in dc motor commutator converted DC to AC 

in DC genator its function is AC to DC that's way we called 
as commutator is a mechanical rectifier

The DC Generator generate the AC supply only in Armature 
not DC Supply.But we want Dc source , so want one converter 
thats why we use commutator.In shortly we can say its 
converts AC source to DC source.

in dc motor commutator converted AC to DC 
it is also used to collect the current from the winding

in d.c motor commutator serves in produsing unidirectional 
torque

its obvious t rectifies ac to dc..bcoz the current produced 
by the dc generator will b of sinusoidal form obviously ac 
since it s a dc generator it should produce only dc 
current..in order 2 convert it to dc v r using this 
commutator.

it the part of dc generator.in dc generator two half rings 
are used the purpose of these rings is to prevent the 
direction of current as in ac thee are caled as commotator

to reverse the current through the rotating coilseach half 
turn or to step up the voltage frm the electrical sourceor 
to convert the motor into an

Dear all please note commutator is not a converter, all are 
telling dc ot ac, we already giving dc supply to motor 
through motor terminals at A1 & A2 (ie.+ & -) it is 
transformed to rotor winding or bars through Brush and 
commutator.

 DC supply - Motor TB A1&A2 - Carbon Brush - Commutator -
             Rotor winding or copper bars

commutator brush is made of carbon material.and it holds in 
the rotor segment, its mainly used to collect current from 
the winding..

in dc motor,rotor provides an alternating flux for the
reason of provides rotating torque so that need for ac
supply to rotor..in order to that comutator converts the ac
from dc supply to rotor... so commutator need for dc motor..

first of all ,it is dc motor not a generator.
We are already giving it a dc current ,so normally there 
should not have been any need of commutator'
but as the armature is rotating, with every half 
rotation ,the direction of rotation is reversed.
so it should be rotating in anti_clock wise direction for 
half cycle and in clock_wise direction for next half cycle.
to cater this ,the direction of current should be reversed 
after every half cycle.
that's why a commutator is used to change the direction of 
current to cater the problem of torque