27/03/2020

Design of DC Machines Interview Question Answer - 1


1
State the applications of DC machines.


Applications of DC machines
  • Air crafts
  • Ships
  • Battery operated vehicles such as fork lift truck, tram car
  • Electrical train
  • Standby power supply
  • Uninterrupted power supply
  • Machines tools
  • Automatic control ( DC servo motor, DC stepper motor )
  • Appliances such as tape recorders, cameras
  • Satellite
  • Small gadgets, hand tools


2
State the applications of miniature motors.


The miniature motors are few power rating motors and they are used for intermittent duty operation.

3
Compare : Homopolar and Hetropolar DC machine


Homo Polar
  • They are DC machines in which permanent magnets or batteries are used control applications. 
  • They are not used for industrial drives.

Hetro Polar
  • They are machines in which excitation is provided by permanent magnet or electromagnets.


4
Describe the function of field winding in the DC machine.


The function of the field winding is to provide excitation.

5
Describe the function of frame in the DC machine.


Function of frame
  • The function of the frame is to provide mechanical support of the machine.
  • It also provides return path for the flux.


6
Why the cast steel is used for yoke material in place of cast iron?


Yoke material – cast steel
  • The saturation flux density of the cast iron is 0.8 weber / meter2 whereas it is 1.6 weber / meter2 in the cast steel.
  • As the working flux density of cast iron is twice to that of cast iron, it requires half cross section for yoke to that of cast iron in the DC machine.
  • The mechanical and magnetic properties of the cast iron are uncertain due to blow holes in the casting. 
  • The cast iron yoke is used only for small rating machines.
  • The silicon steel is generally used as yoke material in modern machines.


7
Whether the poles are laminated in the DC machine? What is its purpose?


Laminated poles
  • The poles are laminated in the DC machine for economy point of view. 
  • This is achieved when the same frame size is used for different rating motors.


8
Whether the pole shoes are laminated in the DC machine? Why?


Laminated pole shoes
  • The pole shoes are laminated in the DC machine in order to reduce pulsation losses in the pole faces.


9
Which material is used for fabrication of pole shoes?


Sheet steel

10
Which material is used for fabrication of inter poles?


Low carbon steel or laminated steel

11
Describe the function of tapered inter poles.


Tapered inter poles
  • The tapered inter poles are used in the large rating machine in order to avoid saturation at the root of the pole during heavy overload conditions.


12
Whether the dimension of pole body and pole shoes are same in the inter poles?


The length and width of pole body and pole shoes are same in the inter poles.

13
Which type of conductor shape is used for shunt field winding?


Rectangular copper wire for high rating machine and circular copper wire for small rating machine

14
Why the series field is placed below shunt field in the compound machine? Why?


Compound machine
  • The series field winding has higher mechanical strength as compared to shunt field winding because of large cross section of the conductors.
  • Therefore the series field winding is place below shunt field winding in the compound machine.


15
Whether the current density in the series field and shunt field is kept same for the DC compound machine?


Current density in the Field winding
  • The current density in the series field winding is kept higher than the shunt filed winding in the DC compound machine because of better cooling condition for series field winding.




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