November 12, 1915.

THE COLLIERY GUARDIAN.

979

Under these conditions motion and pressure, instead
of being positive and negative, simultaneously thereby
always giving a positive product, are of the same sign,
but half the time, so that their product is half the time
positive and half negative. This change in sign of the
product, that is of the power, indicates a reversal of
its direction of flow. Where motion and pressure are
in quadrature the energy flows forward and backward,
instead of moving uniformly forward. The average of
the forward and backward flow is zero, so that such an
alternating flow of energy does not constitute a trans-
mission of power in the ordinary sense.

The phenomenon of motion and pressure in quadrature
occurs wherever a mass moves with a reciprocating
motion. The motion is that of the mass and the pres-
sure that required to accelerate it. The power corre-
sponding to their joint effect may be called accelerating
power. The power transmitted by reciprocating motion
is equal to the product of the mean velocity and mean
pressure when motion and pressure are in phase. The
rule may be extended to apply to accelerating power.

3

ii

P0WEB

AaxierM
flowing I

2

ST

Fig. 3.—Flow of Energy and Accelerating
Power through Engine.

The product of the mean velocity by the mean
pressure, where motion and pressure are in quadrature,
is an amount of power equal to the maximum value
which the accelerating power attains during a cycle.
The momentary flow at all times is proportional to this
maximum. As the flow alternates in direction, giving
an average value over a cycle of zero, it is convenient to
use the maximum value as a measure of the flow where
considering it in connection with the associated flow of
energy over one or more complete cycles.

For reciprocating motion, the product of mean
velocity by mean pressure is the transmitted power if
the two quantities are in phase, but is the accelerating
power if they are in quadrature. Accelerating power is
evidently always necessary where reciprocating motion
is maintained. On the whole, the accelerating power
neither adds to, nor detracts from, the true power trans-
mitted. It may exist where there is no true power
transmitted, and for a given amount of true power the
accelerating power may be greater or less. Finally, it
may be. said that accelerating power is a peculiar form
of power fundamental to reciprocating motion, and that
it may be considered as transmitted independently of
the true power, and according to its own laws.

The steam engine, containing reciprocating parts,
furnishes an example of the transmission of accelerating
power. In a steam engine, whether a simple engine
with a single cylinder and crank, or a compound engine
with several cranks, the motion of all the reciprocating
parts, pistons, piston rods, connecting rods, valves,
valve rods, and eccentric rods, is in proportion to a
single angular velocity, the speed of revolution of the
engine. The distribution of accelerating power in one
engine may form a complex system, .all, however, based
on a single angular velocity. In this respect it is similar
to an alternating current distributing system, which,
however large, has a single frequency.

Where the mean angular velocity is constant the
energising of the reciprocating parts and the de-energis-
ing of the rotating parts is directly proportional to the
amount of accelerating power which flows. For
example, if the weight of a reciprocating part is doubled,
the energy it contains and the accelerating power it
receives are both doubled, while if the stroke is doubled
the energy and accelerating power are each quadrupled.
At constant angular velocity the amount of accelerating
power may be used as a measure of the transfer of
energy, and it may be said that each reciprocating part
takes a flow of accelerating power proportional to the
amount to which it is energised, while each rotating parte
is de-energised in proportion to the flow of accelerating
power which it gives out.

To make a picture of the flow of accelerating power
a second ribbon may be drawn of width equal to the
accelerating power; that is, to the product of the mean
force by the mean velocity. To distinguish between
the two ribbons, that for energy transmission will be
shaded longitudinally, and that for accelerating power
laterally.

Fig. 3 shows the distribution of accelerating power
from a flywheel to the reciprocating parts of an engine.

As reciprocating motion necessitates acceleration of
matter, and that requires the application of accelerating
power, so alternating electric currents necessitate the
acceleration of electricity, and that also requires a
similar kind of power. Mechanically, the accelerating
power is stored intermittently as kinetic energy in the

reciprocating mass, while electrically the power is corre-
spondingly stored in the alternating magnetic field of
the moving electricity. Wherever there is reciprocating
matter there must be mechanical accelerating power,
and wherever there is alternating electricity there must
be electric magnetising power.

