A voltage converter is basically an electric power converter which either changes or converts the voltage of an electrical power source.Voltage converter can be AC to DC or DC to AC. In both cases for conversion we uses a transformer, this can be either step up or step down transformer while the Conversion from one DC voltage to another requires electronic circuitry also. A most common use of the voltage converter is for a device that allows appliances made for the mains voltage of one geographical region to operate in a region with different voltage. Such a device may be called a voltage converter, power converter, travel adapter, etc.
Most single phase alternating-current electrical outlets in the world supply power at 210–240 V or at 100–120 V. A transformer or autotransformer can be used; (auto) transformers are fundamentally reversible, so the same transformer can be used to step the voltage up, or step it down by the same ratio. Means a single reversible transformer can be used for both operations, either step up or step down. Lighter and smaller devices can be made using electronic circuitry, reducing the voltage electronically is simpler and cheaper than increasing it.
On the other hand side Transformers do not change the frequency of electricity and reason for this is simple because in many regions with voltage 100–120 V, electricity is supplied at 60 Hz, and 210–240 V regions tend to use 50 Hz. This may affect operation of devices which depend on mains frequency (some audio turntables and mains only electric clocks, etc. although modern equipment is less likely to depend upon mains frequency). Equipment with high-powered motors or internal transformers designed to operate at 60 Hz may overheat at 50 Hz even if the voltage supplied is correct.
The most important thing is that when it comes to buying a converter transformer then on the back of appliance, we will find a label describing the power requirements. we should see a label describing the Wattage (W) or the Amperage (A) of the appliance. Once noted, choose a voltage transformer / converter which can handle a higher amount of wattage then our device is rated at. Certain devices which are motor based may require additional power to start up then indicated (known as Surge), in this case we should generally add an extra 20% to the power requirements of the device. However some high surge devices have been known to consume 2 or 3 times the wattage indicated.
Reference:
https://en.wikipedia.org/wiki/Voltage_converter
http://www.voltageconverters.com/faq.htm