188 
of the series motor, and the practical results obtained 
with it are certainly very promising. The latest design 
made by Dr. Behn-Eschenburg shows a remarkable 
weight efficiency. His 2500 horse-power electro- 
motives (the power being at a one and a half-hour 
rating) weigh only 108 tons, so that at this rating 
1 horse-power is obtained with a total weight of 
43 kilogrammes. This compares favourably with the 
high-pressure C.C. system, where 50 to 7o kilo- 
grammes per horse-power may be taken as normal 
values. 
The so-called ‘trepulsion motor’? invented by Prof. 
Elihu Thomson has been applied to railway work in 
the slightly modified form due to Mr. Deri, where, 
instead of there being only two brushes per pair of 
poles, double the number is provided, and the adjust- 
ment for speed and torque is made more accurate, 
whilst at the same time the commutation, being split 
up into two steps, becomes easier. In the matter of 
simplicity, an electromotive fitted with Deri motors 
cannot be surpassed by any other arrangement. There 
are no rheostats, contactors, control switches, or other 
gear; all the regulation is effected by mechanical 
transmission of the movement of a hand-wheel placed 
in the driver’s cab to the brushes of the motors. At 
one time it was hoped that this system would win its 
way to a general application; but, unfortunately, the 
motor must run somewhere near synchronous speed, 
and becomes therefore rather heavy with the low 
frequencies alone possible in traction. Moreover, as 
the power-factor obtainable is only about o’8o, that is, 
considerably below the value obtainable with other 
motors, there does not seem to be any great future 
for this system for heavy work, although its great 
simplicity may still turn the balance in its favour on 
lines with a light traffic. For heavy lines the choice 
at present lies between the induction motor, with direct 
rotor excitation, and the straightforward conduction- 
motor, where rotor and stator are traversed in series 
by the same current. The former type of motor— 
also called the Latour-Winter-Eichberg motor— 
depends for its working current in the rotor on electro- 
magnetic induction, which produces the working cur- 
rent in the rotor much in the same way as the current 
in the secondary circuit of a transformer is produced 
by induction. Since the motor has in part the char- 
acter of a transformer its weight would, as is the case 
with any transformer, be unduly augmented by too 
great a reduction in the frequency. Experience has 
shown that a frequency of twenty-five periods per 
second is high enough to render the transformer action 
effective, and at the same time not so high as to 
introduce serious difficulties as regards e.m.f. of self- 
induction and commutation. This frequency has been 
adopted in most cases where electrification of main 
lines has been carried out by motors of this class. 
One valuable feature of this motor is that at a speed 
slightly exceeding synchronism the power-factor may 
be brought up to unity. At this speed the commuta- 
tion takes place under conditions which may be 
described as theoretically perfect. A fair number of 
Continental lines have been electrified by using these 
motors, and they have also been adopted, with very 
satisfactory results, in the electrification of the London, 
Brighton and South Coast lines between Victoria and 
London Bridge and to some distance south of London. 
On this line no locomotives are used, but only motor 
coaches. It is therefore not possible to make a direct 
comparison as to weight efficiency with a locomotive. 
The latter has only to carry the propelling machinery, 
whilst the former has to provide accommodation for 
passengers~ as well. The 600 horse-power motor 
coaches on the Brighton line weigh 50’ tons, or at 
the rate of 83 kilogrammes per horse-power. A 
NO. 2293, VOL. 92] 
NATURE 
[OcTOBER 9, 1913 
1000 .horse-power C.C. electromotive taking: current 
at 1200 volts weighs 74 tons.‘ By making a suitable 
reduction for the extra weight of the passenger 
accommodation in the A.C. coach, its weight per 
horse-power comes out at something like 60 kilo- 
grammes, against 62 kilogrammes in the C.C. engine. 
Series motors are employed on the electrified lines 
of the Midland Company between Heysham, More- 
cambe, and Lancaster. Also in this case motor 
coaches, and not electromotives, are used. At the 
hour-rating a motor coach develops 420 horse-power, 
and as its total weight is about 35 tons, we have here 
the same weight-efticiency as on the Brighton lines— 
‘namely, 83 kilogrammes per horse-power for the whole 
coach. 
Of high-pressure continuous-current lines there are 
many examples, both in Europe and America. The 
term high-pressure does, of course, not imply the 
same order of magnitude as in single-phase A.C. lines. 
There high-pressure may mean anything up to 15,000 
volts, the pressure which is likely to become a standard 
in future electrifications; but in C.C. work one must 
class anything over tooo volts or 1500 volts as high- 
pressure. The general rule is to employ motor 
coaches, and not electromotives; but there is a private 
line belonging to a steel-works in Lorraine, where two 
electromotives, each of 600 horse-power (four C.C. 
motors of 150 horse-power) are working the mineral 
trains under a pressure of 2000 volts. The Southern 
Pacific Railway also employs C.C. electromotives of 
1000 horse-power each, Each engine weighs 74 tons, 
and hauls a train of 270 tons on grades of 4o per 
mille. This is a remarkable performance, rendered 
possible by the fact that with the even torque exerted 
by the electric motor a much large co-efficient of fric- 
tion than is possible in steam traction may safely be 
permitted. Electrical engineers generally base_their 
calculation of the possible tractive effort on a co-efficient 
of o'17, without sand, and as high as 0'25, or even 
0°28, if sand is used. The voltage in the case of the 
Southern Pacific engines is only 1200° volts, taken 
by two motors in series, and there is provision made 
to change over from the overhead wire to third rail, 
with 600 volts, when the motors are all in parallel. 
On European C.C. lines the voltage is higher— 
generally 2000 volts, as on the Chur-Arosa and some 
other Swiss lines—and the tendency is still in the direc- 
tion of higher pressures. Continental makers are now 
prepared to go as far as 1200 volts per motor, so that 
with the usual system of series-parallel control a line- 
pressure of 2400 volts becomes possible. The greatest 
step in advance in this direction has, however, been 
made in England, where Messrs. Dick Kerr, Ltd., 
have adopted a line-pressure of 3500 volts as their 
standard, involving the use of motors constructed for 
1750 volts. After having experimented with this high- 
pressure system for two years, they have undertaken 
the electrification of a short section of the Lancashire 
and Yorkshire Railway with continuous current at 
3500 volts. JI am indebted to the firm for the following 
particulars: The current is collected by pantograph 
from an overhead wire with catenary suspension. The 
train consists of a motor coach and two trailers. The 
motor coach is equipped with four 300 horse-power 
motors, and weighs 62 tons; the trailers weigh each 
26 tons. From these figures it will be seen that the 
weight of the motor coach per horse-power is only 
52 kilogrammes, and thus considerably below what 
the weight of an equivalent single-phase motor coach 
would be. It is especially the saving in weight and 
the avoidance of any telephonic disturbances which 
renders the C.C. system so attractive that, in spite of 
x See Gratzemueller'’s paper read at the Paris meeting of the I. E. E. and 
S. Intern. des Electr. (Paris, May, 1913). 
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