EE 2201 – Electrical
Engineering
2 Mark Questions and
Answers
Unit-II TRANSFORMERS
- What is the function of a transformer?
Transformers are energy converting devices,
converting AC electrical energy with one level of voltage and current, to AC
electrical energy with another level of voltage and current.
- 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.
- What is the purpose of laminating the core in transformers?
Purpose of laminations is to reduce eddy current
loss. Laminations are made from insulated iron stampings.
- Give the emf equation of a transformer and define each term.
Emf induced in primary coil E1 = 4.44 fFmN1
volt. Emf induced in secondary coil E2 = 4.44fFmN2 volt. Where, f is
the frequency of AC input Fm is the maximum value of flux in the core N1, N2
are the number of primary and secondary turns.
- 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.
- Define voltage regulation of a transformer.
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 = (0V2-V2) x 100/0V2
- Full 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
- 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 efficiency =
output in kWh for 24 hrs /input in kWh for 24 hrs.
- Why transformers are rated in kVA ?
Copper loss of a transformer depends on current
and iron loss on voltage. Hence total losses depend on Volt- Ampere and not on
the power factor. That is why the rating of transformers is in KVA and not in
KW.
- 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. Also to avoid sledging of oil i.e. decomposition of
oil. Addition of 8 parts of water in 1000000 reduces the insulations quantity
of oil. Normally silica gel is filled in the breather having pink color. This
color will be changed to white due to continuous use, which is an indication of
bad silica gel, it is normally heated and reused.
- A 1100/400 V, 50 Hz single phase transformer has 100 turns on the secondary winding. Calculate the number of turns on its primary.
We know V1 / V2 = k = N2 / N1 Substituting
400/1100 = 100/N1 N1 = 100/400 x 1100= 275 turns.
- What are the functions of no-load current in a transformer?
No-load current produces flux and supplies iron
loss and copper loss on no-load.
- Can the voltage regulation of a transformer go to negative? If so under what condition?
Yes. If the load has leading power factor.
- What is meant by turns ratio in transformer?
Turns ratio in transformers,K is the ratio of
number of turns in the secondary winding T2 to number of turns in the primary
winding T1 K = T2/T1
- Why are cooling tubes provided in transformer tanks?
By providing cooling tubes, oil circulation and
hence heat dissipation can further be improved by providing cooling tubes in
two or all four walls of the transformer tanks .
- When will a Bucholz relay operate in a transformer?
Bucholz rely is a protective device in a
transformer. If the temperature of the coil exceeds its limit, Bucholz relay
operates and gives an alarm.
- List out general application of transformers.
a. Stepping-up
of voltage
b. Stepping-down
of voltage
c. Instrument
extension
d. Electrical
isolation
e. Impedance
matching
f. Link
between AC and DC systems.
- What is the purpose of conducting open circuit and short circuit tests in transformers?
Open circuit Test : (i) To find out the
equivalent circuit parameters R0 & X0 or no load resistance and reactance
ii) To find out the Iron loss of the transformer Short circuit Test : (i) To
find out the equivalent circuit parameters R01 & X01 or resistance and
reactance of the transformer referred to primary or secondary ii) To find out
the copper loss of the transformer By using these two tests we can find out the
efficiency and regulation of the transformer.