DC Motor Speed Control

$N \propto \frac{V - I_a R_a}{\Phi}$

• Armature voltage control — speeds below base speed at constant torque (modern drives use a PWM chopper or controlled rectifier).
• Field weakening — reduce $\Phi$ to go above base speed (constant power).
• Armature resistance control — simple but wasteful.

Starting: at standstill there is no back-EMF, so $I_a = V/R_a$ could be 10–20× rated current — a starter (series resistance or soft-start electronics) limits it. Never run a series motor without load: its speed can rise dangerously.