Rheostatic Control Method:
Rheostatic Control Method – Series armature-resistance control : Here the applied armature voltage is varied by placing an adjustable resistance Re in series with the armature as shown in Fig. 7.58 along with the speed-torque characteristics.
Some of the limitations of the rheostatic control methods are enumerated below:
(i) Only speeds below the rated value can be obtained. This can be shown by using Eq. (7.71) (armature resistance is negligible) that
(ii) Range of speeds available is limited because efficiency is reduced drastically for large speed reductions. By definition of armature efficiency
(iii) The speed regulation of the method is poor as for a fixed value of series armature resistance, the speed varies directly with load, being dependent upon the resistance voltage drop.
In general, rheostatic controls is economically feasible only for very small motors (fractional kW) or for short-time, intermittent slow-downs for medium-sized motors.