The coils of the field - magnets cannot be constructed of no resistance; thus they always waste some of the energy of the currents in heat. It has been argued that it cannot be economical to use electro - magnets in comparison with permanent magnets of steel, which have only to be magnetized once for all; but certain considerations tell in favour of electro - magnets. For equal power, their prime cost is less than that of steel magnets, which require remagnetizing at intervals. Moreover, as there is a limiting velocity at which it is safe to run a machine, it is important, in order not to have machines of needlessly great size, to use the most powerful field - magnets possible. But if it is more convenient to spend part of the current upon the electro - magnets, economy dictates that they should be so constructed that their magnetism may cost as little as possible. To magnetize a piece of iron requires the expenditure of energy; but when once it is magnetized, it requires no farther expenditure of energy (save the slight loss by heating in the coils, which may be reduced by making the resistance of the coils as little as possible) to keep it so magnetized, provided the magnet is doing no work.
Even if it be doing no work, if the current flowing round it be not steady, there will be loss. If it do work, say, in attracting a piece of iron to it, then there is an immediate and corresponding, call upon the strength of the current in the coils to provide the needful energy. In a dynamo, where, in many cases, are revolving parts containing iron, it is of importance that the approach of a recession of the iron parts should not produce such reactions as these in the magnetism of the magnet. Large slow - acting field - magnets are therefore advisable. The following points embody the conditions for attaining the end desired: -
(a) The body of the field - magnets should be solid. Even in the iron itself currents are induced, and circulate whenever the strength of the magnetism is altered. These self - induced currents tend to retard all changes in the degree of magnetization. They are stronger in proportion to the square of the diameter of the magnet, if cylindrical, or to its area of cross - section. A thick magnet will therefore be a slow - acting one, and will steady the current induced in its field.
(b) Use magnets having in them plenty of iron. It is important to have a sufficient mass, that saturation may not be too soon attained.
,(c) Use the softest possible iron for field - magnets, not because soft iron magnetizes and demagnetizes quicker than other iron (that is here no advantage); but because soft iron has a higher magnetic susceptibility than other iron - is not so soon saturated.
(d) Use long magnets to steady the magnetism, and therefore the current. A long magnet takes a longer time than a short to magnetize and demagnetize. It costs more and requires more copper wire in the exterior coil; but the copper wire may be made thicker in proportion, and will offer less resistance. The magnetism so obtained should be utilized as directly as possible, therefore.
(e) Place the field - magnets, or their pole - pieces, as close to the rotating armature as is compatible with safety in running,
(f) If the field - magnets or their pole - pieces have sharp edges, the field cannot be uniform, and some of the lines of force will run uselessly through the space outside the armature instead of going through it. Theoretically, the best external form to give to a magnet is that of the curves of the magnetic lines of force.
(g) Reinforce the magnetic field by placing iron, or, better still, electromagnets, within the rotating armature. This is done by giving the armature coils iron cores which rotate with them; or the iron cores or internal masses may be stationary. In the former case, is loss by heating; in the latter, are. structural difficulties to be overcome.
(A) In cases where a uniform magnetic field is not desired, but where, as in dynamos of the second class, the field must have varying intensity at different points, it may be advisable specially to use field - magnets with edges or points, so as to concentrate the field at certain regions.
( a ) The pole - pieces should be heavy, with plenty of iron in them, for reasons similar to those urged above.
(6) They should be of shapes adapted to their functions. If intended to form a single approximately uniform field, they should not extend too far on each side. The distribution of the electromotive force in the various sections of the coils on the armature depends very greatly on the shape of the pole - pieces.
(c) Pole - pieces should be constructed so as to avoid, if possible, the generation in them of useless Foucault currents. The only way of diminishing loss from this source is to construct them of laminae, built up so that the mass of iron is divided by planes in a direction perpendicular to that of the currents, or of the electromotive forces tending to start such currents.
(d) If the bed - plates of dynamos are of cast - iron, care should be taken that these do not short - circuit the magnetic lines of force from pole to pole of the field - magnets, Masses of brass, zinc, or other non-magnetic metal may be interposed; but are at best a poor resource. In a well-designed dynamo there should be no need of such devices.
(a) To be of the greatest possible service, the coils of field-magnets should be wound on most thickly at the middle of the magnet, not distributed uniformly along its length, nor yet crowded about its poles. The reason for this is two-fold. Many of the lines of force of a magnet "leak out" from the sides of the magnet before reaching its poles, where they should all emerge if the mass of the magnet were perfectly equally magnetized throughout its whole length. Internally, the magnetization of the magnet is greatest at its centre. At or near the centre, therefore, place the magnetizing coils, that the lines of force due to them may run through as much iron as possible. The second reason for not placing the coils at the end is this: any external influence which may disturb the magnetism of a magnet, or affect the distribution of its lines of force, affects the lines of force in the neighbourhood of the pole far more than those in any other region.
(6) The proper resistances to give to the field-magnet coils of dynamos have been calculated by Sir Wm. Thomson, who has given the following results:-
For series dynamos, make the resistance of the field-magnets a little less than that of the armature. Both should be small compared with the resistance of the external circuit. The ratio of the waste by heating in the machine to the total electric work of the machine will be- waste/ total work=RM + RA/RM+RA+RX and useful work/total work = RX/RM+RA+RX where Rm is the resistance of the magnets, Ra is the resistance of the armature,
Rx is the resistance of the external circuit. For a shunt dynamo the rule is different. The best proportions are when such that
RX=√Rm Ra, or that
RM = R2/x / RA also the ratio of useful work is useful work/total work =1/1+2 √ RA/RM
√ RA/RM must equal 1/20;
As an example of the latter, suppose it was wished that the waste should not be more than 10 Per cent. of the useful work, the ratio of the formula must equal 10/11 or 1 ÷ 1 1/10. Hence or Km the resistance of the field - magnets must be 400 times Ra that of the armature.