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Summary of Characteristics and Applications of D.C. Motors

7.12. Summary of Characteristics and Applications of D.C. Motors

The summary of characteristics and applications of D.C. motors is given in Table 1.

Table 1. Summary of Characteristics and Applications of D.C. motors

S
No.

Type of motor

Characteristics

Applications

1. Separately excited D.C. motors
  • Possible to obtain very accurate speeds.
  • Most suitable for applications requiring speed variation
    from very low value to high value.

    • Paper machines.
    • Steel rolling units.
    • Diesel electric propultion of ships.
 
2. Shunt motors:

(i) Constant speed:

 

  • Starting          torque-medium, usually limited to 250% by a starting resistor but may be increased.
  • Maximum operating torque
    usually limited to about 200% by commutation.
  • Speed control:

-         increase upto 200% speed by field control.

-         decrease by armature voltage control.

  • Employed for constant-speed applications ; may be used for adjustable speed not greater than 2 : 1 range.

Field of applications includes:

-        Lathes

-        Centrifugal pumps;

-        Fans and blowers

-        Machine tools;

-        Wood working machines;

-        Reciprocating pumps;

-        Spinning and weaving machines;

-        Printing presses, etc.

 

S.No. Type of motor Characteristics Applications
  (ii) Adjustable speed:

 

  • Starting          torque-medium, usually limited to 250% by a starting resistor but may be
    increased.
  • Maximum operating torque usually limited to 200% by commutation.
  • Speed regulation – 10 to 15%
  • Speed control: 6 : 1 range by field control.
  • Same as above,
For applications requiring adjustable speeds control, either constant torque or constant output.
3. Series motors:
  • Variable speed.
    • Adjustable varying speed.
    • Starting torque very high upto 500%.
      • Maximum momentary operating torque upto 400%.
      • Speed regulation : Widely
        variable, very high at no-lead.

        • Speed control: By series resistance.
  • Suitable for drives requiring high starting torque and where adjustable, varying speed is satisfactory.
  • Fields of application include:
-        Cranes;

-        Hoists;

-        Trolley cars;

-        Conveyors;

-        Electric locomotives etc.

Loads must be positively connected, not belted.

To prevent overspeed, lightest load should not be much less than 15 to 20% of full-load torque.

4 Compound motors:

(i) Cumulative compound wound:

 

 

 

 

 

 

 

 

 

 

(ii) Differentially compound wound:

  • Variable speed
  • Adjustable varying speed.
  • Starting torque high upto 450% depending upon the degree of compounding.
  • Maximum momentary operating torque higher than shunt,
    upto 350%.
  • Speed regulation: Varying, depending upon degree of compounding, upto 25-30%.
  • Speed control : Usually not used but may be upto 125% by
    field control.
  • Torque and speed almost constant.
  • Tendency towards speed instability with a possibility of
    motor running away and strong possibility of motor starting in wrong direction.

    • Suitable for drives requiring high
      starting torque and only fairly constant speed, pulsuating loads
      with flywheel action.
Fields of application includes:

-        Shears;

-        Punches;

-        Elevators;

-        Conveyors;

-        Rolling mills;

-        Heavy planes etc.

  • Employed for experimental and
    research work.