The correct answer is: D. Any of the above.
The speed of a DC motor can be varied by varying the field current, the applied voltage, or the resistance in series with the armature.
- Varying the field current changes the strength of the magnetic field that the armature rotates in. This in turn changes the speed of the motor.
- Varying the applied voltage changes the amount of current that flows through the armature. This in turn changes the speed of the motor.
- Varying the resistance in series with the armature changes the amount of current that flows through the armature. This in turn changes the speed of the motor.
In general, the speed of a DC motor can be varied by any combination of these three factors. However, there are some practical considerations that need to be taken into account. For example, if the field current is too low, the motor may not start at all. If the applied voltage is too high, the motor may overheat. And if the resistance in series with the armature is too high, the motor may not have enough power to do the job it is supposed to do.
Therefore, it is important to choose the right combination of factors to vary the speed of a DC motor in order to get the desired results.