THEORY OF THE ARMATURE
=Current Distribution in Ring and Drum Armatures.=--In studying
the actions and reactions which take place in the armature, the student
should be able to determine the directions of the induced currents. The
basic principles of electromagnetic induction were given in chapter
X, from which, for instance, the distribution of current in the
gramme ring armature, shown in fig. 279, is easily determined by the
application of Fleming's rule.
Tracing the current from the negative to the positive brush, it
will be seen that it divides, half going through coils 1, 2, 3,
and half through coils I, II, III, these two currents ascend to
the top of the ring, uniting at the positive brush.
=Ques. In the Gramme ring armature (fig. 279) what is the
distribution of armature currents?=
Ans. There are two paths in parallel as indicated in fig. 279.
=Ques. How does the voltage vary in the coils?=
Ans. It varies according to the position of the coils, being least when
vertical and greatest when horizontal in a two pole machine arranged as
in fig. 279.
The upper and lower coils in the right hand half of the ring
armature, fig. 279, will have about the same electromotive
force induced in them, say 2 volts each, while the two coils
between them will have a higher electromotive force, at the
same instant, say 4 volts each, since they occupy nearly the
positions of the maximum rate of change of the magnetic lines
threading through them. These eight coils may be represented by
two batteries connected in parallel, each battery consisting of
two 2 volt cells and two 4 volt cells as shown in fig. 280. The
voltage of each battery then will be
2 + 4 + 4 + 2 = 12 volts
[Illustration: FIG. 279.--Current distribution in a gramme
ring armature. There are two paths for the current between the brushes,
half going up each side of the ring as indicated by the arrows, thus
giving two paths in parallel as indicated in fig. 281.]
[Illustration: FIG. 280.--Battery analogy illustrating current
distribution in a ring armature. The eight coils of the armature, fig.
279, are represented by two batteries of four cells each. The action
of the two units thus connected is indicated by the arrows. In the
external circuit the voltage is equal to that of one battery and the
current is equal to the sum of the currents in each battery.]
The two batteries being connected in parallel, the voltage at
the terminals will be the same, but the current will be the sum
of the currents in each battery.
=Ques. How may the number of paths in parallel be increased?=
Ans. By increasing the number of poles.
For instance, in a four pole machine, as in fig. 283, there are
four paths in parallel. In this case the armature may be used
to furnish two separate currents, though this is not desirable.
[Illustration: FIG. 281.--Diagram showing distribution of
current in the gramme ring armature of fig. 279. The current flows in
two parallel paths as indicated.]
[Illustration: FIG. 282.--Diagram showing current distribution
through armature of a four pole machine with like brushes connected.
There are four paths in parallel, hence the induced voltage will equal
that of one set of coils, and the current will be four times that
flowing in one set of coils.]
=Ques. How are the brushes connected?=
Ans. Usually all the positive brushes are connected together, and all
the negative brushes as in fig. 283, giving four paths in parallel
through the armature as indicated in fig. 282.
[Illustration: FIG. 283.--Brush connections for four pole
dynamo. It is usual to connect all the positive brushes to one terminal
and all the negative brushes to the other which gives four parallel
paths as shown in the diagram, fig. 282. In a four pole machine, two
separate currents can be obtained by omitting the parallel brush
connections.]
=Ques. How does this method of brush connection affect the
voltage?=
Ans. The voltage at the terminals is equal to that of any of the sets
of coils between one positive brush and the adjacent negative brush.
Thus in the four pole machine, fig. 283, the coils of the
four quadrants are in four parallels, which gives an internal
resistance equal to one-sixteenth that of the total resistance
of the entire ring.
When the coils are connected in two circuits or series
parallel, it requires only two brushes at two neutral points on
the commutator, for any number of poles; this arrangement is
shown in fig. 269.
=Ques. In general what may be said about the current paths through an
armature?=
Ans. The paths may be in parallel or series parallel according as the
winding is of the lap or wave type.
[Illustration: FIG. 284.--Morday's method of measuring the
variation of voltage around the commutator by use of a single exploring
brush and volt meter. It consists in connecting one terminal of the
volt meter (preferably an electrostatic one) to one brush of the
machine, and the other terminal to the exploring brush, which can be
moved from point to point, readings being taken at each point.]
