Ques. What may be said of the electromotive force during the second half of the revolution?
Ans. It varies in a similar manner as in the first half of the revolution; that is, the magnetic lines are cut with increasing rapidity during the third quarter, and with decreasing rapidity during the fourth quarter of the revolution, which causes the electromotive force to increase and decrease during these intervals.
The cycle of events just described may be summed up as follows: During the revolution of the loop:
It was stated that, during the revolution of the loop, the magnetic lines were cut “with increasing or decreasing rapidity,” causing the electromotive force to rise or fall. The reason for this is illustrated in fig. 167. The loop is here shown in a horizontal position at right angles to the direction of the magnetic field; the latter, as indicated by the even spacing of the vertical arrows representing the magnetic lines, is assumed to be uniform.
The wire C D of the loop, as it rotates at constant speed, cuts the magnetic lines at the points 0, 1, 2, 3, etc., but the distances 0-1, 1-2, 2-3, etc., between these points, are unequal; that is, the wire C D travels farther in cutting the lines 0 and 1, than it does in cutting 1 and 2, and still less in cutting the lines 2 and 3. After cutting the line 4, which passes through the axis of revolution, the opposite conditions obtain.
If the arcs 0-1, 1-2, etc., of the dotted circle, which are intercepted by the magnetic lines and passed through by the wire, be rectified and laid down under each other, as lines 0-1, 1-2, etc., the time of passage of the wire between successive magnetic lines will vary as the length, since the speed is uniform. Thus the wire in passing from line 0 to line 1, takes much more time than in passing from 1 to 2, as indicated at the left of the figure by 0-1 and 1-2, and still less in passing from 2 to 3; that is, the rate of cutting the lines increases as C D rotates from 0 to 4 and decreases from 4 to 8.
Since similar conditions prevail with respect to A B, for its corresponding movement, it is evident that the number of lines which thread through the loop are decreased with increasing rapidity as the loop rotates through the first quarter of a revolution, and increased with decreasing rapidity during the second quarter of the revolution. Moreover, it must be evident that the reverse conditions obtain for the third and fourth quarters of the revolution.
The Sine Curve.—In the preceding paragraph it was shown that an alternating current is induced in the armature of either an alternator or dynamo; that is, the current: 1, begins with zero electromotive force, 2, rises to a maximum, 3, decreases again to zero, 4, increases to a maximum in the opposite direction, and 5, decreases to zero.
A wave-like curve, as shown in fig. 168, is used to represent these several changes, in which the horizontal distances represent time, and the vertical distances, the varying values of the electromotive force. It is called the sine curve because a perpendicular at any point to its axis is proportional to the sine of the angle corresponding to that point.
Ques. Describe the construction and application of the sine curve.
Ans. In fig. 168, at the left, is shown an elementary armature in the horizontal position, but at right angles to the magnetic field. The dotted circle indicates the circular path described by A B or C D during the revolution of the loop. Now, as the loop rotates, the induced electromotive force will vary in such a manner that its intensity at any point of the rotation is proportional to the sine of the angle corresponding to that point. Hence, on the horizontal line which passes through the center of the dotted circle, take any length, as 08, and divide it into any number of parts representing fractions of a revolution, as 0°, 90°, 180°, etc. Erect perpendiculars at these points, and from the corresponding points on the dotted circle project lines parallel to 08; the intersections with the perpendiculars give points on the sine curve. Thus the loop passes through 2 at the 90° point of its revolution, hence, projecting over to the corresponding perpendicular gives 2 2′, a point whose elevation from the axis is proportional to the electromotive force at that point. In like manner other points are obtained, and the curved line through them will represent the variation in the electromotive force for all points of the revolution.
At 90°, the electromotive force is at a maximum; hence, by using a pressure scale such that the length of the perpendicular 2 2′ for 90° will measure the maximum voltage the length of the perpendicular at any other point will represent the actual pressure at that point.
The curve lies above the horizontal axis during the first half of the revolution, and below it during the second half, which indicates that the current flows in one direction for a half revolution and in the opposite direction during the remainder of the revolution.
The application of the sine curve to represent the alternating cycle, is further illustrated in figs. 169 to 173, which show the position of the armature at each quarter of the revolution.
