Not having a background that involves electrical
(as opposed to electronic) engineering....
In the armatures of motors, each commutator segment
seems to be the end of more than one coil. In addition
to the coil that is currently across the brushes,
another coil is connected; that other coil will terminate
at an unconnected commutator segment from which a third
coil is connected and arrives.......at the other segment in
contact with the other brush.
Now, I can see that this parasitic path will not give rise to
any mechanical output, as the two coils are not well-disposed
towards the magnets, but what effect does the energising of
the parasitic coils have? Are they responsible for the weird
wave-shape identified as "Armature Reaction"?
In the case of an alternator, do these other parasitic
paths cause a loss of output?
DC Armatures are generally one of two basic winding types.
One is called 'LAP' winding. A wire comes from a commutator bar (segment),
stretches over about 1/2 pole pitch (call it CCW), down a slot, back across
one pole pitch (CW), up another slot, then across about 1/2 pole pitch
(again CCW) and connects to the bar directly adjacent to the one where you
started. The next coil starts from this segment, moves the same route, but
through slots that are one-off from the first. This continues all the way
around. So when brushes contact two bars one pole pitch apart (brushes
being spaced 1 pole pitch apart), all the coils between those segments are
in series, and the coil's MMF over'LAP'.
The other type is 'WAVE' winding. A wire comes from a bar, stretches over
about 1/2 pole pitch (again, let's go CCW), down a slot, just as before.
But now, instead of moving CW across the back side, it moves CCW (in the
same direction as when we first left the commutator). It then returns
through a different slot, then continues on (still moving CCW) 1/2 pole
pitch and connects to a commutator bar about 2 pole-pitches away. But not
exactly 2 pole-pitch. Then a similar coil carries on from there (still
moving CCW) 1/2 pole, down slot, 1 pole, up slot, and ~1/2 pole further to
connect to the bar right next to the one that we started with at the very
(each can be further categorized as 'progressive'/'retrogressive', as well
as the exact pitch per coil. Also note that the direction the windings take
moving around the armature from commutator bar to commutator bar has nothing
to do with the direction of rotation of the machine.)
Both of these types of windings have two wires connected to each commutator
slot. Both of these have load current flowing through almost every coil
during most of their rotation. While some coils are poorly positioned to
generate any torque, most are in a position to develop some (depending on
the local air-gap flux). The sum of all the forces from all the coils
provides the total torque. It isn't just one 'active' coil at a time with
the rest being 'parasitic'. In fact, the only 'parasite' is the coil whose
ends terminate at commutator bars directly under the brush, all other coils
are contributing to the total torque developed. Ideally, the coil that is
shorted by having its bars under the brush would have its current reverse
from full load current in one direction, to full current in the opposite
direction before its leading bar loses contact with the brush.
'Armature Reaction' is a term used to describe some of the various affects
caused by the interaction of the MMF of the current-carrying coils of the
armature with the MMF of the fixed field coils. The higher the load (thus
stronger the armature current), the stronger the armature MMF is and the
more distortion of the air gap flux.
When a coil(s) is/are shorted by the brush spanning the gap between two
commutator segments, it is desirable to have as small a voltage induced in
the coil as possible to avoid sparking/burning of the commutator segments.
So the coil sides should be moving in as low an air-gap flux as possible.
This 'neutral' point can be shifted by the distortions of the flux caused by
the armature's MMF. Various schemes exist to counter-act/minimize this
problem so that the neutral point doesn't shift with varying load and thus
sparking/burning of commutator bars is avoided/minimized. (shaded
pole-tips, inter-poles (also called commutating poles), and compensating
windings are schemes used in various situations)
Small machines with few slots per pole often have poor waveforms. This is
due to the heterogenous nature of the magnetic flux the coils pass through,
and the larger voltage steps between commutator bars. Large machines, with
large number of slots and commutator segments have smoother waveforms. The
type of compensation used to improve commutation also plays a role. Very
small machines, with no compensation will have very rough waveforms. And of
course active sparking can generate all sorts of 'noise' including RF.
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