1064

THE COLLIERY GUARDIAN.

Mat 23, 1913.

THE SERIES - WOUND ELECTRIC MOTOR
WORKING AT CONSTANT PRESSURE AND
WITH CONSTANT CURRENT.

By Sydney F. Walker.

The Diffarence in the Working of the Two Machines
under the Different Conditions.

The difference between the working of the series-
wound motor when supplied with current at constant
pressure, as under the usual conditions of service in
mining work, and its working when supplied with
constant current, as developed by M. Thury, is, the
writer believes, of considerable interest to mining
engineers. The most important difference between the
working of what is virtually the same machine, under
the two conditions, is, as M. Thury has been able to
elaborate it, the elimination of the starting resistance
that is necessary when working from a constant pressure
service, and of all regulating, all controlling apparatus,
the whole being accomplished by a simple movement of
the brushes around the commutator of the machine.

The special feature of the series-wound machine,
whether it works as a generator or as a motor, is the
fact that the coils which furnish the excitation of the
field magnets are in the same circuit as the armature
coils, and that the same current which passes through
the armature coils also passes through the field magnet
coils, except in the special case referred to below, in
which the field magnet coils are shunted by a graduated
resistance, for the purpose of controlling the speed of
the machine, when running as a motor.

The series-wound motor, when working upon the
ordinary constant pressure service, varies the current
which passes through its coils, both the field and
armature coils, automatically, in accordance with the
requirements of the load. As the load increases, the
current passing through both coils also increases, except,
again, in the case mentioned above, where the field coils
are shunted for the purpose of regulation, and vice versa.

With the constant current series-wound motor, as the
name implies, the current in both field coils and arma-

ture coils remains the same, no matter what load the
machine is dealing with may be; it meets an increased
load by absorbing an increased pressure, and it decreases
the amount of energy it is taking from the service the
amount of power it is delivering to the axle of its
armature, when the load decreases, by decreasing the
pressure which it absorbs. It will be remembered that
the power which any electric motor is able to convert
from electric to mechanical energy is measured by the
product of the pressure across the terminals of the
motor into the current passing through the machine as
a whole. It will be evident, therefore, that the load to
be dealt with may be varied, either by keeping the
pressure constant and varying the current in accordance
with the variation of the load, or by keeping the
current constant and varying the pressure. The first
arrangement is that which is employed almost univer-
sally at the present time where continuous currents are
used for the distribution of power, and the second is the
arrangement developed by M. Thury. The voltage with
constant pressure services in mines ranges between 200
and 600, 500 to 550 being a favourite figure.

With the ordinary constant pressure service, it will
be remembered, the act of slowing up performed by the
armature when the load increases reduces the back
pressure developed by the armature, thereby increasing
the difference between the pressure of the service and
the back pressure furnished by the machine. It will be
remembered, again, that the armature of every con-
tinuous-current motor when it is revolving furnishes a
pressure in opposition to the pressure of the service, and
it is the difference between this back pressure and that
of the service which allows the necessary current to
pass through the machine as a whole to perform the
work demanded from it. Assuming, for instance, that
the pressure across the terminals of a given motor is
500 volts, and that when performing certain work a
current of 100 amperes is passing through its coils, and
assuming an over-all efficiency 90 per cent., the motor
will be delivering 60 B.H.P. at its axle. This, again,
will mean that the back pressure furnished by the
machine is in the neighbourhood of 450 volts, or that
there is a difference between the service pressure and
that furnished by the machine of 50 volts. If now an
increased load is put upon the machine, say, of 30-horse
power, and assuming for the moment that the efficiency
of the motor remains the same, in order to deal with
that increased load the armature will reduce its speed
until a current of 150 amperes passes through its coils
and those of the field magnets, the machine being
series wound, and, in order to enable this to take place,
the difference in pressure between the service and the
back pressure will be 75 volts.

In practice, the working is not quite so simple as the

above. The pressure between the service mains does
not remain constant; the increase and decrease of the
current taken by the motor itself, unless the service
mains are very large, causes an appreciable decrease and
an increase in the pressure between the service mains.
For practical purposes, however, for comparing the
working of the two systems, the above may be taken.
With the ordinary constant pressure series-wound
motor also the effect of an increase or decrease of
current, caused by a decrease or increase of speed of the
armature, is in a compound ratio. The slowing up of
the armature not only allows more current to pass, but
it increases the strength of the magnetic field furnished
by the field magnets ; and, on the other hand, when the
armature increases its speed, when the load is taken off,
it not only gives rise to a decrease in the current passing
through both armature and field magnets, but also to a
decrease in the strength of the magnetic field. As
mining engineers know by painful experience, the series-
wound motor, working from a constant pressure service,
often plays somewhat sad tricks. It often acts the part
of the willing horse, and will go on decreasing its speed,
and increasing the current passing through its coils,
until the heat liberated by the current, damages the
insulation. On the other hand, when the machine is
suddenly relieved of its load, it has a nasty trick of
racing. With the constant current motor, as developed
by M. Thury, an increase of load may be dealt with by
increasing the speed at which the armature runs ; or, if
preferred, the speed of the armature may remain con-
stant, and the increased pressure obtained by moving
the brushes round the commutator, and a decrease of
load is met by the same operations in the reverse direc-
tion. The method of varying the pressure by moving
the brushes is dealt with below. With the Thury
constant current motor also, when the load decreases,
the condition may be dealt with by decreasing the speed
at which the armature runs, or by decreasing the pressure
delivered to the machine by moving the brushes in the
opposite direction to that for increased load. The
movement of the brushes may be by hand or automatic.

