EE1211 Electrical Machines B.E Question Bank : niceindia.com
Name of the College : Noorul Islam College of Engineering
University : Anna University
Degree : B.E
Department : Electronics and Communication Engineering
Subject Code/Name : EE 1211 – Electrical Machines
Document Type : Question Bank
Website : niceindia.com
Download :https://www.pdfquestion.in/uploads/ni…l_machines.pdf
NICE Electrical Machines Question Paper
Two Marks :
1. Mention the difference between core and shell type transformers? :
In core type, the windings surrounded the core considerably and in shell type the core surround the windings i.e winding is placed inside the core.
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2. What is the purpose of laminating the core in a transformer? :
To reduce the eddy current loss in the core of the transformer.
4. Does transformer draw any current when secondary is open? : Why?:
yes, it(primary) will draw the current from the main supply in order to magnetize the core and to supply for iron and copper losses on no load. There will not be any current in the secondary since secondary is open.
5. Define voltage regulation of a transformer? :
When a transformer is loaded with a constant primary voltage, the secondary voltage decreases for lagging PF load, and increases for leading PF load because of its internal resistance and leakage reactance. The change in secondary terminal voltage from no load to full load expressed as a percentage of no load or full load voltage is termed as regulation.
7. Define all day efficiency of a transformer? :
It is computed on the basis of energy consumed during a certain period, usually a day of 24 hrs. all day efficiency=output in kWh/input in kWh tor 24 hrs.
8. Why transformers are rated in kVA? :
Copper loss of a transformer depends on current & iron loss on voltage. Hence total losses depends on Volt-Ampere and not on PF. That is why the rating of transformers are in kVA and not in kW.
9. What are the typical uses of auto transformer? :
1. To give small boost to a distribution cable to correct for the voltage drop.
2. As induction motor starter.
10. What are the application of step-up & step-down transformer? :
Step-up transformers are used in generating stations. Normally the generated voltage will be either 11kV. This voltage(11kV) is stepped up to 110kV or 220kV or 400kV snd transmitted through transmission lines(simply called as sending end voltage). Step-down transformers are used in receiving stations. The voltage are stepped down to 11kV or 22kV are stepped down to 3phase 400V by means of a distribution transformer and made available at consumer premises. The transformers used at generating stations are called Power transformers.
11. How transformers are classified according to their construction? :
1. core type
2.shell type. In core type, the winding (primary and secondary) surround the core and in shell type, the core surround the winding.
12. Explain on the material used for core construction? :
The core is constructed by sheet steel laminations assembled to provide a continuous magnetic path with minimum of air gap included. The steel used is of high silicon content sometimes heat treated to produce a high permeability and a low hysteresis loss at the usual operating flux densities. The eddy current loss is minimized by laminating the core, the laminations being used from each other by light coat of core-plate vanish or by oxide layer on the surface. The thickness of lamination varies from 0.35mm for a frequency of 50Hz and 0.5mm for a frequency of 25Hz. When will a Bucholtz relay operates and give an alarm.
13.How does change in frequency affect the operation of a given transformer?
With a change in frequency, iron and copper loss, regulation,
efficiency & heating varies so the operation of transformer is highly affected.
14. What is the angle by which no-load current will lag the ideal applied voltage? :
In an ideal transformer, there are no copper & core loss i.e loss free core. The no load current is only magnetizing current therefore the no load current lags behind by angle 900. However the winding possess resistance and leakage reactance and therefore the no load current lags the applied voltage slightly less than 900.
15. List the arrangement of stepped core arrangement in a transformer? :
1. To reduce the space effectively
2. To obtain reduce length of mean turn of the winding
3. To reduce I2R loss.
Why are breathers used in transformers? :
Breathers are used to entrap the atmospheric moisture and thereby not allowing it to pass on to the transformer oil. Also to permit the oil inside the tank to expand and contract as its temperature increases and decreases.
16. What is the function of transformer oil in a transformer? :
1. It provides good insulation
2. Cooling.
17. Can the voltage regulation go –ive? : If so under what condition?:
Yes, if the load has leading PF.
18. Distinguish power transformers & distribution transformers? :
Power transformers have very high rating in the order of MVA. They are used in generating and receiving stations. Sophisticated controls are required. Voltage ranges will be very high. Distribution transformers are used in receiving side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed.
19. What is the purpose of providing Taps in transformer and where these are provided? :
In order to attain the required voltage, taps are provided, normally at high voltages side(low current).
20. What is prime mover? :
The basic source of mechanical power which drives the armature of the generator is called prime mover.