Saturday , October 23 2021

alternator viva questions and answers part-2

Alternator viva questions and answers part-2

 

Q: What is the coil span factor? 

Ans: it is defined as the ratio of the resultant emf to the algebraic of the voltage induced in both the coil sides. 

It is denoted by K which is equal to cos β/2. 

Where. β= 130/3n,  where n=number of slots/poles/phase 

Q: What is the distribution factor? 

Ans: It is denoted by Kd is equal to K_{d}=\dfrac{\sin \dfrac{n\alpha }{2}}{n\sin \dfrac{\alpha }{2}}

Q: What are the n and α in the distribution factor? 

Ans: n = The number of slots per pole per phase,

and α = The angular displacement between the slots or degree per slot. 

Read Alternator viva questions and answers part-1

Q: Will the e.m.f. induced in both coil of an alternator be same? 

Ans: No, it will not be the same in the case of short pitched long pitched windings and may be same in case of whole coil winding i.e full pitched winding. 

Q: Why in both the short pitched and long pitch windings, the a.m.f. induced in both coil sides is same? 

Ans. Because the coil sides are not experiencing the same magnitude of the flux at that speed. As the rate of change of flux is responsible for the production of e.mf. so here speed is naturally same but the magnitude of the flux linking both the coil sides is not of same magnitude, so the e.m.f, induced in both coil sides will not be same, 

Q: What will happen to the em.f. if the frequency is doubled? 

Ans: The generated e.m.f. will be double, 

Q: If you increase the number of conductors what effect will it make on the frequency of generation? 

Ans: No effect, because frequency depends upon speed and number of poles. 

Read Alternator viva questions and answers part-1

Q: What is done to increase the generated e.m.f. of an alternator? 

Ans: increase flux by regulating the field current. 

Q: if there are only two slip rings on the shaft of a three phase alternator, which type of alternator is it? 

Ans: Rotating field type, 

Q: What will you do to increase the output of the alternator? Will you increase the speed, field excitation or fuel? 

Ans: Increase the fuel. 

Q: To increase the field excitation voltage, the rheostat is connected in which circuit? 

Ans: Field circuit of shunt generator, 

Q: In which unit the alternators are rated? 

Ans: kVA or MVA. 

Read Alternator viva questions and answers part-1

Q: Why the rating is in kVA not in kW? 

Ans: Because the power factor of the load is not constant. 

Q: If the power factor of the load is low, how will it effect the winding of the alternator? 

Ans: The winding will carry more current for the same real power at low power factor. 

Q: Name the instrument used for power factor controlling in factories? 

Ans: Minimum power factor demand indicator. 

Q: Why the voltage of an alternator fall down with the increasing of the load? 

Ans: Because of the impedance drop and armature reaction.

Q: What is impedance drop? 

Ans: The voltage drop because of the load current in armature circuit and the impedance of the winding  

Q: What do you mean by the synchronous impedance of the alternator?

Ans: It is the opposition because of the armature effective resistance and synchronous reactance of the winding.

Read Alternator viva questions and answers part-1

Q: What do you mean by the effective armature resistance? 

Ans: It is the resistance offered by the armature winding and A.C. it is increased about 50% because of the skin effect and is known as the effective armature resistance. 

Q: What is leakage reactance

Ans: The flux produced because of the current in armature conductors does not completely cross the air gap but complete its path through the stator, such flux is known as flux leakage and due to this the armature winding is supposed to possess a reactance which is known as leakage reactance. 

Q: What is synchronous reactance? 

Ans: The sum of the leakage reactance and fictitious reactance is known as synchronous reactance. It is denoted by Xs, and unit is ohm. 

Q: The voltage drop in an alternator is because of which factors? 

Ans: These are load current, armature resistance, leakage reactance and the armature reaction. 

Q: The magnitude of the voltage drop is mainly dependent upon…? 

Ans: Armature current. 

Q: Can you name the factor which is responsible for the behavior of this voltage drop.

Ans: The power factor, 

 Q: Define armature reaction? 

Ans: It is the effect of the flux produced by the armature conductors on the distribution of flux under the main poles. 

Read Alternator viva questions and answers part-1

Q: How many types of load is possible in case of alternator?

