DC motors are comprised of an armature and a field. The field can be a magnet or wound like an AC motor. DC motors come in a few different variations. (For another day). The armature receives the DC voltage and the field repels it. That is why it turns. By lowering the DC voltage to the armature the motor will slow down. When the voltage increases to the armature the motor speeds up proportionally to the voltage. This is how you basically control a DC motor.
The DC motor can achieve higher base (nameplate) speeds by whats called field weakening. Lowering the field voltage. This is very basic DC motor technology. This what I know, and I do not know everything.
AC motors are comprised of a rotor and stator. The rotor is built with iron bars and looks like a single barrel of iron. The stator looks very similar to a wound DC motor field depending on the motor.
To achieve suitable speed control of an AC motor you must proportionally adjust hertz and voltage. For example. 460 volt, 1800 rpm, 60 Hz motor. If you apply 460 volts at 60 Hz the motor will run at base (nameplate) speed, 1800. If you apply 240 volts at 30 Hz the motor will run at 900 rpm. If you apply 120 volts at 15 Hz the motor will run at 450 rpm.
This is where Pulse Width Modulation comes into play. AC inverters (VFD's Drives) input AC voltage, then convert the AC to DC. It is stored in the DC bus (800 volts DC at 460 volt AC input) The drive then uses whats called IGBT's, capacitors and logic to transform this DC voltage into a simulated AC signwave. This is PWM at it's most basic description.
Keep in mind all the values above are expressed in general terms and can fluctuate a few volts and rpm's here and there.
Thats all I have my friend. Let me know what you think........John Valdes