Chapter LXVII: Management (3)

Ridiculous as it may appear, the average station attendant
may frequently be seen straining his eyes to read to tenths of
a division on the scale of a meter which, if subjected to test,
would show an inaccuracy of over 2 per cent.

In testing a meter, by comparing it with a standard, in order to
obtain the best results there should be one man at each meter so
that simultaneous readings may be taken on both instruments, and
the man at the standard meter should maintain the voltage constant
while a reading is being taken, by means of a rheostat in the
field circuit of the generator supplying the current.

Each meter should be checked or calibrated at five or six
approximately equidistant points over its scale; the adjustable
resistance being varied each time to give a deflection on the
standard meter of an even number of divisions and the deflection
on the other meter recorded at whatever it may be. Having obtained
the necessary readings, the calculation of the constant or
multiplying factor of the meter undergoing test is next in order.

This may best be shown by taking an actual case in which a 150
scale voltmeter is being tested to determine its accuracy. The
data and calculations are as follows:

Readings on Readings on Constant
standard meter meter tested
150 149.2 150 ÷ 149.2 = 1.005
125 125.0 125 ÷ 125.0 = 1.000
100 98.9 100 ÷ 98.9 = 1.011
75 73.6 75 ÷ 73.6 = 1.019
50 50.0 50 ÷ 50.0 = 1.000
25 24.8 25 ÷ 24.8 = 1.008
------
6.043

Average constant for six readings, 6.043 ÷ 6 = 1.007.

It may be stated in general that before taking the readings for
this test, the zero position of the pointer on the meter tested
should be noted, and if it be more than two-tenths of a division
off the zero mark, the case of the meter should be removed and the
pointer straightened.

Furthermore, it will be noticed from the readings here recorded
that the test is started at the high reading end of the scale;
this is done in order that the pointer may gradually be brought
up to this spot, by slowly cutting out of circuit the adjustable
resistance, and thus show whether or not the pointer has a
tendency to stick at any part of the scale. If the meter seem to
be defective in this respect, it should be remedied either by
bending the pointer or scale, or by renewing one or both of the
jewels, before the comparison with the standard is commenced.

It is obvious from the readings recorded for the 150 scale
voltmeter, that as compared with the corresponding deflections of
the standard, the former are a trifle low.

In order to determine for each observation how much too low
they are, it is necessary to divide each reading on the standard
by the corresponding reading on the meter tested. The result is
the amount by which a deflection of this size on the meter tested
must be multiplied in order to obtain the exact reading. This
multiplier is called a constant, and as shown, a constant is
determined for each of the six observations.

The average constant for the six readings is then found, and
this is taken as the constant for the meter as a whole; that is,
whenever this 150-scale voltmeter is used, each reading taken
thereon must be multiplied by 1.007 in order to correct for its
inaccuracy.

The most convenient and systematic way of registering the
constant of a meter is to write it, together with the number of
the meter and the date of its calibration, in ink on a cardboard
tag and loop the same by means of a string to the handle or some
other convenient part of the meter.

NOTE.--=Transformer polarity test.= A test of
importance in the manufacture of transformers,
and sometimes necessary for the user, is the
so called _banking_ or _polarity_ test. The
transformers from any particular manufacturer
have the leads brought out in such a manner that
a transformer of any size can be connected to
primary and secondary lines in a given order
without danger of blowing the fuses due to
incorrect connections. All manufacturers of
transformers, however, do not bank transformers
in the same way, so that it is necessary in
placing transformers of different makes to test
for polarity. This is done as shown in fig.
2,906. One transformer is selected as a standard
and the leads of the second transformer connected
as indicated in the diagram. If the transformers
be 1,100-2,200 volts to 110-220, two 110 volt
lamps are connected in the secondaries of the
transformers as indicated, while the primary of
the transformer is connected across the line.
In transformers built for two primary and two
secondary voltages, it is necessary to test each
primary and each secondary. The diagram shows the
method of connecting one 2,200 volt coil and one
110 volt coil to the transformer to be tested.
When the primary circuit of the transformer under
test is closed, and if the secondary leads of
the 110 volt coil under test be brought out of
the case properly, the two 110 volt lamps should
be brightly illuminated. If, on the other hand,
the two 110 volt terminals have been reversed,
no current will flow through the lamps. If
these two terminals be found to be brought out
correctly, transfer the secondary leads of the
transformer under test to the second 110 volt
coil. Upon closing the primary circuit, the lamp
should again be brightly illuminated. Repeat this
process with each of the secondary coils and the
other primary coil, and if the lamps show up
brightly in every case on closing the primary
circuit, all leads have been properly brought
out. If on any tests the lamps do not light up
brightly, the leads on the transformer must be so
changed as to produce the proper banking.

