1. Mention the difference between core and shell type transformers.
In core type , the windings surround the core considerably and in shell type the core
surround the winding.
2. What is the purpose of laminating the core in a transformers ?
To reduce eddy current loss.
3. Give the emf equation of a transformer and define each term
Emf induced in primary coil E1 = 4.44 f_ mN1 volt
Emf induced in secondary coil E2 = 4.44f_ mN2 volt
Where f is the frequency of AC input
_ m is the maximum value of flux in the core
N1, N2 are the number of primary and secondary turns.
4. Does the 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 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 power factor 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 .
% regulation down = (0V2-V2) x 100/0V2
% regulation up = (0V2-V2) x 100/V2
6Full load copper loss in a transformer is 1600 watts. What will be the loss at half load ?
If x is the ratio of actual load to full load then copper loss = x2(full load copper loss)
Here Wc = (0.5)2 x 1600 = 400 watts
7.Define all day efficiency of a transformer .
It is the computed on the basis of energy consumed during a certain period , usually a
day of 24 hrs.
_all day = output in kWh /input in kWh for 24 hrs.
8.why transformers are rated in kVA ?
Copper loss of a transformer depends on current and iron loss on voltage . Hence total
losses depends on Volt- Ampere and not on the power factor. That is why the rating of
transformers are in kVA and not in kW.
9.What are the typical uses of auto transformer ?
(i)To give small boost to a distribution cable to correct for the voltage drop.
(ii)As induction motor starters.
10. What are the applications of a step-up and step-down transformers ?
Step-up transformers are used in generating stations. Normally the generated voltage will be either 11 kV .
This voltage(11 KV) is stepped up to 110 kV or 220 kV or 400 kV and transmitted through transmission lines. (In short it may Be called as sending end).
Step-down transformers are used in receiving stations. The voltage are again stepped down to 11 kV or 22 kV and transmitted through feeders.(In short it may be called as receiving end).
Further these 11 kV or 22kV are stepped down to 3 phase 400 V by means of a distribution transformer and made available at consumer premises.
The transformers used at generating stations and receiving stations are called power
transformers.
11. How transformers are classified according to their construction ?
Or
Mention the difference between “CORE” and “SHELL” type transformers.
Or
What are the two types of cores used ? Compare them.
Transformers are classified according to their construction as ,
(i)Core type (ii)Shell type (iii)Spiracore type.
Spirakore type is a latest transformer and is used in big transformers.
In “core” type, the windings(primary and secondary)surround the core and in “shell” type, the core surround the
windings.
12. Explain on the material used for core construction.
The core is constructed of transformer sheet steel laminations assembled to provide a
continuous magnetic path with a 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 insulated from each other by light coat of core-plate vanish or by an oxide layer on the surface .the thickness of laminations varies from 0.35 mm for a frequency of 50 Hz and 0.5 mm for a frequency of 25 Hz.
13. Distinguish between power transformer and distribution transformer .
Power transformers have very high power ratings 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 consumer side. Voltage levels will be medium. Power ranging will be small in order of kVA. Complicated controls are not needed.
14. 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 it is provided at
low voltage side.
BIG QUESTIONS
1.Explain the principle of operation of a transformer.
2.Derive the emf equation of a transformer.
3.With necessary circuit diagram explain in detail about OC and SC test on a transformer.
4.With the help of neat diagrams give the various connection in a three phase transformer.
5.Draw the equivalent circuit of a transformer.
6.What is auto transformer?derive the expression for the volume of copper saved in an auto transformer.
