Chapter LXVII: Management (2)

NOTE.--According to the practice of the General
Electric Co., 2½ degrees of phase difference
from a mean is the limit allowable in ordinary
cases. It will, in certain cases, be possible to
operate satisfactorily in parallel, or to run
synchronous apparatus from machines whose angular
variation exceeds this amount, and in other cases
it will be easy and desirable to obtain a better
speed control. The 2½ degree limit is intended to
imply that the maximum departure from the mean
position during any revolution shall not exceed
2½ ÷ 360 of an angle corresponding to two poles
of a machine. The angle of circumference which
corresponds to the 2½ degree of phase variation
can be ascertained by dividing 2½ by ½ the
number of pole; thus, in a 20 pole machine, the
allowable angular variation from the mean would
be 2½ ÷ 10 = ¼ of one degree.

=Transformers.=--These, as a whole, are simple in construction, high in efficiency, and comparatively inexpensive. Their principles of operation are also readily understood.

The efficiency of a transformer, that is, the ratio between full load primary and full load secondary is greatest when the load on it is such that the sum of the constant losses equals the sum of the variable losses.

In general, transformers designed for high frequencies and
large capacities are more efficient than those designed for
low frequencies and small capacities. As a whole, however, a
transformer leaves but little to be desired as regards efficiency,
a modern 60 cycle transformer of 50 kilowatts capacity or more
possesses an efficiency of approximately 98 per cent. at full load
and an efficiency of about 97 per cent. at half load.

=Ques. How should a transformer be selected, with respect to efficiency?=

Ans. One should be chosen, whose parts are so proportioned that the point of maximum efficiency occurs at that load which the transformer usually carries in service.

In many alternating current installations, comparatively light
loads are carried the greater part of the time, the rated full
load or an overload being occurrences of short durations. For such
purposes special attention should be given to the designing or
selecting of transformers having low core losses rather than low
resistance losses, because the latter are then of relatively small
importance.

=Ques. What kind of efficiency is the station manager interested in?=

Ans. The "all day efficiency."

This expression, as commonly met with in practice, denotes
_the percentage that the amount of energy actually used by the
consumer is of the total energy supplied to his transformer during
24 hours_. The formula for calculating the all day efficiency of
a transformer is based upon the supposition that the amount of
energy used by the consumer during 24 hours is equivalent to full
load on his transformer during five hours and is as follows:

5w
E = -------------
24c + 5r + 5w
where
E = the all day efficiency of the transformer,
w = the full load in watts on the primary,
c = the core loss in watts,
r = the resistance loss in watts.

=Ques. What are the usual all day efficiencies?=

Ans. The average is about 85 per cent. for those of 1 kilowatt capacity, 92 per cent. for those of 5 kilowatts capacity, 94 per cent. for those of 10 kilowatts capacity, and about 94.5 per cent. for those of 15 kilowatts capacity.

=Ques. What becomes of the energy lost by a transformer?=

Ans. It reappears as heat in the windings and core.

This heat not only increases the resistances of the windings and
core, producing thereby a further increase of their respective
losses, but in addition causes in time a peculiar effect on the
iron core which is intensified by the reversals of magnetism
constantly going on within it.

After about two years' service, the iron apparently becomes
fatigued or tired, and this phenomenon is called aging of the
iron. Since the life of the transformer depends to a great extent
upon this factor, the conditions responsible for its existence
should as far as possible be removed. Means must therefore be
provided in the construction to radiate the heat as quickly as it
is generated.

=Ques. What kind of oil is used in oil cooled transformers?=

Ans. Mineral oil.

=Ques. How is it obtained?=

Ans. By fractional distillations of petroleum unmixed with any other substances and without subsequent chemical treatment.

=Ques. What is the important requirement for transformer oil?=

Ans. It should be free from moisture, acid, alkali or sulphur compounds.

=Ques. How may the presence of moisture be determined?=

Ans. By thrusting a red hot iron rod in the oil; if it "crackle," moisture is present.

=Ques. Describe the Westinghouse method of drying oil.=

Ans. It is circulated through a tank containing lime, and afterwards, through a dry sand filter.

=Ques. What is the objection to heating the oil (raising its temperature slightly above boiling point of water) to remove the moisture?=

Ans. The time consumed (several days) is excessive.

=Ques. What effect has moisture?=

Ans. It reduces the insulation value of the oil. .06 per cent. of moisture has been found to reduce the dielectric strength of oil about 50 per cent. "dry" oil will withstand a pressure of 25,000 volts between two 9½ inch knobs separated .15 inch.

=Ques. What is understood by transformer regulation?=

Ans. It is the difference between the secondary voltage at no load and at full load, and is generally expressed as a percentage of the secondary voltage at no load.

=Ques. What governs its value?=

Ans. The resistance and reactance of the windings.

=Ques. How may the regulation be improved?=

Ans. By decreasing the resistances of the windings by employing conductors of greater cross section, or decreasing their reactance by dividing the coils into sections and closely interspersing those of the primary between those of the secondary.

