UNIT 3 - TRANSFORMERS The window space factor k

UNIT 3 - TRANSFORMERS
1. What is meant by window space factor?
The window space factor kw is defined as the ratio of copper area in window.
kw= Ac /Aw= conductor area in window / total area of window
2. What are the advantages of having circular in a transformer?
The excessive leakage fluxes produced during short circuit and over loads , develops severe
mechanical stress in the coils. on circular coils these forces are radial and there is no tendency for
the coil to change its shape but on the rectangular coils the forces are perpendicular to the
conductors and tends to deform the coil in circular.
3. What are the different losses in a transformer?
i) Core or Iron loss ii) copper loss
4. State the merits of three phase transformers over single phase transformer.
i. A three phase transformer occupies less space for same rating, compared to a bank of three
single phased transformers.
ii. It weight less
iii. It cost is less
iv. The core will be of smaller size and the material required for the core is less.
5. Distinguish between shell type and core type transformer.
Core type
Shell type
Simple in design
Comparatively complex
Reduction of leakage reactance is not easily Reduction of leakage reactance is highly
possible
possible
Low mechanical strength
6.
High mechanical strength.
What are the salient features of a distribution transformer?
i. The distribution transformers will have low iron loss and higher value of copper loss
ii. The capacity of transformers will be upto 500KVA
iii. The transformers will have plain walled tanks or provided with cooling tubes or radiators
iv. The leakage reactance and regulation will be low
7. How the distribution transformer differs from that of a power transformer?
i. The distribution transformers are designed to have low iron loss and higher copper loss,
whereas in power transformers the copper loss will be lesser than iron loss
ii. The distribution transformers are designed to have the maximum efficiency at a load much
lesser than full load, whereas the power transformers are designed to have maximum efficiency
at or near full load.
iii. In distribution transformer the leakage reactance is kept low to have better regulation, whereas
in power transformer the leakage reactance is kept high to limit the short circuit current.
8. In transformers, why low voltage winding is placed near the core.
The winding and core made of metals and so insulation have to be placed in between them. The
thickness of insulation depends on the voltage rating of the winding. In order to reduce the insulation
requirement the low voltage winding is placed near the core.
9. The area of the yoke in a transformer is taken 15 to 20% larger than that of the core. Why?
By keeping yoke area 15-20% higher the flux density in the yoke is reduced, resulting into
reduction in iron losses for yoke. The reduced core area results working flux density and needed to
increase the number of turns.
10. What you mean by stacking factor? What its usual value?
In transformers, the core is made of laminations and the laminations are insulated from each
other by a thin coating of varnish. Hence when the laminations are stacked to form the core, the actual
iron area will be less than the core area. The ratio of iron area and total core area is called stacking
factor.
The usual value is 0.9
11. What is tertiary winding?
Some three phase transformers may have a third winding called tertiary winding apart from primary
and secondary. It is also called auxiliary winding or stabilizing winding.
The tertiary winding is provided in a transformer for any one of the following reasons, i) to supply
small additional load at a different voltage ii) to give supply to phase compensating devices such as
capacitors which work at different voltages. iii) to limit short circuit current. iv) to indicate voltage in
high voltage testing transformer.
12. The voltage per turn of a 500KVA, 11KV, ∆/Y three phase transformer is 8.7V calculate the
number of turns per phase of LV and HV windings
Sol:
Phase voltage of LV winding =
= 239.6V (star connected)
Phase voltage of HV winding = 11,000V (Delta connected)
Number of turns in LV winding =
= 27.54=28turn
Number of turns in HV winding= no of turns of LV winding x phase voltage ratio
= 28 x
=1285.5=1286turns
13. What are conservator and breather?
A conservator is a small cylindrical drum fitted just above the transformer main tank. It is used to
allow the expansion and contraction of oil without contact with surrounding atmosphere. When
conservator is fitted in a transformer, the tank is fully filled with oil and the conservator is half filled
with oil.
The breather is a device fitted in transformer for breathing. In small oil cooled transformers some
air gap is provided between the oil level and tank top surface. When the oil is heated, it expands and air
is expelled out of transformer to the atmosphere through breather. When the oil is cooled, it shrinks and
air is drawn from the atmosphere into the transformer through breather. This action of transformer is
called breathing.
14. List out the advantages and disadvantages of stepped core transformer.
Advantages: i) for same area of cross section the stepped cores will have lesser diameter of
circumscribing circle than square cores. This results in reduction in length of mean turn of the winding
with consequent reduction in both cost of copper and copper loss.
Disadvantages: with large number of steps a large number of different sizes of laminations have to
be used. This results in higher labor charges for shearing and assembling different types of laminations.
15. State the various methods of cooling of power transformers.
i) oil natural-air forced ii) oil natural-water forced iii) oil forced-air natural viii) oil forced-air forced
iv) oil forced-water forced
16. How the heat dissipation is improved by the provision of cooling tubes?
The cooling tubes will improve the circulation of oil. The circulation of oil is due to more effective
pressure heads produced by columns of oil in tubes. The improvement in cooling is accounted by
taking the specific heat dissipation due to convection as 35% more than that without tubes.
17. Why is the core of a transformer laminated?
The cores of a transformer laminated in order to reduce the eddy current losses. The eddy current
losses are proportion to the square of thickness of laminations. This apparently implies that the
thickness of laminations should be extremely small in order to reduce the eddy current losses to a
minimum.
18. Write the relation between core area and weight of iron and copper for a single phase
transformer.
Ac = Tp δp + Ts δs
19. What is the cause of noise in transformer?
The cause of noise in the transformer is mainly because of magnetostriction effect and also
loosening of stampings and mechanical forces produced during working.
20. Comment on the hysteresis and eddy current loss when voltage and frequency of a
transformer is doubled.
Hysteresis Loss = Kh f Bm2
 Hysteresis loss is proportional to the supply frequency.
Eddy Current Loss = KCf2 Bm2
 Eddy Current Loss is proportional to the square of the supply frequency.
21. Explain the significance of the ratio r=Φm/AT in the design of transformer.
In transformers, the ratio of specific magnetic and electric loading is specified as r=Φm/AT; rather
than actual value of specific loadings.
22. How the leakage reactance of the transformer is reduced?
In transformers the leakage reactance is reduced by interleaving the high voltage and low
voltage winding.
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