Electrical Mles

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					                       ELECTRICAL MACHINES
                                     UNIT - I

1. What is the use of split rings in DC Machines?
       For making the flow of current unidirectional in the external circuit the slip
rings are replaced by split rings.

2. What are the characteristics of a DC generator?
      1. OCC
      2. External characteristics
      3. Internal characteristics

3. Distinguish between self excited and separately excited dc generator.

           Separately excited                             Self Excited
It the field winding of a generator is It the field winding of a dc machine is
excited by a separate dc supply then the supplied from the armature of the dc
generator is called separately excited.  machine itself then it is called self

4. Write the torque equation of dc motor?
      Ta= 0.159  Ia      N-m.
5. How is an ac converted into unidirectional ct in DC generator?
      Split rings.

6. Name any two methods of a speed control of in DC Shunt motor?
      Control method
      (i)    Rheostatic
      (ii)   Plugging

7. What are the factors governing the speed of DC motor?
      (i) flux/ pole
      (ii) Rheostatic control
      (iii) Applied voltage.

8. The outer frame of a DC machine serves double purpose. What are they?
      i.   It provides mechanical support for the poles and acts as a protecting
           cover for the whole machine.
     ii.   It carries the magnetic flux produced by the poles.
9. Mention the types of electric braking of a dc motor?
      (i) rheostatic
      (ii) plugging
      (iii) regenerative braking

10. What is the necessity of a starter for starting DC motor?
        To avoid flowing of fuses at the time of drawing high current and which
limits the starting current to a Safe value.

11. What is the significance of back emf?
        Eb =  ZN x (P/A) Back emf depends upon the armature speed. If speed is
high Eb is large hence armature current Ia as seen from the above equation is small.
If the Speed is less then E b is Less hence more current flo ws which develops motor
torque . So we find that E b acts like a governor i.e it makes a motor self regulating
so that it draws as much current as is just necessary.

12. What is mean by armature reaction?
       Armature reaction is meant the effect of magnetic field set up by armature
current on the distribution of flux under main poles of generator.

13. State any two causes for the failure of the generator to build up?
       (i) reversed shunt field connection
       (ii) reversed citation
       (iii) reversed residual magnetism.

14. What is the function of dc generator?
        An electrical generator is a machine which coverts mechanical energy into
electrical energy.

15. What is the advantages of Swinburne’s test?

(i) It is convenient and economical because power required to test a large m achine is
small is only no local input power.

(ii) The can be predetermined at any local because constant – losses are known.

16. What is an electrical generator?
      An electrical generator is a machine which mechanical energy to electrical

17. What is the basic principle of a dc generator?
       Basic principle of a dc generator is Faraday’s law electromagnetic induction.
i.e., whenever a conductor is moved in a magnetic field, dynamically induced emf
is produced in that conductor.
18. What are the essential parts of a d.c. generator?
      1. Magnetic frame or Yoke
      2. Poles
      3. Armature
      4. Commutator, pole, shoes, armature windings, interlopes
      5. Brushes, bearings and shaft.

19. What is the purpose of yoke in a d.c. machine?
  i. It acts as a protecting cover for the whole machine and provides mechanical
      support for the poles.
 ii. It carries the magnetic flux produced by the poles. The flux per pole divides
      at the yoke so that, the yoke carries only half the flux produced by each pole.

20. The core of the armature is laminated in DC machines. Justify.
 The armature core is made of laminations, in order to reduce the eddy current

21. What are the types of armature windings available in d.c. machine?
   1. Lap winding
   2. Wave winding.

22. What is the purpose of commutator in a DC generator?
      The commutator converts the alternating emf into unidirectional or direct

23. Write down the emf equation of a d.c. generator.

               Where E g - induced emf in generator,    P – number of poles
                            Z- Total number of conductors in armature, N – Speed
                   in rpm.
      A- Number of parallel path, Φ = Flux per pole in we b

      For lap winding A = P
      For wave winding A = 2.

24. What is a self excited d.c. machine?
      If the field winding of a dc machine is supplied from the armature of the dc
machine itself then it is called a self- excited d.c. machine.

