1. What is the ratio of the net core area to the area of the circumscribing circle in square cores?
a) 0.58
b) 0.64
c) 0.70
d) 0.80
Explanation: 0.64 is the ratio of the gross core area to the area of the circumscribing circle. Net core area is the product of stacking factor and gross iron area.
2. The laminations are manufactured in standard size to minimize the wastage of steel during punching of laminations.
a) True
b) False
Explanation: The laminations are manufactured in the standard size of width, 0.75m to 1 m. This is used to avoid excessively wide assortment of laminations and to minimize wastage of steel during punching of laminations.
3. What is the value of ratio of gross core area to the area of circumscribing circle in stepped cores?
a) 0.71
b) 0.79
c) 0.89
d) 0.91
Explanation: 0.71 is the ratio of net core area to the area of circumscribing circle in stepped cores. The gross core area for stepped cores is 0.618 * d2.
4. What is the net core area for three stepped transformers?
a) 0.45
b) 0.56
c) 0.6
d) 0.62
Explanation: 0.45 is the net core area for the square core transformers. 0.56 is the core area for cruciform or stepped core transformers.
5. What is the relationship between the number of steps to the area of circumscribing circle?
a) number of steps is directly proportional to the area of the circumscribing circle
b) number of steps is indirectly proportional to the area of the circumscribing circle
c) number of steps is directly proportional to square of the area of the circumscribing circle
d) number of steps is indirectly proportional to square of the area of the circumscribing circle
Explanation: As the number of steps increase, the area of the circumscribing circle also increases. But as the area of the circumscribing circle increases, the ratio of the net core area and gross core area to the area of circumscribing circle decreases.
6. What is the formula for the number of turns in primary winding?
a) number of turns of primary winding = Voltage of primary windings * voltage per turn
b) number of turns of primary winding = Voltage of primary windings/voltage per turn
c) number of turns of primary winding = Voltage of secondary windings * voltage per turn
d) number of turns of primary winding = Voltage of secondary windings/voltage per turn
Explanation: For calculating the number of turns of primary windings first we calculate the voltage across the primary windings. Then the voltage per turn is calculated and the ratio gives the number of turns.
7. What is the formula for obtaining the current in the primary winding?
a) current in primary winding = kVA per turn * 103 * primary voltage
b) current in primary winding = kVA per phase * 103 * primary voltage
c) current in primary winding = kVA per turn * 103 / primary voltage
d) current in primary winding = kVA per phase * 103 / primary voltage
Explanation: For obtaining the current in primary winding, the kVA output per phase is obtained. Then the primary voltage is calculated, and the ratio of both gives the current in primary windings.
8. What does the area of conductors in primary and secondary windings depend on?
a) current
b) voltage
c) power
d) current density
Explanation: The area of the conductors is directly dependent on the current density. The area of the conductors are determined after choosing a suitable current density.
9. What does the permissible current density depend upon?
a) local heating
b) efficiency
c) output power
d) local heating and efficiency
Explanation: The permissible current density depends upon the local heating as the heating should not affect the output. It also depends on the efficiency of the transformer
10. What is the relationship between temperature and the current density?
a) current density is directly proportional to the temperature
b) current density is directly proportional to the square of the temperature
c) current density is indirectly proportional to the square of the temperature
d) current density is indirectly proportional to the temperature
Explanation: As the current density increases, the temperature also increases. As the temperature increases, it can cause damage to the insulation.