Objective Questions on D.C. Generator's

How A Prime Movers Works In An Electic Power Plant....?

How A Prime Movers Works In An Electic Power Plant....?
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Electrical Power Plant today is more important than any other, that is because without those power plant operate we cannot do anything. From manufacturing plant, because we are all depend on Electricity. So therefore it a high time to know from the very beginning what is a prime movers, how it works or what is
all about.  To get the plant started, prime movers are typically Diesel Engines, Gas or Steam Turbines, or  Hydro and Wind Turbines. This Prime movers converts oil, gas, coal, wood, uranium, water, wind, etc. into mechanical energy. The mechanical energy is supplied to the shaft  of the generator and the generator in turn transform this mechanical energy to electrical energy. High quality electric power generating systems deliver precise voltage and frequency. The voltage regulator as noted earlier, provides the precise voltage regulation. The prime mover GOVERNOR, by accurately  controlling the speed, provides the precise frequency regulation.
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What is a Governor?
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Govers Define as a device which controls the speed or power of a prime mover in order to convert the power to useful work.When the governor senses the speed of a prime mover and controls the prime mover to maintain its speed (or load) at a desired output.Like for example if  you drive a car. You function as a governor when you control the car’s speed under a varying driving conditions. The driver adjust the accelerator that controls the fuel to maintain the desired speed at a certain level and adjust from time to time should the desired speed varies. Acutually the original  governors were entirely mechanical like a pair of centrifugal flyweights in most ballheads. It is known for its ruggedness, usefulness and sensitivity.
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The  flyweights are rotated by a drive from the prime mover that is directly related to the speed. As the speed changes, the flyweights move out at the top due to  centrifugal force. The centrifugal action causes the weights to increase the force on the thruster that control the pilot valve thereby changing the speed of  the prime mover due to applying or removing load.
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What a Governor Does?
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As the prime mover gets started or a speed of about 25%, a governor mechanism “comes to life”. The governor senses the prime mover rotation and begins to  regulate the linkages controlling the fuel/steam supply of the prime mover.
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But if the prime mover tends to increase speed above the set speed of the  governor, the governor mechanism decreases the controlling lever opening of the fuel/steam supply of the prime mover.Eventually, the prime mover speed will  be controlled so that only enough fuel/steam is being admitted, (power in) to hold the prime mover approximately the speed setting of the governor.

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What Happens as Load is Added to the System?
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As the customer load demand increases, the generator must supply the load instantaneously. The generator slows down as more power is being taken out from the  generator than what is being produced. But as soon as the shaft slows down, the governor mechanism senses and causes to increase the fuel/steam rack opening  it and increasing fuel/steam supply of the prime mover.The power produced by the prime mover will increase to match the generator demand. As the speed will  be a little less than before, since if the speed recover completely, the governor mechanism senses and causes to increase the fuel rack opening increasing
power for the prime mover.
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Therefore, the frequency of the AC produced by the generator will get lower as the generator slows down and load increases. What is a Governor Speed Setting Does? The speeder spring is the part that sets the “desired speed”. Applying more force down on the speeder spring causes the governor to increase fuel/steam supply to the prime mover. This initial force is usually set by the operator for the  desired or “reference’ speed.
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It can be set by an adjusting screw, a knob, a lever, a DC electric motor, an air pressure

D.C. Generators - MCQ and answers

D.C. Generators - MCQ and answers
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1. Laminations of core are generally made of
(a)     case iron
(b)     carbon
(c) silicon steel
(d) stainless steel
Ans: c

2. Which of the following could be lamina-proximately the thickness of lamina-tions of a D.C. machine ?
(a) 0.005 mm
(b) 0.05 mm
(c) 0.5 m
(d) 5 m
Ans: c

3. The armature of D.C. generator is laminated to
(a)     reduce the bulk
(b)     provide the bulk
(c) insulate the core
(d) reduce eddy current loss
Ans: d

4. The resistance of armature winding depends on
(a)     length of conductor
(b)     cross-sectional area of the conductor
(c) number of conductors
(d) all of the above
Ans: d

5. The field coils of D.C. generator are usually made of
(a) mica
(b) copper
(c) cast iron
(d) carbon
Ans: b

6. The commutator segments are connected to the armature conductors by means of
(a) copper lugs    
(b) resistance wires
(c) insulation pads
(d) brazing
Ans: a

