Comparison of the speed control methods of shunt/separately excited dc motor

S. No.	Basis of comparison	Significance	Armature resistance control	Field control	Ward Leonard Control
1.	Limit, range and hardness of speed control	Signifies the ratio of speed with load	The speed in the range of zero or almost crawling to base speeds is possible. The range of speed control depends on load.	Speeds below base speed require field currents above the rated value. Hence it is not employed for speeds below base speed.	An elegant method for smooth variation of speed over a wide range. Speed reversal is possible by reversing the generator field.
2.	Smoothness of speed control	Signifies the number of steady speeds in the operating range.	In large drives speed control is obtained by varying the armature resistance using contactors.	The field circuit is low power one. The resistance in the field circuit can be smoothly varied	Speed control is achieved by controlling the generator field.  The smoothness with which the generator field can be varied decides the smoothness of speed control.
3.	Economics of speed control	Decided by the efficiency of the motor, maintenance and overall cost.	The power loss in the armature resistance reduces the efficiency. The efficiency is very poor, particularly at low speeds.	The method is economical. The efficiency is not very much affected, because the field losses decrease and rotational losses increases. The method is well suited for constant power loads.	High initial cost both for conventional and static. Efficiency is better than armature resistance control.
4.	Stability of operation	The capacity of the drive to return to its original state or a state of new operating conditions following a disturbance in speed or torque	The stability is decided by the torque driven at given speed.	The weak flux conditions in the machine are prone to increase degree of armature reaction. The armature reaction may cause stability problems.	Stability is very good at speeds below base speed. At higher sped he stability is a matter of concern.
5.	Direction of speed control	Signifies the possibility of speed control below and above base speeds.	Only speeds below base speed up to zero are possible.	Only speeds above base speed are possible due to field rating.	Speed variation below base speed by armature voltage control and above base speed by field control.
6.	Permissible load	Signifies Whether the machine is over loaded or under loaded in its speed range and thus utilisation of the machine.	Suitable for constant torque loads	Suitable for constant power loads	Suitable for both constant power & torque loads