Q&A: Electrical Machines_Part 1

1.   What is the function of carbon brush used in D.C generator?
The function of carbon brush is to collect current from the commutator and supply is to the external load circuit and to the field circuit.

2.    Distinguish between lap winding and wave winding used in dc machine
      Lap winding is designed for high current and low voltage machines.
     Wave winding is designed for low current and high voltages.

3.    Write the number of parallel paths in a lap and wave connected windings
In a lap wound machine, the number of parallel paths is equal to the number of poles. But in wave wound machine, the number of parallel paths is always two irrespective of number of poles.

4.    Name the three things required for the generation of emf
o    Presence of armature conductors
o    Presence of magnetic field
o    Relative motion between conductor and magnetic field

5. Define slip of induction motor?
        Slip is descriptive of the way in which the rotor slips back from synchronism. It is usually expressed as percentage of synchronous speed.

6.    What is meant by self excited and separately excited dc generator
•    Self-excited generator are those whose field magnets are energized by the current produced by the generator themselves
•    Separately excited generator are those whose field magnets are energized from an independent external source of dc current

7.    What is the basic difference between dc generator and dc motor
Generator converts mechanical energy into electrical energy. Motor converts electrical energy into mechanical energy. But there is no constructional difference between the two.

8.    Write down the emf equation of dc generator. Give the meaning of each symbol
Emf induced E=?ZNP/60A
?=flux per pole in Weber
Z=total number of armature conductors
N=speed of armature in rpm
P=number of poles
A=number of parallel paths

9.what is pole pitch?
        The periphery of the armature is divided for a number of poles of the generator. the center to center distance between two adjacent poles is called pole pitch
        It is also equal to the number of armature slots or armature conductors per pole.

10.How can the voltage in a DC generator be increased?
                    Increasing the main field flux and the speed of the armature      can increase the voltage in a DC generator.

11.What is critical resistance of a DC shunt generator?
           The value of resistance of shunt field winding beyond which the shunt generator fails to build up its voltage is known as “ critical resistance “.

12.what are the conditions to be fulfilled for a shunt generator to build up Voltage?
a)    There must be some residual magnetism in the field poles.
b)    The shunt field resistance should be less than critical resistance.
c)    The field coils should be connected with the armature in such a way that current flowing through them should increase the EMF induced by the residual magnetism.

13.What do you mean by residual flux in DC generator.
        The magnetic flux retained in the poles of the machine even without field supply is called the residual flux.

14.A DC generator fails to self excite. . List the cause for the failure for the failure.
a)    Residual magnetism may not there in the poles.
b)    Direction of rotation may be wrong.
c)    The field resistance may be more than critical resistance.
d)     There may be disconnection in the field winding.
e)    Brush contact may be poor.
f)    The field coils may be connected with the armature to oppose the EMF due to residual magnetism.

15.What are open circuit characteristics of DC shunt generator.
        It is the relation between the field current and the emf induced in the armature

16. Why a 3-phase synchronous motor will always run at synchronous speed?
                  Because of the magnetic coupling between the stator poles and rotor poles the motor runs exactly at synchronous speed.

17. Mention the methods of starting of 3-phase synchronous motor.
            a.   A D.C motor coupled to the synchronous motor shaft.
            b.     A small induction motor coupled to its shaft.
            c.    Using damper windings as a squirrel cage induction motor.

18.What are the principal advantages of rotating field system type of construction of synchronous machines?
•    Form Stationary connection between external circuit and system of conditions enable the machine to handle large amount of volt-ampere as high as 500 MVA.
•    The relatively small amount of power required for field system can be easily supplied to the rotating field system via slip rings and brushes.
•    More space is available in the stator part of the machine for providing more insulation to the system of conductors.
•    Insulation to stationary system of conductors is not subjected to mechanical stresses due to centrifugal action.
•    Stationary system of conductors can easily be braced to prevent deformation.
•    It is easy to provide cooling arrangement.