25. What are the conditions for buildup of voltage in a shunt generator?
   1. The residual magnetism should be present in the poles.
   2. The field winding should be properly connected with armature.
   3. Under no load condition, the shunt field resistance should be less than the
      critical resistance.
   4. Under loaded condition, the shunt field resistance should be more than the
      critical resistance.

26. Why the air gap is made as small as possible between the armature and the poles
in a dc machine?
        The air gap made as small as possible between the armature and the poles in
a dc machine in order to reduce the reluctance of the magnetic path.

27. List out the main parts of stator of DC machine.
       1. Yoke                      2. Main Poles
       3. Field winding             4. Interpoles

                                    UNIT - II
1. Why is the effectively of transformer higher than other AC machines?
      Since the current of the transformer at portraiture local and pt is defined as
      the o/p by i/p it has higher I.

2. What is the condition for obtaining maximum efficiency of a transformer?
      CU loss = Iron loss

3. What is the main difference between the core type and shell type transformers
      In core type transformers the winding surround considerable part of the core
where as in shell type transformer the core surround a considerable portion of the

4. What are the tests to be conducted to estimate the performance of a transformer?
     Simper Test, Kappa Diagram.

5. Why transformer rating is expressed in KVA?
        CU loss of a transformer depends on current and iron loss on voltage. Hence
total transformer loss on volt ampere KVA and not on phase angle between voltage
That’s why it is transformer is in KVA and not in Kw.

6. Define the term regulation of transformers.
       When the transformer is leashed with a constant primary voltage, the
secondary voltage decreases because of its internal resistance and leakage reactance

       Let oV2 = secondary terminal voltage at No load.
            V2= Second terminal voltage on full load
        The change in secondary terminal voltage from no load to full load is oV2-
V2 . This change is divided by oV2 is Known as regulation down.

7. What is the method used to minimize the Iron loss in transformer?
      The losses are minimized by using steel of high silicon content for the core
and by using very thin lamination.

8. Why is the coreless of a transformer practically same for all load conditions?
      Since there is some at in the primary winding ling core loss is practically.

9. Define the term all-day efficiency of a distribution transformer?
        all – day = Out put
                    Out put +losses.

10. Write the EMF equation of a transformers with refers to primary and secondary
      E1 = 4.44fN1 øm = 4.44fN1BmA
      E2 = 4.44fN2f BmA.

11. List the advantage of core stepping in the design of a transformer?
       Core stepping not only gives high space factor but also results in reduced
length of the mean turn and the consequent I2 R loss.

12. Write short notes on Impedance test?
        Short circuit test meant for finding zo1 Or Zo2 . Xo1 or Xo2 . It also used to
final cu loss at full load.

13. What is an electric motor?

      Electric motor is a machine which converts electrical energy into mechanical

14. Define DC motor.
      DC motor converts d.c. electrical input into mechanical output.

15. What is the basic principle of operation of a dc motor?
      The basic principle of operation of a dc motor is that, a current carrying
conductor placed in a magnetic field experiences a force tending to move it.

16. Define back emf.
       When a motor rotates, the conductors housed in the armature also rotate and
cut the magnetic lines of force. So an emf is induced in the armature conductors and
this induced emf opposes the supply voltage as per Lenz’s law. This induced emf is
called back emf (or) counter emf.

17. What is the nature of the current flowing in the armature conductors of a dc
       Alternating current is following through the armature. This alternating
current is converted into dc by using commutator.

18. State Fleming’s left hand rule.
        The direction of rotation is given by Fleming’s left hand rule. If the thumb,
fore finger and middle finger of the hand are held such that the three fingers are at
mutually perpendicular directions, and if the fore finger indicates direction of the
field, and the middle finger, the direction of current then the thumb points to the
direction of rotation.

19. Write down the voltage equation of a d.c. motor.
      Voltage equation of a DC motor is given by V= Eb + Ia Ra

       where V – applied voltage,                  Eb – back emf
             Ia - armature current,                Ra – armature resistance

20. What are the different types of d.c. motors?
      The types of DC motors are
              Separately excited DC motor
              Self excited DC motor

    Series motor
    Shunt motor
    Compound motor

          a) Long shunt compound motor
          b) Short shunt compound motor

21. What is a separately excited d.c motor?
       In this motor, field winding and armature windings are separated. The field
winding is excited by a separate DC source. That is why it is called separately
excited DC motor.
22. What is a dc series motor?
      In a DC series motor, the field winding is connected in series with the
armature. The field winding should have less number of turns of thick wire.