=Ques. What are the usual remedies applied to a voltmeter to correct a 3 or 4 per cent. error?=

Ans. They consist of straightening the pointer, varying the tension of the spiral springs, renewing the jewels in the bearings, altering the value of the high resistance, and, in the case of a direct current instrument, strengthening the permanent magnet.

=Ques. How is the permanent magnet strengthened?=

Ans. After detaching it from the instrument, wrap around several turns of insulated wire, and pass through this wire for a short time 3 or 4 amperes of direct current in such a direction as to reinforce the magnet magnetism.

=Ques. How may the value of the high resistance of a voltmeter be altered?=

Ans. Determine the resistance of the voltmeter and add or subtract, according as the reading is high or low, a certain length of wire whose resistance is in per cent. of the voltmeter resistance the same as the per cent. of error.

NOTE.--The complete calibration of a two
scale voltmeter does not, as might be supposed,
necessitate that the readings on both scales be
checked with standards, for since the resistance
corresponding to the one scale is always some
multiple of the resistance of the other, the
constants of the two scales are proportional. For
instance, if S = the reading at the end of the
high scale of the voltmeter; S^{1} = the reading
at the end of the low scale of the voltmeter; R =
the resistance in the meter corresponding to the
high scale; R^{1} = the resistance in the meter
corresponding to the low scale; K = the constant
for the high scale, and K^{1} = the constant for
the low scale. Then

SK ÷ R = S^{1}K^{1} ÷ R^{1}

from which

K^{1} = SKR ÷ S^{1}R

That is to say, if the respective resistances corresponding to
the two scales be known, and the constant of the high scale be
determined by comparison with a standard, then by aid of these
known values and the maximum readings on the two scales, the
constant of the low scale may be calculated. It is also possible
to calculate the constant of the high scale if the constant of the
low scale be known, together with the values of the resistances
corresponding to the two scales; for from the equation previously
given.

K = RS^{1}K^{1} ÷ R^{1}S

=Ques. What is a frequent cause of error in an alternating current meter, and why?=

Ans. The deterioration of its insulation, which permits the working parts of the instrument coming in contact with the surrounding metal case.

A convenient method of testing for deterioration of insulation
is shown in fig. 2,905.

=How to Test Generators.=--In the operation of electrical stations, many problems dealing with the generators installed therein can be readily solved by the aid of characteristic curves, which bear a relation to the generators similarly as do indicator diagrams to steam engines.

In steam engineering, a man who did not fully understand the method of taking an indicator diagram would be considered not in touch with his profession, and in electrical engineering the same would be true of one ignorant of the method of obtaining characteristic curves.

The necessary arrangement or connection of the generator from
which it is desired to obtain a characteristic curve, consists in
providing a constant motive power so that the machine may be run
at a uniform speed, and when the field magnets of the generator
are separately excited the field current from the outside source
must also be maintained constant, preferably by a rheostat
connected in the field of the auxiliary exciting machine. It is
also necessary in every case that means be provided for varying
the main current of the generator step by step from zero to
maximum. This may best be done by employing a water rheostat, as
shown in fig. 2,909.

=Ques. What instruments are needed in making a test of dynamo characteristics?=

Ans. A voltmeter, ammeter, speed indicator, the usual switches and rheostats.

=Ques. How is the apparatus connected?=

Ans. It is connected as shown in fig. 2,910.

