Question

State whether True or False :

The speed of DC series motor is directly proportional to armature current.

• A. True
• B. False

Answer 1

The correct option is B False

In case of DC series motor, $I_a=I_f=I_l$

and $N\propto \frac{E_b}{\varphi }$ It is obvious that speed is proportional to back emf $E_b$ and inversely proportional to flux per pole $\varphi$. Now flux per pole is directly proportional to armature current i.e. $\varphi \propto I_a$ because the armature and field are carrying same amount of current. So, speed is inversely proportional to the armature current.