7. In a commutator
(a)  copper is harder than mica
(b)  mica and copper are equally hard
(c) mica is harder than copper
(d) none of the above
Ans: c

8. In D.C. generators the pole shoes are fastened to the pole core by
(a) rivets
(b) counter sunk screws
(c) brazing
(d) welding
Ans: b

9. According to Fleming's right-hand rule for finding the direction of induced e.m.f., when middle finger points in the direction of induced e.m.f., forefinger will point in the direction of
(a) motion of conductor
(b) lines of force
(c) either of the above
(d) none of the above
Ans: b

10. Fleming's right-hand rule regarding direction of induced e.m.f., correlates
(a) magnetic flux, direction of current flow and resultant force
(b) magnetic flux, direction of motion and the direction of e.m.f. induced
(c) magnetic field strength, induced voltage and current
(d) magnetic flux, direction of force and direction of motion of conductor
Ans: b

11. While applying Fleming's right-hand rule to And the direction of induced e.m.f., the thumb points towards
(a) direction of induced e.m.f.
(b) direction of flux
(c) direction of motion of the conductor if forefinger points in the direction of generated e.m.f.
(d) direction of motion of conductor, if forefinger points along the lines of flux
Ans: d

12. The bearings used to support the rotor shafts are generally
(a) ball bearings  
(b) bush bearings
(c) magnetic bearmgs
(d) needle bearings
Ans: a

13. In D.C. generators, the cause of rapid brush wear may be
(a) severe sparking
(b) rough commutator surface
(c) imperfect contact
(d) any of the above
Ans: d

14. In lap winding, the number of brushes is always
(a) double the number of poles
(b) same as the number of poles
(c) half the number of poles
(d) two
Ans: b

15. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f. ?
(a) Lap winding  
(b) Wave winding
(c) Either of (a) and (b) above
(d) Depends on other features of design
Ans: b

16. In a four-pole D.C. machine
(a) all the four poles are north poles
(b) alternate poles are north and south
(c) all the four poles are south poles
(d) two north poles follow two south poles
Ans: b

17. Copper brushes in D.C. machine are used
(a) where low voltage and high currents are involved
(b) where high voltage and small cur-rents are involved
(c)  in both of the above cases
(d)  in none of the above cases
Ans: a

18. A separately excited generator as compared to a self-excited generator
(a) is amenable to better voltage con-trol
(b) is more stable
(c) has exciting current independent of load current
(d) has all above features
Ans: d
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19. In case of D.C. machines, mechanical losses are primary function of
(a) current  
(b) voltage
(c) speed
(d) none of above
Ans: c

20. Iron losses in a D.C. machine are independent of variations in
(a) speed
(b) load
(c) voltage
(d) speed and voltage
Ans: b

21. In D.C. generators, current to the external circuit from armature is given through
(a) commutator    
(b) solid connection
(c) slip rings
(d) none of above
Ans: a

23. Brushes of D.C. machines are made of
(a) carbon
(b) soft copper
(c) hard copper    
(d) all of above
Ans: a

24. If B is the flux density, I the length of conductor and v the velocity of conductor, then induced e.m.f. is given by
(a)Blv
(b)Blv2
(c)Bl2v
(d)Bl2v2
Ans: a

25. In case of a 4-pole D.C. generator provided with a two layer lap winding with sixteen coils, the pole pitch will be
(a) 4
(b) 8
(c) 16
(d) 32
Ans: b

26. The material for commutator brushes is generally
(a) mica
(b) copper
(c) cast iron
(d) carbon
Ans: d

27. The insulating material used between the commutator segments is normally
(a) graphite
(b) paper
(c) mica
(d) insulating varnish
Ans: c

28. In D.C. generators, the brushes on commutator remain in contact with conductors which
(a)     lie under south pole
(b)     lie under north pole
(c) lie under interpolar region
(d) are farthest from the poles
Ans: c

29. If brushes of a D.C. generator are moved in order to bring these brushes in magnetic neutral axis, there will be
(a)     demagnetisation only
(b)     cross magnetisation as well as magnetisation
(c) crossmagnetisation as well as demagnetising
(d) cross magnetisation only
Ans: c