NOTE.--_The term_ ="regulation"= as here used
is synonymous with "drop." The _voltage drop_
in a transformer denotes the drop of voltage
occurring across the secondary terminals of a
transformer with load. This drop is due to two
causes: 1, the resistance of the windings; and
2, the reactance or magnetic leakage of the
windings. On non-inductive load, the reactive
drop, being in quadrature, produces but a slight
effect, but on inductive loads it causes the
voltage to drop, and on _leading current loads_
it causes the voltage to rise. As the voltage
drop of a good transformer is very small even
on inductive load, direct accurate measurement
is difficult. It is best to measure the copper
loss with short circuited secondary by means of
a wattmeter, and at the same time the voltage
required to drive full load current through. From
the watts, the resistance drop can be found, and
from this and the impedance voltage, the reactive
drop may be calculated. From these data a simple
vector diagram will give, near enough for all
practical purposes, the drop for any power
factor, or the following formula may be used
which has been deduced from the vector diagram.
_________________________
D = √(W + X)^{2} + (R + P)^{2} - 100 where R
= % resistance drop; X = % reactive drop; P = %
power factor of load; W = % wattless factor of
________
load (√1 - P^{2}); D = % resultant secondary
drop. For non-inductive loads where P = 100 and W
_____________________
= 0, D = √X^{2} + (100 + R)^{2} - 100. In the
case of leading currents it should be considered
negative.

In transformers where there is a great difference in voltage
between the primary and secondary windings, however, this remedy
has its limitations on account of the great amount of insulation
which must necessarily be used between the windings, and which
therefore causes the distances between them to become such as to
cause considerable leakage of the lines of force.

=Ques. How does the regulation vary for different transformers, and what should be the limit?=

Ans. Those of large capacity usually have a better regulation than those of small capacity, but in no case should its value exceed 2 per cent.

=Ques. What advantages have shell type transformers over those of the core type?=

Ans. They have a larger proportion of core surface exposed for radiation of heat, and a shorter magnetic circuit which reduces the tendency for a leakage of the lines of force into the air.

Both types have advantages and disadvantages as compared with
the other. In the shell type, there is less magnetic leakage, but
also less surface exposed for radiation, and greater difficulty in
providing efficient insulation between the two circuits; in the
core type there is more surface exposed for radiation and less
difficulty in insulating the windings, but there is also a great
leakage of the lines of magnetic force into the outer air.

=Ques. How are the windings usually arranged?=

Ans. As a rule, there is only one primary winding but the secondary winding is generally divided into two equal sections, the four terminals of which are permanently wired to four connection blocks which may be connected so as to throw the secondary sections either in parallel or in series with each other at will.

=Ques. What is necessary for satisfactory operation of transformers in parallel?=

Ans. They must be designed for the same pressures and capacities, their percentages of regulation should be the same and they must have the same polarity at a given instant.

One may satisfy himself as to the first of these conditions by
examining the name plates fastened to the transformers, whereon
are stamped the values of the respective pressures and capacities
of each.

Although equal values of regulation is given as one of the
conditions to be satisfied, transformers may be operated in
parallel when their percentages of regulation are not the same.
Ideal operation, however, can be attained only under the former
state of affairs. Suppose, for instance, a transformer having a
regulation of two per cent. be operated in parallel with another
of similar size and design but having a regulation of one per
cent. The secondary pressures of these transformers at no load
will of course be the same, but at full load if the secondary
pressure of the one be 98 volts, that of the other will be 99
volts. There will, therefore, be a difference of pressure of one
volt between them which will tend to force a current backward
through the secondary winding of the transformer delivering 98
volts. This reversed current, although comparatively small in
value, lowers the efficiency of the installation by causing a
displacement of phase and a decrease in the combined power factor
of the transformers.

=Ques. Describe the polarity test.=

Ans. The test for polarity consists in joining together by means of a fuse wire, a terminal of the secondary winding of each transformer, and then with the primary windings supplied with normal voltage, connecting temporarily the remaining terminals of the secondary windings. The melting of the fuse wire thus connected indicates that the secondary terminals joined together are of opposite polarities, and that the connections must therefore be reversed, whereas if the fuse wire do not melt, it shows that the proper terminals have been joined and that the connections may be made permanent.

The object of this test is, obviously, not to determine the
exact polarity of each secondary terminal, but merely to indicate
which of them are of the same polarity.

=Motor Generators.=--In motor generator sets, either the shunt or series wound type of motor may be employed at the power producing end of the set, but the field of the generator is either shunt or compound wound, depending upon whether or not it is desired to maintain or to raise the secondary voltage near full load. In either case a rheostat introduced in the shunt field winding of the generator will be found very essential. Both generator and motor are so mounted on the base that their respective commutators are at the outer ends of the set. By this means ample space surrounds all of the working parts, and repairs can readily be made.

Motor generators are frequently used as boosters to raise or boost the voltage near the extremities of long distance, direct current transmission lines. Of these, electric railway systems in which it is desired to extend certain of the longer lines, form a typical example.

Owing to the great cost of changing such a system over to one employing alternating current, or storage batteries, or of constructing an additional power station, these solutions of the problem are usually at variance with good judgment and the amount of money at hand. The choice then remains between the purchase of additional wire for feeders, the connection of a booster in the old feeders, or the installation of both larger feeders and a booster. Of these, it is generally found that either the second or the third mentioned alternative meets the conditions most satisfactorily.

A booster installed in a railway system for the purpose just
mentioned, would have a series wound motor, and the conditions
to which it must conform would be as follows: The motor having a
series winding must provide for the full feeder current passing
through both armature and field windings.