23. What is a dc shunt motor?
       In a dc shunt motor, the field winding is connected across the armature. The
shunt field winding has more number of turns with less cross – sectional area.

24. Why a dc shunt motor is also called a constant flux motor or constant speed
       In shunt motor, flux produced by field winding is proportional to the field
current Ish .
       i.e., ( α Ish)
       Here, the input voltage is constant and so the flux is also constant. Therefore,
DC shunt motor is also called a constant flux motor or constant speed motor.

25. What is a compound motor?
      A dc motor consisting of both series and shunt field windings is called a
compound motor.

26. Why series motor cannot be started without any load?
       In dc series motor, flux is directly proportional to armature current. i.e., α
Ia. Under no load condition, the armature current is very low and flux also be less.
By using the formula N α , here is less, the motor speed will be very high. Due to
this motor will be damaged. Hence DC series motor should always be started with
some load on the shaft.
                                    UNIT - III
1. What are the applications of damper winding in synchronous motor?
       When a synchronous motor is used pr deiving a fluctuating load the rotor
starts oscillating about its new position of equilibrium corresponding to the new
load. To prevent heating dampers are employed. This is the application of dampers

2. Why is the single phase induction motor made as self starting?
       The starting method of single phase induction motor is very simple. An
auxillary winding in the stator is provided in addition to the main winding. Then the
induction motor. Starts as a too phase motor.

        The main winding axis and auxiliary winding axis are displaced by 90 o
electrical degrees. The impedance of the windings differ and class in the main and
auxiliary windings are phase shifted from each other. As a result of this a rotating
stator field is produced and the rotor rotates.
3. What are the reasons for the variation of terminal voltage of an alternator on
      1. Voltage drop due to Ra
      2. Voltage drop due to XL
      3. Voltage drop due to armature reaction.

4. What are the various method starting of synchronous motor?
      1. By dc source

5. List the torque associated with a synchronous motor?
        1. Starting Tongue 2. running Torque 3. Pull in torque 4. Pull out torque.

6. List out the application of synchronous motor
        1. Power factor correction
        2. Constant speed, constant load drives
        3. Voltage regulation

7. Write the emf equation of an alternator?
       EMF/phase = 4.4 4 Kc kd f ø T

8. Define voltage regulation Name t wo methods used to determine voltage
regulation of alternators.
       Voltage regulation of an alternator is defined as the rise in voltage when full
load is removed ctield excitation and speed remaining the same divided by the rated
terminal voltage.
       1. EMF method
       2. MMF method

9. Calculate the distribution factor for a 36 slots 4 pole single layer three phase
winding of a alternator?
       n= 36/4 =9           B= 180 o n/9 > 20 o
              m= 36 / 4 x 3= B
      Sinm / 2 Sin3x20 0 / 2
Kd                           0.96
      mSin / 2 3Sin20 0 / 2

10. Define hunting.
      When a synchronous motor is used for driving a varying load then condilion
known as hunting is produced.

11. Write the expression for distribution factor of a winding?
      Kd = emf with winding in3 slots / pole /phase
            emf with minding in 1slot / pole /phase
12. What are the different types of single phase motor?
   i. Single phase induction motors
  ii. Single phase synchronous motors
 iii. Single phase series (or universal) motors

13. What are the different types of single – phase induction motor?

  i.   Resistance – start (split – phase) induction motor.
 ii.   Capacitor – start induction motor.
iii.   Capacitor – run induction motor.
iv.    Capacitor – start capacitor run induction motor.
 v.    Shaded pole induction motor.