30. Armature reaction of an unsaturated D.C. machine is
(a) crossmagnetising
(b) demagnetising
(c) magnetising    
(d) none of above
Ans: a

31. D.C. generators are connected to the busbars or disconnected from them only under the floating condition
(a) to avoid sudden loading of the primemover
(b) to avoid mechanicaljerk to the shaft
(c) to avoid burning of switch contacts
(d) all above
Ans: d

32. Eddy currents are induced in the pole shoes of a D.C. machine due to
(a) oscillating magnetic field
(b) pulsating magnetic flux
(c) relative rotation between field and armature
(d) all above
Ans: c

33. In a D.C. machine, short-circuited field coil will result in
(a)    odour of barning insulation
(b)    unbalanced magnetic pull producing vibrations
(c)  reduction of generated voltage for which excitation has to be increased to maintain the voltage
(d)  all above
Ans:

34. Equilizer rings are required in case armature is
(a) wave wound    
(b) lap wound
(c) delta wound    
(d) duplex wound
Ans: b

35. Welding generator will have
(a) lap winding    
(b) wave winding
(c) delta winding
(d) duplex wave winding
Ans: a

36. In case of D.C. machine winding, number of commutator segments is equal to
(a) number of armature coils
(b) number of armature coil sides
(c) number of armature conductors
(d) number of armature turns
Ans: a
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37. For a D.C. machines laboratory following type of D.C. supply will be suitable
(a)     rotary converter
(b)     mercury are rectifier
(c) induction motor D.C. generator set
(d) synchronous motor D.C. generator set
Ans: c

38. The function of pole shoes in the case of D.C. machine is
(a) to reduce the reluctance of the magnetic path
(b) to spread out the flux to achieve uniform flux density
(c) to support the field coil
(d) to discharge all the above functions
Ans: d

39. In the case of lap winding resultant pitch is
(a) multiplication of front and back pitches
(b) division of front pitch by back pitch
(c) sum of front and back pitches
(d) difference of front and back pitches
Ans: d

40. A D.C. welding generator has
(a) lap winding    
(b) wave moving
(c) duplex winding
(d) any of the above
Ans: a

41. Which of the following statement about D.C. generators is false ?
(a)     Compensating winding in a D.C. machine helps in commutation
(b)     In a D. C. generator interpoles winding is connected in series with the armature winding
(c)  Back pitch and front pitch are both odd and approximately equal to the pole pitch
(d)  Equilizing bus bars are used with parallel running of D.C. shunt generators
Ans: d

42. The demagnetising component of armature reaction in a D.C. generator
(a) reduces generator e.m.f.
(b) increases armature speed
(c) reduces interpoles flux density
(d) results in sparking trouble
Ans: a

43. Magnetic field in a D.C. generator is produced by
(a) electromagnets
(b) permanent magnets
(c) both (a) and (b)
(d) none of the above
Ans: a

44. The number of brushes in a commutator depends on
(a)     speed of armature
(b)     type of winding
(c)  voltage
(d)  amount of current to be collected
Ans: d

45. Compensating windings are used in D.C. generators
(a)     mainly to reduce the eddy currents by providing local short-circuits
(b)     to provide path for the circulation of cooling air
(c) to neutralise the cross-magnetising effect of the armature reaction
(d) none of the above
Ans: c

46. Which of the following components of a D.C, generator plays vital role for providing direct current of a D.C. generator ?
(a) Dummy coils  
(b) Commutator
(c) Eye bolt
(d) Equilizer rings
Ans: b

47. In a D.C. generator the ripples in the direct e.m.f. generated are reduced by
(a)     using conductor of annealed copper
(b)  using commutator with large number of segments
(c)  using carbon brushes of superior quality
(d)  using equiliser rings
Ans: c

48. In D.C. generators, lap winding is used for
(a) high voltage, high current
(b) low voltage, high current
(c) high voltage, low current
(d) low voltage, low current
Ans: b

49. Two generators A and B have 6-poles each. Generator A has wave wound armature while generator B has lap wound armature. The ratio of the induced e.m.f. is generator A and B will be
(a) 2 : 3
(b) 3 : 1
(c) 3 : 2
(d) 1 : 3
Ans: b

50. The voltage drop for which of the following types of brush can be expected to be least ?
(a) Graphite brushes
(b) Carbon brushes
(c) Metal graphite brushes
(d) None of the above
Ans: c