Owing to the varying loads on a railway system, due to the
frequent starting and stopping of cars, the feeder current varies
between zero and some such value as 150 amperes. This fluctuation
of current through the field winding will, in ordinary cases, vary
the magnetization of the pole pieces from zero almost to the point
of saturation; that is, the maximum feeder current will so nearly
fill the magnet cores with lines of force that it would be quite
difficult to cause more lines of magnetic force to pass through
them.

So long as the point of saturation is not reached, however, the
proportion of current to field strength remains constant, and
therefore the ratio of amperes to volts will not vary.

The severe fluctuations of the feeder current would, if the
motor were shunt or compound wound, cause most serious sparking
and various other troubles, but in a series motor where the back
ampere turns on the armature that react on the field vary in
precisely the same proportion as the ampere turns in the field,
there exists at all times a tendency to balance the active forces
and produce satisfactory operation. If, however, the field magnet
cores be very large, they cannot so quickly respond, magnetically,
to changes in the strength of the current, and there is then
greater liability of the armature reaction momentarily weakening
the field and thereby producing temporary sparking.

=Ques. Are motor generators always composed of direct current sets?=

Ans. No.

=Ques. Describe conditions requiring a different combination.=

Ans. For purposes where for instance direct currents of widely different voltages are to be obtained from an alternating current circuit, and it is desired to install but one set, a motor generator consisting of an alternating current motor such as an induction motor, and a dynamo must necessarily be employed.

In such sets, it is common to find both motor and dynamo
armatures mounted on a common shaft, and the respective field
frames resting on a single base, although for connection on a
very high pressure alternating current circuit, separate armature
shafts insulated from each other but directly connected together,
and separate bases resting on a single foundation, are usually
employed to afford the highest degree of insulation between the
respective circuits of the two machines.

=Ques. What is the objection to a set composed of alternating current motor and alternator?=

Ans. The commercial field that would be naturally covered by such a set is better supplied by a transformer.

=Ques. Why?=

Ans. Because a transformer contains no moving parts, and is therefore simpler in construction, cheaper in price, and less liable to get out of order.

=Dynamotors.=--A dynamotor differs from a motor generator in that the motor armature and the generator armature are combined into one, thereby requiring but one field frame. Since the motor and generator armature windings are mounted on a single core, the armature reaction due to the one winding is neutralized by the reaction caused by the other winding. There is, consequently, little or no tendency for sparking to occur at the brushes, and they therefore need not be shifted on this account for different loads.

=Ques. How is a dynamotor usually constructed?=

Ans. It is usually built with two pole pieces which are shunt wound.

=Ques. Why does the voltage developed fall off slightly under an increase of load?=

Ans. Because a compound winding cannot be provided.

=Ques. Describe the armature construction and operation.=

Ans. It consists of two separate windings; one of which is joined to a commutator mounted on one side of the armature for motor purposes, and the other to the commutator on the other side of the armature for generator purposes.

By means of two studs of brushes pressing on the motor
commutator, current from the service wires is fed into the winding
connected to this commutator, and since the shunt field winding
is also excited by the current from the service wires, there is
developed in the generator winding on the rotating armature a
direct voltage which is proportional to the speed of rotation of
the armature in revolutions per second, the number of conductors
in series which constitute the generator winding, and the total
strength of the field in which the armature revolves. This
pressure causes current to pass through the generator winding and
the distributing circuit when the distributing circuit to which
this winding is connected by means of its respective commutator,
brushes, etc., is closed.

=Ques. How is a dynamotor started?=

Ans. It is connected at its motor end and started in the same manner as any shunt wound motor on a constant pressure circuit.

=Ques. What precautions should be taken in starting a dynamotor?=

Ans. The necessary precautions are, to have the poles strongly magnetized before passing current through the motor winding on the armature; to increase gradually the current through this winding, and not to close the generating circuit until normal conditions regarding speed, etc., are established in the motor circuit.

=Ques. How is the current developed in the machine regulated?=

Ans. It can be regulated by the introduction of resistance in one or the other of the armature circuits, or by a shifting of the brushes around the commutator.

=Ques. Are dynamotors less efficient than motor generators of a similar type?=

Ans. No, they are more efficient.

=Ques. Why?=

Ans. Because they have only one field circuit and at least one bearing less than a motor generator.

A motor generator has at least three bearings, and occasionally,
four, where the set consists of two independent machines directly
connected together.

=Rotary Converters.=--An important modification of the dynamotor is the rotary converter. This machine forms, as it were, a link between alternating and direct current systems, being in general a combination of an alternating current motor and a dynamo.

It has practically become a fixture in all large electric railway systems and in other installations where heavy direct currents of constant pressure are required at a considerable distance from the generating plant. In such cases a rotary converter is installed in the sub-station, and being simpler in construction, higher in efficiency, more economical of floor space, and lower in price than a motor generator set consisting of an alternating current motor and a dynamo which might be used in its place, it has almost entirely superseded the latter machine for the class of work mentioned.

=Ques. What is the objection to the single phase rotary converter?=

Ans. It is not self-starting.

=Ques. What feature of operation is inherent in a rotary converter?=

Ans. A rotary converter is a "reversible machine."

