short-circuited, if not promptly discovered, are likely to result
in burned-out armatures, as the weakening of the field reduces the
counter-electromotive force of the motor, so that an abnormally
large current flows through the armatures. Cars with partially
short-circuited fields are likely to run above their proper speed,
though, if only one motor on a four-motor equipment has defective
fields, the motor armature is likely to burn out before the defect is
noticed from the increase in speed.
[Illustration: Fig. 96.]
=Defects of Armature Windings.= Defects in armature windings probably
cause one-third the maintenance expenses of electrical equipment
of cars. Almost all repair shops have men continually employed in
repairing them. The most frequent trouble with armatures is through
failure of the insulation of the coils and consequent “grounding.”
This term is used in connection with armatures and fields and other
electrical apparatus where a direct path exists to ground. As the
armature core is electrically connected to the ground through its
bearings and the motor casing, a break down of the insulation of the
coils in the slots permits the current to pass directly to ground.
This shunts the current around the fields and an abnormal current
flows because of their weakness. The circuit breaker or fuse is
placed in circuit to protect the apparatus in such an emergency, but
usually before such devices break the circuit, several of the coils
of the armature are burned in such a manner as to make their removal
necessary. The coils are so wound on top of one another that in order
to replace one coil alone, one-fourth of the coils of the armature must
be lifted.
[Illustration: Fig. 97.]
With the armature of No. 1 motor grounded the car will not operate and
if the resistance points be passed over, the fuse will usually blow.
When No. 2 motor is grounded the action of No. 1 motor is not impaired
and this latter motor will pull the car until the controller is thrown
to the multiple position. But if the motors are thrown in multiple, the
path through the ground of No. 2 motor shunts motor No. 1. A study of
Fig. 18 will make this evident.
[Illustration: Fig. 98.]
Next to grounding, open circuits are the most serious defects of
armatures. These are usually caused by burning in two of the wires in
the slot, or where they cross one another in passing to the commutator.
Sometimes the connections where the leads are soldered to the
commutator become loose.
The effect of an open circuit is shown in Fig. 96. The circuit is open
at n. The brushes are on segments _a_ and _d_. By tracing out the
winding it will be found that no current flows through the wires marked
in heavy lines. Whenever segments _c_ and _d_ are under a brush the
coil with the open circuit is bridged by the brush and current flows
as in a normal armature. As segment _c_ passes out from under the brush
the open circuit interrupts the current in half the armature and a long
flaming arc is drawn out.
In Fig. 97 is shown the result of a short circuit between two coils.
The short circuit is at _b_, _c_, the two leads coming in contact with
each other when they cross. The effect is to short-circuit all of the
winding indicated by the heavy lines.
[Illustration: Fig. 99.]
=Mistakes in Winding Armatures.= The armature winder is given very
simple rules as to how to wind the armature, but the great number of
leads each to be connected to their proper commutator segment sometimes
so confuse him that misconnections are made. The effect of getting two
leads crossed is shown in Fig. 98. The leads to segments _b_ and _c_
from the right are shown interchanged. This short-circuits the coils
shown in heavy lines. The abnormal current resulting in these would
usually cause them to burn out.
Fig. 99 shows the results of placing all of the top leads, or all of
the bottom leads one segment beyond the proper position. This causes
the circuit starting from _a_ and traveling counter clockwise around
the armature to return on segment _m_ instead of on segment _b_ as is
the case in Fig. 97.
The only result of such connections is to change the direction of
rotation of the armature. It may be noticed by comparing the two
figures that with the positive brush on segments _a_ the arrows show
the currents to be in opposite directions in coils similarly located
with reference to the position of the brushes. Some armatures are
intended to be wound as in the last case mentioned.
=Sparking at the Commutator.= As railway motors are made to operate,
and usually do operate, almost sparklessly, sparking at the brushes may
be taken as a sign that something is radically wrong.
The pressure exerted by the spring in the brush holder may not hold the
brush firmly against the commutator.
