592 TRANSACTIONS OF SECTION G. 
matically kept constant, notwithstanding any difference that may exist in the 
size of the drivers on the two electromotives. Incidentally, it may be mentioned 
that this method of liquid rheostat control has also the advantage 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 in- 
ability 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 speeds 
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 starting 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 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 contrary to the accepted rules for 
safe working, which requires 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 automatically 
controlled liquid rheostat the braking comes in quite gradually, and is through- 
out 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 are permitted to run at 
forty-five kilometres per hour. This concession is not extended to passenger 
trains. Nevertheless tht 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 