19.Write down the equation for frequency of emf induced in an alternator.
       F    = PN / 120 Hertz
Where P    = No. Of poles
           N    = Speed in rpm.

20.What are the advantages of salient pole type of construction used for synchronous machines?
•    They allow better ventilation.
•    The pole faces are so shaped radial air gap length increases from the pole center to the pole tips so that flux distribution in the air gap is sinusoidal in shape which will help to generate sinusoidal emf.
•    Due the variable reluctance, the machine develops additional reluctance power, which is independent of excitation.

21Why do cylindrical rotor alternators operate with steam turbines?
     Steam turbines are found to operate at fairly good efficiency only at high speeds. The high-speed operation of rotor tends to increase mechanical losses, so the rotors should have smooth external surface. Hence smooth cylindrical type rotors with less diameter and large axial length are used for synchronous generators driven by steam turbines with either 2 or 4 poles.

22.Which type of synchronous generators are used in Hydroelectric plants and why?
As the speed of operation is low, for hydro turbines used in hydroelectric plants, salient pole type synchronous generators are used. These allow better ventilation and also have other advantages over smooth cylindrical type rotor.

23.What is the relation between electrical degree and mechanical degree?
Electrical degree ?e and mechanical degree are related to one another by the number of poles P, the electrical machine has, as given by the following equation.
?e = (P/2) ?m

24. What is the meaning of electrical degree?
Electrical degree is used to account the angle between two points in rotating electrical machines. Since all electrical machines operate with the help of magnetic fields, the electrical degree is accounted with reference to the polarity of magnetic fields. 180 electrical degrees is accounted as the angle between adjacent North and South poles

25. Why short-pitch winding is preferred over full pitch winding?
Advantages: -
•    Waveform of the emf can be approximately made to a sine wave and distorting harmonics can be reduced or totally eliminated.
•    Conductor material, copper is saved in the back and front-end connections due to less coil span.
•    Fractional slot winding with fractional number of slots/phase can be used which in turn reduces the tooth ripples.
•    Mechanical strength of the coil is increased.

  
26. Write down the formula for distribution factor.
Kd = sin (mß/2)    or     Kdn = sin (mnß/2)
          msin(ß/2)                 msin(nß/2)
where
        m    - number of slots/pole/phase
        ß    - angle between adjacent slots  in electrical degree
        n    - order of harmonics.

27. Define winding factor.
    The winding factor Kw is defined as the ratio of phasor addition of emf induced in all the coils belonging to each phase winding of their arithmetic addition.   

28. Why are alternators rated in kVA and not in kW?
    The continuous power rating of any machine is generally defined as the power the machine or apparatus can deliver for a continuous period so that the losses incurred in the machine gives rise to a steady temperature rise not exceeding the limit prescribed by the insulation class.
    Apart from the constant loss the variable loss incurred in alternators is the copper loss, occurring in the 3-phase winding, which depends on I2R, the square of the current delivered by the generator. As the current is directly related to apparent power delivered by the generator, the alternators have only their apparent power in VA/kVA/MVA as their power rating

29. What are the causes of changes in voltage of alternators when loaded?
•    Voltage variation due to the resistance of the winding R.
•    Voltage variation due to the leakage reactance of the winding X1.
•    Voltage variation due to the armature reaction.

30. What is meant by armature reaction in alternators?
  The interaction between flux set up by the current carrying armature conductors   and the main field flux   is defined as the armature reaction.

31.  What do you mean by synchronous reactance?
    It is the sum of the leakage reactance X1 and armature reactance Xa 
        Xs = X1 + Xa

32. What is synchronous impedance?
The complex addition of resistance R and synchronous reactance jXs is synchronous impedance Zs.
Zs = (R+jXs)    = Zs??
Where ? = tan –1(Xs/R)
|Zs| = v(R2+jXs2)   

33. What is meant by load angle of an alternator?
    The phase angle introduced between the induced emf phasor E and terminal voltage phasor V during the load condition of an alternator is called load angle. The load angle increases with increase in load. It is positive during generator operation and negative during motor operation.
 

