I thought this was a cool depiction of a variable frequency drive.



Sometimes when systems are dumbed down like this, they are easier to comprehend.

There was actually a good intro to variable frequency drives that accompanied the diagram, so here it is:

Define the Inverter / Variable Freq. Drive

The term inverter and variable frequency drive are related and somewhat interchangeable. An
electronic drive for an AC motor controls the motor's speed by varying the frequency of the
power sent to the motor.

An inverter, in general, is a device that converts DC power to AC power. The diagram shows how the variable-frequency drive employs an internal inverter.

The drive first converts incoming AC power to DC through a rectifier bridge, creating an internal DC bus voltage. Then the inverter circuit converts the DC back to AC again to power the motor. The special inverter can vary its output frequency and voltage according to the desired motor speed.

The simplified diagram of the inverter shows three double-throw switches. The switches are actually IGBTs (Isolated Gate Bipolar Transistors). Using a commutation algorithm, the microprocessor in the drive switches the IGBTs ON and OFF at a very high speed to create the desired output wave forms. The inductance of the motor windings helps smooth out the pulses.

More good basic info on inverters can be found here Inverter FAQ's

The basic frequency drive diagram reminds me of an electrical one-line diagram.