27/03/2020

Design of DC Machines Interview Question Answer - 5



61
Comment : The conductor area is directly proportional to square of armature diameter.


Conductor area – Armature diameter
  • Let we assume that conductor area is directly proportional to square of armature diameter.
  • This is true if depth and width of slot increases linearly with armature diameter.
  • The width of slot increases with diameter of armature but depth of the slot cannot make to vary directly with armature diameter.
  • As the deep depth of slot result in high leakage reactance giving rise to high reactance voltage which greatly affects the armature reaction.
  • Therefore the conductor area is not directly proportional to square of armature diameter because it is limited by depth of slot.


62
Describe the relation between ampere conductor per meter and armature diameter.


Ampere conductor – Armature diameter
  • There is linear relation between ampere conductor and armature diameter only if there is no change in the flux density in teeth.


63
Describe the effect of ampere conductors on the armature reaction of the DC machine.


Ampere conductors – Armature reaction
  • As the ampere conductors increase, the armature flux becomes stronger than the field flux.
  • This will result in distortion of field flux increases.
  • However the distortion effect is reduced by increasing the field flux.
  • The field flux increases by increasing the field turns which increase the cost of machine.
  • If the ampere conductors of the armature increase, the cost of machine increases due to effect of armature reaction.


64
Describe the effect of high value of ampere conductors on the commutation of DC machine.


Ampere conductors – Commutation
  • The ampere conductors per meter for DC machine is given       by = IZ / πD
  • The ampere conductors per meter are increased by increasing number of armature conductors or decreasing armature diameter D.
  • The inductance of the coil is directly proportional to square of armature turns. Therefore the high value of ampere conductors per meter results in high inductance.
  • If the diameter of the armature decreases, it is not possible to use wider slots. 
  • If the slots are become wider, the thickness of teeth becomes smaller giving rise to high flux density in them.
  • Therefore the deeper slots are used to accommodate armature conductors which also increase the inductance of coil.
  • The inductance of the coil increases in both of the above cases. The reactance voltage of coils is directly proportional to inductance of coil. 
  • Therefore the high value of reactance voltage delays commutation condition.
  • It is finally conclude that the higher value of ampere conductors per meter adversely affects the commutation condition.


65
State the advantages and disadvantages of high value of electrical loading.


Advantages
  • Volume and size of machine is reduced
  • Weight is reduced
  • Reduce cost

Disadvantages
  • Armature and field copper loss increases
  • Reactance voltage increases which worsens commutation condition
  • Temperature of machine increases


66
State the advantages and disadvantages of high value of magnetic loading.


Advantages
  • Volume and size of machine is reduced
  • Weight is reduced
  • Reduce cost

Disadvantages
  • No load current increases
  • Field copper losses increases
  • Iron loss increases
  • Noise increases
  • Flux density in the tooth increases
  • Magnetic saturation of iron parts increases


67
Explain : ‘ The specific electrical loading and specific magnetic loading are interdependence ’.


Specific electrical loading – Specific magnetic loading
  • The output of the DC machine is directly proportional to product of specific electrical loading and specific magnetic loading.
  • The value of specific electrical loading and specific magnetic loading are interdependent for same output rating.
  • The specific electrical loading of the machine increases if the lower value of the specific magnetic loading is chosen and vice versa.


68
Comment : The tooth width affect the specific magnetic loading of the DC machine.


Tooth width – Specific magnetic loading     
  • The flux density in any part of the tooth does not allow increasing beyond 2.2 weber / m2 in the DC machines.
  • The specific magnetic loading of the DC machines increases if the tooth width allows to increases but keeping the slot width same as earlier.
  • Therefore we conclude that higher value of specific magnetic loading allows for wide tooth and vice versa.


69
Comment : The tooth width affect the specific electrical loading of the DC machine.


Tooth width – Specific electrical loading
  • The flux density in any part of the tooth does not allow increasing beyond 2.2 weber / meter2 in the DC machines.
  • If the tooth width increases for specific slot pitch, the width of slot decreases. Therefore there is small space left to accommodate armature conductors. This is only possible by smaller value of ampere turns per meter.
  • Therefore we conclude that as the tooth width increases, the specific electrical loading of machine reduces.   


70
Describe the effect of yoke area on the number of poles in the DC machine.


Yoke area – Number of poles
  • The flux carries by the yoke is inversely proportional to number of poles. 
  • As the number of poles increases, the yoke area decreases and vice versa.


71
Why the iron losses do not occur in the yoke of the DC machine?


Iron losses in the yoke
  • The yoke carries steady flux therefore there is no iron losses in the yoke of the DC machine.


72
Describe the effect of number of poles on iron losses of the core and weight of armature core.


Number of poles – Iron losses, Weight of armature core
  • As the number of poles increases, the iron losses of the core increase due to increase frequency of flux reversals.
  • The weight of armature core decreases as the number of poles increases.


73
Describe the effect of number of poles on the eddy current loss in the DC machine.


As the number of poles in the DC machine increases, the eddy current loss does not affected for the same core area.

74
Describe the effect of number of poles on the hysteresis loss in the DC machine.


As the number of poles in the DC machine increases, the hysteresis loss decreases for the same core area.

75
Define : Active conductors, Inactive conductors


Active conductors
  • The conductors which are responsible for production of induced emf or torque production in the DC machines is called as active conductors.
  • The conductors in side slots area are called as active conductors.

Inactive conductor
  • The overhang portion of the conductor is called as inactive conductors. 
  • The overhang portion of the conductor provides connection between active conductors.
  • The inactive conductors do not contribute any emf.



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