Synchronous Machines topics include: Physical concepts, Emf Polygons, power angle and power factor control of synchronous machines, excitation systems for synchronous machines, flux and mmf phasors in synchronous machines,synchronous machine applications and measurement of xd and xq. A synchronous machine is an electrical machine that operates at synchronous speed. It is an AC machine that uses a rotating magnetic field to generate power.  Here are some characteristics of synchronous machines: Rotation speed: The rotor of a synchronous machine rotates at the same speed as the rotating... Show more

Synchronous Machines topics include: Physical concepts, Emf Polygons, power angle and power factor control of synchronous machines, excitation systems for synchronous machines, flux and mmf phasors in synchronous machines,synchronous machine applications and measurement of xd and xq.

A synchronous machine is an electrical machine that operates at synchronous speed. It is an AC machine that uses a rotating magnetic field to generate power. 

Here are some characteristics of synchronous machines:
Rotation speed: The rotor of a synchronous machine rotates at the same speed as the rotating field.
Magnetic field: The rotor side of a synchronous machine has a fixed magnetic field.
Power supply: Synchronous machines require an AC supply to work.

Power system applications: Synchronous machines are used in many power system applications, including:
Power generators
Power factor correction
Driving constant speed mechanical loads

Household appliances: Synchronous machines are used in small household appliances such as air coolers, refrigeration, fans, and air conditioners. 

Synchronous machines have several advantages and disadvantages:

Advantages
Synchronous motors have several advantages and disadvantages.
Synchronous motors can operate under any power factor, lagging as well as leading.
Synchronous motors are not self-starting and cannot be started on load.

Disadvantages
Synchronous motors have a higher cost per kW output than induction motors.
Synchronous motors require collector rings and brushes, which increases maintenance.
Synchronous motors cannot be used for variable speed applications or applications requiring frequent starting. 

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Electrical Machines Practice Test: Synchronous Machines

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25 Questions

1. Consider the principle that the torque in a rotating machine is proportional to peak of stator MMF(F_s), rotor MMF(F_r) and the angle between them(δ). Following statements relate to F_s,F_r and δ in different machine. Which of these statements are correct?

(i) δ is fixed in DC machine
(ii) F_s is fixed in DC shunt machine
(iii) δ is variable in induction machine
(iv) F_r is fixed in synchronous machne
(v) F_r is variable in DC machine
(vi) F_r is fixed in induction motor
(vii) resultant of F_s and F_r is fixed in synchronous motor

(ii), (iii), (v), (vi) and (vii)(i), (ii), (iii), (iv) and (vi)(i), (ii), (iv), (v) and (vii)(i), (ii), (iii), (v) and (vii)

2. If the synchronous machine is connected to an infinite bus of constant voltage Vt, through a transformed, a transmission line of reactance ‘X’. Then power for a cylindrical machine is?

P = (Ef*Ef sin2δ)/(Xs + X)P = (Vt*Vt sinδ)/(Xs + X)P = (Ef*Vt sinδ)/(Xs – X)P = (Ef*Vt sinδ)/(Xs + X)

3. Load angle δ is the angle between ____________

rotor MMF Fr and resultant MMF FRstator MMF Fs and resultant MMF FR (or) rotor MMF Fr and resultant MMF FRstator MMF Fs and resultant MMF FRstator MMF Fs and rotor MMF Fr

4. A synchronous machine which is synchronized with an infinite bus. If it is desired to obtain a condition when the machine delivers the real power to the IBB without changing the field excitation, then __________

no reactive power flow will take placereactive power will be consumed by the machinereactive power will be delivered by the machinenone of the mentioned

5. For obtaining maximum current when we conduct the ‘Slip Test’ on a synchronous machine, its armature field will align along __________

q-axis45° to d-axis45° to q-axisd-axis

6. In all rotating electrical machine, electrical torque is developed when relative speed between stator field and rotor field is ____________

zeroequal and opposite to rotor speedequal to rotor speeddependent upon the type of electrical machine