In fig. 179, the loop A B C D is in the vertical position at the beginning of the revolution. At this instant the electromotive force is zero, hence the sine curve as shown begins at E, the zero point—that is, on the axis or line of no pressure.
As soon as the loop rotates out of the vertical plane, the electromotive force rises and the current begins to flow in the direction indicated by the arrows, going out to the external circuit through brush M, and returning through brush S.
Continuing the rotation, the electromotive force increases in proportion to the sine of the angle made by the plane of the loop with the horizontal, until the loop comes into the horizontal position illustrated in fig. 170. This increase is indicated by the gradual rise of the sine curve from E to F. The loop has now made one quarter of a revolution and the electromotive force reached its maximum value.
As the loop rotates past the horizontal position of fig. 170, the electromotive force gradually decreases in intensity, reaching the zero point at the end of the second quarter—that is, when the loop has turned one half revolution. This is indicated by the gradual fall of the curve from F to G.
When the loop turns out of the vertical position shown in fig. 171 the current reverses, because the movement of A B and C D is reversed; at this instant the brush M becomes negative, and S positive. This reversal of current is indicated by the curve falling below the axis from G to I.
During the second half of the revolution, figs. 171 to 173, the changes that occur are the same as in the first half, with the exception that the current is in the reverse direction; these changes are as shown by the curve from G to I.
How the Dynamo Produces Direct Current: The Commutator.—The essential difference between an alternator and a dynamo is that the alternator delivers alternating current to the external circuit while the dynamo delivers direct current. In both machines, as before stated, alternating currents are induced in the armature, but the kind of current delivered to the external circuit depends on the manner in which the armature currents are collected.
In the case of an alternator, the method is quite simple. As previously explained, each end of the loop is connected with an insulated collector ring carried by the shaft, the current being collected by means of brushes which bear against the rings. This principle, rather than the actual construction, is shown in the preceding illustrations. Its important point, as distinguished from other methods of collecting the current, is that each end of the loop is always in connection with the same brush.
Ques. How is direct current obtained in a dynamo?
Ans. A form of switch called the commutator is placed between the armature and the external circuit and so arranged that it will reverse the connections with the external circuit at the instant of each reversal of current in the armature.
Ques. How is a commutator constructed?
Ans. It consists of a series of copper bars or segments arranged side by side forming a cylinder, and insulated from each other by sheets of mica or other insulating material.
Ques. Where is the commutator placed?
Ans. It is attached to the shaft at the front end of the armature.
Ques. What are inductors?
Ans. The insulated wires wound on the armature core, and in which the electric current is induced.
Ques. How are the inductors connected to the commutator?
Ans. The ends of each conducting loop or coil must be connected with the commutator segments in a certain order to correspond with the type of winding.
Ques. Explain in detail how direct current is obtained in a dynamo.
Ans. It will be easily seen by the aid of a series of illustrations just how the alternating armature currents are transformed into direct current. Figs. 174 to 178 show, in several positions, a single loop of wire with its ends joined to a commutator; the latter has only two segments, one for each end of the loop. In fig. 174 the loop is shown in the vertical position, and it should be noted that the division between the two segments forming the commutator is in the same plane as the loop. When the loop is in the vertical position, as shown in fig. 174, brush M is in contact with segment F, and S with G. As the armature rotates, the current flows for one half revolution in the direction A B, through segment F and out to the external circuit through brush M as shown in figs. 174 and 175, returning through brush S and segment G. At the beginning of the second half of the revolution, fig. 176, the current in the loop reverses and flows in the opposite direction B A as indicated by the arrows. At this instant, however, the brushes M and S pass out of contact with segments F and G, and come into contact with G and F respectively; that is, M leaves F and contacts with G, while S leaves G and contacts with F. The effect of this is to reverse the connections with the external circuit at the instant the alternation or reversal of current in the armature takes place, thus keeping the current in the external circuit in the same direction.
Ques. How is this indicated by the sine curve?
Ans. The sine curve, instead of falling below the axis, as in figs. 169 to 173, again rises as in the first half of the period, that is G′H′I′ is identical with E′F′G′.