Starting and Controlling the Speed of Series - wound
Motors.

The question of starting motors from rest, and
controlling their speed when at work, is one of very
great importance. Owing to the fact that the working
of the machine depends upon the development of a back
pressure by the armature, it is absolutely necessary to
reduce the pressure delivered to the terminals of the
machine at starting, on the constant-pressure system;
because otherwise a powerfully-damaging current would
pass through both armature and field coils in the case
of a series-wound machine, and the machine itself could
not start, owing to the armature field overpowering that
furnished by the field magnets. To meet the difficulty,
the well-known device, by which a comparatively large
resistance is inserted in the circuit when the machine is
first started up, and is cut out step by step, has been
introduced. As mining engineers know also to their
cost, the starting resistance, whatever may be its form
is a source of trouble and expense; and its elimination
should be a great boon. It is eliminated in the
constant current system, by moving the brushes as
described below.

In the matter of the regulation of the speed of the
series-wound motor, taking current from a constant
pressure service, there are two methods available, both
demanding a graduated resistance. In one method—
that employed with the tramcar services, and with some’
pumping plants in mines—a graduated resistance is
interposed between one supply main and one terminal
of the motor, and is varied according to the requirements
of the service. Increasing the resistance decreases the
pressure delivered to the motor, and decreases its speed;
while decreasing the resistance increases the speed of
the motor. In the other method a graduated resistance
is placed as a shunt across the field coils, and its
quantity is varied, according to the requirements of the
service. Decreasing the resistance of the shunt,
decreases the current passing through the field coils,
and allows of the speed of the machine being increased,
and vice versa. There are important objections to the
use of both of these arrangements. If a resistance is
interposed in the circuit, to reduce the pressure delivered
to the motor, a certain amount of energy is wasted in
heating the resistance. And trouble has often arisen in
connection with the regulation of the speed of electric
motors in mines, owing to starting resistances being
used for regulating the speed of the motor. The starting
resistance is only made to carry the current passing
through it, for the very short period during which the
motor is getting up speed, while the regulating resis-
tance has to carry the current for as long as it is
employed in the office of regulation. Hence the starting
resistance is made very much smaller than the regulating

resistance, and if it is used to regulate, it heats up, joints
become disconnected, arcs following, and so on.

The method of regulating the speed by the graduated
shunt across the field magnets has the same objection
as the starting resistance—the contacts are apt to give
trouble. In addition, in order that more power may be
obtained from the motor, at the higher speed at which it
runs when its field magnet coils are shunted, a larger
current has to pass through the armature coils, and the
troubles mentioned below follow.

How the Constant Current Motor Deals with Starting
and Regulating.

As mentioned above, the constant - current motor
deals with both of these problems, by merely rocking the
brushes. Fig. 1 is a diagrammatic representation of a
ring-wound armature, having 20 coils, connected to
20 segments of the commutator. The ring winding is
chosen because it enables the action of the constant-
current motor to be explained better than the drum
winding; and a two-pole machine is chosen also for the
sake of simplicity. It is assumed also, for the purpose
of the diagram, that the well-known reaction of the
armature current upon the field magnets is absent, and
that, therefore, the points of commutation, the points
where the brushes are to be placed—for least sparking,
and for obtaining the best results from the machine—
are at opposite ends of a diameter, at right angles to
that passing through the centres of the pole pieces. The
commutator and armature are shown also with a small
number of coils, and a small number of segments, for
reasons of simplicity. When the machine runs as a
motor, and assuming that the top brush is the positive,
the current enters at the top brush and divides equally

IRON CORE

ARMATURE COIL

Fig. 1.

POLE
PIECE

POLE
PIECE

COMMUTATOR

BRUSH

Diagram showing a Ring-wound Armature, Commutator,
and Brushes. The diagram is intended to illustrate
the rocking to and fro of the brushes on a constant
current motor.

between the two halves of the armature on each side,
passing out by the negative brush at the bottom. The
two halves of the armature—the ten coils on each side—
are performing an equal amount of work in causing the
armature to revolve. If, now, the brushes are moved
one segment away from the vertical line, the condition
of things is completely altered. Instead of the 10 coils on
each side of the armature performing equal work, nine
coils on each side will perform work in the same manner as
the 10 do when the brushes are in the vertical position,but
one on each side will be acting in the opposite direction—
that is to say, eight coils will be performing useful
work only. If the brushes are moved still farther in
the same direction, over another segment of the
commutator, eight coils on each side will be performing
work in the same manner as the original 10 did, and two
coils on each side will be operating in the opposite direc-
tion, so that the net work will be performed by six coils.
This means, as will easily be understood, that the work
performed by the motor is less and less, as the brushes
are moved farther and farther from the vertical line;
and when they reach the line passing through the centres
of the pole pieces, there will be five coils on each side
doing useful work, or that would do useful work if they
were able to, and five coils on each side opposing them,
the result being that no work is performed. M. Thury
has called these points, where the brushes lie when no
work is being performed, the neutral points. With four
pole, six-pole, and multipolar machines generally, the
neutral points for each set of brushes will lie halfway
between the positions of the positive and negative
brushes adjoining each other. In the Thury system,
when a motor is at rest, performing no work, the brushes
are at what he has called the neutral position. With
his constant-current system also, any motor which is
not performing work is switched out of circuit, and by
the operation of switching is insulated from the service.
When the motor is to be put into service, it is first
switched into the circuit, and then the brushes are