51. The e.m.f. generated by a shunt wound D.C. generator isE. Now while pole flux remains constant, if the speed of the generator is doubled, the e.m.f. generated will be
(a) E/2
(b) 2E
(c) slightly less than E
(d) E
Ans: b

52. In a D.C. generator the actual flux distribution depends upon
(a) size of air gap
(b) shape of the pole shoe
(c) clearance between tips of the ad¬jacent pole shoes
(d) all of the above
Ans:

53. The armature core of a D.C. generator is usually made of
(a)     silicon steel    
(b)     copper
(c) non-ferrous material
(d) cast-iron
Ans: a

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                      ( Sai Saikumar Jn)

SYNCHRONOUS MACHINES-MCQ with Answers

SYNCHRONOUS MACHINES-MCQ
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1. The winding of a 4-pole alternator having 36 slots and a coil span of 1 to 8 is short-pitched by........ degrees.

A.140
B. 80
C. 20
D. 40

Ans: D

2. A 50 HZ alternator will run at the greatest possible speed if it is wound for ...... poles

A. 8
B. 6
C. 4
D. 2

Ans: D

3. An electric motor in which both rotor and stator fields rotates with same speed is called a/an ............. motor

A. DC
B. charge
C. reluctance
D. universal
E. synchronous

Ans: E

4. A synchronous motor can be started by

A. a pony motor
B. dc compound motor
C. providing damper winding
D. any of the above

Ans: D

5. An alternator is also called as _________ generator.

A. asynchronous
B. synchronous
C. Rosenberg
D. dc

Ans: B

Synchronous Generator : Concepts -- 3 ( sai saikumar jn)

Synchronous Generator : Concepts

Synchronous Generator: BLONDELS TWO REACTION THEORY:

In case of cylindrical pole machines, the direct-axis and the quadrature axis mmfs act on the same magnetic circuits, hence they can be summed up as complexors. However, in a salient-pole machine, the two mmfs do not act on the same magnetic circuit. The direct axis component F_ad operates over a magnetic circuit identical with that of the field system, while the q-axis component F_aq is applied across the interpole space, producing a flux distribution different from that of F_ad or the Field mmf.

The Blondel's two reaction theory hence considers the results of the cross and direct-reaction components separately and if saturation is neglected, accounts for their different effects by assigning to each an appropriate value for armature-reaction "reactive" respectively X_aq and X_ad .

Considering the leakage reactance, the combined reactance values becomes

X_ad = X + X _ad and X _sq = X _aq

X_sq < X_sd as a given current component of the q-axis gives rise to a smaller flux due to the higher reluctance of the magnetic path.


Let l_q and I_d be the q and d-axis components of the current I in the armature reference to the phasor diagram in Figure. We get the following relationships

I_q= I cos (σ+θ) I_a = I cosφ

I_d = I sin (σ+ φ) I_r = I sinφ

And I = √(I_d² + I_q²)= = √(I_d² + I_r²)
 
where I_a and I_r are the active and reactive components of current I.

Voltage Regulation of synchronous generator:

voltage regulation of an alternator is defined as "the rise in voltage when full load is removed (field excitation and speed remaining unaltered) divided by the rated terminal voltage. Thus

% regulation =( E₀ – V ) / V x 100

In case of leading load pf the regulation is negative.

Parallel Operation of Synchronous Generators:

A stationary synchronous generator should not be connected to five bus bars because, stator induced e.mf. being zero, a short circuit will result. For proper paralleling of Generators the following three conditions must be satisfied :

1. The terminal voltage of the incoming generator must be same as bus-bar voltage.

2. The speed of the incoming generator must be such that its frequency (PN/120) equal bus-bar frequency.

3. The phase of the synchronous generator voltage must be identical with the phase of the bus voltage.

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Synchronous Generator : Concepts -- 2 ( sai saikumar jn)

Synchronous Generator : Concepts

 

(3) Load Characteristics of Synchronous Generator:

While the exciting current and the speed remain constant, the terminal voltage changes with the load current in the armature and the relationship between the terminal voltage and load current of an alternator is known as its load characteristics.

When the armature current increases, the terminal voltage drops. This is mainly due to

(a) Resistance and reactance of armature winding, and
(b) Armature reaction.

