Deck 3: Transformer Design and Electromagnets

Question 1

The design of electromagnets is based upon which of the following fundamental equations ?

A)Force equation
B)Magnetic circuit equation
C)Heating equation
D)All of the above

Answer 1

All of the above

Question 2

When the two coil sides forming a coil are spaced exactly one pole pitch apart they are said to be of

A)short pitch
B)full pitch
C)either of the above
D)none of the above

Answer 2

full pitch

Question 3

are always double layer type.

A)Closed windings
B)Open windings
C)Either of the above
D)None of the above

Answer 3

Closed windings

Question 4

The distance between the starts of two consecutive coils measured in terms of coil sides is called

A)front pitch
B)winding pitch
C)commutator pitch
D)back pitch

Question 5

The winding where dummy coils are used is sometimes called

A)duplex winding
B)triplex winding
C)forced winding
D)none of the above

Question 6

Dummy coil should not be used in

A)small machines
B)large machines
C)either (a) or (b)
D)none of the above

Question 7

Power transformers have rating

A)equal to 50 kVA
B)equal to 100 kVA
C)above 200 kVA
D)none of the above

Question 8

Power transformers should be designed to have maximum efficiency

A)at one-fourth load
B)at one-half load
C)at or near full load
D)any of the above

Question 9

In transformers using hot rolled steel, the cross-section of the yoke is made about _____ greater than that of the core

A)5 percent
B)10 percent
C)15 percent
D)30 percent

Question 10

Yokes with rectangular cross-section are used for

A)small capacity transformers
B)medium capacity transformers
C)large capacity transformers
D)any of the above

Question 11

The cold rolled grain oriented steel has ______ permeability in the direction of the grain orientation.

A)minimum
B)maximum
C)nil
D)none of the above

Question 12

Cylindrical windings using circular conductors, employed in transformers, are

A)single layered
B)double layered
C)multi-layered
D)none of the above

Question 13

Helical windings are used in

A)distribution transformers
B)power transformers
C)shell type transformers
D)none of the above

Question 14

Multi-layer helical windings are commonly used in the transformers as high voltage windings

A)upto 20 kV
B)upto 50 kV
C)upto 80 kV
D)for 110 kV and above

Question 15

Disc windings are primarily used in

A)short capacity transformers
B)medium capacity transformers
C)high capacity transformers
D)any of the above

Question 16

The heat dissipating capability of transformers of ratings higher than 30 kVA in increased by providing which of the following ?

A)Corrugations
B)Fins
C)Tubes
D)All of the above

Question 17

Transformers with a capacity of upto _____ have a cooling radiator system with natural cooling

A)2 MVA
B)5 MVA
C)7.5 MVA
D)10 MVA

Question 18

The forced oil and air circulation method is usual one for transformers of capacities

A)upto 5 MVA
B)upto 10 MVA
C)upto 20 MVA
D)30 MVA upwards

Question 19

The flash point of transformer oil should be higher than

A)40°C
B)60°C
C)80°C
D)104°C

Question 20

The voltage control in electric supply networks in required on account of which of the following reasons ?

A)Adjustment of voltage at consumers premises within statutory limits
B)Control of active and reactive power
C)Adjustment of short period daily and seasonal voltage variations in accordance with variations of load
D)All of the above

Question 21

D.C. windings are

A)sometimes 2-layer type
B)never 2-layer type
C)always 2-layer type
D)none of the above

Question 22

The usual values of maximum flux densities for distribution transformers using hot rolled silicon steel are

A)0.5 to 0.8 Wb/m²
B)0.8 to 1.0 Wb/m²
C)1.1 to 1.35 Wb/m²
D)1.4 to 1.8 Wb/m²

Question 23

For 275 kV transformers, using cold rolled grain oriented steel, which of the following values of flux density may be used?

A)1.0 Wb/m²
B)1.1 Wb/m²
C)1.3 Wb/m²
D)1.6 Wb/m²

Question 24

For large power transformers, self oil cooled type or air blast type which of the following values of current density may be used ?

A)1.0 to 1.2 A/mm²
B)1.5 to 2.0 A/mm²
C)2.2 to 3.2 A/mm²
D)3.2 to 4.2 A/mm²

Question 25

A current density of _____ is ilsed for large power transformers with forced circulation of oil or with water cooling coils

A)1.5 to 2.5 A/mm²
B)3.5 to 4.5 A/mm²
C)4.0 to 5.0 A/mm²
D)5.4 to 6.2 A/mm²