Power frequency converters take electrical input power at one frequency and voltage and provide electrical output power at a different frequency, and at a different or equal voltage. There are two basic types of power frequency converters: rotary (motor-generator set) and solid state.  The major difference between the two is the method used to enact the frequency conversion.  Either of these two types of power frequency converters is acceptable for most applications but there are a number of distinct differences in their capabilities and features.  These differences may determine that one type of converter is better suited to a specific application.  When in doubt, check with the product manufacturer.