That is to say, if it be supplied with direct current of the
proper voltage at its commutator end, it will run as a direct
current motor and deliver alternating current to the collector
rings. While this feature is sometimes taken advantage of in
starting the converter from rest, the machine is not often used
permanently in this way, its commercial application being usually
the conversion of alternating currents into direct currents.

=Ques. How does a rotary converter operate when driven by direct current?=

Ans. The same as a direct current motor, its speed of rotation depending upon the relation existing between the strength of the field and the direct current voltage applied.

If the field be weak with respect to the armature magnetism
resulting from the applied voltage, the armature will rotate at a
high speed, increasing until the conductors on the armature cut
the lines of force in the field so as to develop a voltage which
will be equal to that applied.

Again, if the field be strong with respect to the armature
magnetism, resulting from the applied voltage, the armature will
rotate at a low speed. If, therefore, it be desired to operate the
converter in this manner and maintain an alternating current of
constant frequency, the speed of rotation must be kept constant by
supplying a constant voltage not only to the brushes pressing on
the commutator, but also to the terminals of the field winding.

=Ques. How does it operate with alternating current drive?=

Ans. The same as a synchronous motor.

=Ques. What is the most troublesome part and why?=

Ans. The commutator, because of the many pieces of which it is composed and the necessary lines along which it is constructed, its peripheral speed must be kept within reasonable limits.

=Ques. What should be the limit of the commutator speed?=

Ans. The commutator speed, or tangential speed at the brushes should not exceed 3,000 feet per minute.

=Ques. Name another limitation necessary for satisfactory operation.=

Ans. The pressure between adjacent commutator bars should not exceed eight or ten volts.

If the commutator bars be made narrow in order to obtain
the necessary number for the desired voltage with the minimum
circumference and therefore low commutator speed, the brushes
employed to collect the current are liable to require excessive
width in order to provide the proper cross section and yet not
cover more than two bars at once.

=Ques. How can the commutator speed be kept within reasonable limits, other than by reducing the width of the commutator bars?=

Ans. By using alternating current of comparatively low frequency.

For a rotary converter delivering 500 volt direct current, the
proper frequency for the alternating current circuit has been
found to be 25 cycles per second.

=Ques. When a rotary converter is operated in this usual manner on an alternating current circuit, how can the direct current be varied?=

Ans. It may be varied (from zero to a maximum) by changing the value of the alternating pressure supplied to the machine, or it may be altered within a limited range by moving the brushes around the commutator, or in a compound wound converter by changing the amount of compounding.

Under ordinary conditions, varying the voltage developed by
changing the voltage at the motor end is not practical, hence
the voltage developed can be varied only over a limited range.
In addition to this, the voltage developed at the direct current
end bears always a certain constant proportion to the alternating
current voltage applied at the motor end; this is due to the same
winding being used both for motor and generator purposes. In all
cases the proportion is such that the alternating current voltage
is the lower, being in the single phase and in the two phase
converters about .707 of the direct current voltage, and in the
three phase converter about .612 of the direct current voltage.
It is thus seen that whatever value of direct current voltage be
desired, the value of the applied alternating current voltage must
be lower, requiring in consequence the installation of step down
transformers at the sub-station for reducing the line wire voltage
to conform to the direct current pressure required.

=Ques. What is the efficiency of a rotary converter?=

Ans. It may be said to have approximately the same efficiency as that in the average of the same output, although in reality the converter is a trifle more efficient on account of affording a somewhat shorter average path for the current in the armature, reducing in consequence the resistance loss and the armature reaction.

=Ques. May a converter be overloaded more than a dynamo of the same output, and why?=

Ans. Yes, because there is usually less resistance loss in the armature of the converter than in the armature of the dynamo.

Thus, a two phase converter may be overloaded approximately 60
per cent., and a three phase converter may be overloaded about 30
per cent. above their respective outputs if operated as dynamos.

=Ques. Describe how a converter is started.=

Ans. There are several methods any one of which may be employed, the choice in any given case depending upon which of them may best be followed under the existing conditions.

If it be found advisable to start the converter with direct
current, the same connections would be made between the source of
the direct current and the armature terminals on the commutator
side of the converter as would be the case were a direct current
shunt motor of considerable size to be started; this naturally
means that a starting rheostat and a circuit breaker will be
introduced in the armature circuit.

The shunt field winding alone is used, and this part of the
wiring may be made permanent if, as is usually the case, the same
source of direct current is used normally for separate field
excitation.

The direct current may be derived from a storage battery, from a
separate converter, or from a motor generator set installed in the
sub-station for the purpose.

An adjustable rheostat will, of course, be connected in the
field circuit for regulation. Before starting the converter,
however, it is necessary to do certain wiring between the
terminals on the collector side of the machine and the alternating
current supply wires, in order that the change over from direct
current motive power to alternating current motive power may be
made when the proper phase relations are established between
the alternating current in the supply wires and the alternating
current in the armature winding of the converter.

In order that proper phase relations exist, the armature
of the converter must rotate at such a speed that each coil
thereon passes its proper reversal point at the same time as the
alternating current reverses in the supply wires. This speed may
be calculated by doubling the frequency of the supply current and
then dividing by the number of pole pieces on the converter, but a
far more accurate method of judging when the converter is in step
or in synchronism with the supply current consists in employing
incandescent lamps as shown in fig. 2,872.