14. What are the applications, characteristics of split-phase motors?
      i) Fans                                     ii) Blowers
      iii) Centrifugal pumps                      iv) Washing machines
      i) The starting torque is 100% to 250% of the rated value.
      ii) The breakdown torque is up to 300%.
      iii) The power factor of the motor is 0.5 to 0.65.
      iv) The efficiency of the motor is 55% to 65%.
      iv) The power rating of this motor is in the range of ½ to 1 HP

15. What are the applications and characteristics of capacitor-start motor?
        i) Compressors
       ii) Conveyors
      iii) Air conditioning equipment
      iv) Pumps
       v) Refrigerator
      vi) Washing machine

        i) The starting torque is 250% to 400% of the rated value.
       ii) The breakdown torque is up to 350%.
      iii) Power factor of the motor is 0.5 to 0.65.
      iv) The power rating of the motor is ⅛ to 1 HP
       v) The efficiency of the motor is 55% to 65%.

16. What are the main advantages of capacitor run motor?
        i) High power factor at full –load
       ii) High full-load efficiency
       iii)     Increased pull-out torque
       iv)      Low full-load line current

17. What are the applications and characteristics of capacitor run-motor?
        i) Fans
       ii) Blowers
      iii) Centrifugal pumps
        i) The starting torque is 100% to 200% of the rated value.
       ii) The breakdown torque is up to 250%
      iii) The power factor of the motor is in the range of 0.75 to 0.9.
      iv) The efficiency of the motor is 60 to 70%
       v) The power rating of the motor is ⅛ to 1 HP.

18. What are the advantages of capacitor – start capacitor run motor?
      i) High starting torque                    ii) High efficiency
      iii) High power factor

19. What are the applications and characteristics of capacitor- start capacitor run
        i) Compressors                   ii) Pumps
        iii) Conveyors                   iv) Refrigerators
      i) The starting torque is 200% to 300% of the rated value.
     ii) The rated breakdown torque is up to 250%.
    iii) The power factor of the motor is 0.75 to 0.9.
    iv) The efficiency of the motor is 60 to 70%.
     v) The power rating of the motor is ⅛ to 1 HP.
20. Compare the performance and applications of resistance split phase and
permanent capacitor single induction motors.

         Features                Split phase induction       Permanent capacitor
                                         motor                induction motors.

1. Starting torque             Low or medium               High

2. Power factor                Low                         High

3. Efficiency                  Low                         High

4. Applications                Fans, blowers               Compressors, pumps
                                                           conveyors, refrigerators.
21. Name any two applications of shaded pole induction motors.
      i) Turn tables                   ii) Hair driers
      iii) Fans                        iv) Blowers

22. What are the drawbacks of the presence of the backward rotating field in a
single phase induction motor?
      i) Net flux will be zero
     ii) No starting torque in the motor.

23. List out four applications of shaded pole induction motor.
       i) Fans                             ii) Blowers
       iii) Turn tables                    iv) Hair driers

24. What is the use of shading coil in the shaded pole motor?
        The copper shading coil is used to produce rotating magnetic field movi ng
from the unshaded to shaded portion of the pole. Hence, the motor produces a
starting torque.
                                   UNIT - IV
1. What is an universal Motor?
       A universal motor is defined as a motor which may be operated either on
direct or single phase ac supply at approximately the same speed and output.

2. What are the uses of skewed rotor slots in a squirrel-cage induction motor?
       (i) It helps to make the motor rum quietly by reducing the magnetic hum and
       (ii) It helps in reducing the locking tendency of the rotor i.e the tendency of
the rotor teeth to remain under the stator teeth else to direct magnetic attraction
between the two.

3. The single phase induction motor is not a self starting motor why?
        When a single phase supply is fed to the stator winding it produces only an
alternating flux only is one which alternates along on space axis only. Due to this
starting torque will be Zero. Hence the motor does not rotate.

4. State any two applications of hysterisis motor?
          i) Used for driving an electric clock
         ii) It is used in gear train for reducing speed?

5. Name the advantage of brushless alternator?
      Brushes, sliprings and commutators are eliminated.

6. What are the types of rotors employed in induction motros?
    i) Squirrel cage rotor.
    ii)    Phase wound.

7. On what condition maximum tongue of an 7m can be obtained?
      R2 = X2

8. Define the term slip?
       Diff between Ns and actual speed of the rotor is slip.

9. What are the App of split phase motor?
      Fan, Blowers.

10. Why is IM not self starting motor?
      When feal from a 1 phase supply its stator relay produces a flux which is
only alternating i.e one which alternates along one space axis only. It is not a
synchronously revolving flux as in the case of 2 or 3 phase stator well tied from 2 or
3 phase supply. Now an alternating or pulsating flux acting on a stationary squirrel
cage motor can now produce rotation. That is why 1 phase motor is not self starting.

