Electrical Engineering ⇒ Topic : Armature Resistance Control

Figure shows the circuit for controlling the speed of a dc motor by means of a variable resistor connected in series with the armature. The electrical connections for a controller are exactly the same as for a starter.However,the wattage rating is different.The starting resistance should only be inserted in the circuit for a short interval,whereas the speed control resistance can remain in the circuit indefinitely. For a given armature current, the larger the resistance of the controller, the smaller will be voltage drop across the armature and hence the speed will be lower.

The main disadvantages of this type of speed control are (a) it is relatively more costly, (b) the speed varies greatly with variation of load due to changes in the voltage drop across the controller, (c) the power losses in the resistors are large at low speeds resulting in low efficiency of the motor and (d) the speed can not be increased. The main advantage is that the speed can be varied easily and the method is chiefly used for controlling the speed of cranes, hoists, trains etc.