7. Squirrel cage bars placed in the rotor pole faces of a 3-phase alternator help in reducing hunting.

IIInone of the mentionedI, III, II, III

8. A 3 phase 400V salient pole alternator is running at no-load. Suddenly the excitation is removed then the net power in the machine is proportional to _____________

cos 2dsin d0sin 2d

9. When the excitation voltage is increased from 1 to 1.3 pu of a 3-phase synchronous motor. Then load angle for the constant power operation __________

must be decreaseddecreasedcan also be increasedincreased

10. The armature winding of a 2-pole, 3-phase alternator for each phase is distributed in a number of slots per phase. The RMS value of the voltage per phase is less than the RMS value of the voltage per coil multiplied by the number of coils in series because the _________

equal RMS voltages in different coils of the phase has mutual phase differencemaximum value of the induced voltage in different coils of the phase are differentRMS value of the voltage in different coils of the phase is differentdifferent coils of the phase pass through different saturated regions of the magnetic circuit

11. The brushless excitation and conventional DC excitation systems are comparable but one significant change is _______

a three phase rectifier is fed by main exciterno direct rectification is provided to the pilot exciterpilot exciter is brushless permanent magnet motorthree phase rectifier feeds directly to main exciter

12. The operating frequency and voltage of an isolated alternator ________ if the prime mover input to motor is increased.

remains constantno relation exists between the power and frequencyincreasesdecreases

13. The magnetization current drawn from an AC supply a synchronous motor is used to _________

set up magnetizing armature reactioncompensate core lossesset up flux in magnetic circuit of deviceall of the mentioned

14. Consider a 3-phase alternator with negligible armature resistance and high rotational inertia delivering power to an isolated load when the armature terminals got short circuited. After sometime, the mechanical power input to the shaft of the machine is terminated at time t0. The short-circuit current circulating in the armature will change till the machine comes to halt at ‘ts’. Which one of the curves depicts this phenomena?

15. For a power system having induction motor loads, an overexcited synchronous motor is also attached. Then the over all power factor __________

becomes upfdegradesremains sameimproves

16. In the design of electromagnetic devices, the maximum value of MMF is limited from the considerations of ____________

power ratingtorque production risetemperature riselimitation on flux density in teeth

17. The internal power factor angle is given for a lagging load of a 3- phase alternator.

ψ = δ + θψ = δ – θψ = -δ -θψ = δ -θ

18. The static excitations mainly comprises of _______

brushless and thyristor based excitation systemthyrister based excitation systemsynchronous motor excitation systembrushless excitation system

19. The armature current is ________ at UPF when compared to leading power factor for a motor.

equalminimumnone of the mentionedmaximum

20. Which of these statements are correct regarding the design of distributed armature winding in a 3-phase alternator?

(i) it reduces the phase-belt harmonics
(ii) it increases the utilization of the armature iron and copper
(iii) it increases rigidity and mechanical strength of the winding
(iv) it reduces copper in the overhang of the winding

(ii), (iii), (iv)(i), (iii), (iv)(i), (ii), (iii)(i), (ii), (iii), (iv)

21. Floating of synchronous machine on infinite bus means ___________

Fa = 0 and machine is not excitedFa = 0 and machine neither generator nor motorFa is not zero and machine neither generator nor motorfrequency is zero

22. The slip test is used to determine ____________

XdXqNone of theseXd and Xq

23. for a synchronous motor, its power factor __________

increases with loading for a given excitationimproves with increase in excitation and may even become leading at higher excitationsdecreases with increase in excitationis independent of its excitation

24. If the internal power factor angle of synchronous motor is 30 degree. Then the direct-axis component of the armature current will be __________

0.866 pu1.73 pu00.5 pu

25. The V-curves of synchronous motor is plotted between ________

If Vs Ia with constant shaft loadpower factor vs Ifpower factor vs IaIa Vs If with constant shaft load