here is a little info that might help you better understand it
DC current is determined meaning that it is steady, a strait line, so there is no phase.
Here are a couple graphs I did really quick that should clear any of that up.
AC in your house: (single phase AC, it is called that because you only have one wire that is 'phased')
Note that voltage is defined as the energy difference between two points so when you have -120v on the 'hot' wire and 0 on your neutral you still have 120 volts worth of energy.
12v DC: (no Phase)
Here is a little animation to show a 3 phase brushless motor like we use.
Hope this helps, if you have any questions about it feel free to ask.