Deck 4: Transformers: Efficiency, Design, and Testing

Question 1

Power transformers are generally designed to have maximum efficiency around

A)no-load
B)half-load
C)near full-load
D)10% overload

Answer 1

near full-load

Question 2

Which of the following is the main advantage of an auto-transformer over a two winding transformer ?

A)Hysteresis losses are reduced
B)Saving in winding material
C)Copper losses are negligible
D)Eddy losses are totally eliminated

Answer 2

Saving in winding material

Question 3

During short circuit test iron losses are negligible because

A)the current on secondary side is negligible
B)the voltage on secondary side does not vary
C)the voltage applied on primary side is low
D)full-load current is not supplied to thetr

Answer 3

the current on secondary side is negligible

Question 4

Two transformers are connected in parallel. These transformers do not have equal percentage impedance. This is likely to result in

A)short-circuiting of the secondaries
B)power factor of one of the trans¬formers is leading while that of the otherlagging
C)transformers having higher copper losses will have negligible core losses
D)loading of the tr

Question 5

The changes in volume of transformer cooling oil due to variation of atmospheric temperature during day and night is taken care of by which part of transformer

A)Conservator
B)Breather
C)Bushings
D)Buchholz relay

Question 6

An ideal transformer is one which has

A)no losses and magnetic leakage
B)interleaved primary and secondary windings
C)a common core for its primary and secondary windings
D)core of stainless steel and winding of pure copper metal

Question 7

When a given transformer is run at its rated voltage but reduced frequency, its

A)flux density remains unaffected
B)iron losses are reduced
C)core flux density is reduced
D)core flux density is increased

Question 8

In an actual transformer the iron loss remains practically constant from noload to fullload because

A)value of transformation ratio remains constant
B)permeability of transformer core remains constant
C)core flux remains practically constant
D)primary voltage remains constant

Question 9

An ideal transformer will have maximum efficiency at a load such that

A)copper loss = iron loss
B)copper loss < iron loss
C)copper loss > iron loss
D)none of the above

Question 10

If the supply frequency to the transformer is increased,"the iron loss will

A)not change
B)decrease
C)increase
D)All of the above

Question 11

Negative voltage regulation is indicative that the load is

A)capacitive only
B)inductive only
C)inductive or resistive
D)none of the above

Question 12

Iron loss of a transformer can be measured by

A)low power factor wattmeter
B)unity power factor wattmeter
C)frequency meter
D)any type of wattmeter

Question 13

When secondary of a current transformer is open-circuited its iron core will be

A)hot because of heavy iron losses taking place in it due to high flux density
B)hot because primary will carry heavy current
C)cool as there is no secondary current
D)none of above will happen

Question 14

The transformer laminations are insulated from each other by

A)mica strip
B)thin coat of varnish
C)paper
D)All of the above

Question 15

Which type of winding is used in 3phase shell-type transformer ?

A)Circular type
B)Sandwich type
C)Cylindrical type
D)Rectangular type

Question 16

During open circuit test of a transformer

A)primary is supplied rated voltage
B)primary is supplied full-load current
C)primary is supplied current at reduced voltage
D)primary is supplied rated kVA

Question 17

Open circuit test on transformers is conducted to determine

A)hysteresis losses
B)copper losses
C)core losses
D)eddy current losses

Question 18

Short circuit test on transformers is conducted to determine

A)hysteresis losses
B)copper losses
C)core losses
D)eddy current losses

Question 19

For the parallel operation of single phase transformers it is necessary that they should have

A)same efficiency
B)same polarity
C)same kVA rating
D)same number of turns on the secondary side.

Question 20

The transformer oil should have volatility and viscosity.

A)low,low
B)high,high
C)low,high
D)high,low

Question 21

The function of breather in a transformer is

A)to provide oxygen inside the tank
B)to cool the coils during reduced load
C)to cool the transformer oil
D)to arrest flow of moisture when outside air enters thetr

Question 22

The secondary winding of which of the following transformers is always kept closed?

A)Step-up transformer
B)Step-down transformer
C)Potential transformer
D)Current tr

Question 23

The size of a transformer core will depend on

A)frequency
B)area of the core
C)flux density of the core material
D)(a) and (b) both

Question 24

N atural air coo ling is generally restricted for transformers up to

A)1.5 MVA
B)5 MVA
C)15 MVA
D)50 MVA

Question 25

A shell-type transformer has

A)high eddy current losses
B)reduced magnetic leakage
C)negligibly hysteresis losses
D)none of the above