If brushes are burned or broken so that they do not make good contact
on the commutator, they should be renewed or should be sandpapered to
fit the commutator.
A dirty commutator will cause sparking.
A commutator having uneven surface will cause sparking, and should be
polished off or turned down.
[Illustration: Fig. 100.]
Sometimes the mica segments between commutator bars do not wear as fast
as the bars and when this is the case, the brushes will be kept from
making good contact when the commutator bars are slightly worn. The
remedy is to take the armature into the shop, and groove out the mica
between the commutator bars for a depth of about ¹⁄₆₄-inch below the
commutator surface.
A greenish flash which appears to run around the commutator,
accompanied by scoring or burning of the commutator at two points,
indicates that there is an open-circuited coil at the points at which
the scoring occurs as in Fig. 100.
The magnetic field may be weakened by a short circuit in the field
coils, as before explained, and this may give rise to sparking.
Short circuits in the armature may give rise to sparking, but will also
be made evident by the jerking motion of the car and the blowing out of
the fuse.
=Failure of Car to Start.= The failure of the car to start when the
controller is turned on may be due to any of the following causes:
Opening of the circuit breaker at the power house.
Poor contact between the wheels and the rails owing to dirt or to a
breaking of the bond wire connections between the rail on which the car
is standing and the adjacent track.
One controller may be defective in that one of the contact fingers
may not make connection with the drum. In this case try the other
controller if there is another one on the car.
The fuse may be blown or the circuit breaker opened. The occurrence
of either of these, however, is usually accompanied by a report which
leaves little doubt as to the cause of the interruption in current.
The lamp circuit is always at hand for testing the presence of current
on the trolley wire or third rail. If the lamps light when the lamp
circuit is turned on, it is a tolerably sure sign that any defect is
somewhere in the controllers, motors, or fuse boxes, although in case
the cars are on a very dirty rail enough current might leak through
the dirt to light the lamps, but not sufficient to operate the cars.
In such a case, the lamps will immediately go out as soon as the
controller is turned on. Ice on the trolley wire or third rail will
have the same effect as dirt on the tracks.
LOCATING DEFECTS IN MOTOR AND CONTROLLER WIRING.
Defects in the wirings are those due to (1) open circuits, (2) short
circuits. Open circuits make themselves evident by no flow of current,
short circuits usually by a blowing of the fuse or opening of the
breaker. The point of the short circuit or “ground” can be located
roughly by noting on what point the fuse is blown. Accurate location
can be made by cutting out the motors, disconnecting, etc., according
to directions in the following pages. The tests outlined apply
particularly to the K type of controller with two-motor equipment.
OPEN-CIRCUIT TESTS.
No current:
On 1st point,
Open circuit but not located.
On 1st point multiple,
Motors most probably O. K.
On series-resistance points after trying 1st point multiple,
Open circuit outside controller and equipment wiring.
With an open anywhere between trolley and ground no current will flow
on the first point. Opens are most likely to occur in the motors and
these may be tested first. However, as will be explained later, one
open in an armature will not stop the current. To test the motors open
the breaker and put the controller on the first point multiple. Then
flash the breaker quickly. Current flowing indicates that one or the
other of the motors has an open circuit. In the series position this
open prevented the flow but in multiple the current flows through
the other motor. Which one is at fault can be quickly determined by
returning the controller to the off position and cutting out one or the
other of the motors by means of the cut-out switch and then trying for
current. The car can in any event be run on the remaining motor. On
returning to the shop the open can be determined definitely by the use
of the lamp bank.
But should no current flow when the breaker is flashed on the 6th point
it is reasonable to presume that the motors are O. K. and that the open
is elsewhere. The ground for such a supposition is that as there is a
path through each motor normally, there would necessarily be an open
in each one to stop the current. It is hardly probable that such a
coincidence would occur.
After failure to find fault with the motors, doubt as to the resistance
may be removed. The controller should be placed on progressive
series-resistance points and the breaker flashed on each one. If
current is obtained on any point, the open is in the resistance or the
resistance lead just behind the one being used. Special care should be
used to flash the breaker quickly for otherwise the fuse may be blown.