=Variation of Voltage Around the Commutator.=--There are numerous
ways of determining the value of the induced voltage in an armature
at various points around the commutator. In the method suggested by
Morday, it can be measured by the use of a single exploring brush and a
volt meter as shown in fig. 284.
In this method, one terminal of the volt meter is connected
to one of the brushes of the dynamo, and the other terminal
is joined by a wire to a small pilot brush which can be
pressed against the commutator at any desired part of its
circumference. With the machine running at its rated speed, the
exploring brush is placed in successive positions between the
two brushes of the machine. In each position a reading of the
volt meter is taken and the angular position of the exploring
brush noted.
[Illustration: FIG. 285.--Cross magnetization. This is defined
as lines of magnetic force set up in the windings of a dynamo armature
which oppose at right angles the lines of force created between the
poles of the field magnet. The figure shows this cross flux which is
due to the armature current alone.]
=Ques. How does the voltage vary between successive pairs of
commutator segments?=
Ans. The variation is not constant.
=Cross Magnetization; Field Distortion.=--In the operation of
a dynamo with load, the induced current flowing in the armature
winding, converts the armature into an electromagnet setting up a field
across or at right angles to the field of the machine. This cross
magnetization of the armature tends to distort the field produced by
the field magnets, the effect being known as _armature reaction_. To
understand the nature of this reaction it is best to first consider the
effect of the field current and the armature current separately.
Fig. 285 represents the magnetic flux through an armature at rest,
where the field magnets are separately excited. If the armature be
rotated clockwise, induced currents will flow upward through the two
halves of the winding between the brushes, making the lower brush
negative and the upper brush positive.
=Ques. If, in fig. 285, the current in the field magnet be shut off,
and a current be passed through the armature entering at the lower
brush, what is the effect?=
Ans. The current will divide at the lower brush, flowing up each side
to the top brush. These currents tend to produce north and south poles
on each half of the core at the points where the current enters and
leaves the armature. Hence, there will be two north poles at the top of
the ring and two south poles at the bottom.
=Ques. What effect is produced by the like poles at the top and
bottom of the ring?=
Ans. The external effect will be the same as though there were a single
north and south pole situated respectively at the top and bottom of the
ring.
=Ques. In the operation of a dynamo, how do the poles induced in the
armature affect the magnetic field of the machine?=
Ans. They distort the lines of force into an oblique direction as shown
exaggerated in the diagram fig. 286.
[Illustration: FIG. 286.--Distortion of magnetic field due
to cross magnetization. For clearness, the effect is shown somewhat
exaggerated. A drag or resistance to the movement of the armature is
caused by the attraction of the north and south poles on the armature
and pole pieces respectively.]
=Ques. What effect has the presence of poles in the armature on the
operation of the machine?=
Ans. In fig. 286, the resultant north pole _n_, _n_, _n_, where the
lines emerge from the ring, attracts the south pole, _s_, _s_, _s_,
where the lines enter the field magnet, hence a load is brought upon
the engine, which drives the dynamo, in dragging the armature around
against these attractions. The stronger the current induced in the
armature, the greater will be the power necessary to turn it.
=Ques. Why does this reaction in the armature require more power to
drive the machine?=
Ans. The effect produced by the armature reaction is in accordance
with Lenz's law which states that: _In electromagnetic induction,
the direction of the induced current is such as to oppose the motion
producing it._
[Illustration: FIG. 287.--Actual distortion of field resulting
from cross magnetization, as shown by iron filings.]
=Remedies for Field Distortion.=--Since the distortion of the
magnetic field of a dynamo causes unsatisfactory operation, numerous
attempts have been made to overcome this defect, as for instance, by:
1. Experimenting with different forms of pole piece;
The reluctance of the pole piece should be increased in the
region where the magnetic flux tends to become most dense.
The trailing horn of the pole piece may be made longer than
the advancing horn and cut farther from the surface of the
armature, so as to equalize the distribution of the magnetic
flux.
2. Lengthening the air gap;
This increases the reluctance, and also necessitates more
ampere turns in the field winding. The field distortion,
however, will not be so great, as it would be if the magnetic
field of the machine were weaker.
3. Slotting the pole pieces;
Both longitudinal gaps and oblique slots have been tried. The
reduction of cross section of the pole piece causes it to
become highly saturated and to offer large reluctance to the
cross field.