Electric currents in quadrature with the electrical
pressure have been called idle or wattless currents
because they transmit no power in the ordinary sense,
and the product of these currents by the pressure has
been called wattless power, for the same reason. As
the power taken by reactive coils is almost wholly
wattless, the power is also called reactive power, with
the advantage that there is no implication that it is
not really power. As .the idle currents produce
magnetisation they are also called magnetising currents,
and the name magnetising powTer seems to give the
simplest view of the function of reactive power. All of
these electrical terms describe a special form of power
which alternates in direction, and is otherwise identical
with mechanical accelerating power which exists in all
reciprocating machines.

When the magnetism is uniform in direction, the
magnetisation is produced once for all at the beginning,
and no further magnetising power is required. The
same distinction occurs in mechanics; uniform motion,
say, of a flywheel, requires accelerating power only at
the time of starting; after reaching speed it will run
indefinitely without further acceleration. A permanent
magnet may be compared to a frictionless flywheel which
will run indefinitely when once started. The field
current or exciting current merely overcomes electrical
friction, that is resistance, and conserves the magnetisa-
tion put in at the first instant.

In practically all commercial machines for interchang-
ing mechanical and electrical power, that is, generators
and motors, the mechanical forces are due to the attrac-
tion and repulsion of magnets. With alternating current
machines the magnetism of these magnets is produced in
whole or in part by this magnetising power.

As we may picture mechanical accelerating power
flowing out from the flywheel to each reciprocating part

Pump

induction

Motor „ t

Magnetization



FULL LOAD

Efficiency 9i%

Power Factor 83%

£

q 100 K	K w Energy

-j LossniOKW

1

50K.W

Magnetization
45K.VA

HALF LOAD

G Efficiency 89%

56 y.w. Energy q Power Factor 78%

Uj

----------------Mechanics! Power reverses here —•-------------------

NO LOAD

Efficiency 0%

Monetization
.	38 KVA

NO LOAD

HALF LOAD

Engine

enzdnon <

}55K.y.A Q: EULL LOAD

Magnetization

\ 45K.VA

asoD uj

38 KVA. >J

Energy^
W.

i

Fig. 4.—Induction Motor and Induction

Generator Operation at Varying Loads.

of an engine, so may we picture magnetising power flow-
ing out from the generator to each motor, transformer,
or other part containing an alternating-current magnet.
The alternating-current magnets are magnetised at the
expense of the direct-current magnets. The gain of
magnetisation of one exactly equals the loss of magnet-
isation of the other. Alternating-current machinery,
such as ordinary generators, synchronous motors, and
synchronous converters, contains both direct and
alternating-current magnets, while other important
electric machinery, such as transformers, induction
motors, induction regulators, and induction generators,
contains alternating-current magnets only.

Magnetising power being but an electrical variety of
accelerating power, its flow may be shown by a similar
picture. A ribbon may be drawn for it with a width
equal to the product of the mean current by the mean
potential. Magnetising power may be measured in the
same units as accelerating or other powder, though in
electrical work magnetising power is measured in
kilovolt-amperes, instead of in kilowatts. The two units
are of -the same dimensions, a kilowatt being the same
as a kilovolt-ampere, except that its application is
limited by an additional convention that the current and
pressure are in phase. Arrows may be placed on the
ribbon to show the direction in which magnetisation has
been displaced. The demagnetising of the field magnets
of an alternator, and the magnetisation of the fields of
an induction motor being proportional to the magnetis-
ing power, and due to its flow, there is a natural basis
for describing the flow as being from the point where
demagnetising takes place, and to the point where the
magnetism reappears.

