each generator with its step-up transformers as a unit and to connect
the generator permanently with its bank of transformers, and to
control this unit by a single three-pole machine-operated oil switch.
In this case there are no switchboard switches between generators and
transformers, and this simplifies the switchboard considerably. There
must be switches on the high-tension side of the transformers in any
event. The switchboard for rotary converters in the substations is, of
course, a combination of alternating and direct-current apparatus. The
direct-current ends of the rotary converters are treated almost exactly
like direct-current railway generators; and their switchboard panels
are similarly equipped, except that usually there is a rheostat that
can be connected in series with the armature whereby a rotary converter
can be brought up to speed from a state of rest by connecting it with
the direct-current bus bars of the substation.
The alternating-current end of the rotary converter is supplied
through switches in the alternating-current leads from the step-down
transformers. A rotary converter can be started from a state of rest
by connecting it to the alternating-current leads through the medium
of compensating coils which reduce the voltage. A very heavy current
is required to do this, as the motor thus starts as a very inefficient
induction motor with a very low power factor.
[Illustration: Fig. 92. Connection of Substations.]
There are usually but two direct-current feeder panels in a substation
of an interurban electric road. One of these feeders is to supply the
trolley or third rail extending in one direction from the substation,
and the other feeds that extending in the other direction from the
substation. The trolley or third rail has a section insulator directly
at the substation. When both feeders are connected to the bus bars,
it is evident that this section insulator is short-circuited through
the medium of the substation bus bars, every substation on the line
being connected in this way, as indicated in Fig. 92. It is seen that,
should a short circuit occur on any section, it would open the circuit
breakers at the substations at both ends, and that section would not
interfere with the balance of the road. At the same time, when the
road is in normal operation and there is an unusually heavy load
between any two substations, the other substations along the line can
help out those nearest to the load by feeding through the bus bars of
the nearest substation. The high-tension apparatus at a substation
consists usually of a bank of high-tension lightning arresters;
high-tension switches, for shutting off the high-tension current; and
step-down transformers, for reducing from the high transmission voltage
to the 370 volts commonly fed to the alternating-current end of railway
rotary converters.
=Storage Batteries in Stations.= Storage batteries are frequently
used both in substations and in direct-current power stations. They
may be connected directly across the line and allowed to “float,” as
it is termed; or they may be used in connection with storage-battery
boosters, which will cause the storage battery to take the fluctuations
in the load and to give a constant load on the rotary converters or
power station. The action of storage-battery boosters which cause the
storage battery to be charged automatically at light loads and to
discharge and assist the station at heavy loads, is explained in the
paper on “Storage Batteries.”
ALTERNATING-CURRENT SYSTEMS.
So far this paper has been devoted almost entirely to electric railway
systems employing 500-volt direct-current motors on the cars, since
this is the system almost universally employed on electric railways
at the present time. There are, however, several systems employing
alternating-current motors on cars, which have already been used
experimentally and to some extent commercially. Some of these give
promise of coming into extensive use.
=Three-Phase Motors.= On several roads in Europe three-phase induction
motors are employed. These induction motors are operated by three-phase
alternating current taken direct from the trolley wires. As three
conductors are necessary, two trolley wires are used, with the rails
as the third conductor. The two principal objections to the system are
the necessity of two trolley wires, and the fact that the induction
motor operates very much like a direct-current shunt motor in that it
is a constant-speed motor and not adapted to variable-speed work. The
power factor is low in starting; that is, a great volume of current
is taken, although, owing to the voltage and the current not being in
phase, the actual energy consumed is small.
