Generators and electric motors work on same principle..
In electric motor we have to change the direction of current after every half cycle of the axle and we do this by using commutators which rotate the axle.. That is for DC current
When we use AC current it doesn't need commutator because it changes its direction of flow that's why we don't need commutator..
Probably generators do this..
It is because of the method in which electricity is most easily generated from mechanical energy.
Electrical energy is most easily generated from mechanical energy via a connection between electricity and magnetism called Faraday's law of induction. Faraday's law of induction states that when a coil captures a changing magnetic field (or flux rather), there is an induced voltage in the coil.
"changing" is the key word. The only way to really change the magnetic field is to move/align the coil either closer or further from the permanent magnet. If you try to make a direct DC generator, you will eventually run out of magnetic field as you continuously separate the magnet and coil to keep the induced voltage constant.
So, we do the next best thing. We move the coil toward and away from the magnet in cycles, and this naturally produces AC signals in the coils.
Hence, AC is most easily produced in a generator. DC requires us to be creative, either with a commutator or a rectifier.
This is one reason why AC dominates the distribution grid. The other reason is transformers only work with AC, and transformers are of interest to act as "electrical gears" to step-up and step-down voltage. We desire high voltage for long distance transport (more efficient) and low voltage for households (safety concern).
DC flows in only one direction (Direct Current) whereas AC is changing its direction (Alternating Current)
a commutators keeps the flow of current flowing in the same direction (same wire ) as the generator goes through its positive and negative cycles. so you have a positive side or wire and a negative side or wire.
AC is the constant change of positive and negative cycles. DC is not.
the positive and negative are in Reference to ground which is zero.
Because DC generators generates ac. so it need commutators. that is why every dc genarator have commutators. the direction of current in1st half rotation,(which is generate by induction effect )is reversed in the 2nd half rotation. And for ac generator is by default generate ac,with time period = period of 1 rotation.