4. The use of auxiliary poles.
These are small poles placed between the main poles and so
wound and connected that their action opposes that of the cross
field.
=Normal Neutral Plane.=--This may be defined as _a plane passing
through the axis of the armature perpendicular to the magnetic field of
the machine when there is no flow of current in the armature_, as shown
in fig. 288. It is the plane in which the brushes would be placed to
prevent sparking when the machine is in operation were the field not
distorted by armature reaction, and there were no self-induction in the
coils.
=Commutating Plane; Lead of the Brushes.=--It has been found that
in order to reduce sparking to a minimum, the brushes must be placed in
certain positions found by trial and designated as being located in the
_neutral plane_.
When the brushes are in the neutral plane, they are in contact with
commutator segments connecting with coils that are cutting the lines of
force at the minimum rate.
=Ques. Define the term "commutating plane."=
Ans. This is a plane passing through the axis of the armature and
through the center of contact of the brushes as shown in figs. 289 and
300.
=Ques. What is the angle of lead?=
Ans. The angle between the normal neutral plane and the commutating
plane.
In the operation of a dynamo since the field, on account of
armature reaction, is twisted around in the direction of
rotation, the proper position for the brushes is no longer in
the normal neutral plane, but lies obliquely across, a few
degrees in advance. Hence, for sparkless commutation, the
commutating plane is a little in advance of the normal neutral
plane, the lead being measured by the angle between these
planes, as stated in the definition.
[Illustration: FIG. 288.--Normal neutral plane. This is
a reference plane from which the lead is measured. As shown, the
normal neutral plane lies at right angles to the lines of force of an
undistorted field.]
=Ques. What may be said with respect to the angle of lead?=
Ans. _For sparkless commutation, the angle of lead varies with the
load._
If the field be much altered at full load, it is evident that
at half or quarter load it will not be nearly so much twisted,
hence the necessity for mounting the brushes on some kind of
rocking device which will allow them to be shifted in different
positions for different loads. A desirable point, then, in
dynamo design is to make the angle of lead at full load so
small that it will not be necessary to shift the brushes much
for variation of load. This can be accomplished by making the
field magnet field considerably more powerful than the armature
field.
[Illustration: FIG. 289.--Diagram illustrating the
demagnetizing effect of armature reaction. This results from
the forward lead given the brushes in order to secure sparkless
commutation.]
=Demagnetizing Effect of Armature Reaction.=--In the operation
of a dynamo, as previously explained, the position of the brushes for
sparkless commutation must be varied with the load; that is, for light
load they should occupy a position practically midway between the poles
and for a heavy load they must be moved a few degrees in the direction
of rotation. In other words, the commutating plane must be more or less
in advance of the normal neutral plane as shown in fig. 289.
=Ques. What is the effect of lead?=
Ans. It produces a demagnetizing effect which tends to weaken the field
magnets.
=Ques. Describe this demagnetizing effect in detail.=
Ans. Tracing the armature currents, in fig. 289 according to Fleming's
rule, it will be seen that current in inductors 1 to 18 flow _from_
the observer indicated by crosses representing the tails of retreating
arrows and in inductors 19 to 36, _toward_ the observer from the back
of armature, indicated by dots representing the points of approaching
arrows. In determining these current directions the inductors to the
right of the neutral line are considered as moving downward, and those
to the left as moving upward. The current in inductors 1 to 15 and 19
to 33, tends to cross magnetize the magnetic field of the machine, but
the current in inductors 34 to 36 and 16 to 18 tends to produce north
and south poles as indicated. These poles are in opposition to the
field poles and tend to demagnetize them. Hence, the inductors lying
outside the two upright lines are known as _cross magnetizing turns_,
and those lying inside, as _demagnetizing turns_.
The breadth of the belt of demagnetizing turns included between
the two upright lines is clearly proportional to the angle of
lead; therefore, the demagnetizing effect increases with the
lead.
=Eddy Currents; Lamination.=--Induced electric currents, known
as eddy currents, occur when a solid metallic mass is rotated in a
magnetic field. They consume considerable energy and often occasion
harmful rise in temperature. Armature cores, pole pieces, and field
magnet cores are specially subject to these currents.
