Q. Explain why equalizing connection are used in lap winding and dummy coils are some time used in wave winding.
Sol. For batteies operating in parallel, the circulating current will flow if their e.m.fs are unequal. Similarly, in lap - wound machine, the circulating current will exist if in the emfs generated various parallel paths (equal to the no .equal. The existence of circulating current in lap - wound machine may due to any one or all of the following reason :
(i) The air gap under different poles may not be the same, due to wear of the bearings or faulty assembly. As a result of it, some poles carry more flux than the other poles and, therefore, different e.m.fs. are generated in the various parallel paths.
(ii) The joints b/w pole cores and yokes or b/w pole - cores and pole - faces may not be identical for all the poles. This result in different reluctance for the various magnetic circuit. Even if the air gap is uniform, the pole flux will be different under the various pole and, therefore, different e.m.fs are generated in the parallel circuits.
(iii) The different reluctance for each magnetic path may also be due to the impurities or imperfections in the materials constituting the magnetic circuit.
The circulating current result in overheating of the armature.
In order to avoid overloadind and sparking at the brushes, it is essential to take some steps so that circulating currents are not handled by the brushes. These steps consists in providing equalizer rings or equalizer connections. An equalizer ring has a low resistance conductor wire, which connects togther the points in the armature winding which should be at the same potentials. The functiom of equalizer ring is to cause the circulating current to flow within the armature winding itself, without letting them pass through the brushes.
The function of equalizer rings are
(i) to save the brushes from handling the circulating currents and
(ii) to reduce the magnetic flux unbalance that cause sthe potential difference in various parallel paths.
It is usual to provide equalizer rings at the back of the armature i.e. at the other side of the commutator.
Equalizer rings are not require for simplex wave winding even if there is magnetic unbalance. This is due to the fact that coil sides in two parallel paths are disturbed undr all the poles. In view of this, both the paralle circuits are affected equally and there is therefore no need of equalizer rings in simplex wave windings.
Dummy Coils : From economy point of view standard armature lamination are used for different ratings of the commutator machines. When simplex wave windings are used. If y = C ± 1/P/2 relation is not fulfilled, then two ends of one (or more) coil may be cut off, insulated and not connected to the commutator segments. This unused and inactive coil, though placed in the armature slots for mechanical balance is called a dead or dummy coil. In order to have a symmetrical armature winding, a dummy coil should be avoided as far as possible. However, it may be used in the low - rating commutator machines where armature core with exsiting no. of slots have to used in order to avoid extra expenditure on new punching tools etc. If dummy coils are to be avoided, then S/P/2 should be an intger.