=Ques. How is a polyphase converter started with alternating current?=

Ans. This may be done by applying the alternating pressure directly to the collector rings while the armature is at rest. There need be no field excitation; in fact the field windings on the separate pole pieces should be disconnected from each other before the alternating voltage is applied to the armature, else a high voltage will be induced in the field windings which may prove injurious to their insulation. The passage of the alternating current through the armature winding produces a magnetic field that rotates about the armature core, and induces in the pole pieces eddy currents, which, reacting on the armature, exert a sufficient torque to start the converter from rest and cause it to speed up to synchronism.

=Ques. How much alternating current is required to start a polyphase converter?=

Ans. About 100 per cent. more than that required for full load.

=Ques. How may this starting current be reduced?=

Ans. Transformers may be switched into circuit temporarily to reduce the line wire voltage until the speed become normal.

In conjunction with this method, the method of synchronizing
shown in fig. 2,872 may be used, thus, in starting, there is
an alternating current between the brushes which pulsates very
rapidly, but when synchronism is approached, the pulsations
become less rapid until finally with the converter in step with
the alternator the pulsations entirely disappear.

The light given by the lamps thus connected indicates accurately
the condition of affairs at any one time, varying from a rapidly
fluctuating light at the beginning to one of constant brilliancy
at synchronism.

=Ques. If the armature of the starting motor have a starting resistance, how must this be connected?=

Ans. It should be connected in series with the armature inductors before the alternating voltage is applied.

As the motor increases in speed, the starting resistance is
gradually short circuited until it is entirely cut out of circuit.

NOTE.--Some converters are provided
with a small induction motor for starting
mounted on an iron bracket cast in the converter
frame, and whose shaft is keyed to that of the
converter. Allowing for a certain amount of
slip in the induction motor, the field of this
machine must possess a less number of magnet
poles than the converter in order to enable the
latter machine to be brought to full synchronism.
To start the induction motor, it is simply
necessary to apply to its field terminals the
proper alternating voltage. The bracket, and
therefore the motor, is usually mounted outside
the armature bearing on the collector side of the
converter.

=Ques. Describe the usual wiring for the installation of a rotary converter in a sub-station.=

Ans. Commencing at the entrance of the high pressure cables, first there is the wiring for the lightning arresters, then for the connection in circuit of the high tension switching devices, from which the conductors are led to bus bars, and thence to the step down transformers.

On a three phase system the transformers should be joined in
delta connection, as a considerable advantage is thereby gained
over the star connection, in that should one of the transformers
become defective, the remaining two will carry the load without
change except more or less additional heating. Between the
transformers and rotary converter the circuits should be as short
and simple as possible, switches, fuses, and other instruments
being entirely excluded. The direct current from the converter is
led to the direct current switchboard, and from there distributed
to the feeder circuits.

=WATTMETER ERROR FOR A LOAD OF 1,000 VOLT-AMPERES=
(For a lag of 1 degree in the pressure coil)

+------------+----------+-------+-------------------+
| | | |Error of indication|
|Power factor|True watts| Error | in per cent |
| | | | of true value |
+------------+----------+-------+-------------------+
| 1. | 1,000 | .3 | 0.03 |
| .9 | 900 | 7.6 | 0.85 |
| .8 | 800 | 10.5 | 1.31 |
| .7 | 700 | 12.5 | 1.78 |
| .6 | 600 | 13.9 | 2.32 |
| .5 | 500 | 15.1 | 3.02 |
| .4 | 400 | 15.9 | 3.98 |
| .3 | 300 | 16.6 | 5.54 |
| .2 | 200 | 17.1 | 8.55 |
| .1 | 100 | 17.3 | 17.30 |
+------------+----------+-------+-------------------+

NOTE.--In the iron vane type instrument when
used as a wattmeter, the current of the series
coil always remains in perfect phase with
the current of the circuit, provided series
transformers are not introduced. The error,
then, is entirely due to the lag of the current
in the pressure coil, and this error in high
power factor is exceedingly small, increasing as
the power factor decreases. In the above table
it should be noted that the value of the error
as distinguished from the per cent. of error,
instead of indefinitely increasing as the power
factor diminishes, rapidly attains a maximum
value which is less than 2 per cent. of the power
delivered under the same current and without
inductance. It should also be noted that the
above tabulation is on the assumption of a lag of
1 degree in the pressure coil. The actual lag in
Wagner instruments for instance, is approximately
.085 of a degree, and the error due to the lag
of the pressure coil in Wagner instruments is,
therefore, proportionally reduced from the
figures shown in the above tabulation.

=Ques. In large sub-stations containing several rotary converters how are they operated?=

Ans. Frequently they are installed to receive their respective currents from the same set of bus bars; that is, they may be operated as alternating current motors in parallel. They are also frequently operated independently from single bus bars, but very seldom in series with each other.

=Ques. How may the direct current circuit be connected?=

Ans. In parallel.

