Transformers Board Exam ( EE ) Reviewer

79問 • 3年前

問題一覧

What is the main use of a zigzag transformer?

C. to stabilize transformer sequence currents

A power transformer is rated at 50,000 kVA, 34.5 kV/13.8 kV is connected wye-wye, what are the line currents at full-load?

B. 836.7 amperes, 2,092 amperes

A three-phase transformer has a primary voltage rating of 34.5 kV, wye-connected with two 2.5% taps above and two 2.5% taps below the rated voltage. The secondary winding is rated 460 volts and is delta connected. If it has 100 turns, what will be the effective number of turns in the primary winding if voltage tap us set at minus 2.5%?

C. 4,222 turns

Two single-phase transformers each rated 75-kVA are connected in V-V or open-delta, to serve a three-phase load of 120 kW at 0.80 power factor lagging. To prevent the overloading of transformers, determine the size of the capacitor in kVAR.

B. 40

A transformer composed of two or more coils which are wound around a non-metallic core

D. Air core transformer

An autotransformer contains how many coils?

B. One

Find unloaded output of a power supply having a transformer regulation of 11.10% and 900 V load output

C. 999.9 V

A transformer consists of the following?

D. Two coils wound on a common core

How much is the secondary voltage of a power transformer whose parameters are as follows, primary voltage is 240 Volts, 140 turns in the primary and 7 turns in the secondary.

A. 12 V

It is a transformer with two or more windings on a laminated iron core used to supply stepped up or stepped down values of voltages to various circuits of an equipment.

B. Power transformer

A transformer with 100 turns in the primary winding and 25 turns in the secondary winding is which of the following?

C. A step-down transformer

Find the primary voltage of a power transformer under the following conditions 3 turns for primary, 6 turns for secondary and 120 volts for secondary voltage.

B. 60 volts

The current that flows in the primary of a transformer with the secondary open (or no load is attached).

A. Source current

What type of transformer that is used to protect technicians and operates from deadly electrical shock?

D. Isolation transformer

Components of the transformer which receives energy from the primary winding and delivers it to the load.

C. Secondary winding

Which of the following does not contribute to losses in a transformer?

B. Self-inductance

A transformer composed of two or more coils which are wound around a non-metallic core

C. Air core transformer

A transformer will work on ____.

A. a.c. only

The primary and secondary of a transformer are ____ coupled.

B. magnetically

A transformer is an efficient device because it ____.

A. is a static device

A transformer transfers electrical energy from primary to secondary usually with a change in ____.

C. voltage

The voltage per turn of the primary of a transformer is ____ the voltage per turn of the secondary.

C. the same as

The winding of the transformer with greater number of turns will be ____.

A. high-voltage winding

A transformer does not possess ____ changing property.

D. power

The iron-core is used to ____ of the transformer.

B. provide tight magnetic coupling

If a transformer core has air gaps, then ____.

C. magnetizing current is greatly increased

The maximum flux produced in the core of a transformer is ____.

B. inversely proportional to supply frequency

The flux in the core of a single-phase transformer is ____.

A. purely alternating one

A transformer is so designed that primary and secondary have ____.

C. tight magnetic coupling

When the primary of a transformer is connected to a d.c. supply, ____.

D. primary may burn out

An ideal transformer is one which ____.

A. has no losses and leakage reactance

The no-load primary current Io is about ____ of full-load primary current.

A. 3 – 5%

The no-load p.f. of a transformer is small because ____

B. magnetizing component of Io is large

The no-load input power to a transformer is practically equal to ____ loss in the transformer.

A. iron

Two things which are same for primary and secondary of a transformer are ____.

A. ampere-turns and voltage per turn

A transformer operates poorly at very low frequencies because ____.

B. magnetizing current is abnormally high

If a power transformer is operated at very high frequencies, then ____.

C. core losses will be excessive

The primary leakage flux links ____.

A. primary winding only

The effect of leakage flux in a transformer is to ____.

C. cause voltage drop in the windings

Leakage flux in a transformer occurs because ____.

B. air is not good magnetic insulator

The mutual flux in a transformer remains constant at all loads because ____.

A. applied voltage and frequency are constant

transformer are built up to nearly circular crosssection in order to reduce ____

D. copper loss

Which of the following connection is best suited for 3-phase, 4- wire service?

C. – Y

Leakage fluxes of a transformer may be minimized by

D. sectionalizing and interleaving the primary and secondary windings

The type of load for which the voltage regulation of a transformer is negative is

A. capacitive

For given effective applied voltage of constant frequency eddy current losses

B. are independent of the wave shape of the applied voltage

When 240 V D.C. supply is given to an unloaded 220 V, 50 Hz transformer

B. primary will carry hvvy current and may possibly burn

Which of the following 3-phase connections of transformer create disturbances in communication sys terns

C. star/star

The path of a magnetic flux in a transformer should have

D. low reluctance

While conducting short-circuit test on a transformer the following side is short-circuited

B. Low voltage side

The no load current in a transformer lags behind the applied voltage by an angle of about

D. 75°

Greater the secondary leakage flux

A. less will be the secondary induced e.m.f.

The chemical used in breather for transformer should have the quality of

B. absorbing moisture

The highest voltage for transmitting electrical power in India is

D. 400 kV

Delta/star transformer works satisfactorily when

C. on balanced as well as unbalanced loads

A transformer can have zero voltage regulation at

A. leading power factor

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

B. Saving in winding material

Negative voltage regulation is indicative that the load is

A. capacitive only

The transformer oil should have____ volatility and ____ viscosity.

A. low, low

A transformer can have regulation closer to zero

C. on leading power factor

The secondary of a current transformer is always short-circuited under operating conditions because it

A. avoids core saturation and high voltage induction

The noise produced by a transformer is termed as

B. hum

The voltage transformation ratio of a transformer is equal to the ratio of

C. secondary induced e.m.f. to primaryinduced e.m.f.

In an ideal transformer, on no-load, the primary voltage is balanced by

B. the secondary induced e.m.f.

Which of the following statements is correct ?

C. A transformer operateat a power factor depending on the power factor of the load

In a power transformer the efficiency should be maximum at

D. full load

The colour of fresh dielectric oil for a transformer is

C. pale yellow

Which of the following methods is used for cooling of transformers upto a capacity of 10 MVA ?

C. Oil natural cooling

Buchhalz's relay is used in

D. transformer protection

Which of the following is not a fitting on a transformer ?

A. Commutator

A transformer transforms

D. power

In a transformer the resistance between its primary and secondary should be

D. infinity

Which of the following loss in a transformer is zero even at full load ?

B. Friction loss

Part of the transformer which is most subject to damage from overheating is

C. winding insulation

A transformer, as compared to an amplifier, cannot increase

C. the output power

For minimum weight of a transfoirmer, the weight of iron should be

A. equal to the weight of copper

In a transformer an insulating material may fail due to

D. any of the above

The secondary winding of a current transformer whose primary is carrying current should

A. not be open-circuited

The iron-core is used to ______ of the transformer.

b.) provide tight magnetic coupling

Machine Design ( 1st Set )

Jacky Shane Luyas Buhia · 20問 · 2年前

Machine Design ( 1st Set )