There are three considerations to ponder when choosing a Variac power supply. It is important to have a basic understanding of Variacs before deciding to purchase one. There are many great articles available via the internet with this type of information. The first thing to do when picking a Variac, is to determine which characteristics and the line frequency your applications require. Once theses are known it is time to begin searching.
First check the line frequency of the devices. Most Variacs will be for a 50-60 Hz range. However, there are some that are designed to operate from 500 to 1200 HZ. In most cases a line frequency of that level is not needed, but it may be for your project.
Now it will be time to decide on a phase level for your Variac. There are two types of phases found on Variacs, one and three phase. A single phase will most likely be 120 or 240 volts, depending on the manufacturer. If a voltage of 480-560 volts is needed there is always the option of using two Variacs that are rated at 240V. The coils would need to be connected in series across the line, then the load connected to one side of the outputs.
The load rating is the final step. Load ratings are specified in three ways; maximum current, rated current in amperes, and power in kVA. They are all very similar, however choosing the right one is important. The kVA rating is a great way to determining which Variac will meet your power requirements.