=Single-Phase Motors.= The Westinghouse Electric & Manufacturing
Company has brought out a railway motor adapted to operate on
single-phase alternating-current circuits. This motor is very similar
in construction to the ordinary series-wound 500-volt direct-current
railway motor. It has, however, more field poles than the ordinary
direct-current motor; and the pole pieces are laminated to avoid
heating of the iron by eddy currents caused by the influence of
the alternating current. There are also other special features
in the design that reduce the sparking at the commutator, which
sparking was for several years the greatest obstacle to the use of
alternating-current motors of this kind. In the Westinghouse system
the current is taken from the trolley wire at high potential, and is
reduced by an auto-transformer on the car. This auto-transformer is
connected with an induction regulator so arranged that a low voltage
can be supplied to the motor in starting or for slow running, and
this voltage increased to increase the speed. There is thus no need
to reduce the trolley voltage by wasting part of it in a rheostat,
as is the case with direct-current motors; and the efficiency during
acceleration is, therefore, higher with this alternating system than
with the direct current. Several other single-phase railway motors
are also being worked out at the present time, including that of the
General Electric Company.
=Alternating-Current Motor Advantages.= There are two great advantages
secured by the use of an alternating-current railway motor. The first
is a reduction in investment and operating expenses by doing away
with substations containing rotary converters. Such substations are
necessary on long lines of railway operating with direct-current
motors. The second advantage is that, owing to the fact that a high
tension current can be used on the trolley wire and reduced by a
transformer on the car, the difficulties of collecting a large amount
of energy from a trolley wire are much reduced.
First, in regard to the substations, it will be seen that with the
alternating-current motor system, high-tension current can be conducted
from the power house to substations along the line which contain
nothing but static transformers. Since these transformers have no
revolving parts they do not require the constant attendance that a
rotary converter does. Furthermore, the investment in rotary converters
is entirely dispensed with, and this makes a considerable reduction in
the total cost of the distribution plant. With the alternating-current
system, current is fed direct to the trolley wire from the secondary
terminals of the transformers at the substations.
As regards the advantages of carrying a high voltage on the trolley
wire, it will readily be seen that, since the amount of power, or the
watts required by a car, is equal to the product of the voltage and
current, an increase in the voltage reduces the volume of current
necessary. By having high voltage on the trolley wire, even a large car
can be operated with a small volume of current, and this current can be
taken through an ordinary trolley wheel without difficulty. Where 500
volts is the pressure used on the trolley wire, there is considerable
flashing and burning of trolley wheel and wire when large cars and
locomotives are run, owing to the heavy current conducted; and this has
been one of the principal reasons for the adoption of the third rail
instead of the trolley on certain roads. Even with the third rail, the
volume of current that must be conducted to large electric locomotives
involves some difficulties in the way of heated contact shoes and
considerable loss of energy. The use of high voltage on the trolley
wire, with transformers on the car to reduce the voltage to a safe
pressure for use on the motors, overcomes many of the difficulties that
would otherwise be found in the use of electricity for heavy railroad
work.
OPERATION.
=Power Taken by Cars.= The amount of power required in the practical
operation of a car depends upon so many variable elements that many of
the calculations sometimes given for determining the power required
by a car are of little value. The theoretical horsepower required to
maintain a car at a certain speed on a level, is evidently the tractive
effort in pounds multiplied by the speed in feet per minute and divided
by 33,000. What the tractive effort per ton of car will be, depends
on the condition of the rail and on several other uncertain factors.
For street-railway motor cars, 20 pounds per ton is the usual tractive
effort assumed as necessary. A calculation of this kind, however, takes
no account of the losses in the motors and gears, nor of the fact
that the greater part of the power required to propel a street car in
practical service is used in accelerating the car from a state of rest
to full speed. In interurban service, of course, the power required
in acceleration is not so great a proportion of the whole.
[Illustration: Fig. 93. Plotted Data of Road Test.]
The safest figures to use in engineering calculations as to the amount
of power required, are those taken from actual results obtained in
everyday commercial service. The power required by an eight-ton car in
service in a large city like Chicago, is in the neighborhood of one
kilowatt hour per car-mile run. On outlying lines this figure may be
reduced to .7 kilowatt hour, and in the down-town districts may run up
to 1.5 kilowatt hours per car mile. Double-truck cars in city service,
weighing from 20 to 25 tons, take from 2½ to 4 kilowatt hours per car
mile at the power station. Interurban cars around Detroit, weighing
about 32 tons, in interurban service, making 25 miles per hour,
including stops, in level country, and geared to 43 miles per hour,
take about 3 kilowatt hours per car mile at the power station. However,
interurban railway conditions are extremely variable.
