acted by the slip-adjustment of the motors. 
motives. 
ee eee a ee le SS 
OcrToBER 9, 1913] 
ence originally existing will diminish in service. For 
the same reason, any tendency to wear unequally, 
say, in consequence of unequal material, is counter- 
This point 
has been tested practically by the makers of the 
Simplon three-phase electromotives. It was found that 
if originally a slight difference in diameter of the 
drivers had been permitted to exist, after a short time 
this had vanished. That is as regards the condition 
on one electromotive; but if we come to the case of 
a train being hauled by two engines, then a sensible 
difference in the size of their wheels may exist. In 
this case it is necessary artificially to adjust the slip 
so as to make each motor take half the load. 
This problem has been solved by Mr. v. Kando in the 
electromotives which he designed for the Italian State 
railways. In these engines only liquid resistances are 
used in the rotor circuit for starting and speed regula- 
tion. The liquid is raised or lowered in the rheostat 
chambers. so as to cover more or less of the contact 
plates, and the level of the liquid is controlled by a 
solenoid under the influence of the working current. 
The working current, and therefore also the tractive 
effort exerted by each motor, is thereby automatically 
kept constant, notwithstanding any difference that may 
exist in the size of the drivers on the two electro- 
Incidentally, it may be mentioned that this 
method of liquid rheostat control has also the advan- 
tage of a perfectly constant acceleration during the 
starting period—a point which makes for comfort of 
travel in a three-phase train. 
The third objection advanced by theorists against 
three-phase traction is against the waste of energy 
consequent on rheostatic speed control and the inability 
to run at more than synchronous speed so as to make 
up for lost time. The obvious remedy for the last- 
named difficulty is to fix the time-table so that the 
synchronous speed should be high enough for making 
up lost time and to employ motors which can run 
economically at less than synchronous speed. As a 
matter of practical experience, three-phase trains are 
not more unpunctual than any other kind, steam not 
excluded. A train pulled by a series motor (C.C. or 
A.C.) runs slower on an up-grade or if abnormally 
heavy; this is one of the characteristics of the series 
motor, and it is valuable, because it limits the excess 
load thrown on to the source of power; but it is 
clearly not a condition making for good time-keeping. 
With a series motor time lost cannot be recovered on 
an up-grade, whilst with a three-phase motor the 
speed on an up-grade may be kept practically the same 
as on the level or on down-grades, so that the process 
of gaining time is not restricted to the easy parts of 
the line. 
The problem of speed control without rheostatic 
waste has been solved in various ways. One of the 
simplest and generally adopted solutions is that of 
cascade and single working. If the two motors are 
put into cascade connection the speed is halved. The 
cascade is used in starting and on heavy grades (unless 
time has to be made up), and on the easy grades or 
on down-grades the motors.work singly—that is to 
say, in simple parallel connections. Intermediate 
sneeds may be obtained by some pole-changing device. 
Ordinarily, such devices have to be applied to stator 
and rotor, but in some of the Simplon electromotives 
only the stator is arranged for pole-changing, the 
rotor being a squirrel cage. In this arrangement the 
advantage of cascade-working has to be given up, 
but the system has the merit of great simplicity. The 
number of poles may be changed from twelve at 
startine to eight, six, and four at top speed. Thus, 
four different speeds, all without rheostatic waste, are 
possible. The single bars in the squirrel cage rotor 
NO. 2293, VOL. 92] 
NATURE 
187 
are connected at their ends by resistance-connectors 
made of an alloy having a high temperature co- 
efficient. At starting the rotor current is large and 
heats up these strips, thus automatically providing 
what is technically termed a_ starting-resistance. 
When the motor is running the current is less, and 
by reason of the fanning action of the connecting- 
strips these get cooled so as to bring their resistance 
down to a permissible amount. Thus the efficiency of 
the motor when running under load is only a few per 
cent. less than that of a motor with a wound rotor. 
A valuable feature of the three-phase system is the 
automatic recuperation of current whenever the speed 
exceeds synchronous speed by a few per cent.: and, 
connected with this property is the further advantage 
that it is impossible for a train to race on a down- 
grade. Obviously recuperation can only take place if 
power is given to the motor. This is provided partly 
by the electromotive itself and partly by the train 
pushing it on a down-grade. This means that the 
train is braked in front only, and railway engineers 
have raised the objections that such a method is con- 
trary to the accepted rules for safe working, which 
require that even on a down-grade all the couplings 
should remain in tension, which means that each coach 
must be independently braked. Here we have again 
a case where the theorists’ objections have been proved 
to be without foundation in actual practice. It is no 
doubt objectionable to brake a train in front only, if 
the braking action is jerky; but with the automatic- 
ally controlled liquid rheostat the braking comes in 
quite gradually, and is throughout so even that it has 
been found possible to permit a higher down-grade 
speed with recuperation than with ordinary braking. 
On the Italian State railways the regulation permits 
on heavy down-grades a speed of thirty kilometres per 
hour for steam trains, but the electric goods trains on 
the Giovi line are permitted to run at forty-five kilo- - 
metres per hour. This concession is not extended to 
passenger trains. Nevertheless the economic effect is 
considerable. Recuperation saves 17 per cent. on the 
coal bill, and this amount is sufficient to provide for 
interest and sinking fund on the electrical plant at 
the generating station. 
One advantage of three-phase traction over steam 
traction is the lessened weight of the locomotive in 
comparison with its tractive force and power. As an 
example, we may take the Giovi line in Italy where 
steam trains, consisting of 310 tons of rolling-stock 
and 202 tons of locomotive (one in front and the other 
at the back), have been replaced by three-phase trains, 
consisting of 380 tons of rolling-stock and two electro- 
motives, each weighing 60 tons (also placed front and 
rear). Thus there has been a saving in total weight 
of 12 tons, and at the same time an increase in useful 
weight hauled of 70 tons. The average grade of this 
line, over which passes the whole traffic between the 
Port of Genoa and the Plain of Lombardy, is 27 per 
mille, and the maximum is 35 per mille This traffic 
is now worked with forty electromotives, each of 
60 tons weight. These engines have five driving- 
wheels connected to two eight-pole motors by gear- 
wheels and rods. The pressure on each driving-axle 
is 12 tons. Each electromotive develops 2000 horse- 
power at the hour-rating; thus 1 horse-power is 
obtained for each 30 kilogramme weight of engine. 
The number of patented designs for single-phase 
traction motors is verv large; but, notwithstanding 
considerable difference in matters of detail, all motors 
which have been successfully applied in practice may 
be ranged under three great groups—namely, the so- 
called repulsion type, the repulsion tvne with addi- 
tional excitation of the rotor, and the straightforward 
series motor. The present tendency is rather in favour 