[Illustration: FIG. 290.--Arago's experiment illustrating
eddy currents. Arago found that if a copper disc be rotated in its
own plane underneath a compass needle, the needle was dragged around
as by some invisible friction. The explanation of this phenomenon,
known as _Arago's rotations,_ is due to Faraday, who discovered that
it was caused by induction. That is, a magnet moved near a solid mass
of metal, induces in it currents, which, in flowing from one point to
another, have their energy converted into heat, and which, while they
last, produce (in accordance with Lenz's law) electromotive forces
tending to stop the motion. Thus, in the figure, there are a pair of
eddies in the part passing between the poles, and these currents oppose
the motion of the disc. Foucault showed by experiment the heating
effect of eddy currents, but such currents were known years before
Foucault's experiments, hence they are incorrectly called Foucault
currents.]
=Ques. Describe the formation of eddy currents.=
Ans. In fig. 291, a bar inductor is seen just passing from under the
tip of the pole piece N of the field magnet. Noting the distribution of
the lines of force, it will be seen that the edge _c d_ is in a weaker
field than the edge _a b_, hence, since the two edges move with the
same velocity, the electromotive force induced along _c d_ will be less
than that induced along _a b_. This gives rise to whirls or current
eddies in the copper bar as shown.
[Illustration: FIG. 291.--Formation of eddy currents in a
solid bar inductor. On account of its appreciable size, the field
is sometimes weaker at one point than another, hence the unequal
electromotive forces thus produced will induce eddy currents.]
=Ques. What should be noted in seeking a remedy for eddy currents?=
Ans. It should be noted that eddy currents are due to very small
differences of pressure and that the currents are large only because of
the very low resistance of their circuits.
=Ques. What is the best means of reducing eddy currents?=
Ans. Lamination.
=Ques. Explain this mode of construction with respect to the bar
inductor fig. 291.=
Ans. In the case of a large bar inductor such as shown in fig. 291, it
could be replaced by a number of small wires soldered together only at
the ends. The layer of dirt or oxide on the outside of the wires will
furnish sufficient resistance to practically prevent the eddy currents
passing from wire to wire.
[Illustration: FIG. 292.--Eddy currents induced in a solid
armature core. Eddy currents always occur when a solid metallic mass
is rotated in a magnetic field, because the outer portion of the metal
cuts more lines of force than the inner portion, hence the induced
electromotive force not being uniform, tends to set up currents between
the points of greatest and least potential. Eddy currents consume
a considerable amount of energy and often occasion harmful rise in
temperature.]
=Ques. How should an armature core be laminated to avoid eddy
currents?=
Ans. It should be laminated at right angles to its axis.
Fig. 292 shows the induced eddy currents in a solid armature
core, and fig. 293 shows the manner in which the paths of these
currents are interrupted and the losses due to their effect
diminished by the use of laminated cores.
[Illustration: FIG. 293.--Armature core with a few laminations
showing effect on eddy currents. In practice the core is made up
of a great number of thin sheet metal discs, about 18 gauge, which
introduces so much resistance between the discs that the formation of
eddy currents is almost entirely prevented.]
In fig. 293, only five laminations or plates are indicated,
so as to show the sub-division of the eddy currents, but in
practical armatures, the number of laminations or punchings
ranges from 40 to 66 to an inch, and brings the eddy current
loss down to about one per cent. A greater increase in the
number of laminations per inch is not economical, however,
owing to the difficulties encountered in the punching and
handling of extremely thin sheets of iron, and the loss of
space between the plates.
Armature cores constructed of the number of plates stated,
and forced together by means of screws and heavy hydraulic
pressure, contain from 80 to 90 per cent. of iron, and have a
magnetic flux carrying capacity only from 5 to 15 per cent.
less than when they are made of an equal volume of solid iron.
=Magnetic Drag on the Armature.=--Whenever a current is induced
in an armature coil by moving it in the magnetic field so as to cut
lines of force, the direction of the induced current is such as to
oppose the motion producing it. Hence, in the operation of a dynamo,
considerable driving power is required to overcome this magnetic drag
on the armature.
[Illustration: FIG. 294.--Circular concentric magnetic field
surrounding a conductor carrying a current. If this conductor be moved
across a magnetic field, as between the poles of a magnet, the lines of
force will be distorted as in fig. 295, which will oppose the motion of
the conductor.]