The reports of several Indiana electric railways show an average power
consumption of 1.48 kilowatt hours per car mile for city cars and 5.18
kilowatt hours for interurban cars, including line and distribution
losses.
An interurban car weighing 31½ tons and equipped with two 150
horsepower motors, on a test run of 50 miles at an average speed of
39 miles per hour consumed 2.20 kilowatt hours per car mile. This car
made 18 stops. A similar car under the same conditions made the same
run at an average speed of 26 miles per hour with 44 stops, consumed
2.44 kilowatt hours and a third car, making 12 stops and at a speed of
33 miles per hour, consumed 2.10 kilowatt hours per car mile. These
individual car test figures are from measurements taken at the car and
do not include line losses.
=Road Tests of Electric Cars.= Of late considerable attention has been
given to making road tests of electric cars. The results of the tests
are usually plotted in the form shown in Fig. 93. Time is plotted
horizontally in seconds, while volts, amperes, speed and per cent grade
are plotted vertically. The diagram referred to is the result of a
continuous run of 6 minutes of a 32.5 ton car equipped with two motors.
The line voltage, motor consumption and other readings may be obtained
for any instant of time. The acceleration in miles per hour per
second may be obtained by noting the increase in height of the speed
curve in one second. In making such a test the necessary instruments,
voltmeters, ammeters, wattmeters and speed indicators are mounted
direct on the car and are read at intervals of a few seconds.
The curve of motor consumption gives an idea of the abnormal current
required to get the car under headway.
=Economy in Power.= As already stated, a large part of the energy
taken by a car in city service is used in accelerating the car. Much
of this energy must be destroyed or used up in the brake shoes at the
next stop. The energy stored up in a car by process of acceleration is
represented by the formula:
Mass in lbs. × (Velocity in ft. per sec.)²
Energy in ft. lbs. = ——————————————————————————————————————————, which
2
is the formula for kinetic or live energy, the derivation of which is
found in any Instruction Paper on Mechanics. In performing any given
schedule with frequent stops, the more rapid the acceleration the lower
the maximum speed required to make the schedule, and the less the
energy required in acceleration. For city street and elevated service,
therefore, rapid acceleration and low maximum speeds are desirable
because not only more economical but safer.
For economical operation with any given equipment and schedule, it
is important to use as much of the energy stored up in the car as
possible, before wasting it by applying the brakes. Motors are built
of a size to yield the large horsepower required in acceleration, and
consequently are lightly loaded when operating the car at maximum
speed. To economize in power, current should be shut off as soon as
possible after the car has attained full speed; and the car should be
allowed to drift without current as long as possible before the brakes
are applied. In this way the energy stored in the car will propel it
at nearly maximum speed for a considerable distance between stops;
there will be the smallest possible waste of energy in the brake shoes;
and the losses of energy which take place when the current is in the
motors will be prevented as far as possible. Practical tests as well
as theoretical calculations show a possibility of very material saving
in energy in the operation of an electric railway car or train, by the
observance of this simple rule of drifting as much as possible and
using the brakes as little as possible. Whatever energy is used up in
the brake shoes is necessarily wasted. The smaller this waste can be
kept while performing a given service, the greater the economy secured.
=Cost of Power.= The reports of 85 per cent of the railway power
generating stations in Indiana show the average cost at the station per
kilowatt hour to be .755 cent. This was divided as follows: Fuel .526
cents, labor .158 cents, lubricants and miscellaneous supplies .032
cents, repairs .039 cents. The lowest cost reported was .505 cents.
