum voltage regulation?
a) 0
b) 1
c) Leading
d) Lagging
Answer: d
Clarification: At lagging power factor the voltage regulation is given by I*(Rcosφ+ Xsinφ). Thus, at a particular condition of angle φ we will get maximum voltage regulation. While in leading power factor case we have – sign in the above formula.
5. Which is the correct phasor equation indicating the transformer voltages lagging?
a) V1 = V2 + I*(R cosφ+ X sinφ)
b) V2 = V1 + I*(R cosφ+ X sinφ)
c) V1 = V2 + I*(X cosφ+ X sinφ)
d) V1 = V2 + I*(R cosφ+ R sinφ)
Answer: a
Clarification: According to the phasor diagram drawn for lagging current, we will have positive sign in the voltage regulation formula thus, V1 = V2 + I*(R cosφ+ X sinφ) gives the correct relation, while V1 indicates the primary voltage.
6. Which is the correct phasor equation indicating the transformer voltages leading?
a) V1 = V2 + I*(R cosφ- X sinφ)
b) V2 = V1 + I*(R cosφ- X sinφ)
c) V1 = V2 + I*(X cosφ- X sinφ)
d) V1 = V2 + I*(R cosφ- R sinφ)
Answer: a
Clarification: According to the phasor diagram drawn for leading current, we will have negaitive sign in the voltage regulation formula thus, V1 = V2 + I*(R cosφ- X sinφ) gives the correct relation, while V1 indicates the primary voltage.
7. What is the correct formula to get power factor angle in leading condition?
a) tan φ= X/R
b) tan φ= R/X
c) cos φ = R/√(R2+X2)
d) cos φ= R/X
Answer: b
Clarification: For leading condition derivative of voltage regulation with respect to φ is obtained and solved for the power factor angle calculations we’ll get tan φ = R/X for leading condition, for lagging condition we’ll get tan φ= X/R.
8. What is the correct formula to get power factor angle in lagging condition?
a) sin φ= X/R
b) tan φ= R/X
c) cos φ = R/√(R2+X2)
d) cos φ= R/X
Answer: c
Clarification: For lagging condition derivative of voltage regulation with respect to φ is obtained and solved for the power factor angle calculations we’ll get tan φ = R/X for leading condition, for lagging condition we’ll get tan φ= X/R. In terms of cosine function, we’ll get cos φ = R/√(R2+X2).
9. Zero voltage regulation of a transformer is achieved at 1 pf leading.
a) True
b) False
Answer: b
Clarification: Though zero voltage regulation occurs at leading power factor condition, it is not occurring at unity power factor leading. As at unity power factor leading, cos term will be equal to 1. Hence, we’ll get some non-zero VR at unity power factor.