Q-1 Operation of a 50Hz transformer on a 25 Hz supply. Other things remaining the same?
Ans:
1. If the Transformer is operated with the frequency and voltage changed in the same proportion, the flux density will remain unchanged, and apparently, the no-load current will also remain unaffected.
2. The transformer can be operated safely at a frequency less than the rated one with correspondingly reduced voltage.
3. In this case iron losses will be reduced. But if the electrical transformer is operated with more voltage and frequency in the same ratio, the core losses may increase to an very high level.
4. Increase in frequency with constant supply voltage will cause a reduction in hysteresis loss and leave eddy current losses unaffected.
5. Some increase in voltage could, therefore, be tolerated at higher frequencies, but exactly how much depends on the relative magnitude of the hysteresis eddy current losses and the grade of iron used in the transformer core.
Q-2 Why do copper losses occur in a DC machine ?
Ans: Copper losses occur in a dc machine due to power consumed in forcing a current against the resistance od winding and connections.
Q-3 What is meant by GTO?
Ans: Gate turn off switch is a special thyristor that can be turned on by a positive gate signal and can be turned off by a negative gate signal.
Q-4 Why all-day efficiency of a transformer is normally carried on a low voltage side?
Ans:
1. There are certain types of transformers whose performance can not be judged by ordinary or comercial efficiency.
2. For instance, distribution transformer are energized for 24 hours, but they deliver very light loads for major portion of the day.
3. Thus iron or core loss occur for the whole day but the copper loss occurs only when the transformer is loaded.
4. The performance of such transformer must be judged by its all day efficiency, also called energy efficiency or operational effieciency which is computed on the basis of energy consumed during the whole days.
5. The all day edfficiency is defined as the ratio of energy(kwh) output over 24 hours to the energy input over the same period.
All day efficiency = output of machine in kwh / input of machine in kwh
6. Since the distribution transformer does not supply the rated laod for the whole day so the all day efficiency of such a transformer will be less than ordinary or commercial efficiency.
7. For determination of all day efficiency of a transformer, it is necessary, of course, to know the load varies from hour to hour during the day
Q-5. What role does the grid play in the grid controlled mercury-arc rectifiers ?
Ans: The fucntion of grid control system is to permit the establishment of a current arc on the anode at some pre-determined instant in the anode voltage cycle by applying positive potential to the grid and to prevent or delay the re-establishment of an arc at some other instant by applying a negative potential to it.
Q-6 What is an interphase transformer ?
Ans: Interphase trasformer is like a centre tapped smoothing choke with a closed iron core. it provides not only a cansiderable smoothing effect but also better and more economical loading of the transformer secondary windings.