The load characteristics of an alternator is shown in the figure.

Phasor diagram of synchronous generator under three types of leading conditions :

Simplified equivalent AC circuit (per phase) for synchronous generator:

AT POWER FACTOR LAGGING:

(A) When R_A is very small:

α = torque angle
P = 3 V_φE₀ sin α/ X_s

Torque induced,
T _ind = 3 V_φE₀ sin α/ X_s ω_m

where ω_m = speed.

(B) General case:

P = 3 E₀/Zs [ E₀cosθ – V (cosθ + α) ]
where cosθ = R_a / Z_s

:. Small R_a implies θ = 90.

For maximum power output:

cosφ = E₀/√ (E²₀ +V²φ)

α = 90⁰

3 P _max = (3 Vφ I _max E₀)/√ (E²₀ +V² φ) = 3 V_φ E₀ / X<sub>s

from ur's -- Bellapuri saikumar
                         ( sai saikumar jn)</sub> 

Synchronous Generator : Concepts -- 1

Synchronous Generator : Concepts

Induced E.M.F in synchronous Generator:

e = B l v volts
where

B = flux density Wb/sqm
v = velocity (m/s) of movement
l = length of conductors in meters.

Frequency of Synchronous Generator:

The frequency of the voltage generated is given by

f = NP/120

where P is the total number of poles and N is the speed in r.p.m.

Breadth factor of Synchronous Generator:

(K_b) = Voltage obtained in multi-slots winding / Voltage obtained if the windings were all concentrated in one slot

Thus breadth factor is always less than unity.
Mathematically,

K_b= (sin δ n/2 )/ (π sin δ/2)

where n is the number of slot and is the slot pitch.

Pitch factor of Synchronous Generator:

Shortening the pitch of the coil has the same effect as the distribution of the winding. When the turns of the windings do to span a complete pitch there occurs a slight loss in the induced emf. A pitch factor ( K_p ) is given by

K_p = cos θ/2

for a coil which extends over (180° - θ) instead of 180°.

Magnitude of Induced emf in alternators / phase:

E_RMS = 4.44 Kp Kb φ f.T. volts

Synchronous Reactance:

X_s = X_L + X_A

where
X_L = Leakage reactance;
X_A = Armature reactance.

Synchronous Impedance:

Z_s = (R² + X²_s )^1/2

SYNCHRONOUS GENERATOR CHARACTERISTICS:

(1) Open Circuit Characteristics of Synchronous Generator:

(Magnetisation Curve).

(2) Short Circuit Characteristics of Synchronous Generator:

(Terminal Voltage vs Current)



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Q:What is the power factor of an alternator at no load? ( sai saikumar jn )

Q:What is the power factor of an alternator at no load? 

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 A:At no load Synchronous Impedance of the alternator is responsible for creating angle difference. So it should be zero lagging like inductor. 

Difference Between Electric motor and Electrical Generator..? ( sai saikumar jn )

 Hello ..... Engineer's

MCQ:- What is the difference between Electric Motor and Electric Generator?

MUST READ & SHARE WITH YOUR FRIEND'S

Ans:- • Generator converts mechanical energy to electrical energy, while motor converts electrical energy to mechanical energy.

• In a generator, shaft attached to the rotor is driven by a mechanical force and electrical current is produced in the armature windings, while the shaft of a motor is driven by the magnetic forces developed between the armature and field; current has to be supplied to the armature winding.

• Fleming's left-handrule (for motors), and Fleming's right-handrule (for generators ).

Donot forget to  share it.

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Electrical Engg PICS................ ( sai saikumar jn)

          
                                                      Power generation from COAL












                 

         
                                                                  ALTERNATOR

 
                                                        TYPES OF CAPACITORS


            
              
           

  
                                           Difference between Circuit breaker, Relay and Isolater

                                                  
                                                                THERMISTOR  

               
                                                                WIND TURBINE

                                                                PLATE EARTHING

                                                          LIGHTING ARRESTORS

 
                                                                 ROD EARTHING

                                            

                                                                   Electrical mosquito killer

                                                    D.C. VOLTAGE & CURRENT FILTER 

                                                          Working of LASER PRINTER 




 MUST SHARE WITH UR FRIEND'S.......................................


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                                                                                                        ( sai saikumar jn)