=Ques. Describe the test.=

Ans. Having completed the preliminaries as in fig. 2,910, the test should be started with the main circuit of the generator open. Then, in the case of the shunt machine, the speed should be made normal and the field rheostat adjusted until the voltmeter reading indicates the rated voltage of the machine at no load and readings taken. The electrodes of the water rheostat should be adjusted for maximum resistance and main circuit closed, and a second set of readings taken. Several sets of readings are taken, with successive reductions of water rheostat resistance. The results are then plotted on coordinate paper giving the characteristic curve shown in fig. 2,908.

=Ques. What does the characteristic curve (fig. 2,911) show?=

Ans. An examination of the curve shows that the highest point of the curve occurs at no load or 0 amperes; that as the current is increased, the voltage drops, first slightly to the point B and then rapidly until the point E is reached, when any further lowering of resistance in the main circuit to increase the current causes not only a rapid decline in the voltage but also of the current until both voltage and current become approximately zero.

In some generators, a very slight current results even when the
terminals of the machine are actually short circuited; that is,
due to residual magnetism in the pole pieces, the lower portion of
the curve often terminates, not exactly at zero, but at a point
some distance along the current line.

The working portion of the curve is from A to C, at which time
the machine is supplying a fairly constant voltage. From C to E
shows a critical condition of affairs, while the straight portion
D O represents the unstable part of the curve caused by the field
current being below its proper value.

The position of the point C determines the maximum power the
machine is capable of developing, being in this case (47.5 × 25) ÷
746 = 1.59 horse power.

=Ques. How may the commercial efficiency of a generator be determined?=

Ans. To obtain the commercial efficiency, the _input_ and _output_ must be found for different loads.

The input may be found by running the generator as a motor
at its rated speed, loading it by means of a Prony brake. The
generator must be stripped of all belting or other mechanical
connections, supplied with its normal voltage and full load
current, and the pressure of the Prony brake upon its armature
shaft or pulley adjusted until the rated speed of the armature is
obtained. The data thus obtained is substituted in the formula.

2π L W R
input in brake horse power = ---------- (1)
33,000

in which

L = length of Prony brake lever;
W = pounds pull at end of lever;
R = revolutions per minute.

The output or electrical horse power for the same load is easily calculated from the formula

amperes × volts
output in electrical horse power = --------------- (2)
746

After obtaining value for (1) and (2) the commercial efficiency for the load taken is obtained from the formula

output
commercial efficiency = ------ (3)
input

Having obtained the commercial efficiency, the difference between the ideal 100 per cent. and the efficiency found will be due to certain losses in the generator. These losses may be classified as

1. Mechanical.
2. Electrical.

The mechanical losses are the friction of the bearings and
brushes, and air friction. The electrical losses consist of the
eddy current loss, hysteresis loss, armature resistance loss, and
field resistance loss.

In testing for these losses, the generator to be tested should
be belted to a calibrated motor which latter machine should
preferably be of the constant pressure, shunt wound type.

The friction of the bearings and belt of the generator are
determined together by raising the brushes off its commutator and
running it at the rated speed by means of the calibrated motor.

The amount of power as ascertained from the calibration curve of
the motor for the voltage and current used therein when driving
the generator as just explained, is a measure of these two losses.
The power thus used is practically constant at all loads and is
about 2 per cent. of that necessary to drive the generator at full
load.

The friction of the brushes can very conveniently be determined
next by lowering them on the commutator and giving them the proper
tension.

The increase in power resulting from the greater current that
will now be taken by the motor to run the dynamo at its rated
speed, will be a measure of this loss. In general, its value will
be about .5 per cent. of the total power required to drive the
dynamo at full load, and this also will remain constant at all
loads.

The friction of the air upon the moving armature of the dynamo
cannot be determined experimentally, but theoretically this loss
is small and may be estimated as .5 per cent.; it is also constant
at all loads.