10. What   are the app of universal motor.
      i)    Sewing m/c
     ii)    Camera
    iii)    Hair dryer.

11. Define a transformer.
       A transformer is a static device which changes the alternating voltage from
one level to another.

12. What are turns ratio and transformation ratio of transformer?
       Turns ratio =
                            E2   I
Transformation ratio =         = 1 =K
                            E1   I2

13. What is staggering in the construction of transformers?
       In transformer, the joints in the alternate layers are staggered in order to
avoid the presence of narrow gaps right through the cross-section of the core.

14. What determines the thickness of the lamination or stampings?
      1. Frequency                2. Iron loss

15. Classify the transformer according to the construction.
       1. Core type transformer 2. Shell type transformer       3.    Berry      type
16. What are the two component in transformer’s no load current?
      1. Active or working component (I w)
      2. Reactive or magnetizing component (I  )

17. Why is the core of transformer laminated?
      The core of transformer is laminated in order to minimize eddy current loss.

18. Why transformer rating is expressed in terms of kVA?
       Copper loss depends on current and iron loss depends upon voltage. Hence
the total loss in a transformer depends upon volt-ampere (VA) only not on the phase
angle between voltage and current i.e. it is independent of load power factor. That is
why the rating of a transformer is given in kVA and not in kW.

19. What are the properties of an ideal transformer?
       The ideal transformer has the following properties.
1. No winding resistance. i.e., purely inductive.      2. No magnetic leakage flux.
3. No I R loss i.e, no copper loss.                    4. No core loss.

20. What is the application of equivalent circuit of a single phase transformer.
      1. To determine circuit parameters.
      2. To predetermine efficiency and voltage regulation.

                                                                               V2  N
21. Explain why in a real transformer V1 : V2  N1 : N2                i.e.,      = 2
                                                                               V1  N1
                                   N2                                  V
But in case of real transformer,      ratio is fixed but voltage ratio 2 is varied
                                   N1                                  V1
because of voltage drop due to resistance and leakage reactance. That is why, the
real transformer V1 : V2  N1 : N2.

22. Explain the term percentage impedance as applicable to transformer.

                                                 I 1 Z 01         I Z
       Percentage impedance at full load %Z =             x 100 = 2 02 x 100
                                                   V1              V2
%Z =    (%R 2  % X 2 )

23. Mention the properties of oil used in transformers.
      i) High dielectric strength.
      ii) Free from inorganic acid, alkali and corrosive sulphur to prevent injury to
      the conductor or insulation.
      iii) Low viscosity to provide good heat transfer.
      iv) Free from sludging under normal operating conditions.
      v) Good resistance to emulsion so that the oil may throw down any moisture
      entering the tank instead of holding it in suspense.
24. Define regulation up and regulation down for a transformer.
       The change in secondary terminal voltage from no-load to full load is V2NL –
V2 . This change is divided by V2NL is known as regulation down. If this change is
divided by V2 , then it is called regulation up.

                                   V2 NL  V2
         % Regulation down =                  x 100
                                     V2 NL

                               V2 NL  V2
         % Regulation up =                x 100
25. What is an ideal transformer?
      100% efficiency of the transformer is called ideal transformer.

26. State the condition for maximum efficiency of a transformer. Then what is the
corresponding output current?
       Iron loss = copper loss (or) constant loss = Variable loss …
       Hence efficiency of a transformer will be maximum when copper losses are
       equal to iron losses.
       The load current corresponding to maximum efficiency is given by

         I2 =

27. Define “all day efficiency” of a transformer.
       The ratio of output in kwh to input in kwh of a transformer over a 24 hour
period is known as all-day efficiency.
              kwhoutputi 24hours
 all-day =
               kwhinputin hours

28. Define regulation and efficiency of a transformer.
      The regulation of a transformer is defined as reduction in magnitude of the
terminal voltage due to load, with respect to the no-load terminal voltage.