Ques. Is the direct current indicated by the sine curve in figs. 174 to 178 continuous?
Ans. No; it is properly described as a pulsating current, or one, constant in direction, but periodically varying in intensity so as to progress in a series of throbbings or pulsations instead of with uniform strength.
Ques. What is generally understood by the word “continuous” as applied to the current obtained from a dynamo?
Ans. It is usually accepted as meaning a steady or non-pulsating direct current; one that has a uniform pressure and constant direction of flow as opposed to an alternating current.
Ques. Is a continuous current ever obtained with a dynamo?
Ans. No.
It should be clearly understood at the outset that it is impossible to obtain a continuous current with a dynamo. The so-called continuous current which it is said to produce is in reality a pulsating current, but with pulsations so minute and following each other with such rapidity that the current is practically continuous, and as such is generally called continuous.
Ques. How is the so-called continuous current produced by a dynamo?
Ans. In order to obtain a large number of small pulsations per revolution of the armature instead of two large pulsations, as with the single loop armature, the latter must be replaced by one having a great number of loops properly connected to commutator segments and so arranged that the successive loops begin the cycle progressively.
The difficulties encountered in connecting up numerous loops were overcome by Gramme, who, in 1871 invented a “ring” armature. His method consists in winding a ring with a continuous coil of wire, connections being made at suitable intervals with the commutator.
In order to understand the action of such an arrangement, it will be well to first consider four separate coils wound on a ring as shown in fig. 184. These coils are all similar, but at the moment occupy different magnetic positions on the ring. The rotation being clockwise, 1 is about to enter the field adjacent to the north pole, while 2 is emerging from the field in the region of the south pole. Again, 3 is approaching the south pole and 4 receding from the north pole.
Ques. Describe in detail the action of the four coils wound around the ring as in fig. 184.
Ans. According to the laws of electromagnetic induction, pressures are set up at the ends of the coils such as tend to produce currents in the directions indicated by the arrows. Now, assuming the electromotive forces in coils 1 and 2 to be equal, if the adjacent ends be joined, no flow of current will take place, but the junction will be at a higher pressure than the loose ends of the coils and if a wire be attached to this junction, and the necessary circuits completed, a current will flow along the wire outward from the junction. Similarly, if the adjacent ends of coils 3 and 4 be joined, there will be no flow of current, but the junction will be at a lower pressure than the loose ends, and if a wire be attached to the junction and the necessary circuits completed, current will flow from the junction around the coils.
Ques. What may be said with respect to the four coil Gramme ring armature shown in fig. 185?
Ans. According to the laws of electromagnetic induction, with the north pole of the field at the left and clockwise rotation, the induced currents flow upward on both sides of the ring, hence, the electromotive forces oppose each other at only two of the junctions, namely: at the one connected to brush M where the pressures on either side are both directed toward the junction and the other at the junction connected to brush S, at which the pressures are both directed from the junction.
It is evident, then, that the pressure at M is higher than at S; that is, M is positive and S negative; consequently, the current flows from M to the external circuit and returns through S.
Ques. In what other way may the four coils of the armature in fig. 185 be regarded?
Ans. They may be considered as two pairs A A′ and B B′, the action of either pair being identical with the two coil armature shown in fig. 183; this, in turn, produces the same effect as the one coil armature of fig. 182, with the exception that the amplitude of the current generated with two coils is twice as great as that with one coil of the same number of turns.
Again considering the action of the four ring coil shown in fig. 185, and starting at the beginning of the revolution, the variation of electromotive force induced in coils AA′ is indicated by the dotted sine curve 1, and of BB′ by dotted curve 2. It will be seen that 1 begins at the axis or line of no pressure, and 2 at maximum pressure.
The two curves overlap each other, and in order to determine the effect of this it is necessary to trace the resultant curve, 3. This is easily done, as the resultant electromotive force induced at any point in the revolution of the armature is equal to the sum of the pressures induced in AA′ and BB′. Thus, at the beginning of the revolution the pressure induced in AA′ is at zero point, and in BB′ at its maximum J, hence, the resultant curve begins at the point J. Again, for any point in the revolution, as N, the height of the resultant curve is equal to NP + NT = NV. For 45° or 1⁄8 revolution, the resultant curve reaches its amplitude, which is equal to 2 × RZ = RW, and at 90° it again reaches its minimum, XY.
