ACTION OF THE COMMUTATOR
A commutator performs two important functions in a DC. machine-the first one is to convert alternating quantities to direct quantities or vice-versa and the second function is to keep the rotor or armature m.m.f. stationary in space. The first commutator function of rectifying the alternating quantities in the armature winding to direct quantities at the output brushes Ofa D.C. generator, has already been explained in Art. 3.4.4. In case of a DC. motor, the direct quantities at the brushes are converted to alternating quantities (voltage and current) in the armature winding. Thus a commutator can act both as a convertor or rectifier (AC. to DC.) and as an inverter (DC. to A.C.)..
The stator of the DC. machine carries salient poles, which produce stationary air gap flux. For the development of electromagnetic torque, the armature m.m.f. wave must also be stationary in space. The commutator in a DC. machine carries out this second important function of rendering the armature m.m.f. wave stationary in space, inspite of the armature rotation. In the 'present article, this action of the commutator is examined in detail.
Fig. 4.3 shows a 2-pole D.C. machine, with four full-pitched coils housed in 4 slots. The four commutator segments, numbered 1 to 4, are insulated from each other and from the shaft. The centre line of the poles around which the field coil (or coils) is wound, is called the direct axis of the machine. The shape of the coil end-connections, makes it necessary to place the two carbon brushes along the direct axis or field axis, as depicted in Fig. 4.3.
In an actual D.C. machine, there are a large number of coils and slots and the number of poles is usually greater than 2, except for small D.C. machines. Therefore the DC. machine illustrated in Fig. 4.3 differs much from a real machine, but it serves well in explaining the commutator action. It may also be noted that the brushes make contact with the outside periphery of the commutator, though in Fig. 4.3, these are shown inside the commutator segments, for the sake of clarity.
Coil sides in the slots are represented by small circles with crosses and dots in them, indicating currents away and towards the reader, respectively. In Fig. 4.3 (a), there are two parallel paths between the two brushes. One parallel path may be traced as follows-terminal A to right hand carbon brush and commutator segment 1, upper coil-side in slot 1, back end connection 1, lower coil-side in slot 3, commutator segment 2, upper coil-side in slot 2, coil 2, lower coil-side in slot 4, commutator segment 3 and back to the terminal B. The second parallel path is from commutator segment 1, lower coil-side in slot 2, back coil end-connection 4, top coil-side in slot 4, commutator segment 4,10wer coil-side of slot 1 and so on till terminal B is reached. The current entering at the right hand carbon brush divides equally between the two parallel paths described above. The effect of current in the armature coils is to set up an armature m.m.f. that is vertically upward along an axis 90° away from d-axis. This axis, which is space displaced by an angle of 90° from the d-axis, is called the quadrature axis. At the rotor surface, armature m.m.f. creates N-pole at the top and S-pole at the bottom of the rotor. The S-pole on the stator, attracts rotor N -pole and repels rotor S-pole. Similarly N -pole on the stator attracts rotor S-pole and repels rotor N -pole. The torque produced by the interaction of field and armature poles is called electromagnetic torque. If this machine is working as a motor, the rotor would rotate in the direction of electromagnetic torque, i.e. in the clockwise direction in Fig. 4.3. In case the machine is working as a generator, then the rotor must be driven by the prime-mover against the direction of electromagnetic torque for proper energy conversion. In Fig. 4.3, this direction of rotation for generator operation must be anticlockwise.
Fig. 4.3 (b) illustrates the conditions when the rotor has turned through 45° clockwise There are again two parallel paths, one through coil 1 and the second through coil 3. It maybé seen that coils 1, 3 (housed in slots 1, 3) have maximum e.m.fs. generated in them ; because these coils are cutting the maximum flux. Coils 2, 4 are short-circuited by the brushes and carry no currents, because e.m.fs. induced in these coils are zero. Currents in coil-sideg pertaining to slots 1, 3 in Fig. 4.3 (b), again establish armature m.m.f. that is direc Vertically upward. Note that coil current in each parallel path is again Ia/2 as it is in Fig. 4.3(a).
Fig. 4.3 (c) depicts the condition when the rotor has turned through by further 45° from the position of Fig. 4.3 (b). One parallel path is now through coils 4, 1 and the second parallel path is through coils 3, 2. Coils in each parallel path carry again Ia/2 as it is in Fig. 4.3 (a) and (b). The armature m.m.f. is again directed vertically upward. Hence it can be concluded that armature m.m.f. is always stationary in space and is directed along the interpolar 0r quadrature axis, even though the armature rotates. For a motor, the electromagnetic torque is always acting in the clockwise direction, (Fig. 4.3) and the rotation continues. For a generator, the armature must be driven in anticlockwise direction (Fig. 4.3), for the necessary energy conversion from' mechanical to electrical.
An examination of Fig. 4.3 reveals that just before the coils 2, 4 are short-circuited by the brushes, these coils carry current Ia/2 (or in general Ia/a) in one direction. Soon after the short-circuit is over, these two coils carry current Ia/2 (or in general Ia/a) in the reversed direction as shown in Fig. 4.3 (c). Here a is the number of parallel paths and in Fig. 4.3, a is equal to 2. Thus during the time of short-circuit, the currents in the short-circuited coils, in general, must be reduced from Ia/a to zero and then built up to Ia/a. This process of current reversal in the coils short-circuited by the brushes is called commutation and the time during Which it takes place is called commutation period. If the current variation with time is uniform from Ia/a to Ia/a as depicted in Fig. 4.4, a linear commutation is obtained.
Another important conclusion from the foregoing analysis is that the magnitude of current in the coils under each pole, remains constant and equal to Ia/a, (Fig. 4.4). But the magnitude of e.m.fs. induced in the coils under each pole is not constant coil under the pole centre has maximum e.m.f. induced in it, whereas the other coils have their e.m.f. magnitudes proportional to the flux density wave present there.
The coils short-circuited by the brushes during commutation process must have their coilsides in the zero flux density region so that e.m.f.s induced in them are zero. In other words, the coils undergoing commutation must have their coil sides in the magnetic neutral axis of the field poles, i.e. in the interpolar or quadrature axis. For example, coils 2 and 4 undergoing commutation in Fig. 4.3 (bmm in the magneW neutral axis or interpolar axis. The shape of the coil end connections is such that the brush' axis is aligned along the field pole or direct axis. For convenience in the schematic diagram of a DC. machine, the brushes are shown along the quadrature axis, i.e. in the position of the cons which are short-circuited by the brushes and are undergoing commutation-this schematic diagram for a 2-p01e D.C. machine is shown in Fig. 4.5 (a). Hence it may be stated that the armature m.m.f. is always directed along the brush axis, which now coincides with the interpolar or quadrature axis of the DC. machine. The circuit representation, more commonly employed for a DC. machine with any number of poles is as illustrated in Fig. 4.5 (b). In this figure, circular symbol represents the armature and two small rectangles or squares at the opposite sides of circle represent the two brushes.
