The method of transforming by direct current without the aid of
batteries is not practically at work; but, as the advantages are so
obvious and its development is only a question of time, a description
of the system may not be considered out of place.
The electrical exhibition at Philadelphia in 1884 contained a dynamotor
which was exhibited by the Van de Poele Electric-Light Company, but,
as far as could be ascertained, was not worked, and, as it was simply
described as an induction machine for distributing currents for the use
of incandescent and other lights, it attracted little attention.
The advantages of an alternating current transformer system of
distribution, Class II., has been put forward in these pages,
especially that of simplicity and cheapness. An alternating current
dynamo for a given output is cheaper than a direct, and it takes less
labour to look after it, because it has no commutator.
An alternating transformer is also an exceedingly simple piece of
apparatus. If originally made with due care and kept in a dry place, it
never breaks down, as it has no moving parts, and so there is nothing
to go wrong.
The alternating system of distribution has, however, some very serious
disadvantages. In the first place, it is most important that motors
should be driven during the day when the lights are not in use. In the
second, batteries cannot be used in an alternating current system, so
any immunity from breakdown that they might ensure is wanting; and
steam must be kept up all day and all night.
If motors are wanted during the day, so that the load on the engine is
nearly constant, batteries are not so valuable, except with a view of
preventing a breakdown; but, if batteries cannot be used, the advantage
of using motors becomes enormous, as the plant has to be large enough
to supply the maximum load, and would otherwise be idle during the day.
In alternating current systems there are two difficulties in the way
of using motors. It is difficult to make an alternating current motor
that will start, and, if that difficulty is surmounted, it is difficult
to make an alternating current motor that will work on varying loads
without great waste of power. The question of the efficiency of
alternating current motors has never been really practically studied
yet; and, until these difficulties are overcome, we must regard
alternating current motors as non-existent. Several methods of working
alternating current transformers off direct currents by commutating the
primary have been proposed at different times; but they all seem to be
impracticable, and it seems impossible to get over the difficulties
that arise from sparking when it is attempted to break a high-tension
circuit.
One of the first methods of distribution over large areas proposed was
by means of motors and dynamos combined. For instance, suppose, in
order to keep down the size of the leads, 2,000 volts are used in the
mains, a motor capable of working with 2,000 volts is put down where
the lights are wanted, and this is made to drive a dynamo giving 100
volts and a large current. Instead of having a separate motor and a
dynamo connected by a belt or by coupling the spindles together, it is
simpler to make one machine with two armatures, or to have only one
armature with two circuits on it. One circuit is wound with fine wire
and takes the 2,000 volts and tends to turn the armature round. The
other circuit is wound with thick wire giving 100 volts and a large
current, and tends to stop the armature, thus absorbing the power
supplied by the high-pressure circuit. The direct-current transformer
or dynamotor is thus a sort of double dynamo, or dynamo and motor
combined. If it gets 2,000 volts and 10 ampères, it would, if there
were no waste, give 100 volts and 200 ampères; with a waste of 10 per
cent., it will give 100 volts and 180 ampères.
In the United States it is usual to place an alternating current motor
in each house to be lighted; but the conditions are quite different
there, overhead wires being used extensively. In this country this
system is not likely to find favour, and local sub-stations will be
used, the high pressure, which is always dangerous to life, will
thus be kept out of private houses and offices. There is, then,
very little difference in the cost of maintenance of alternating
current transformers and dynamotors, and the advantages possessed
by alternating current transformers in this respect are more than
counter-balanced by the use of motors on direct current circuits.
Dynamotors have not come into general use yet because no stations have
been started in this country of the size which demands them. No central
station with sub-stations is in operation, but there is every reason to
expect several will be soon; and it is very necessary to discuss the
various methods, not only in use at this moment but coming into use in
the immediate future. The dynamotor itself needs no working out, as
any maker of direct-current dynamos can, of course, make them. Messrs.
Paris and Scott of Norwich showed some in operation at the Newcastle
Exhibition in 1887; the most successful type is that recently invented
by Mr. Jas. Swinburne, illustrated by Fig. 29. The backward main round
primary or motor magnet is shown on the left, and the forward main
round the secondary or dynamo magnet on the right, the outside coil
round both magnets is the shunt.
[Illustration: FIG. 29.]
The dynamotor may be made with two circuits on one armature as
already explained, or it may have two armatures in separate fields,
still making up one machine. The first arrangement has two grave
disadvantages. There is difficulty about securing perfect insulation
between the two circuits, and this leads to chances of danger in the
houses. A dynamotor with two circuits on one armature cannot be
compounded, that is to say, it cannot be made to give constant
electrical pressure on the mains if the number of lamps is varied. A
Swinburne double armature machine can be compounded, not only to give
constant pressure with a varying load, but to give constant electrical
pressure even if both the load and the pressure on the primary circuit
vary. This makes a considerable difference in the copper of the primary
leads, as in large and complicated districts it is almost impossible
to arrange leads, even when working with high electrical pressure and
small currents, so that the electrical pressure remains constant,
or even nearly so. A very small variation of the pressure on an
incandescent lamp makes an enormous difference in the amount of light
it gives, and in its duration. It is, therefore, most important that
the E. M. F. on the lamps should be kept absolutely constant.
This difficulty is, of course, insurmountable in the case of
alternating current transformers. Alternating current transformers
cannot be made to compound, and the loss in leads cannot be corrected
by them, so that the lamps burn dull at full load.
If secondary batteries are used at the sub-stations, the reduction of
pressure might be effected by them. A number would be charged in series
and discharged in parallel. This arrangement needs at least two sets
of cells, and cells are expensive; and it is difficult to preserve the
insulation of cells with such electrical pressure as 2,000 volts. If
cells are used for the purpose of equalising the load or as a safety
reserve, it is better to charge them by means of a dynamotor.