Figure 8-12 shows a lap-winding coil. The conductors shown on the left side lie in the top side of the rotor slot. Those on the right side lie in the bottom half of another slot approximately one pole pitch away. At any instant the sides are under adjacent poles, and voltages induced in the two sides are additive.
Other coil sides fill the remaining portions of the slots.
The coil leads are connected to the commutator segments, and this also connects
the coils to form the armature winding. This is shown in Fig. 8-13.
The pole faces are slightly shorter than the rotor core.
Almost all medium and large dc machines use simplex lap
windings in which the number of parallel paths in the armature winding equals
the number of main poles. This permits the current per path to be low enough to
allow reasonable-sized conductors in the coils.
Conductors in the upper layers are shown as full lines, and
those in the lower layers as dotted lines. The inside end connections are those
connected to the commutator bars. For convenience, the brushes are shown inside
the commutator.
Note that both windings have the same number of useful
conductors but that the Gramme-ring winding requires twice the number of actual
conductors and twice the number of commutator bars. Figure 8-15 shows a 6-pole
simplex lap winding. Study of this reveals the six parallel paths between the positive
and negative terminals. The three positive brushes are connected outside the
machine by a copper ring T# and the negative brushes by T#.
The two sides of a lap coil may be full pitch (exactly a
pole pitch apart), but most machines use a short pitch (less than a pole pitch
apart), with the coil throw one-half slot pitch less than a pole pitch. This is
done to improve commutation.
Equalizers. As shown in Fig. 8-15, the parallel paths of the
armature circuit lie under different poles, and any differences in flux from
the poles cause different voltages to be generated in the various paths. Flux
differences can be caused by unequal air gaps, by a different number of turns
on the main-pole field coils, or by different reluctances in the iron circuits.
With different voltages in the paths paralleled by the
brushes, currents will flow to equalize the voltages. These currents must pass through the brushes and
may cause sparking, additional losses, and heating. The variation in pole flux is minimized by careful
manufacture but cannot be entirely avoided.
To reduce such currents to a minimum, copper connections are
used to short-circuit points on the paralleled paths that are supposed to be at
the same voltage. Such points would be exactly two pole pitches apart in a lap
winding.
Thus in a 6-pole simplex lap winding, each point in the
armature circuit will have two other points that should be at its exact
potential. For these points to be accessible, the number of commutator bars and
the number of slots must be a multiple of the number of poles divided by 2.
These short-circuited rings are called “equalizers.”
Alternating currents flow through them instead of the brushes. The direction of
flow is such that the weak poles are magnetized and the strong poles are
weakened. Usually, one coil in about 30% of the slots is equalized. The
crosssectional area of an equalizer is 20% to 40% that of the armature
conductor.
Involute necks, or connections, to each commutator bar from
conductors two pole pitches apart give 100% equalization but are troublesome
because of inertia and creepage insulation problems. Figure 8-15 shows the
equalizing connections behind the commutator connections. Normally they are
located at the rear coil extensions, and so they are more accessible and less
subject to carbon-brush dust problems.
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