Methods of Improving Commutation

Methods of Improving Commutation Points : Methods of Improving Commutation Improvement of commutation means, the current from, the rotating armature winding to the external stationary circuit or vice versa should be made to pass through the commutator and brushes directly and not as a spark. The passage of current in any electric circuit, according to ohms law, is governed by emf and resistance. Hence methods of improving commutation can be classified as. Emf commutation and Resistance commutation Emf Commutation In emf commutation, the coil undergoing commutation is made to have an additional emf induced in it. This additional emf should be equal in magnitude and opposite in direction to that of the reactance emf. The additional emf neutralizes the reactance emf and hence the current in the coil can easily reverse to its full value. The magnetic field required to induce this additional emf is called the commutating field. One of the method to obtain the commutating field is to shift the brush axis, from magnetic neutral axis. The brush-axis is to be shifted along the direction of rotation of armature for generator operation and in opposite reaction for motor operation. The coil undergoing commutation is thus subjected to commutating field and this field will induce an emf in the coil canceling the reactance emf, thus sparkles commutation can be obtained. The emf commutation by brush shifting method has the following disadvantages: Resistance Commutation The second method of obtaining satisfactory commutation is by uses of high resistance brushes. When the current from coil C reaches near the commutator bar Q at the time of commutation, there exist two parallel path for the flow of current towards the brush. The first path is straight from coil C through the commutator segment ‘Q’ to the brush. The second path is from coil C through coil B then through segment P to the brush. If copper brushes which are having low contact resistance is used there is no inducement for the current to take the second path mentioned above. This will not assist satisfactory commutation. With carbon brushes, which are having high contact resistance the current 1 from coil C tends to take both the parallel paths