The tests indicated are sufficient for the motors, controllers and
resistance wiring. If no current is obtained on either of them, the
trouble is evidently caused by a bad rail contact, ground wire off if
both motors are grounded through the same wire, an open in the blow-out
coil, at the lightning arrester, circuit breaker or on top of the car.
None of the tests applied locate the open definitely, but this can
easily be done in the shop or wherever a lamp bank is at hand. Connect
one terminal of the lamp bank to the trolley just behind the circuit
breaker and the controller on the 1st point series, then with the other
terminal begin at ground and trace backwards up the circuit until the
lamps fail to light. The path in a K type of controller is readily
traced with the help of Fig. 22.
SHORT-CIRCUIT TESTS.
The location of short-circuits is much more tedious. The blowing of the
fuse or opening of the breaker will locate them as shown below. The
separate tests can then be followed until location is definite.
These tests it must be kept in mind are more especially adapted to
cases on the road or where no facilities for testing are at hand.
Rather than blow fuses as frequently as indicated it would in most
cases be better to place a lamp bank across the open circuit breaker
and note the flow of the current by the lights.
=Fuse Blows=:
I. When overhead is thrown on may be due to:
1. Grounded controller blow-out coil.
2. Grounded trolley wire or cable.
3. Grounded lightning arrester.
II. On first point:
1. Grounded resistance near R 1.
2. Grounded controller cylinder.
3. Bridging between the insulated sections of cylinder.
III. Near last point series:
1. Grounded resistance near R 3, R 4 and R 5.
2. No. 1 motor grounded.
IV. Near last point multiple:
1. No. 2 motor grounded.
2. Bridging between lower sections of cylinder.
3. Armature defective.
CASE I.
=Fuse Blows= when overhead is thrown on:
1. Grounded controller blow-out coil.
2. Grounded trolley wire or cable.
3. Grounded lightning arrester.
The blowing of the fuse immediately on closing the overhead switch or
circuit breaker, when the controller is on the off position, indicates
that the fault exists somewhere between the overhead and the upper or
trolley finger of the controller.
Should the defect occur during a thunderstorm, it may be presumed at
once that lightning has grounded the blow-out coil of the controller.
CASE II.
=Fuse Blows= on first point:
1. Grounded resistance near R 1.
2. Grounded controller cylinder.
3. Bridging between sections of cylinder.
When the controller is on the first point all of the wiring of
the system with the exception of the ground wire for No. 1 motor
is connected with trolley. But a defect in the wiring beyond the
resistance will not show itself on the first point by an abnormal rush
of current because the resistance of the rheostats is sufficient to
prevent any excessive flow of current.
[Illustration: Fig. 101.]
[Illustration: Fig. 102.]
The resistance and leads and the controller cylinder are the only parts
to be tested when the fuse blows on the 1st point.
CASE III.
=Fuse Blows= on 3rd or 4th point:
1. Grounded resistance near R 4 or R 5.
2. No. 1 motor grounded.
With either of the above defects the car will most probably refuse to
move as the current is led to ground before passing through the motors.
[Illustration: Fig. 103. Plan of Car Shop.]
No. 1 motor may be tested by cutting it out of service by means of its
cut-out switch. If this removes the ground, the motor is at fault.
CASE IV.
=Fuse Blows= near last point multiple:
1. No. 2 motor grounded.
2. Either armature short-circuited.
The fact that the fuse did not blow on the series positions excludes
the resistances and No. 1 motor from investigations for grounds.
Cut out both motors. If the ground still exists the controller is
defective. If not, the fault may be located in either one of the motors
by cutting out first one and then the other.
ARMATURE TESTS FOR GROUNDS.
With a lamp bank at hand tests for grounded armature can be made as
follows:
Throw the reverse on center. Attach one terminal of the lamp bank
to the trolley. Put the other terminal on the commutator of the
armature to be tested. No current shows the armature O. K. If
current flows remove brushes and try again, to be certain that the
ground is not in the leads.
