According to ODO, and other dictionaries, convertor is a variant of the more common
converter and is used for all its different meanings:
(Computing) A program that converts data from one format to another.
A device for altering the nature of an electric current or signal, especially from AC to DC or vice versa, or from analogue to digital or vice versa.
The double suffix may derive from the etymology of the term: Convert : c. 1300, from Old French convertir, from Vulgar Latin convertire, from Latin convertere "turn around, transform,"
English agent noun ending, corresponding to Latin -or. In native words it represents Old English -ere (Old Northumbrian also -are) "man who has to do with,"
Generally used with native Germanic words. In words of Latin origin, verbs derived from past participle stems of Latin ones (including most verbs in -ate) usually take the Latin ending -or, as do Latin verbs that passed through French (such as governor); but there are many exceptions (eraser, laborer, promoter, deserter; sailor, bachelor), some of which were conformed from Latin to English in late Middle English.
- Its usage, as shown in Ngram is much less common the converter, but it is a valid term.