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
Type of motor
||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.
(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:
- Centrifugal pumps;
- Fans and blowers
- Machine tools;
- Wood working machines;
- Reciprocating pumps;
- Spinning and weaving machines;
- Printing presses, etc.
||Type of motor
||(ii) Adjustable speed:
- Starting torque-medium, usually limited to 250% by a starting resistor but may be
- 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.
- 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:
- Trolley cars;
- 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.
(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,
- Speed regulation: Varying, depending upon degree of compounding, upto 25-30%.
- Speed control : Usually not used but may be upto 125% by
- 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:
- Rolling mills;
- Heavy planes etc.
- Employed for experimental and