Currents induced in armature conductors of a d.c. generator are alternating. To make their flow unidirectional in the external circuit, we need a commutator. Moreover, these current flow in one direction when armature conductors are under N - pole and in the opposite direction when they are under S - pole. As conductors pass out of the influence of an N - pole and enter that of S - pole, the current in them is reversed. The reversal of current take place along magnetic neutral axis or brush axis i.e. when the brush spans and hence short circuit that particular coil undergoing reversal of current through it. This process by which current in the short circuited coil is reversed while it crosses the M.N.A. is called commutation. The brief period during which coil remains short - circuited is known as commutation period Tc.
If the current reversal i.e. the change from +1 to zero and then zero to -1 is completed by the end of short circuit or commutation period, then the commutation is ideal. If current reversal is not complete by that time, then sparking is produced b/w the brush and the commutator which results in progressive damage to both.
Let us discuss the process of commutation of current reversal where ring winding has been used for simplicity. The brush width is equal to the width of one commutator segment and one mica insulation. Coil B is about to be short circuited because brush is about to come in touch with commutator segment 'a'. It is assumed that each coils carries 20 A, so that brush current is 40 A. It is so because every coil meeting at the brush supplies half the brush current whether lap wound or wave wound. Prior to the beginning of short circuit, coil b belongs to the group of coils lying to the left of the brush and carries 20 A from left to right. Coil B has entered its period of short - circuit and is approx. at one - third of this period. The current through coil B has reduced down from 20 A to 10 A because the other 10 A flows via segment ('a'). As area of contact of the brush is more with segment ('b') than with the segment a, it receives 30 A from the former, the total again being 40 A.
Coil B in the middle of its short - circuit period. The current through it has decreased to zero. The two currents of value 20 A each, pass to the brush directly from coil A and C. The brush contact areas with the two segment 'b' and 'a'are equal.
Coil B has become part of the group of coils lying to the right of the brush. It is seen that brush contact area with segment 'b' is decreasing rapidly whereas that with segment 'a' is increasing. Coil B now carries 10 A in the reverse direction which combines with 20 A supplied by coil A to make up 30 A that passes from segment 'a' to the brush. The other 10 A is supplied by coil c and passes from segment 'b' to the brush, again giving a total of 40 A at the brush.
Depicts the moment when coil b is almost at the end of commutation or short - circuit period. For ideal commutation, current through it should have reversed by now but it is carrying 15A only (instead of 20 A). The difference of current b/w coils C and B i.e. 20 - 15 = 5 A in this case, jumps directly from segment b to the brush through air thus producing spark.
If the changes of current through coil B are plotted on a time base., it will be represented by a horizontal line AB i.e. a constant current of 20 A up to the time of beginning of commutation. From the finish of commutation, the current will be represented by another horizontal line CD. Now, again the current value is FC = 20 A, although in the reversed direction. The way in which current changes from its positive value of 20 A (=BE) to zero and then ti its negative value of 20 A (= CF) depends on the conditions under which the coil B undergoes commutation. If the current varies at a uniform rate i.e. if BC is a straight line, then it is reffered to as linear commutation. However, due to the production of self - induced e.m.f. in the coil (discussed below) the variation follow the dotted curve. It is seen that, in that case, current in coil B has reached only a value of KF = 15 A in the reversed direction, hence the difference of 5 A (20 A - 15 A) passes as a spark.
So, we conclude thar sparking at the brushes, which results in poor commutation is due to the inability of the current in the short - circuited coil to reverse completely by the end of short - circuit period.
The main cause which retards or delays this quick reversal is the production of self - induced e.m.f. in the coil undergoing commutation. It may be pointed out that the coil possesses appreciable amount of self inductance because it lies embedded in the armature which is built up of a material of high magnetic permeability. This self - induced e.m.f. is known as reactance voltage whose value is found. This voltage, even though of a small magnitude, produces a large current through the coil whose resistance is very low due to short circuit. It should be noted that if the brushes are set so that the coils undergoing short - circuit are in the magnetic neutral plane, where they are cutting no flux and hence have no e.m.f. induced in them due to armature rotation, there will still be the e.m.f. of self - induction which causes severe sparking at the brushes.
Value of Reactacne Voltage
Reactace voltage = coefficient of self - inductance × rate of change of current
It should be remembered that the time of short - circuit or commutation is the time required by the commutator to move a distance equal to the circumferential thickness of the brush minus the thickness of one insulating plate or strip of mica.
Let Wb = brush width in cm; = Wm = width of insulation in cm
V = peripheral velocity of commutator segments in cm/second
Then Tc = time of commutation or short - circuit = Wb - Wm second
If brush width are given in terms of commutator segments, than commutator velocity should also be converted in terms of commutator segments per second.
If I is the current through a conductor, then total change during commutation = I - ( - 1) = 21
Self induced or reactance voltage
= L × 2I/Tc = if commutation is linear
= 1.11 L × 2I/ Tc = if commutation is sinusoidal
The reactance e.m.f. hinders the reversal of current. This means that there would be sparking at the brushes due to the failure of the current in the short - circuited coil to reach its full value in the reversed direction by the end of short - circuit. This sparking will not only damage the brush and the commutator but this being a cumulative process, it may worsen and eventually lead to the short - circuit of the whole machine by the formation of an around the commutator from brush to brush.