i am unable to understand dc motor commutation can anybody help please
There are many diagrams on the web - take a look.
Here's one http://www.pc-control.co.uk/dc-motors.htm.
Referring to the picture at the top of the link: the forces on the two lengths of coil parallel to the shaft are in different directions, one up and one down (because the flow of current is in two different directions through each leg of the coil). This will cause the coil to turn. However, it will only be turned until it lies at right angles to the magnetic field lines and the coil lies in a vertical plane. In order to make it keep turning clockwise, rather than keeping being pulled into the vertical position, you need to reverse the direction of the current so that the top section is pulled down and the bottom bit is pulled up. (The momentum will carry it past the exact vertical position). You could do this switching yourself by changing the connections to the battery every time the coil completes half a turn. But, if you use a commutator, which rotates with the coils the switching is done for you automatically at exactly the right time. The segments of the commutator are arranged to make the switchover so that current will always be going left to right on the S pole side and right to left on the N pole side.
The maximum torque is when the coil is horizontal. More complicated motors have 'multipoles', coils wound at all angles. This gives a good torque at all angles of the motor and the commutator provides switching when each of the multiple coils is vertical.
This is only one of dozens of animations:
Actually commutation is just nothing but changing the direction of the current. in electrical fields, the force direction is mainly depends upon how the current cuts the magnetic field. Let us take the diagram of the first link which is given above by sophiecentaur. The field direction goes from right to left (from north to south) and current flows towards outside near north pole same but current is in opposite direction near south pole. now the force acting near the north pole is equal to F = I*a*B (I-current,a-length of the wire,b-magnetic field) please note I and B are vectors. so the force experienced at the north pole is downwards. at the south pole the force is same but direction is directly opposite to north pole because current direction is opposite. this creates the torquewhich makes the coil to rotate.
Here comes your question, if you don't use the commutator (changing the current direction), still this will work up to 90 degree it may even go beyond 90 degree due to inertia but after this the coil will start oscillate back and forth then finally it will halt at 90 degree (where force and torque both are zero) due to change in force directions. if you use commutator, once the coil goes beyond 90 degree the current direction changes which makes forces to continue in the same direction so your motor never comes to halt it continue rotation in the same direction.
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