How many kinds of electric fan speed regulation methods are there? Speed regulation principle?

Generally, PWM or SPWM is used for speed regulation, which saves electricity and has high efficiency. Principle: The core component of the electric fan is the motor, and the principle of speed regulation first depends on the principle of the motor. Since there are inductors and capacitors, pwm pulse width modulation technology is needed for speed regulation. Slowing down saves electricity. But the electric fan should be driven fast. Fast gear saves electricity than slow gear. Hope to adopt, thank you ~ ~