A conductor carrying a current is surrounded by a circular concentric
magnetic field. If now such a conductor, with current flowing toward
the observer as in fig. 294, be placed in a uniform magnetic field, a
distortion of the magnetic lines will occur as shown in fig. 295. The
resulting mechanical actions are easily determined by remembering
that _the magnetic lines act like elastic cords tending to shorten
themselves_. There is in fact a tension along the magnetic lines and a
pressure at right angles to both, proportional at every point to the
square of their density.
[Illustration: FIG. 295.--Illustrating drag on armature
inductors. In moving a wire carrying a current through a magnetic
field, the lines of force are distorted, and the effect on the wire
is the same as though the magnetic lines were elastic cords tending
to shorten themselves. They, therefore, oppose the motion of the
wire; hence, in dynamo operation, more or less power is absorbed in
overcoming this drag on the numerous inductors. In the figure the
inductor is being moved upward against the "drag" due to the magnetic
field.]
It is evident by inspection of the lines in fig. 295, that there is a
drag upon the conductor in the direction shown by the arrow.
=Smooth and Slotted Armatures.=--The inductors of an armature may
be placed on a smooth drum or in slots cut in the surface parallel to
the axis.
In the first instance, the magnetic drag comes on the inductors and in
the case of slots, upon the teeth.
The effect of embedding the armature inductors in slots is
to distort the magnetic field as shown in fig. 296. Most of
the lines of force pass through the teeth, thus, not only are
the inductors better placed for driving purposes, but, being
screened magnetically by the teeth, the forces acting on them
are reduced, the greater part of the magnetic drag being taken
up by the core.
It should be noted that, although screened from the field, the
inductors in a slotted armature cut magnetic lines precisely
as if they were not protected. The effect is as though the
magnetic lines flashed across the slots from tooth to tooth,
instead of passing across the intermediate slot at the ordinary
angular velocity.
=Comparison of Smooth and Slotted Armatures.=--The slotted
armature has the following advantages over the smooth type:
1. Reduced reluctance of the air gap;
2. Better protection for the winding;
3. Inductors held firmly in place preventing slippage;
4. No magnetic drag on inductors;
5. No eddy currents in inductors;
6. Better ventilation;
7. Opposition to armature reaction.
Due to increased density of flux through the teeth.
The disadvantages of slotted armatures may be stated as follows:
1. Tendency of the teeth to induce eddy currents in the pole pieces;
2. Increased self-induction of the armature coils;
3. Greater hysteresis loss on account of denser flux in the teeth;
4. Leakage of lines of force through the core, especially in the case
of partially enclosed slots.
[Illustration: FIG. 296.--Effect of slotted armature. The
teeth, as they sweep past the pole face, cause oscillations of the
magnetic flux in the iron near the surface because the lines in the
pole piece PP tend to crowd toward the nearest teeth, and will be
less dense opposite the slots. This fluctuation of the magnetic lines
produce eddy currents in the pole faces unless laminated. The armature
inductors, being screened from the field, are relieved of the drag
which is taken by the teeth.]
=Magnetic Hysteresis in Armature Cores.=--When the direction
or density of magnetic flux in a mass of iron is rapidly changed a
considerable expenditure of energy is required which does not appear as
useful work. For instance, when an armature rotates in a bipolar field,
the armature core is subjected to two opposite magnetic inductions in
each revolution; that is, at any one instant a north pole is induced
in the core opposite the south pole of the magnet and a south pole in
the core opposite the north pole of the magnet as indicated in fig.
297 by _n_ and _s_. Accordingly, if the armature rotate at a speed of
1,000 revolutions per minute, the polarity of the armature will be
changed 2,000 times per minute, and result in the generation of heat at
the expense of a portion of the energy required to drive the armature.
This loss of energy is due to the work required to change the position
of the molecules of the iron, and takes place both in the process of
magnetizing and demagnetizing; the magnetism in each case lagging
behind the force.
=Core Loss or Iron Loss.=--These terms are often employed to
designate the total internal loss of a dynamo due to the combined
effect of eddy currents and hysteresis, but as the losses due to the
former are governed by laws totally different from those applicable to
the latter, special analysis is required to separate them.
The eddy current loss per pound of iron in the armature core diminishes
with the thinness of the laminated sheets, and may be made indefinitely
small by the use of indefinitely thin iron plates, were it not for
certain mechanical and economical reasons.