Ques. State the conditions upon which the steadiness of the current depends.
Ans. It depends on the number of coils and the manner in which they are connected.
Comparing curves 1 and 3, in fig. 185, it will be noted that with four coils the variation of pressure or amplitude of the pulsations is less than half that obtained with two; moreover, with four coils the number of pulsations per cycle is doubled.
In order to further observe the approach to continuous current obtained by increasing the number of coils, the effect of a six coil armature is shown in fig. 186, the resultant curve being obtained in the same manner as just explained. For comparison, the curves for the three cases of two, four, and six coils are reproduced under each other in fig. 187.
As the number of coils is further increased, the amplitude of the pulsations decreases so that the resultant curve approaches nearer the form of a straight line.
In the actual dynamo there are a great many coils, hence the amplitude of the pulsations is exceedingly small; accordingly, it is customary to speak of the current as “continuous,” although as previously mentioned such is not the case.
In order to adapt the dynamo to the varied conditions of service, its design is modified in numerous ways, giving rise to the different “types.” These may be classified with respect to:
The first division relates to the number of magnetic poles, as unipolar, bipolar, and multi-polar dynamos; also inter-polar dynamos. Under the second division are included the following:
1. Self-exciting machines of which the magneto is the simplest. Its magnetic field is obtained from permanent magnets, hence the electromotive force generated is comparatively small. The more important type of self-exciting machine is provided with electromagnets in which the field of force is “built up” from the residual magnetism of the soft iron or steel cores of the field magnets of the dynamo itself. Nearly all commercial types of dynamo are of this class.
2. Separately excited machines in which the field magnets are magnetized when the machine is in operation by current supplied from a separate source such as a battery or magneto generator.
With respect to the third division, based on the field winding, dynamos are classed as:
In addition to the foregoing there are further distinctions with respect to the mechanical features. Most dynamos have a revolving armature and stationary field magnets; however, in some cases, both the armature and field magnets are stationary, a revolving iron inductor being provided to intercept the magnetic lines intermittently which produces the same effect as is obtained in cutting the magnetic lines by a revolving armature.
Ques. What may be said of bipolar and multi-polar dynamos?
Ans. Dynamos with bipolar field magnets were universally used prior to 1890, but since that time machines of this type are only made in very small sizes; the multi-polar dynamo is the type now in general use.
Ques. State some of the features of the multi-polar dynamo.
Ans. In this class of machine, the armature and field magnets are surrounded by a circular frame, or ring yoke to which the field magnets are attached. This ring arrangement has the advantages of strength, simplicity, symmetrical appearance, and minimum magnetic leakage, since the pole pieces have the least possible surface and the path of the magnetic flux is shorter.
Ques. What important advantage is gained by the use of multi-pole field magnets?
Ans. Commercial voltages are obtained at moderate armature speed.
The difficulty experienced with bipolar machines is that, with a dynamo of large output, the speed at which its armature would have to rotate to generate commercial voltages would be excessive.
It is evident that with two or more magnetic fields, secured by increasing the number of poles, the armature inductors revolving between them cut more magnetic lines in one revolution than with a single field, hence, a given voltage is obtained with less speed of the armature than in the bipolar machine.
For instance, if a bipolar dynamo be required to run at say 900 revolutions per minute to generate 125 volts, a four pole machine of equal output will require only 450 revolutions, and one of eight poles only 225 revolutions per minute.
Ques. What is a self-exciting dynamo?
Ans. A machine in which the initial excitation of the field is due to the residual magnetism retained by the cores.
Ques. What may be said of the field due to this residual magnetism?
Ans. It presents a very weak field, and the voltage that could be generated by the armature revolving in such a field would be only about two to ten volts.
Ques. How then can commercial voltages such as 100 or more volts be obtained with a self-exciting dynamo?
Ans. Part or all of the current induced in the armature is passed through the windings of the field magnets, thus strengthening the field. The voltage, therefore, will “build up,” increasing until the maximum has been reached.
