Expatriates or long-term visitors to the United States, Canada and parts of Latin America who arrive with 220-volt appliances cannot use them with 110-volt local current unless they are converted. Rewiring an appliance or device or installing an internal transformer is almost impossible. Therefore, the most practical option is to convert them using a step-up AC power transformer. Such transformers usually have European style two-prong round or British style three-prong flat outlets for plugging in appliances, and a standard US style power cord that you plug into the wall outlet.
Make sure the appliance or device you need to convert does not have a voltage switch and that it is not rated for both 110 and 220 volts. Many desktop computers, shavers and hair dryers have voltage switches; if yours does, simply change the setting to 110 volts. Laptop power packs and cellphone adapters are often rated for both 110 and 220 volts.
Check the wattage on the appliance you need to convert and obtain a step-up transformer that is rated for the correct wattage or higher. An appliance will not run on a transformer with too low a wattage and a higher wattage will not cause any damage.
Plug the power cord of the appliance into the outlet of the transformer. You may need an adapter plug if the outlet is not the usual type for the plug of your appliance. However, some transformer outlets are universal even if they appear to be British or Australian standard; check before buying extra adapter plugs.
Plug the power cord of the transformer into the wall outlet. You can now turn the appliance or device on and use it as you normally would. The difference in frequency between American and European electricity may affect speeds of motorized appliances, but you usually will not notice any difference.