29. Define regulation and efficiency of a transformer.
The regulation of a tr ansformer is defined as reduction in magnitude of the terminal
voltage due to load, with respect to the no-load terminal voltage.
         % regulation =

30. Name the factors on which hysteresis loss depends.
1. Frequency        2. Volume of the core       3. Maximum flux density

31. Why the open circuit test on a transformer is conducted at rated voltage?
       The open circuit test on a transformer is conducted at rated voltage because
core loss depends upon the voltage. This open circuit test given only c ore loss or
iron loss of the transformer.
                                     UNIT - V
1. What is a repulsion motor?
       Repulsion motors are similar to series motors except that the rotor and the
stator windings are inductively coupled i.e., the rotor current is obtained by
transformer action from the stator.

2. What are the disadvantages of repulsion motor?
      i) Speed variations with the variations in load dangerously high at no load.
     ii) Low power factor, except at high speeds.
    iii) Requires frequent maintenance.
    iv) Higher cost.
     v) Sparking at the brushes.

3. What is a universal motor?
       A universal motor is a specially designed series wound motor, that operates
at approximately the speed and output on either dc or ac of approximately same

4. What are the applications of universal motor?
       i) Sewing machines
      ii) High speed vacuum cleaners
     iii) Electric typewriters
     iv) Electric shavers
      v) Motion picture projectors
     vi) Portable hand tools
    vii) Hair dryers
   viii) Cameras
     ix) Food mixers

5. What is a hysteresis motor?
       It is a single phase motor whose operation depends upon the hysteresis effect
(i.e., magnetization produced in a ferromagnetic material lags behind the
magnetizing force) and on the presence of continuously revolving magnetic flux.

6. What are the applications of hysteresis motor?
        Due to their quiet operation and ability to drive high- inertia loads, hysteresis
motors are used for driving i) electric clocks ii) timing devices iii) tape – decks iv)
turn tables and other precision equipment.
7. Write some advantages of Hysteresis motor over induction motor.
  i) Here, hysteresis torque remains constant from locked rotor to synchronous
       speed, a hysteresis motor is able to synchronize any load.

 ii)         Due to its smooth cylindrical rotor, the motor operates quietly and does not
             affected from magnetic pulsations cause by slots/ salient poles that are
             present in the rotors of other motors.

8. What is switched Reluctance Motor?
       The switched reluctance motor is a double salient, singly-excited motor. This
means that it has salient poler on both the rotor and the stator, but only one member
carries windings. The rotor has no windings, magnets (or) cage winding. It works on
variable reluctance principle.

9. Why rotor position sensor is essential for the operation of switched reluctance
       It is normally necessary to use a rotor position sensor for commutation and
speed feed back. The turning ON and OFF operation of the various devices of power
semiconductor switching circuit are influenced by signals obtained from rotor
position sensor.

10. List the disadvantages of a switched reluctance motor?
      i) Stator phase winding should be capable of carrying magnetizing current.
     ii) For high speed operation developed torque has undesirable ripples is a
           result develops undesirable noises (or) acoustic noises.
    iii) For high speed current wave form has undesirable harmonics to suppress
           this effect large size capacitor is to be connected.
    iv) It requires a position sensor.

11. What       are the advantages of switched reluctance motor?
      i)        Construction is simple and robust.
     ii)        Rotor carries no windings, no slip rings, no brushes, less maintenance.
    iii)        There is no permanent magnet.
    iv)         Ventilating system is simpler as losses takes place mostly in the stator.
     v)         Power semiconductor switching circuitry is simpler.
    vi)         No shoot through fault likely to happen power short circuits.
   vii)         Developed torque doesn’t depend upon the polarity of current in the phase
  viii)         The operation of the machine can be easily changed from motoring mode
                to generating mode by varying the region of conduction.
       ix)      It is possible to get very high speed.
        x)      Depending upon the requirement T-w characteristics can be achieved.
       xi)      It is the self starting machine.
   xii)    Energy stored in the phase winding is fed back to the supply through the
           feedback diodes during off period.

