Torque Speed Characteristics of DC Shunt Motor

How does varying armature current affect the flux, speed, and torque in the DC Shunt Motor?

A constant applied voltage V is assumed across the armature. As the armature current Ia varies the armature drop varies proportionally and one can plot the variation of the induced emf E. The MMF of the field is assumed to be constant. The flux inside the machine however slightly falls due to the effect of saturation and due to armature reaction. Knowing the value of E and flux one can determine the value of the speed. Also knowing the armature current and the flux, the value of the torque is found. This procedure is repeated for different values of the assumed armature currents and the values are plotted. From these graphs, a graph indicating speed as a function of torque or the torque-speed characteristics is plotted. As seen from the figure the fall in the flux due to load increases the speed because the induced emf depends on the product of speed and flux. Thus the speed of the machine remains more or less constant with the load. 

How does series winding provide stability to DC Shunt Motor?

With highly saturated machines the on-load speed may even slightly increase at overload conditions. This effect gets more pronounced if the machine is designed to have its normal field ampere-turns much less than the armature ampere-turns. This type of external characteristic introduces instability during operation and hence must be avoided. This may be simply achieved by providing a series-stability winding that aids the shunt field MMF.

Load Characteristics

Torque-Speed Curve