Synchronous machines best question

1➤ Higher synchronous reactance is preferred in the present day alternator, beacuse one can have

Reduce transient current Reduce harmonics current Higher voltage regulation with load All of these

2➤ The short circuit characteristics of alternator is

Always linearNone of these Always non Linear Both a and c

3➤ The rating of synchronous machine is usually governed by its

Temperature rise SpeedWeightNone of these

4➤ How the reactive power delivered by a synchronous generator be controlled

By changing the prime mover input By changing direction of rotation By changing the excitation By changing the prime mover speed

5➤ A synchronous generator is feeding a zero power (lagging) load at rated current. The armature reaction is

MagnetizingCross magnetizing Demagnetizing Both a and b

6➤ When the load (inductive) is thrown off the terminal voltage will

Remains constant Increase DecreaseNone of these

7➤ The leakage reactance of three phase alternators is determined by performing

Short circuit and slip testOpen and short circuit testOpen circuit and zero power factor test Zero power factor and slip test

8➤ In an alternators short circuit current is limited by the

Saturated synchronous impedance Unsaturated synchronous impedance Both a and bAll of these

9➤ While starting a synchronous motor it's field winding should be

Short circuitedKept open Connected to a DC source None of the above

10➤ The maximum power developed by a synchronous motor depends on

Supply voltage Excitation current Both a and b None

11➤ A synchronous motor is cheaper than 3 phase induction motor for_________

High speed high output Low speed High output High speed low outputAny of the abice

12➤ When Synchronous motor is pulled out of synchronism the stator current ________

Becomes 0Is Decrease too muchRemains unchanged Is increased too much

13➤ The mechanical power developed by synchronous motor is independent of________

SpeedTorque angleField excitation Applied voltage

14➤ Small synchronous motors are started by

Variable frequency sourcePony motorsDamper windingNone

15➤ Synchronous motor are generally used in application requiring

Infrequent startingSudden application of heavy loadVariable speed Frequent stopping

16➤ The stator of modern alternators are wound for______phase groups

180°240°120°60°

17➤ It is desirable to eliminate 5th harmonic voltages from the phase volatage of an alternators. The coil should be short pitched by an electrical angle of

30 °72°36°18°

18➤ In a synchronous Machine hunting is predominantly damped by

Copper losses in rotorMechanical losses in the rotorCopper losses in the stator Iron lossess in rotor

19➤ Modern alternators are designed to have poor regulations as it

Increase the value of short circuit current Limit the value of short circuit current Both a and bIncrease efficiency of operation

20➤ The synchrounous reactance is the

Combine reactance due to leakage flux and armature reaction Reactance due to leakage fluxReaction due to armature reaction of machineBoth a and b

21➤ In a synchronous generator delivering lagging power factor load

The excitation emf leads terminal volatage by the power angle Excitation emf leads the terminal voltage by the power factor angle Both a and bThe excitation emf lags the terminal voltage by power angle

22➤ For maximum current during slip test on a synchronous machine the armature mmf align along

45°to q-axis45°to d-axisq-axisd-axis

23➤ In which of the following is the reactance power developed

Squirrel cage induction motor Transformer Salient pole alternator Cylindrical pole alternator

24➤ In a 3-phase synchrounous motor the magnitude of field flux

Remains constant at all loadsVaries with loadVaries with power factor Varries with speed

25➤ A 3 phase synchrounous motor needs dc supply for excitation

At the starting instant onlyContinuouslyOf stator None of these

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