NOTE.--In motor testing, by the methods
illustrated in the accompanying cuts, it is
assumed that the motor is loaded in the ordinary
way by belting or direct connecting the motor
to some form of load, and that the object is to
determine whether the motor is over or under
loaded, and approximately what per cent. of full
load it is carrying. All commercial motors have
name plates, giving the rating of the motor and
the full load current in amperes. Hence the
per cent. of load carried can be determined
approximately by measuring the current input
and the voltage. If an efficiency test of the
apparatus be required, it becomes necessary to
use some form of absorption by dynamometer, such
as a Prony or other form of brake. The output of
the motor can then be determined from the brake
readings. The scope of the present treatment
is, however, too limited to go into the subject
of different methods of measuring the output of
the apparatus, and is confined rather to methods
of measuring current input, voltage, and watts.
The accuracy of all tests is obviously dependent
upon the accuracy of the instruments employed.
Before accepting the result obtained by any test,
especially under light or no load, correction
should be made for wattmeter error. See table of
wattmeter error on page 2,075.

=Ques. What provision should be made against interruption of service in sub-stations?=

Ans. There should be one reserve rotary converter to every three or four converters actually required.

=Ques. Why does a rotary converter operate with greater efficiency, and require less attention than does a dynamo of the same output?=

Ans. There is less friction, and less armature resistance, the latter because the alternating current at certain portions of each revolution passes directly to the commutator bars without traversing the entire armature winding as it does in a dynamo; there is no distortion of the field and consequently no sparking, or shifting of the brushes, since the armature reaction resulting from the current fed into the machine and that due to the current generated in the armature completely neutralizes each other.

=What electrical difficulty is experienced with a rotary converter?=

Ans. Regulation of the direct current voltage.

=Ques. How is this done?=

Ans. It can be maintained constant only by preserving uniform conditions of inductance in the alternating current circuit, and uniform conditions in the alternator.

While changes in either of these may be compensated to a certain
extent by adjustment of the field strength of the converter, they
cannot be entirely neutralized in this manner; it is therefore
necessary that both the line circuit and the alternator be
given attention if the best results are to be obtained from the
converter.

=Ques. What mechanical difficulty is experienced with rotary converters?=

Ans. Hunting.

=Ques. What is the cause of this?=

Ans. It is due to a variation in frequency.

The inertia of the converter armature tends to maintain a
constant speed; variations in the frequency of the supply circuit
will cause a displacement of phase between the current in the
armature and that in the line wires, which displacement, however,
the synchronizing current strives to decrease. The synchronizing
current, although beneficial in remedying the trouble after it
occurs, exerts but little effort in preventing it, and many
attempts have been made to devise a plan to eliminate this trouble.

NOTE.--Three phase motor test; polyphase
wattmeter method. This is identical with the test
of fig. 2,882, except that the wattmeter itself
combines the movement of the two wattmeters.
Otherwise the method of making the measurements
is identical. If the power factor be known to be
less than 50 per cent., connect one movement so
as to give a positive deflection; then disconnect
movement one and connect movement two so as to
give a positive deflection. Then reverse either
the pressure or current leads of the movement,
giving the smaller deflection, leaving the
remaining movement with the original connections.
The readings now obtained will be the correct
total watts delivered to the motor. If the power
factor be known to be over 50 per cent., the same
methods should be employed, except that both
movements should be independently connected to
give positive readings. An unloaded induction
motor has a power factor of less than 50 per
cent., and may, therefore, be used as above
for determining the correct connections. For a
better understanding of the reasons for the above
method of procedure, the explanation of the two
wattmeter method, fig. 2,882, should be read. The
power factor may be calculated as explained under
fig. 2,882. Connect as shown in fig 2,882. The
following check on connection may be made. Let
the polyphase induction motor run idle, that is,
with no load. The motor will then operate with a
power factor less than 50 per cent. The polyphase
meter should give a positive indication, but
if each movement be tried separately one will
be found to give a negative reading, the other
movement will give a positive reading. This can
be done by disconnecting one of the pressure
leads from the binding post of one movement. When
the power factor is above 50 per cent. then both
movements will give positive deflection.

=Ques. What are the methods employed to prevent hunting?=

Ans. 1, the employment of a strongly magnetized field relative to that developed by the armature; 2, a heavy flywheel effect in the converter; 3, the increasing of the inductance of the armature by sinking the windings thereon in deep slots in the core, the slots being provided with extended heads; and 4, the employment of damping devices or amortisseur winding on the pole pieces of the converter.

=Ques. What method is the best?=

Ans. The damping method.

The devices employed for the purpose are usually copper shields
placed between or around the pole pieces, although in some
converters the copper is embedded in the poles, and in others it
is made simply to surround a portion of the pole tips.

In any case its action is as follows: The armature rotating at a
variable speed has a field developed therein which is assumed to
be also rotating at a variable speed; the magnetism of this rotary
field induces currents in the copper which, however, react on the
armature and oppose any tendency toward a further shifting of the
magnetism in the armature and therefore prevent the development
of additional currents in the copper. Since copper is of low
resistance, the induced currents are sufficient in strength to
thus dampen any tendency toward phase displacement, and so exert a
steadying influence upon the installation as a whole.

=Electrical Measuring Instruments.=--In the manufacture of most measuring instruments, the graduations of the scale are made at the factory, by comparing the deflections of the pointer with voltages as measured on standard apparatus. The voltmeters in most common use have capacities of 5, 15, 75, 150, 300, 500 and 750 volts each, although in the measurement of very low resistances such as those of armatures, heavy cables, or bus bars, voltmeters having capacities as low as .02 volt are employed.

