Electrical Machines- Transformers

This are some IAS previous year  questions.......

1. When a short-circuit test is conducted on a single-phase transformer, 30% of the rated voltage is required to allow full-load current The short-circuit power factor is found to be 0.2. The percentage regulation at UPF is [IAS-1993]
a.57.7% b. 100% c. 50% d.75%

2. transformer designed for operation on 60 Hz supply is worked on 50 Hz supply system without changing its voltage and current ratings. When compared with full load efficiency at 60 Hz,the transformer efficiency on full load at 50 Hz would be [IAS-1994]
a. increase marginally
b. Increases by a factor 1.2
c. Remains unaltered
d. Decreases marginally

3. The series equivalent impedance of a single phase transformer referred to its secondary side is Z= R+jX, The voltage regulation of this transformer will be zero when the power factor of the load is [IAS-1995]
a. Unity                   b. [Z/X] leading c.[Z/X] lagging         d. [R/Z] leading

4.If the height to width ratio of the window of core type transformer is increased, then[IAS-1997]
a. its leakage reactance and percentage voltage regulation will decrease
b. its leakage reactance and percentage voltage regulation will increase
c.its leakage reactance will be unaffected but total space in the window will increase to accommodate the windings better d.efficiency will increase and temperature rise will decrease

5. Consider the following statements :
The use of Delta-connected tertiary windings in star-star connected power transformers
1.makes available supply for single-phase loads.
2.suppresses harmonic voltages. 3.allows flow of earth fault current for operation of protective devices 4.provides low-reactance paths for zero- sequence currents. Of the statements : [IAS-1998]
a. 1,2 and 3 correct
b. 1,3 and 4 correct
c. 2 and 4 correct
d. All are correct

6. A six -pulse non-reversible thyritorised 4I5 V ac converter Is controlling a 440 V dc motor. If total drop In the circuit Is limited to 10%, then the converter must be locked at an angle of [IAS-1999]
a. 30 b. 60 c. 45 d. 90

7. Two transformers X and Y with identical ratings and dimensions have 0.8 mm and 1.2 mm thick laminations respectively. If Rm and Xm are the magnetizing branch parameters in the equivalent circuit, then [IAS-2000]
a. Rm values in both are likely to be equal, but Xm of X is likely to be higher than of Xm of y.
b. Xm values in both are likely to be equal, but Rm of X is likely to be higher than of Rm of y.
c. Xm values in both are likely to be equal, but Rm of X is likely to be lesser than of Rm of y.
d. Rm values in both are likely to be equal, but Xm of X is likely to be lesser than of Xm of y.

8. Three units of 1 : 5 transformers are connected in to supply a 3-phase load from a 400 V 3-phase source. The line voltage on the load side is [IAS-2001]
a.1000  b. 80.    c. 3464    d. 803

9. Consider the following statements concerning the ferromagnetic materials :
1. There are easy and hard magnetization directions in the ferromagnetic Crystal.
2. Eddy current losses are smaller for materials having low electrical conductivity values.
3. All ferromagnetic materials can be used at high frequencies.
Which of the statements given above are correct ? [IAS-2004]
a. 1,2&3 b. 1&3 c. 2&3 d. 1&2

10. a transformer has a reactance of 0.1 p.u. and negligible resistance. What is its voltage regulation at full load, and 0.8 power factor lagging ? [IAS-2006]
a. 8%       b.6%     c.- 6%       d. -8%

11. An open circuit test was done on a transformer with rated voltage at rated frequency. If the open circuit test is now done on the transformer with double the rated voltage with double the rated frequency, then which one of the following is correct ? [IAS-2008]
a. Both current and power factor increase
b.  Both current and power factor decrease
c. Current decreases but power factor increases
d. Current increases but power factor decreases

12. The main purpose of the conservator in a (Driving system) transformer is to
a. store extra oil to compensate for the loss of oil due to leakage
b. achieve better cooling of the transformer
c. take up the expansion of oil due to heating
d. have the bucholtz relay fitted