The maximum voltage will depend upon the capacity of the field magnets as determined by the construction, and upon the strength of current used to excite them.
Ques. How long does the process of “building up” require?
Ans. The time required to fully excite the field magnets is from ten to twenty seconds, the rise in field strength being indicated on the voltmeter or by the gradual increase in the brilliancy of the pilot lamp.
Ques. Name three important classes of dynamo.
Ans. Series wound, shunt wound, and compound wound.
Ques. Describe the winding of a series dynamo.
Ans. In this machine, the field magnets are wound with a few turns of thick wire joined in series with the armature brushes as shown in fig. 190.
Ques. What is the effect of this arrangement?
Ans. All of the current generated by the machine passes through the coils of the field magnets to the external circuit. The current in passing through the field magnets, energizes them and strengthens the weak field due to the residual magnetism of the magnet cores, resulting in the gradual building up of the field.
Ques. For what service is the series dynamo adapted?
Ans. It may be used for series arc lighting, series incandescent lighting, and as a booster for increasing the pressure on a feeder carrying current furnished by some other generator.
Ques. What is the effect of the series winding in the operation of the machine?
Ans. Its characteristic is to furnish current at an increased voltage as the load increases. If sufficient current be drawn to overload the machine, the voltage will drop.
Since the armature coils, field magnets and external circuits are in series, any increase in the resistance of the external circuit lessens the power of the machine to supply current, because it diminishes the current in the coils of the field magnets and therefore diminishes the effective magnetism. Again, a decrease in the resistance of the external circuit will increase the voltage because more current will flow through the field magnets. Accordingly, when the external circuit has lamps in series (as is common in an arc light circuit) the switching on of an additional lamp both adds to the resistance of the circuit and diminishes the power of the machine to supply current. When the lamps are in parallel, the switching on of additional lamps not only diminishes the resistance of the circuit, but causes the field magnets to be further excited by the increased current, so that the greater the number of lamps put on, the greater becomes the risk of inducing too much current.
The series dynamo has also the disadvantage of not starting action until a certain speed has been attained, or unless the resistance of the external circuit be below a certain limit.
Regulation of Series Dynamos.—The series dynamo is ordinarily used for operating arc lamps connected in series. The current generally consumed is about 10 amperes, and it is necessary that it should remain at this strength to keep the lights burning steadily. If it increase, the lights will be too bright, and if it decrease, they will be too dim or flicker.
With all the lamps connected in series it is evident that the resistance of the circuit will vary widely as they are turned on or off, the resistance increasing as the lamps are turned on, and decreasing as they are turned off. It is necessary, therefore, that some means of regulation be provided to enable the dynamo to increase or decrease the voltage in proportion to the load. There are several methods of regulation, as by:
Whatever method be used the necessary regulation should be accomplished by automatic devices, as it would not be practical to station a man in constant attendance to regulate the voltage every time one or more lamps were thrown on or off.
Ques. When is the first method of regulation used?
Ans. It is only used in special cases, as for constant load; if the voltage be not just right to give the required current, it may be adjusted by changing the speed of the engine.
Ques. What may be said of the second method?
Ans. In both the “ring” and “drum” types of armature, rotating in a bipolar field, there are two points situated at opposite extremities of a diameter of the commutator, at one of which the potential is a maximum and at the other a minimum, and it is at these points that the brushes must be placed in order to obtain the greatest difference of pressure, the difference being less at other points. Hence, by rocking the brushes around the commutator the pressure at the terminals of the machine may be varied and regulated as required.
Ques. What difficulty is experienced in rocking the brushes to regulate the voltage?
Ans. Sparking takes place at the brushes when they are moved any considerable distance from the neutral position.
Special dynamos have been designed to overcome this objectionable feature, still this method of regulation is not extensively used.
Ques. What may be said of the third method of regulation?
Ans. The third method, that of variation of field strength, is the one in general use.
Ques. How is the field strength varied?
Ans. This may be done by the two path method, or by the variable field coil method.
Ques. Describe the two path method of field regulation.
Ans. An adjustable resistance or rheostat is connected in parallel with the field winding as shown in fig. 191. This shunts more or less of the current from the field winding according to the amount of resistance made active by the lever, L.