The difference between the design of direct current voltmeters of different capacities lies simply in the high resistance joined in series with the fine wire coil. This resistance is usually about 100 ohms per volt capacity of the meter, and is composed of fine silk covered copper wire wound non-inductively on a wooden spool.

In the operation of an instrument, if the pointer when deflected do not readily come to a position of rest owing to friction in the moving parts, it may be aided in this respect by gently tapping the case of the instrument with the hand; this will often enable the obstruction, if not of a serious nature, to be overcome and an accurate reading to be obtained.

=Ques. Describe a two scale voltmeter.=

Ans. In this type of instrument, one scale is for low voltage readings and the other for high voltage readings; on these scales the values of the graduations for low voltages are usually marked with red figures, while those for high voltages are marked with black figures. A voltmeter carrying two scales must also contain two resistances in place of one; a terminal from each of these coils must be connected with a separate binding post, but the remaining terminal of each resistance is joined to a wire which connects through the fine wire coil with the third binding post of the meter. The two first mentioned binding posts are usually mounted at the left hand side of the meter and the last mentioned binding post and key at the right hand side.

The resistance corresponding to the high reading scale is
composed of copper wire having the same diameter as that
constituting the resistance for the low reading scale, but as the
capacity of the former scale is generally a whole number of times
greater than that of the latter scale, the resistances for the two
must bear the same proportion.

=Ques. How is a two scale voltmeter connected?=

Ans. In the connection of a two scale voltmeter in circuit, the single binding post is always employed regardless of which scale is desired. If, then, the voltage be such that it may be measured on the low reading scale, the other binding post employed is that connected to the lower of the two resistances contained within; if, however, the pressure be higher than those recorded on the low reading scale, the binding post connected to the higher of the two resistances contained within is used.

NOTE.--Three phase alternator load test. By
means of the connection shown in fig. 2,888,
readings of armature current and field amperes
can be obtained with any desired load. The field
current can be varied also so as to maintain
constant armature voltage irrespective of load;
or the field current may be kept constant and
the armature voltage allowed to vary as the load
increases. The connections may also be used to
make a temperature test on the alternator by
loading it with an artificial load. In some cases
after the alternator is installed the connection
may be used to make a temperature test, using
the actual commercial load the alternator is
furnishing.

Inasmuch as the capacities of the scales are
usually marked on or near the corresponding
binding posts, there will generally be no
difficulty in selecting the proper one of the two
left hand binding posts.

=Ques. How is a two scale voltmeter connected when the binding posts are not marked?=

Ans. If only an approximate idea is possessed of the voltage to be measured, it is always advisable to connect to the binding post corresponding to the high reading scale of the meter in order to determine if the measurement may not be made safely and more accurately on the low reading scale. In any case, some knowledge must be had of the voltage at hand, else the high reading portion of the instrument may be endangered.

_Too much care cannot be taken to observe these precautions_
whenever the voltmeter is used, for the burning out or charring
of the insulation either in the fine wire coil or in the high
resistance of the meter by an excessive current, is one of the
most serious accidents that can befall the instrument.

If a voltmeter has been subjected to a voltage higher than that
for which it was designed, yet not sufficiently high to injure the
insulation, but high enough to cause the pointer to pass rapidly
over the entire scale, damage has been done in another way. The
pointer being forced against the side of the case in this manner,
bends it more or less and so introduces an error in the readings
that are afterward taken.

The same damage will be done if the meter be connected in
circuit so the current does not pass through it in the proper
direction, although in this case the pointer is not liable to
be bent so much as when it is forced to the opposite side of
the meter by an abnormal current, since then it has gained
considerable momentum which causes a severer impact. The extent
of the damage may be ascertained by noting how far away from the
zero mark the pointer lies when no current is passing through
the instrument. If this distance be more than two-tenths of a
division, the metal case enclosing the working part should be
removed and the pointer straightened by the careful use of a pair
of pinchers.

=Ques. What should be noted with respect to location of instruments?=

Ans. If they be placed near conductors carrying large currents, the magnetic field developed thereby will produce a change in the magnetism of the instruments and so introduce an error in the readings.

=Ques. How should portable instruments be wired?=

Ans. The wires must be firmly secured to the supports on which they rest, so as to reduce the possibility of their being pulled by accident, and so causing the instruments to fall.

A fall or a rough handling of the meter at once shows its effect
on the readings, for as much harm is done as would result from a
similar treatment of a watch.

The hardened steel pivots used in all high grade voltmeters are
ground and polished with extreme care so as to secure and maintain
a high degree of sensitiveness. The jewels on which the moving
parts revolve are of sapphire, and they too must necessarily be
made with skill and carefulness; if, therefore, the jewels become
cracked and the pivots dulled by careless handling, the meter at
once becomes useless as a measuring instrument.

=Ques. How should readings be taken?=

Ans. The deflection of the pointer should be read to tenths of a division; this can be done with considerable accuracy, especially after a little practice.

For very accurate results, a temperature correction should be
applied to compensate the effect which the temperature of the
atmosphere has upon the resistance of the meter when measurements
are being taken. In ordinary station practice the temperature
correction is negligible, being for resistance corresponding to
the high scale in first class meters, less than one-quarter of 1
per cent. for a range of 35 degrees above or 35 degrees below 70
degrees Fahrenheit.

=Ques. What attachment is sometimes provided on station voltmeters used for constant pressure service?=

Ans. A normal index.

