1.Vary the dc power supply and the motor speed will follow the applied voltage.
2.Pulse width modulation.
Constant amplitude voltage pulses of varying widths are provided to the motor.The wider the pulse,the more energy transferred to the motor.
The frequency of the pulses is high enough that the motors inductance average them and it runs smooth.
3.A single transistor and diode can control the speed of a dc motor.
The motor speed (Voltage) is proportional to the transistor ON duty cycle.
Positive torque only - Passive breaking.
4.An H-bridge power circuit enables four quadrant control.
Forward and reverse motion and breaking.
Complementary PWM signal applied to the high and low side switches in the bridge.