AC inverters (often referred to as just "inverters") and VFD inverters have some common features, but also have many important differences that affect their ideal usage.
What does an AC inverter do?
The AC inverter is an essential component in the vast majority of solar installations. The inverter converts the power generated by solar panels from direct current (DC) to alternating current (AC), which is the standard used in the modern electricity grid.
Due to the way they are designed, and the unreliable intensity of sunlight, AC inverters cannot power appliances with just solar. Instead, power from the electricity grid or batteries must be available in order to provide a constant voltage (typically 220) and frequency (50 Hertz).
How is a VFD inverter different?
Many motors, such as those used in flour mills and water pumps, will adjust the speed based on the frequency of the power provided. VFD ("Variable Frequency Drive") inverters take advantage of this, and adjust the frequency based on the power that is generated from solar panels. When the solar generation is high, the speed of the motors will be at maximum. Conversely, when the solar generation is low (due to weaker sunlight), the speed of the motors will be slower.
While this results in an inconsistent speed of the motors, this method of operation enables VFD inverters to be significantly cheaper, and to operate without the presence of the electricity grid and batteries.
For more information, find out more about powering motors with VFD inverters.