Ans: Resistive, inductive, capacitive and mixed. 

Q: What is the effect of the purely resistive load? 

Ans: The main field is distorted and weakened. 

Q: How will you experience about the effect of the purely inductive load?

Ans: It is purely demagnetizing 

Q: Is there any kind of load which has the magnetizing characteristic? 

Ans: Yes, purely capacitive load. 

Q: What is the power factor in case of purely resistive, inductive and capacitive loads? 

Ans: Unity, zero lagging and zero leading respectively.

Read: Alternator viva questions and answers part-1

Q: What is voltage regulation? 

Ans: It is defined as the rise in voltage shown in the percentage of full load voltage, when the full load is thrown off. 

Q: In which unit the regulation is expressed? 

Ans: It is a ratio hence no unit. 

Q: In the case of purely resistive load, will there be any difference between the voltage induced (e.m.f.) and the terminal voltage (V). 

Ans: Yes, the e.m.f. will be more than the potential difference V and e.m.f. will be leading the terminal voltage by some angle.

Q: Can the e.m.f. induced be less than the terminal voltage? 

Ans: Yes, in case of purely capacitive load. 

Q: What is the formula for finding out the generated e.m.f. at unity power factor, lagging power factor and leading power factor? 

Ans: 

E= \sqrt{( V\cos \phi +I_{a}R_{a}}) ^{2}+\sqrt{\left( V\sin \phi \pm X_{s}I_{a}\right) ^{2}}

Where, E = induced voltage in volts.

V = Terminal voltage in volts 

Φ = Angular displacement 

Ia = Armature current in amperes 

Ra= Effective armature resistance in ohms

Xs = Synchronous reaction in ohms 

Q: why in the portion (V\sin \phi \pm I_{a}X_{s}), sign \pm is used? 

Ans: It is a common formula, + sign is used for lagging and resistive loads

but – sign is used for leading power factor. 

Read Alternator viva questions and answers part-1

Q: How the power factor can be improved? 

Ans: By using the synchronous capacitor and bank of capacitors, 

Q: What is the synchronous capacitor?

Ans: The overexcited synchronous motor behaves like a synchronous capacitor. 

Q: How actually the power factor correction is made? 

Ans: Just by reducing the reactive component which will certainly improve the power factor. 

Q: If the load on a power station is increased, what will be done to meet the increasing demand of the load? 

Ans: Another alternator is put in parallel to meet the increasing demand, which is known as synchronizing of the alternator. 

Q: What is the utility of synchronizing the alternators? 

Ans: Better efficiency, periodic overhauling, general repair, continuation of electric supply in case of any fault in the generating station etc. 

Q: What are the conditions to be fulfilled for ideal synchronizing? 

Ans:

  1. The voltage of the incoming machine must be the same as that of the busbar voltage.
  2. The phase sequence must be the same.
  3. The frequency must be the same. 

Read Alternator viva questions and answers part-1

Q: How all these conditions are satisfied? 

Ans: By means of voltmeter, phase sequence indicator and frequency-meter, respectively. 

Q: What are the different methods of synchronizing the alternators? 

Ans: Dark lamp method, bright lamp method, dark and bright lamp method and synchroscope method. 

Q: What should be the indications of lamps in case of dark lamp method and how these are connected? 

Ans: Lamps should be dark and these are connected across the same phases. 

Q: What are the drawbacks of this method? 

Ans: In this method it is difficult to judge the zero value of voltage because even at less voltage the lamp may not glow and may cause some error. 

Q: How the lamps are connected in bright lamp method? 

Ans: Across the opposite phase. 

Read Alternator viva questions and answers part-1

Q: Which method is generally preferred for synchronizing the alternator?

Ans: Dark and light lamp method.

Q: How the lamps are connected in case of dark and bright lamp method? 

Ans: Dark lamp across the same phase and bright lamps across the opposite phases. 

Q: If the voltage of generation and frequency is alright, what conditions of lamps will be suitable for synchronizing the alternators? 

Ans: The dark lamp must be dark and bright lamps ha bright for connected synchronizing 

Q:If the required condition is not achieved what should be down? 

Ans: I can be because of wrong connections.Change it and observe that the same conditions are achieved. 

Q: On what basis does the synchroscope work? 