34. Define the term voltage regulation of alternator.
    It is defined as the change in terminal voltage from no load-to-load condition expressed as a function or terminal voltage at load condition, the speed and excitation conditions remaining same.
% Regulation = (E-V)/V x 100

35. What is the necessity for predetermination of voltage regulation?
    Most of the alternators are manufactured with large power rating and large voltage ratings. Conduction load test is not possible for such alternators. Hence other indirect methods of testing are used and the performance can be predetermined at any desired load currents and power factors.

36. Why is the synchronous impedance method of estimating voltage regulation is considered as pessimistic method?
    Compared to other methods, the value of voltage regulation obtained by this method is always higher than the actual value and therefore is called pessimistic method.

37. Why is the MMF method of estimating the voltage regulation is considered as the optimization method?
    Compared to EMF method, MMF method involves more number of complex calculation steps. Further the OCC is referred twice and SCC is referred once while predetermining the voltage regulation for each load condition. Reference of OCC takes core saturation effect. As this method requires more effort, the final result is very close to the actual value. Hence this method is called the optimistic method.

38.What does hunting of synchronous motor mean?
   When the load applied to the synchronous motor is suddenly increased or decreased, the rotor oscillates about its synchronous position with respect to the stator field.  This action is called hunting.

39.What could be the reasons if a 3-phase synchronous motor fails to start?
                   It is usually due to the following reasons
              a.  Voltage may be too low.
              b.  Too much starting load.
              c.  Open circuit in one phase or short circuit.
              d.  Field excitation may be excessive

40. What is synchronous condenser?
                     An over-excited synchronous motor used for the improvement of power factor is called as synchronous condenser because, like a capacitor it takes a leading current.

41.Write the applications of synchronous motor.
               a.  Used for power factor improvement in sub-stations and in industries.
               b.  Used in industries for power applications.
               c.  Used for constant speed drives such as motor-generator set, pumps and compressors.

42. What is an inverted 'V' curve?
                   For a constant load, if the power factor is plotted against various values of field exciting current, the curve formed is known as inverted 'V' curve.

43. How the synchronous motor is made self-starting?
                   By providing damper windings in the pole faces, it will start and run like a squirrel cage induction motor.

44.  Asynchronous motor starts as usual but fails to develop its full torque.  What could it be due to?
               a.  Exciter voltage may be too low.
               b.  Field spool may be reversed.
               c.  There may be either open-circuit or short-circuit in the field.          

45.What are the two types of 3-phase induction motor?
a.    Squirrel cage induction motor.
b.    Slip ring induction motor.

46.Write the two extra features of slip ring induction motors.
a.    Rotor is having 3-phase winding.
b.    Extra resistance can be added in the rotor circuit for improving in the power factor with the help of three slip rings.

47.Can we add extra resistance in series with squirrel cage rotor? State the reason?
        We cannot add extra resistance in series with the rotor because all the copper bars of the rotor are short circuited in both the sides by copper end rings to have a closed circuit.

48.Why an induction motor is called rotating transformer?
        The rotor receives electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why an induction motor can be called as a rotating transformer i.e., in which primary winding is stationary but the secondary is free to rotate.

     49.Why an induction motor will never run at its synchronous speed?
        If it runs at synchronous speed then there would be no related speed between the two, hence no rotor emf, no rotor current so no rotor torques to maintain rotation. That is why the rotor runs at its synchronous speed.

      50.What are slip rings?
        The slip rings are made of copper alloys and are fixed around the shaft insulating it. Through these slip rings and brushes the rotor winding can be connected to external circuits.

     51.State the difference between slip ring rotor and cage rotor of an induction motor?
        Slip ring rotor has 3-phase windings. Three ends of which are stared and the other three ends are brought up and connected to 3 slip rings mounted in the shaft. Extra resistance can be added in the rotor circuit. Squirrel cage rotor has short-circuited copper bars.  Extra resistance can’t be added as slip ring rotor.