Exam Question

### (a) Speed Control Methods of DC Series Motors
The speed of a DC motor is governed by the relationship $$N \propto \frac{E_b}{\phi}$$, where N is the speed, E_b is the back EMF, and Φ is the magnetic flux. The back EMF is given by $$E_b = V - I_a R_a$$, where V is the terminal voltage, I_a is the armature current, and R_a is the armature circuit resistance. Therefore, speed can be controlled by varying either the flux (Φ), the armature circuit resistance (R_a), or the applied voltage (V).
For DC Series motors, the primary methods are:
**1. Flux Control Method (Field Control)**