12. What   are the applications of SRM?
      i)    Washing machines.
     ii)    Vacuum cleaners
    iii)    Fans
    iv)     Future auto mobile applications
     v)     Robotics control applications

13. What are the types of permanent magnet DC motor?
     i) PMBL square wave motor
    ii) PMBL sine wave motor

14. What is permanent magnet DC commutator motor?
       A dc motor consists of permanent magnet in the stator and armature winding,
commutator in the rotor. This motor is called permanent magnet DC commutator

15. What are the advantages and disadvantages of brushless dc motor drives?
   i) There is no field winding so that field copper loss is neglected.
  ii) Length of the motor is very small as there is no mechanical commutator, so
      that size becomes very small.
 iii) Better ventilation because of armature accommodated in the stator.
 iv) Regenerative braking is possible.
  v) Speed can be easily controllable.
 vi) Motor can be designed for higher voltages subjected to the constraint caused
      by the power semi conductor switching circuit.
vii) It is possible to have very high speeds.

  i) Motor field cannot be controlled
 ii) Power rating is restricted because of the maximum available size of
      permanent magnets.
iii) It requires a rotor position sensor.
iv) It requires a power semi conductor switching circuit.

16. List the various permanent magnet materials.
   i) Alnico
  ii) Rare – earth magnet
iii) Ceramic magnet
 iv) NdFeB magnet
  v) Sm2CO17 magnet
17. Mention some applications of PMBL DC motor.
   i) Power alternators
  ii) Automotive applications
iii) Computer and Robotics applications
 iv) Textile and Glass industries

18. Compare conventional DC motor and PMBLDC motor.

      Features            Conventional DC motor                PMBL DC motor
Mechanical structure   Field magnets on the stator      Field magnets on the rotor
Maintenance            Maintenance is high              Low maintenance
Winding connection     Ring connection                  The highest grade: D or star-
                       The simplest: Delta connection   connected 3-  connection.
                                                        Normal y-connected three phase
                                                        winding with grounded neutral
                                                        point or four-phase connection.
                                                        The simplest:
                                                        Two-phase connection.

Commutation method Mechanical contact between           Electronic switching using
                   brushes and commutator               power semi conductor devices
                                                        i.e transistors, MOSFETS.

Detecting method       Automatically detected by        Rotor position can be detected
                       brushes                          by using sensor i.e.,

Reversing method       By a reverse of terminal         Hall sensor; optical encoder.
                       voltage.                         Rearranging logic sequencer

19. Why is the PMBLDC motor called electronically commutated motor?
        The PMBL DC motor is also called electronically commutated motor
because the phase windings of PMBL DC motor is energized by using power
semiconductor switching circuits. Here, the power semiconductor switching circuits
act as a commutator.

20. What are the classification of BLPM dc motor?
      1. BLPM square wave motor                2. BLPM sine wave motor
21. Compare PMBLDC motor and switched reluctance motor.

             PMBLDC motor                               Switched Reluctance Motor
1. Rotor is a permanent magnet.                 No permanent magnet in the rotor.
2. High cost.                                   Cost is less compared with PMBLDC motor
3. Torque = 4B grlITph                             1 L
                                                T  i2
4. More efficient                                  2 
                                                Less efficient.

22. What are the features of permanent magnet synchronous motor?
      i) Robust, compact and less weight.
     ii) No field currents or rotar current in PMSM, unlike in induction motor.
    iii) Copper loss due to current flow which is largest loss in motors is about half
          that of induction motor.
    iv) High efficiency.

23. What are the applications of PMSM?
     i) Used as a direct drive traction motor.
    ii) Used as high speed and high power drives for compressors, blowers,
          conveyors, fans, pumps, conveyors, steel rolling mills, main line traction,
          ship-propulsion, aircraft test facilities.

24. What are the merits and demerits of PMSM?
i) It runs at constant speed.
ii) No field winding, no field loss, better efficiency.
iii) No sliding contacts. So it requires less maintenance.

i) Power factor of operation cannot be controlled as field winding cannot be
ii) It leads to losses and decreases efficiency.
25. Clearly explain the differences between synchronous reluctance motor and PM
synchronous motor.

   Synchronous Reluctance Motor                PM Synchronous Motor
1. Rotor has no permanent magnet.       Rotor has permanent magnet.
2. Less cost.                           High cost.
3. Low efficiency.                      High efficiency.

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