7.11. Comparison of D.C. Motor Characteristics
Fig. 73 shows the comparison among shunt, series and compound operation for the same motor with its armature being acted upon by either or both fields. It is worth noting here that the outputs vary at the same armature current.
- Fig. 74 compares shunt, series and compound performance when the three are rated at the horse power (or kilowatts) and rpm (or radians/see). In this type of plot all the characteristics cross at the design load point.
- Figs. 73 and 74 are drawn with the motors operating on their full rated voltage. Different voltages will move the curves, but the relative shapes will remain similar.
Fig. 74. Speed and torque relations for shunt, series and compound fields. Motors of same rated speed, torque and current.
The following may be deduced from Fig. 74.
(i) The curves diverge between no-load and full load. If the load torque were around mid-load, the shunt motor would draw the least current and the series the most.
(ii) At overloads the opposite is true. The series motor can develop an overload torque of around double the rated torque without drawing excessive current.
Fig. 73 shows that the speed characteristics for the same armature current are very much different. Therefore, the ability to take increasing torque loads that is the special contribution of a series field is accompanied by a much greater speed change.