Difference of Variable frequency drive and Frequency converter

A variable frequency drive is not a typical converter though it has conversions inside it. A converter would typically convert a DC signal to an AC signal vice versa. You do get variable frequency drives that can convert single phase AC to 3 Phase AC then in that case it is a converter but not a typical one.

What is Variable Frequency Drive?

A variable frequency drive runs a motor in such a way that you can increase or decrease the velocity of rotation by varying the frequency of the input signal i.e incoming AC voltage. In effect it converts a standard AC frequency that is typically 50 hz or 60 Hz to anything between 0 Hz and either 50 or 60 Hz.

With a variable frequency drive you can have different configurations to control a motor. You can control the speed (rpm) of a motor by changing the frequency and you can do this in different ways. You can use preset speeds, you can change preset speeds to increase speed depending on your application, you can also control speed in a variable way.

It's more than frequency conversion... you have a first stage that rectifies incoming AC, and charges a DC capacitor bank called the "DC Bus". Then, the energy from the DC Bus is modulated as a PWM output (usually 4KHZ carrier) using a microprocessor... this is what controls how much energy is sent to the motor, and allows you to control its speed and torque from the variable frequency drive.
Also, variable frequency drives are sometimes called "Inverters" because of that last stage... commonly referred to as a "power inverter" because you're generating AC power, using DC as the power source... (and hence, you're "inverting"...)