Fig. 4 shows the flow of energy through an induction
motor at varying loads. The motor receives from the

generator two independent kinds of power. The ordinary
flow of energy through it is similar to that through a
direct-current motor, and requires no further comment.
The magnetising power which flows into the motor and
goes no further is the peculiar feature. The width of
this power ribbon does not vary much from full load
to no load. Motors of different sizes take magnetising
power approximately in proportion to their horse-power
rating, though small motors take a somewhat greater
proportion due to less perfect design. Slow speed
motors take more than high speed motors of the
same rating, because the speed being low, the maximum
torque must be high, and as the diameter of the
armature is not increased sufficiently to maintain the
same surface speed, a -high torque requires more
magnetic attraction, and consequently more magnetism.
Subject -to considerable variation due to such causes,
the figures shown may be taken as typical of any induc-
tion motor.

An induction motor, like other motors, is reversible in
its function, and may operate as a generator, and is
then called an induction generator. The operation of

Exciter

Alternating

Exciter Generator

Synchronous
Motor

Norma! Excitation



____a_____

Under Exated

Energy flowing to Motor

Normal Excitation

Fig. 5.—Flow of Energy and Magnetisation at
Varying Excitations.

Energy flowing to Motor

the motor as a generator amounts to a reversal of the
flow of energy, precisely as with a direct-current
machine. However, the flow of magnetising power
does not and cannot reverse. An induction generator
cannot produce magnetising power.

An alternating-current generator can operate as an
alternating-current motor, and is then called a synchron-
ous motor. The synchronous motor differs from the
induction motor in being completely reversible; that is,
reversible as regards the flow of magnetisation as well
as of energy. Where an alternator transmits power to
a synchronous motor there are three cases, as shown
in fig. 5. The simplest case is where, as in direct-
current transmission, no magnetising power is -required.
Each machine has its magnetisation furnished locally
by direct currents. The equality of excitation is
indicated 'in the figure by drawing the exciters of the
generator and motor as of the same size.

Above this case is shown the one where magnetisation
as well as energy is being transmitted from the generator
to the motor. This indicates that the motor has insuffi-
cient magnetisation; it is, therefore, said to be under-
excited. The less the direct-current excitation of the
motor the greater the magnetising power absorbed by
it. This magnetising power produces a useful magnet-
isation in the motor just sufficient to supplement the
inadequate direct-current magnetisation. If the direct-
current excitation decreases, the magnetising power
increases, so that finally, if the direct-current magnetisa-
tion fails entirely, the magnetising power alone may
furnish approximately normal magnetisation. A syn-
chronous motor may, therefore, run without direct-
current field excitation; -in fact, this principle is used
in starting synchronous motors and converters. Such
an unexcited motor is similar to an induction motor, but
is -so imperfectly designed for unexcited operation that it
takes a much greater proportion of magnetising power.
An unexcited synchronous motor is, like an induction
motor, reversible in regard to flow of energy, and may
be used as a generator. However, an unexcited
generator or motor must absorb, and cannot produce
magnetising power.

At the bottom of the figure is shown the case where
the motor has an excessive amount of magnetisation,
or is over-exicted. The magnetising power now flows
from the motor to the generator. The effect of this flow
is to demagnetise -the motor, the amount of flow being
just sufficient to bring the magnetisation of the motor
down to equality with that of the generator.

Magnetising power may, therefore, be imagined as a
current or flow of magnetisation from points of surplus
to those of deficient magnetisation. The surplus
magnetisation is to be found in those machines strongly
excited by direct currents, the deficient magnetisation
in those weakly excited, and more especially in trans-
formers, induction motors, and other apparatus having
no direct-current excitation at all. If the excitation of
a generator and a motor -are equally strong or equally
weak, no flow of magnetising power between them is
necessary to preserve a balance. The voltage which a
machine tends to have, due to its direct-current excita-
tion alone, furnishes perhaps the simplest measure of
the strength of excitation from the present point of view.

Where several alternating-current machines having
direct-current excitation are connected together the
increase in the excitation of any one machine tends to
raise its voltage, but this tendency is counteracted by a
magnetising current which flows away from it to the
other machines, demagnetising it and magnetising them.
The relative excitation of the machines (generators and
motors) on a system governs the flow of magnetisation,