The core loss may be determined experimentally by exciting
the field magnets of the dynamo with the normal full load field
current through the magnet coils, and noting the increase of power
required by the motor to maintain the rated speed of the dynamo
thus excited under no load, over that necessary under the same
conditions with no field excitation. This increase of power will
be the value of the core loss. The core loss is approximately 3
per cent. of the power required to operate the dynamo at full
load, and it is constant at varying loads. If it be desired to
divide the core loss into its component parts, it is necessary
also to run the dynamo under the same conditions as before with
field excitation but at half its rated speed. If, then,

H = the power lost in hysteresis at rated speed,
E = the power lost in eddy currents at rated speed,
T = the power lost in hysteresis and eddy currents at rated speed,
S = the power lost in hysteresis and eddy currents at half speed.

there may be formed the two following equations:

H E
T = H + E, and S = --- + ---,
2 2

from which the elimination of H will give E = 2T - 4S.

The value of the eddy current loss thus found will be about 1½
per cent., and constant at all loads.

Having previously ascertained the power lost in both eddy
currents and hysteresis, and knowing now the power lost in eddy
currents alone, it is easy to find that lost in hysteresis by
simply subtracting the latter known value from the former. The
value of the hysteresis loss is therefore approximately 1½ per
cent., and it is constant at different loads.

There yet remains to be determined the armature resistance
loss and the field resistance loss. As for the calibrated motor,
this may be disconnected from the dynamo, as it need not be used
further in the test.

The armature resistance is the resistance of the armature
winding of the dynamo, between the commutator bars upon which
press the positive and negative brushes. Assume that the value
of the armature resistance be known, call this value R ohms,
together with that of the full load armature current, which is
also known and which call I amperes, this is sufficient data for
calculating the armature resistance loss at full load. It is
evident that to force the full load current I through the armature
resistance R will require a pressure of R volts, and that the
watts lost in doing so will be the voltage multiplied by the
current. The armature resistance is consequently

IR × I = I^{2}R watts

or, expressed in horse power it is

I^{2}R ÷ 746

At full load it is usually about 2 per cent. of the total
power required to drive the generator fully loaded. The armature
resistance loss varies in proportion to the load, in fact, as the
last expression shows, it increases as the square of the armature
current.

The field resistance loss is calculated in the same manner as
just explained for the armature resistance loss, it being equal
in horse power to the square of the full load field current
multiplied by the resistance of the field winding and divided by
746. In a shunt dynamo it is practically constant at 2 per cent.
of the total power at full load, but in a series or in a compound
generator it will vary in proportion to the load.

HAWKINS PRACTICAL LIBRARY OF ELECTRICITY

IN HANDY POCKET FORM PRICE $1 EACH

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=ELECTRICAL GUIDE, NO. 1=
Containing the principles of Elementary Electricity, Magnetism,
Induction, Experiments, Dynamos, Electric Machinery.

=ELECTRICAL GUIDE, NO. 2=
The construction of Dynamos, Motors, Armatures, Armature
Windings, Installing of Dynamos.

=ELECTRICAL GUIDE, NO. 3=
Electrical Instruments, Testing, Practical Management of Dynamos
and Motors.

=ELECTRICAL GUIDE, NO. 4=
Distribution Systems, Wiring, Wiring Diagrams, Sign Flashers,
Storage Batteries.

=ELECTRICAL GUIDE, NO. 5=
Principles of Alternating Currents and Alternators.

=ELECTRICAL GUIDE, NO. 6=
Alternating Current Motors, Transformers, Converters, Rectifiers.

=ELECTRICAL GUIDE, NO. 7=
Alternating Current Systems, Circuit Breakers, Measuring
Instruments.

=ELECTRICAL GUIDE, NO. 8=
Alternating Current Switch Boards, Wiring, Power Stations,
Installation and Operation.

=ELECTRICAL GUIDE, NO. 9=
Telephone, Telegraph, Wireless, Bells, Lighting, Railways.

=ELECTRICAL GUIDE, NO. 10=
Modern Practical Applications of Electricity and Ready Reference
Index of the 10 Numbers.

=Theo. Audel & Co., Publishers. 72 FIFTH AVENUE=,
=NEW YORK.=