The loss due to hysteresis per pound of iron in the core, does not vary
with the thinness of the core plates; it can be reduced only by the use
of a material having a low hysteretic coefficient.
=Dead Turns.=--The voltage generated in a dynamo with a given
degree of field excitation is not strictly proportional to the speed,
but somewhat below on account of the various reactions. That is, the
machine acts as though some of its revolutions were not effective in
inducting electromotive force.
The name _dead turns_ is given to the number of revolutions by which
the actual speed exceeds the theoretical speed for any output.
Again, this term is sometimes used to denote that portion of the wire
on an armature which comes outside the magnetic field and is therefore
rendered ineffective in inducing electromotive force. The number of
dead turns is about 20% of the total number of turns.
[Illustration: FIG. 297.--Magnetic hysteresis in armature
core. Unlike poles are induced in the core opposite the poles of the
field magnet. Since on account of the rotation of the core the induced
poles are reversed a thousand or more times a minute, considerable
energy is required to change the positions of the molecules of the iron
for each reversal, resulting in the generation of heat at the expense
of a portion of the energy required to drive the armature.]
=Self-induction in the Coils; Spurious
Resistance.=--Self-induction opposes a rapid rise or fall of an
electric current in just the same way that the inertia of matter
prevents any instantaneous change in its motion. This effect is
produced by the action of the current upon itself during variations in
its strength.
In the case of a simple straight wire, the phenomenon is almost
imperceptible, but if the wire be in the form of a coil, the adjacent
turns act inductively upon each other upon the principle of the mutual
induction arising between two separate adjacent circuits.
=Ques. What effect has self-induction on the operation of a
dynamo?=
Ans. It prevents the instantaneous reversal of the current in the
armature coils. That is, the current tends to go on and in fact does
actually continue for a brief time after the brush has been reached.
[Illustration: FIG. 298.--Distribution of magnetic lines
through a ring armature. Since the lines follow the metal of the ring
instead of penetrating the interior, no electromotive force is induced
in that portion of the winding lying on the interior surface of the
ring. There is, therefore, a large amount of dead wire or wire that
is ineffective in inducing electromotive force; this is the chief
objection to the ring type of armature.]
=Ques. What becomes of the energy of the current at reversal?=
Ans. The energy of the current in the section of the winding undergoing
commutation is wasted in heating the wire during the interval when it
is short circuited, and as it passes on, energy must again be spent in
starting a current in it in the reverse direction. There is, then, a
lagging of the current in the armature coils due to self-induction.
=Ques. What is spurious resistance?=
Ans. This is an apparent increase of resistance in the armature
winding, which is proportional to the speed of the armature, and due to
the lagging of the current.
[Illustration: FIG. 299.--Distribution of magnetic lines
through solid drum armature of a four pole machine.]
=Armature Losses.=--The mechanical power delivered to the pulley
of a dynamo is always in excess of its electrical output on account of
numerous mechanical and electrical losses. Mechanical losses result
from:
1. Friction of bearings;
2. Friction of commutator brushes;
3. Air friction.
The electrical losses may be classified as those due to:
1. Armature resistance;
2. Hysteresis;
3. Eddy currents.
=Ques. How do the mechanical and electrical losses compare?=
Ans. The mechanical losses are small in comparison with the electrical
losses.
=Ques. What may be said with respect to friction?=
Ans. The bearing friction varies with the load. In calculating this
loss not only must the weight of the armature be considered but also
the belt tension and magnetic attraction in order to get the resultant
thrust on the bearing. Friction of the brushes is very small and may be
neglected. A small loss of power is caused by the friction of the air
on the armature. The latter, since it revolves rapidly, acts to some
extent as a fan, and in some machines this fan action is made use of
for ventilation and cooling.
=Ques. How are the other losses determined?=
Ans. The loss of power due to armature resistance is easily found
by Ohm's law, but the hysteresis and eddy current losses, known
collectively as _iron losses_, are not so easily determined. If the
magnetization curve of the particular quality of iron used for armature
plates be known, the hysteresis loss may be calculated approximately.
Eddy current losses are the most important, especially in large
machines. As previously explained, in all the moving metal masses
unless laminated, there will be eddy currents set up if they cut
magnetic lines. Power may be lost from this cause even in the metal of
the shaft if there be leakage of magnetic lines into it.