A Variac is a variable auto transformer. That means it has only one winding which serves as both the primary and the secondary windings. The primary is usually fed in on two fixed terminals, a neutral and a hot. The output is taken from the neutral and from a rotating brush that travels across the windings. That brush can usually travel from the zero Voltage point (at the neutral connection) to some distance past the hot input. So it can deliver any Voltage from zero to 125 to 150 percent of the input Voltage or even more.
A 2000 Watt unit can handle more power than a 500 Watt unit. But there will also be a current rating for that winding and power equals Voltage X current. So at low Voltages you may reach the current limit before you reach the power limit, perhaps a lot sooner and your 2000 Watt unit may only be able to deliver 100 Watts or less. At higher Voltages the power rating will take over as the limiting factor. You need to read the spec sheet carefully, but in general it should be able to handle the rated power at the one to one point where the output Voltage equals the line Voltage.
What will the 2000 Watt one do that the 500 Watt one will not. Well, it will allow you to control larger motors. Both of them will handle a motor that consumes 500 Watts, at least up to the one to one point that I mentioned above. With a 1000 Watt motor you will not be able to run at full speed with the 500 Watt unit without overheating the Variac and possibly damaging it. The only warning you would get before failure would be the amount of heat it gives off. In general, when going past the ratings, the larger the motor, the more limited you will be with a Variac.
You will probably be on the safe side if the power rating of the Variac is 1.5 to 2 times that of the motor. Otherwise, you can do a careful calculation of the Voltage, power, and current at various settings. That would allow you to minimize the size of the Variac for a given motor and load.
Paul A.
Make it fit.
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