Ans: On the production of the rotating magnetic field. 

Read Alternator viva questions and answers part-1

Q: How many windings are there and how these are connected? 

Ans: Two windings, both are fed, one from the incoming machine and other from the busbar or already running machine. 

Q: How is the correct position is achieved? 

Ans: There is a making on the disc of the synchroscope, if it stops and mark is standstill then correct position is obtained. 

Q:If the disc rotates in either direction what does it indicate? 

Ans: The machines are not in synchronizing state. The frequencies are not the same. 

Q: What do you mean by the floating of alternators on Busbar? 

Ans: While the alternator is neither delivering nor drawing any power from the busbar, the position is known as floating on the busber. 

Q: What is to be adjusted to share the load? 

Ans: Increase the field excitation of the machine to be loaded. Q: If an alternator is to be shut down, what should be done? Ans: First of all reduce the load current to minimum possible value say zero by decreasing the field excitation of the machine, than off the main switch or circuit or breaker and finally stop the prime mover. 

Read Alternator viva questions and answers part-1

Q: How the voltage of an alternator is controlled? 

Ans: By manual controlling and by automatic controlling, 

Q: What are the types of automatic regulators? 

Ans: Vibrating type and sliding type. 

Q: Name the vibrating type automatic regulator

Ans: Tirill regulator. 

Read Alternator viva questions and answers part-1

Q: Can you tell any example of the sliding contact type automatic regulator? 

Ans: The silver stud regulator and Brown Bovery regulator. 

Q: Can you tell how actually all types of regulator control the voltage? 

Ans: By controlling the field excitation of the alternator 

Q: What is open circuit Characteristic? 

Ans: The relationship between the voltage induced and the field excitation is known as open circuit characteristic. 

Read Alternator viva questions and answers part-1

Q: What do you understand by the short circuit characteristic?

Ans: It is the relationship between the short circuit current and the field current of the alternator.

Q: What are the different methods for finding out the voltage regulation? 

Ans:

  • The synchronous impedance method or EMF method
  • The ampere-turn or mmf method
  • Potier method or zero power factor method

Q: What are the switch board equipment for an alternator? 

Ans: Circuit breaker, voltmeter M.I type (three in number), voltmeter M.C type (One number), ammeter M.I type (three in number), ammeter M. C type (one number) energy meter (one number), power factor meter, frequency meter, wattmeter, synchroscope, Field regulator, speedometer(One each) 

Q: What are the possible data you can obtain from the nameplate of an alternator? 

Ans: Capacity in kVA, voltage in volts, kW at specific power factor, frequency in c/s, current in amps, speed in r.p.m., number of phase, number of poles, current in amps, field voltage in volts, temperature rise, type of insulation and serial number of the set etc 

Q: What do you mean by hunting? 

Ans: it is defined as the oscillation due to sudden loading and switching off of the load.

Q: Where this winding is housed? 

Ans: in the pole shoes near pole faces and are short circuited as that of the squirrel cage winding in induction motors. 

Read Alternator viva questions and answers part-1

Q: What are constant losses? 

Ans: The losses which are constant from no load to full load are known as constant losses. 

Q: What is efficiency? 

Ans:

Efficiency %η=inuput/output x 100 

Q: Is cooling necessary in the alternators? 

Ans: Yes, because of load current heat is developed which certainly required to be dissipated. 

Q: What provision is made for normal cooling? 

Ans: A fan is provided to circulate air, which helps in dissipating the heat developed. 

Q: In big alternators which cooling media is generally provided? 

Ans: Hydrogen cooling, 

Read Alternator viva questions and answers part-1

Q: Is there any special attention provided for the rotor and stator construction of the big alternator for cooling? 

Ans: Yes, some ventilating ducts are provided, horizontally and vertically for better and effective coolant circulation even through the cores.

Q: If any alternator is down for overhauling, will the shaft of the alternator be left to stand still? 

Ans: No, simultaneously a small motor generally comes in operation and will keep the rotor running at a low speed, so that the thermal stresses may not cause any defect or deformation in the shaft or rotor of the alternator.

Check Also

Viva questions for transformer part 3

Viva questions for transformer part 3

Viva questions for transformer part 3   Q: What is a P.T?  Ans: It is …