it is typical to use alternating north-south pole in an alternator to generate alternating current.my question is would an alternator generate d.c. voltage if identical or similar poles ( either north poles only or south poles only) are used and not alternating north-south pole

 

Monopoles have been discussed here before. If you ever come up with one alternator is the least of the applications, since none are known to exist at this time.

 

Is this what you are asking about? http://upload.wikimedia.org/wikipedia/commons/1/19/Faraday_disk_generator.jpg

Or, are you asking about a conventional permanent magnet, DC generator with commutator?

The reference for above is: http://en.wikipedia.org/wiki/Homopolar_generator