Yes, It is based on the brushes used in M/c. But the 
primary wave generated by m/c is always A/C

It is by virtue of rotation of alternator through 360 
degree and Fleming's Left hand rule that we get Sine wave. 
We cannot generate other wave forms by Alternators. For 
other form of Wave Inverters can be used.

alternator is designed to rotate through an angle of
360degrees during operation one of the phase will be
reaching max and other will be in zero position so by this
the wave form will be pulsating and the first wave generated
will be ac in every machine so we cannot produce other
waveforms....to produce other wave forms u can go for invertors

i think we can produce other wave forms also.it depends on 
the shape of the pole, in a salient pole motor we get a 
sine wave due to its shape,we may also get a flat topped 
wave.