During 1901 the average cost of power generated at the power house of
the Indiana Union Traction Company was .443 cents per kilowatt hour at
the switchboard. Distributed from the substations it was .765 cents per
kilowatt hour. Natural gas was used for fuel. On occasions when this
failed, coal at $1.50 per ton was burned.
=Sliding and Spinning Wheels.= In accelerating a car, however, there
is no economy in turning on current so rapidly as to spin the wheels.
As mentioned in the section on “Brakes,” the tractive effort between
wheels and rails falls off about two-thirds when the wheels begin to
slip; and this slipping of wheels, therefore, reduces the chance of
securing the acceleration which is possible. For the same reason, in
making emergency stops either by the use of brakes or by reversing the
motors, care should be taken not to slide the wheels, as by so doing
the time required to stop the car is much increased.
In the ordinary straight air-brake equipment used on heavy electric
cars, there is much higher pressure carried in the storage reservoir
than it is permissible to turn into the brake cylinder, since, if the
full pressure were turned into the brake cylinder, it would result in
sliding of the wheels—which, it has just been shown, is something to
be avoided, not only on account of making flat spots on the wheels,
but also because of the reduction in the braking force as soon as
the wheels begin to slide. An experienced motorman can tell from the
feeling of the car when the wheels are sliding, and will instantly
release the brake sufficiently to allow the wheels to begin to revolve
as soon as he notices that this has taken place.
The friction between brake shoes and car wheels decreases as the speed
increases. A certain pressure applied to the brake shoes upon a car
running 50 miles per hour, therefore, exerts much less retarding force
than the same pressure at ten miles per hour. In order to give the
same braking or retarding force at higher speeds, the brakes must be
applied harder than at the lower speeds. If they are applied at the
maximum pressure possible without sliding the wheels at higher speeds,
it is evident that this pressure must be reduced as the speed of the
car is reduced, or the wheels will be “skidded.” In the Westinghouse
high-speed automatic air brake used on steam roads, this reduction of
pressure is automatically accomplished.
TESTING FOR FAULTS.
[Illustration: Fig. 94. Bond Testing.]
=Bond Testing.= It is important to test the conductivity of rail bonds
from time to time in order to determine if they have deteriorated so
as to reduce their conductivity and introduce an unnecessary amount
of resistance into the return circuits. One way of doing this is to
measure the drop in potential over a bonded joint as compared with
the drop in potential of an equal length of unbroken rail. To do
this, an apparatus is employed whereby simultaneous contact will be
made bridging three or more feet of rail and an equal length of rail
including the bonded joint, as shown in Fig. 94, which illustrates the
connections of a common form of apparatus where two milli-voltmeters
are employed that measure the drop in voltage of the bonded and
unbonded rail simultaneously. If the current flowing through the
rail due to the operation of the cars were constant, of course one
milli-voltmeter might be used, being connected first to one circuit
and then to the other. The current in the rail, however, fluctuates
rapidly, so that two instruments are necessary for rapid work. The
resistance of the bonded joint is usually considerably more than that
of the unbroken rail, and the milli-voltmeter used to bridge the joint
consequently need not be so sensitive as that bridging the unbroken
rail.
In another form of apparatus, a telephone receiver is used instead
of the milli-voltmeter, the resistance of a long unbroken rail being
balanced against that of the bonded joint, as in a Wheatstone bridge,
until, upon closing the circuit, these two resistances when balanced
give no sound in the telephone receiver.
Bond tests of this kind can be made with satisfaction only when a
considerable volume of current is flowing through the rails at the
time of the test, because the drop in voltage is dependent on the
current flowing, and in any event is small. It has sometimes been found
necessary or advisable to fit up a testing car equipped with a rheostat
which will itself use a considerable volume of current, so as to give
a current in the rail which will give an appreciable drop of potential
across a bonded joint. Some of the latest forms of testing cars carry
motor generators which will pass a large current of known value through
a bonded joint, and so cause a drop of potential across the joint large
enough to be easily measured.