FIELD TESTS FOR GROUNDS.
Disconnect field leads and put test point of the lamp bank on one side
of the terminals. No current indicates that the fields are O. K.
REVERSED FIELDS.
In placing new fields in the shell it often happens that one or more
are wrongly connected. Reversed fields make themselves known by
excessive sparking at the brushes in each case.
In Fig. 101 all of the fields are connected correctly. The flow of
magnetism is in one pole and out of the adjacent one. Some of the
magnetism leaks out of the shell and affects a compass held near the
outside. The direction taken by the compass needle in the different
positions is shown. The needle should point in opposite directions over
adjacent coils and should lie parallel to the shell in positions half
way between two coils.
Figure 102 shows the flow of magnetism when one field is reversed. In
such a case the compass will take the position shown. The field marked
“X” is the one reversed.
With one reversed field a machine will usually operate, as the
magnetism in three of the poles is in the normal direction. But an
excessive flow of current that has no effect in turning the armature
will take place on that side of the armature next to the reversed field.
CAR REPAIR SHOPS.
Every electric railway system has a repair shop in which the cars are
overhauled. Hardly two shops are built alike. In those shops where only
a few cars are cared for, the work is sometimes all done in one room.
The shop plan shown in Fig. 103 was presented to the American Railway
Mechanical and Electrical Association by W. D. Wright. It contains the
idea upon which the larger shops are now being constructed, having a
transfer table between the separate departments on either side. In the
general design of shops the blacksmith shop, machine shop and truck
shop or equipping shop should be close together as a great deal of
heavy material is carried between these departments. The paint shop
should be separated as much as possible from the other departments in
order that flying dust and dirt be avoided. The wood shop may occupy a
position at a considerable distance from the other departments as no
heavy material is carried from this shop to them.
The tracks of the motor and truck repair shop are usually provided
with pits so that trucks and electrical equipment may be repaired and
inspected from below. The tracks in shops are usually about 15 or 16
feet between centers. This gives a clearance of about 6 or 8 feet
between cars when adjacent tracks are occupied.
A large portion of the work done in the average shop consists of the
repairing of trucks and the motors mounted on them. With the smaller
car, especially those with single trucks, much of this work is done
from the pit below while the trucks are in position under the cars. In
this case the armatures are either removed by letting them down with
the lower half of the motor shell by means of a pit jack, or the lower
half of the armature shell is swung down by the use of a chain and
block placed in the car and the armature rolled out on a board.
The trucks of double truck cars are usually taken out from under the
car body when repairs are to be made. In this case the motor leads, the
sand box connections and the brake rigging are disconnected and the
car body either raised or the trucks lowered from it. Several methods
of raising the car body are in use. Where no special apparatus is at
hand, this is done by means of jacks, hydraulic or mechanical, placed
under the side sills of the car near the end to be raised. Sometimes an
overhead crane is employed to lift the car body. A special apparatus
to raise the body is employed by the St. Louis Transit Company. This
consists of four screw jacks located below the floor of the shop. An
I-beam extends over the tops of the two located on the same side of
the car. The jacks are motor driven by means of one sprocket chain so
that they rise at the same speed. When a car is to be raised it is run
on the track between the jacks, bars are placed under the car resting
across the I-beams and the jacks raise the car off the trucks. The
trucks are then rolled out from under the car and the repairs made.
Sometimes, as has been stated, the trucks are dropped from the car
body. In this case the car is so placed that the truck rests on an
elevator or section of track that drops to the floor below. After the
car is blocked up the trucks are dropped and the repairs made. This
method is also used in changing wheels in small shops. The old pair of
wheels is dropped by a hand-operated drop section of track. A new pair
is then elevated into position. This saves jacking up one end of the
car.
[Illustration: ONE OF THE SINGLE-PHASE LOCOMOTIVES ON THE NEW YORK,
NEW HAVEN & HARTFORD RAILROAD CO.
Note the two pantograph bow trolleys for collecting the current.]
