What are the different methods of DC motor speed control?

DC motor speed control methods include: armature voltage control (varying voltage to armature, provides speed below base speed, constant torque), field weakening (reducing field current, provides speed above base speed, constant power), armature resistance control (inserting resistance, inefficient but simple). Modern methods use PWM-based DC drives for precise armature voltage control. Speed equation: N = (V - IaRa)/(K x phi) shows that speed varies with voltage (Va), resistance (Ra), and flux (phi).