=Motor-Coil Testing.= Testing for faults in the motor armature and
field coils is done in a great variety of ways. The resistance of these
coils can be measured by means of a Wheatstone bridge employing a
telephone receiver in place of the galvanometer used in such bridges in
laboratory practice; but other less delicate tests are also in use.
Another method is to pass a known current through the coil to be tested
and to measure the drop in the voltage between the terminals of the
coil, the voltage divided by the current equaling the resistance.
A simple method, and one which involves no delicate instruments, has
lately been introduced into railway shop practice very successfully.
This is known as the _transformer test_ for short-circuited coils. It
requires an alternating current which can easily be supplied either
by a regular motor generator or by putting collecting rings onto an
ordinary direct-current motor and connecting these rings to bars of
opposite polarity on the commutator.
The method of testing for short-circuited armature coils employed in
the shops of the St. Louis Transit Company is indicated in diagram
in Fig. 95. A core built up of soft laminated iron is wound with 28
turns of No. 6 copper wire. This coil is supplied with alternating
current from a 110-volt circuit. The core has pole pieces made to fit
the surface of the armature. When one side of a short-circuited coil
in the armature is brought between the pole pieces of this testing
transformer, as in Fig. 95, the short-circuited armature coil becomes
like the short-circuited secondary of a transformer, and a large
current will flow in it. This current will in time manifest itself by
heating the coil; but it is not necessary to wait for this, as a piece
of iron held over that side of the coil not enclosed between the pole
pieces, as indicated in. Fig. 95, will be attracted to the face of the
armature if held directly over the coil, but will be attracted at no
other point.
[Illustration: Fig. 95. Method of Testing for Short-circuited Armature
Coils.]
This testing can be done very rapidly, and does not require delicate
instruments or skilled operators.
Tests for short circuits in field coils can be made in a similar
manner, by placing the coils on a core which is magnetized by
alternating current. The presence of a short circuit, even of one
convolution of a field coil, will be apparent from the increase in
the alternating current required to magnetize the core upon which the
field coil is being tested. The insulation resistance of armatures
and fields is frequently tested by means of alternating current, about
2,000 volts being the common testing voltage for 500-volt motor coils.
One terminal of the testing circuit is connected to the frame of the
motor, and the other to its windings. Any weakness in the insulation
insufficient to withstand 2,000 volts will, of course, be broken down
by this test. Alternating current is generally used for such tests
because it is usually more easily obtained at the proper voltage, as it
is a simple matter to put in an alternating transformer which will give
any desired voltage and which can be controlled by a primary circuit of
low voltage.
Open circuits in the armature can be easily detected by placing
the armature in a frame so that it can be rotated, the frame being
provided with brushes resting 90° apart on the commutator. If either an
alternating or direct current be passed through the armature by means
of these brushes, and the armature be rotated by hand, a flash will
occur when the open-circuited coils pass under the brushes. A large
current should be used.
The tests just mentioned are among the best of the methods used by
electric-railway companies for systematic work in the location of
certain classes of faults. A large number of other methods of testing
have also been evolved.
The following are some of the most common faults experienced with
electric railway car equipments:
=Grounds.= As one side of the circuit is grounded, any accidental
leakage of current from the car wiring or the motors to ground will
cause a partial short circuit. Such a ground on a motor will manifest
itself by blowing the fuse or opening the circuit breaker whenever
current is turned into the motor. In case the fuse blows when the
trolley is placed on the wire and the controller is off, it is a
sign that there is a ground somewhere in the car wiring outside of
the motors. Moisture and the abrasion of wires are the most common
causes of grounds in car wiring. In motors, defects are usually due to
overheating and the charring of the insulation.
=Burn-Outs.= Burning out of motors is due to two general causes: First,
a ground on the motor, which, by causing a partial short circuit,
causes an excessive current to flow; second, overloading the motor,
which causes a gradual burning or carbonizing of the insulation until
it finally breaks down.