THE SINGLE-PHASE ELECTRIC RAILWAY.
In no other line of electrical activity have developments during the
last few years been so rapid as in that of electric railway work, and
from all indications the limit has not yet been reached.
Until recent years all electric traction has been dependent upon direct
current as a motive power. This is due principally to the fact that the
series direct-current motor is admirably adapted for such work, and no
alternating-current motor had been developed which could be substituted
for it. One of the great advantages possessed by the direct-current
series motor is its large starting torque, which may be several times
greater than that required to propel a car at full speed. This type
of motor is also essentially a variable speed machine, and lends
itself very well to wide variations in speed control; consequently,
for many years, in this country at least, all advance was made along
direct-current lines.
The trolley voltage used at first was from 450 to 500 volts, this
being supplied directly to the cars by means of a trolley wire, the
rails being used for the return circuit. It is evident from the
outset that the comparatively low voltage, necessitating as it did a
correspondingly large current for a given amount of power, would place
a definite limitation on the use of such a system for anything other
than purely local distribution. To overcome this difficulty as far as
possible, the trolley voltage was gradually raised to 600 or 650. This
of course decreased the required current, thus increasing the scope
of the system accordingly. The limit of increase of direct-current
voltage on the trolley was reached at about this point, and the fact
was recognized that some means must be devised for using a still higher
voltage, since there are difficulties to increasing the trolley voltage
beyond 600 or 700, due to flashing of the motors, which seems to
increase directly with the voltage.
It may be mentioned in passing that one prominent electric traction
expert has stated that a direct-current trolley voltage of 1500 can
be used, but it remains to be proven whether or not he is correct. A
very satisfactory solution of the problem for large city street railway
systems and long interurban roads, consists in the use of a combination
alternating-current direct-current system in which three-phase high
tension alternating current is generated and distributed on high
tension lines to substations along the road. It is here stepped down
by means of transformers, and then changed to direct current by rotary
converters, and supplied to the trolley wire as direct current at the
usual voltage of say 600. This system has many advantages, as there is
but small loss in the high-tension lines, and these lines can be made
comparatively small, thus effecting a considerable saving in investment
for copper.
The above mentioned system of distribution is very generally used, and
has been found quite satisfactory. The substations can be located at
frequent intervals, and the distance that the 600-volt current must be
conducted to supply the cars is not great. By this means current can
be distributed over wide areas with a small loss, where it would be
impossible to use the straight direct-current system of distribution.
While, as stated, this furnishes a fairly satisfactory solution of the
problem, it is far from perfect, as it necessitates the intervention of
the rotary converter substation, in which the investment must be large;
and moreover the cost of operation is high, as such a station requires
skilled attendance on account of the somewhat intricate nature of the
rotary converter. The ideal system, therefore, is one which does away
altogether with the use of direct current, the power being generated,
distributed, and utilized by the motors, as alternating current.
Three-phase induction motors have been used quite extensively and with
considerable success in Europe for many years past. The three-phase
motor, however, is not entirely adapted for railway work, since it
possesses the characteristics of the shunt rather than of the series
motor, being a constant speed, not a variable speed machine. Moreover,
two trolley wires are necessary instead of one, and still another
disadvantage consists in the low power-factor of the three-phase
induction motor at starting.
The recent application of the single-phase alternating current to
railway work has opened up a new field, which bids fair to supplant all
other forms of distribution to a great extent at least, and it is
impossible to predict at the present time just what its limitations may
or may not prove to be. This has been made possible by the development
of a _practical commercial_ single-phase motor, which permits of the
use of alternating current on the trolley wire with all its advantages,
and yet sacrifices few, if any, of the advantages of the direct-current
series motor on the car.
[Illustration: INTERIOR OF SUB-STATION SHOWING ROTARY CONVERTER AND
TRANSFORMERS.
The three-phase current is delivered to the transformers where it is
stepped down to the voltage required for the rotary converter. In
this machine it is transformed to direct current and delivered to the
trolley wire.]