CHAPTER XIX

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

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.

=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.

=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.

=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.

=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.

=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._

=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.

=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.

=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.

=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.

=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.

=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.

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.

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.

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.

=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.

=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.

=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.

=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.