=Ques. What precaution must be taken in connecting station voltmeters?=

Ans. Care must be taken to guard against any short circuiting of the voltmeter, which, would mean a short circuiting of the generator, and as a result the probable burning out of its armature.

The high resistance of the voltmeter prevents any such
occurrence when it is connected in the proper way, but should one
side of the circuit be grounded to the metal case or frame of the
meter, a careless handling of the lead connected with the other
side of the circuit would produce the result just mentioned.

=Ques. Why do station voltmeters indicate a voltage slightly lower than actually exists across the leads?=

Ans. Since they are usually connected permanently in circuit; a certain amount of heat is developed in the wiring of the instrument.

The effect of this heat increases the voltmeter resistance and
consequently reduces the current below that which otherwise would
pass through the meter; since the deflections of the pointer
are governed by the strength of the current, station voltmeters
invariably indicate a voltage slightly lower than that which
actually exists across their leads.

NOTE.--=Checking up of a recording wattmeter.=
This may conveniently be done by noting the
deflections at short intervals on an ammeter
connected in circuit, and also the readings
on the dial of the recording wattmeter during
this period. If this test be continued for an
appreciable time, the product of the pressure
in volts, the current in amperes, and the time
in hours, should equal the number of watthours
recorded on the counters of the dial.

NOTE.--=Transformer testing.= In the early
days of transformer building, before the
commercial wattmeter had been perfected, leakage
or exciting current was the criterion of good
design. After the introduction of the wattmeter,
core loss became the all important factor, and
for a long time the question of leakage current
was lost sight of. With the introduction of
silicon steel, leakage or exciting current again
assumed prominence. Keeping in mind the fact
that all characteristics of a transformer are of
more or less importance, it is essential that
the user of such apparatus have at hand the
necessary facilities for making tests of all such
variable quantities. The tests which all users
of transformers should make, are given in this
chapter.

=Ques. Can direct current be measured by an alternating current voltmeter?=

Ans. Yes.

NOTE.--=Transformer copper loss test.= The
usual and best method of obtaining copper
losses is to separately measure the primary and
secondary resistance and calculate from these
the primary and secondary copper losses. For
general diagram of connections and discussion
of the drop method, see fig. 2,875. The current
should be kept well within the load current of
the transformer to avoid temperature rise during
the test. In other words, the resistance of the
coil is the voltage across its terminals divided
by the current. The resistance of the primary
coil can be measured similarly. The copper loss
in watts in each coil will then be the product
of the resistance and the square of the rated
current for that coil. The total copper loss will
be the sum.

=Ques. What would be the effect of placing a direct voltmeter across an alternating current circuit, and why?=

Ans. There would be no deflection of the pointer owing to the rapid reversals of the alternating current.

=Ques. What are the usual capacities of alternating current voltmeters?=

Ans. They are 3, 7.5, 10, 12, 15, 20, 60, 75, 120, 150, 300 and 600 volts, but these capacities may each be increased by the use of a multiplier.

=Ques. How are station voltmeters usually attached to the switchboard?=

Ans. They are usually bolted to the switchboard by means of four iron supports mounted on the back of the instrument; two of these are fastened near each side of the case.

Under certain conditions, however, as in paralleling of
alternators, it is convenient to have the alternating current
voltmeter mounted on a swinging bracket at the side of the
switchboard. The voltmeter may then be swung around in any desired
direction so as to enable the attendant to keep informed of the
voltage while switching in each additional alternator.

=Ques. How should an ammeter be operated to get accurate readings, and why?=

Ans. It should be cut out of circuit except while taking a reading, because of the error introduced by the heating effect of the current.

In an ammeter having a capacity of 50 amperes, the error thus
introduced will be less than 1 per cent. if connected continuously
in circuit with a current not exceeding three-quarters this
capacity.

An ammeter of 100 amperes capacity may be used indefinitely
in circuit with less than 1 per cent. error up to one-half its
capacity, and for five minutes at three-quarters capacity without
exceeding the 1 per cent. limit.

The 150 scale ammeter may be left in circuit for an indefinite
length of time at one-third its full capacity, and for three
minutes at one-half its full capacity, with a negligible error.

Ammeters of 200 and of 300 ampere capacities must not
continuously carry more than one-quarter of these loads
respectively if the readings are to have an accuracy within 1 per
cent. nor more than one-half these respective number of amperes
for three minutes if the same degree of accuracy be desired.

In order to cut or shunt the ammeter out of circuit when not
in use, it is customary when wiring the instrument in place, to
introduce a switch as a shunt across it; this switch is kept
closed except when a measurement is being taken.

When currents larger than 300 amperes have to be measured,
ammeter shunts are generally employed, although ammeters up to
500 amperes capacity are manufactured.

=Ques. What is used in place of instrument shunts for high pressure alternating current measurements?=

Ans. Instrument transformers.

=Ques. What important attention should be periodically given to measuring instruments?=

Ans. They should be frequently tested by comparison with standards that are known to be correct.

Electrical measuring instruments, owing to the nature of their
construction and the conditions under which they must necessarily
be used, are subject to variations in accuracy. This feature is an
annoying one on account of the difficulty of detecting it; a meter
may, as far as appearances go, be in excellent working order and
yet give readings which are not to be relied upon.