MCQs on Electrical Machines

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01․ The use of higher flux density in the transformer design
reduces the weigh per KVA
increases the weigh per KVA
has no relation with the weight of transformer
none of the above

As flux density is inversely proportional to area, weight will reduces for higher flux density.

02․ Transformer core is laminated to reduce
eddy current loss.
hysteresis loss.
both 1 and 2.
copper loss.

Transformer core is laminated to reduce eddy current loss. Each lamination will act as individual core to form a single core. This will increase the eddy resistance and reduce the eddy current. The size and volume of eddy current loops become smaller. Therefore, eddy current losses will be reduced when core is laminated.

03․ Transformer oil is used as
an insulator.
a coolant.
both A and B.
inert medium.

Small transformers less than 25 KVA are naturally air cooled and insulation type. But in large transformers, transformer oil is used. It has two functions. 1. Cooling and 2. Insulating It should contain high dielectric strength.

04․ Conservator tank is used
to take up the expansion of oil due to temperature rise.
to act as an oil storage.
for better cooling of transformer.
none of the above.

The main tank is completely filled with oil ensuring no air gap, because air has less dielectric strength than oil and sensitive to flash over, which inturn produces sludging. It again destroys dielectric strength of oil and affects the performance by settling down on the core winding, tank etc. and when transformer operates at varying load conditions, to adjust pressure variation, tank is extended around 10% of its volume as a conservator tank located on the top which is connected through vent pipe and breather.

05․ stepped cores are used in transformers in order to reduce
volume of iron.
volume of copper.
iron loss.
reluctance of core.

As the ratings are increased stepped or multi-stepped cores are preferred, which improve mechanical stability, reduce the length of mean turn and result in copper savings besides increasing efficiency and improving regulation.

06․ Which is the arrangement of windings in a core type single phase transformer?
Half lv inside and half hv outside on each core limb
Lv on one core limb and hv on the other
Sandwiched lv and hv discs on each core limb
Half hv inside and half lv outside on each core limb

In core type the winding is concentric and sectionalized. The lv and hv windings are divided into two parts generally and placed on the same limb. Lv is placed first to reduce insulation requirement and hv winding is placed over the lv winding. This design is also suitable for tappings. Due to adjacent lv and hv section any leakage flux links with other and participate in energy transfer and it is no more called as leakage flux.

07․ A shell type transformer has
high eddy current loss
reduced magnetic leakage
neglegible hysteresis loss
none of the above

The core surrounds major portion of windings. A 1-phase shell type core is a 3 legged core. Both lv and hv windings are placed in the central limb known as sandwich winding or interleaved winding. This design offer very less leakage flux. Lv winding is placed near to the yoke to reduce the insulation requirement.

08․ A 100 KVA transformer is operating at 60 Hz, 240 V supply. The same transformer is operating at 50 Hz, 200 V supply. The new rating is
100 KVA
50 KVA
83.33 KVA
123.3 KVA

09․ A transformer core is made up of a silicon steel laminations which has maximum flux density 1 Tesla. If it was cold rolled grain oriented its flux density increases to 1.6 Tesla. What is the saving in core size or area due to cold rolled grain oriented process?

In transformer flux density Bm ∝ 1/A

10․ Which of the following statements is/are correct statements?
For ideal transformer efficiency is 100%
For ideal transformer voltage regulation is 100%
both 1 and 2
none of the above

For any ideal device efficiency should be 100% and its voltage regulation should be zero means there should not be any internal losses are there and no change in secondary voltage due to load. Therefore only statement 1 is correct.