Thus, if the current in the armature and main circuit be 10 amperes and the resistance of the field winding 10 ohms, a resistance of 40 ohms in parallel with the winding would cause the current to split in the ratio of 40 to 10, or 4 to 1; 2 amperes would pass through the resistance and 8 amperes through the field.
Ques. Describe the variable field coil method of field regulation.
Ans. This consists in dividing the field winding into a number of sections and throwing the sections in and out of circuit as shown in fig. 192.
Since the strength of any magnet depends on the number of ampere turns in its field winding, reducing or increasing the number of turns will respectively reduce or increase the field strength, the current being kept constant.
Ques. What is the objection to this method?
Ans. This arrangement is undesirable for magnets of large size, because of the tendency to flashing at the contacts of the regulating switch.
The Shunt Dynamo.—The shunt wound dynamo differs from the series wound machine, in that an independent circuit is used for exciting its field magnet. This circuit is composed of a large number of turns of fine insulated copper wire, which is wound round the field magnets and connected to the brushes, so as to form a shunt or “by pass” to the brushes and external circuit, as shown in fig. 193. Two paths are thus presented to the current as it leaves the armature, between which it divides in the inverse ratio of the resistance. One part of the current flows through the magnetizing coils, and the other portion through the external circuit.
In all well designed shunt dynamos, the resistance of the shunt circuit is always very great, as compared with the resistance of the armature and external circuit, the strength of the current flowing in the shunt coils being very small even in the largest machines.
Ques. For what service is the shunt dynamo adapted?
Ans. It is used for constant voltage circuits, as in incandescent lighting.
Ques. In the operation of a shunt dynamo what is its characteristic feature?
Ans. The voltage at the dynamo remains practically unchanged, and the current varies according to the load.
Ques. Does the voltage remain constant for all loads?
Ans. There is a certain maximum load current that the shunt dynamo is capable of supplying at constant voltage; beyond this, the voltage will decrease, the machine finally demagnetizing itself, and ceasing to generate current.
Ques. Why does the voltage not remain constant for all loads?
Ans. Because there is a drop in the voltage in forcing the current through the armature windings which increases with the load.
Ques. What is the usual method of regulation for shunt dynamos?
Ans. The method of varying the current through the field coils by means of a rheostat inserted in series with the field winding as shown in fig. 194.
Moving the lever L of the rheostat to the right increases the resistance in series with the field winding, and this reduces the amount of current in that winding, thus reducing the strength of the magnet and consequently the voltage at the brushes. The contrary movement of the lever, by cutting out the resistance, produces the opposite effect.
The Compound Dynamo.—This class of generator is designed to automatically give a better regulation of voltage on constant pressure circuits than is possible with a shunt machine. It possesses the characteristics of both the series and shunt machines, of which it is in fact a combination.
The field magnets of the compound dynamo, as shown in fig. 195, are wound with two sets of coils, one set being connected in series, and the other set in parallel, with the armature and external circuit. The purpose of the series winding is to strengthen the magnets by the current supplied from the armature to the circuit, and thus automatically sustain the pressure. If the series winding were not present, the pressure at the terminals would fall as the load increased. This fall of pressure is counteracted by the excitation of the series winding, which increases with the load and causes the pressure to rise. The number of turns and relative current strengths of the series and shunt windings are so adjusted that the pressure at the terminals is maintained practically constant under varying loads.
With respect to the ratio between the number of turns of the two field windings, the dynamo is spoken of as:
Ques. What is the difference between a compound and an over compounded dynamo?
Ans. In the first instance, there are just enough turns in the series winding to maintain the voltage constant at the brushes for variable load. If a greater number of turns be used in the series winding than is required for constant voltage at the brushes for all loads, the voltage will rise as the load is increased, and thus make up for the loss or drop in the transmission lines, so that a constant voltage will be maintained at some distant point from the generator. The machine is then said to be over compounded.
Ques. For what service is over compounding desirable?
Ans. For incandescent lighting where there is considerable length of transmission lines.
Ques. What is the usual degree of over compounding?
Ans. Generally for a rise of voltage of from five to ten per cent.