THE COLLIERY GUARDIAN

AND

JOURNAL OF THE COAL AND IRON TRADES.

Vol. CXVI.	FRIDAY, AUGUST 9, 1918.	No. 3006.

Notes on the Three=phase Induction Motor.

By L.

The remarkable success which has attended the intro-
duction of the three-phase induction motor for colliery
work has now made it a machine of indispensable value.
The simplicity of operation and control for the majority
of colliery drives, together with the robust construction,
places it far above all other types of motors where the
service is severe and reliability an important factor.
It is surprising, however, that the majority of those’
responsible for such plant have a vague (if any) idea as

Fig. 1.	Fig. 2.	Fig. 3.

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A	B

Fig. 4.



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ductor remains stationary, is immaterial—all that is
required being a relative movement at right angles
between flux and conductor.

(3)	Further, the conductor and electro-magnet may
remain stationary, only the strength of the field being
varied. In this case the induced E.M.F. depends upon
the rate of charge of the magnetic flux.

(4)	Finally, it is necessary to understand the phase
relationship existing between the three-phase currents



through the figure. All values reckoned above the
horizontal line are positive, and all values below the
line negative, whilst the line itself is zero.

The mere lengths of the vectors must be disregarded
as long as they are all equal, because the value of a
vector at any instant is equal to a line drawn from its
extremity to the horizontal line to which it would be at
right angles. This is shown in fig. 1, where B is at its
maximum value, whilst the other two vectors A and C
are both 50 per cent, of B—as will be found by com-
paring the vertical dotted lines from the extremities of
A and C with the length of B.

Now it will be obvious that the three vectors are
balanced about the point O, since B is maximum and
positive, while A and C are each 50 per cent, and
negative. Since the three vectors are at a relative
angle of 120 degs., it will be seen that as they rotate
they will be balanced all the time. For instance, take
the position in fig. 2, where B is at zero value, and
A and C are about 86 per cent, positive and negative
respectively and are therefore balanced.

Further, take the ease of a three-phase star-connected
winding (fig. 3) connected to the supply, and at the
instant indicated in fig. 1.

If we consider the direction of current to be positive ’
when flowing from the star point and negative when
flowing towards it, then, at every instant, the current
which enters the three windings is equal to that which
leaves it. For instance, in fig. 3, if we consider the
maximum value of the current to be 100 amperes at
a given instant, there is a current of 50 amperes in
phase A and C flowing towards the star point, with
one of 100 amperes flowing from that point through B,
and the current entering and leaving the windings is
the same.

Reference has already been made to the fact that the
construction of an induction motor tends to obscure its
principle of working. For instance, there are no well-
defined magnetic poles similar to those found on poly-
phase generators or direct-current machinery. The
magnetic flux of the induction motor is constantly
changing in polarity and position from instant to
instant; but in spite of this, as will be shown, there are
distinct poles to every induction motor—the resultant
field at any instant being produced by a combination of
poles, which will be explained later.

The stator windings of an ordinary induction motor
are placed in slots running from one side to the other
of the stator iron, so that the latter is left with a com-

to the principles of operation of this class of machinery.
To look after any kind of apparatus intelligently, a
knowledge of fundamental principles is important, and
also adds interest to the work.

Moreover, in the case of an induction motor, there are
constructional details that tend to obscure and make
difficult the answers to many questions which come
naturally to those who wish to improve themselves by
knowing the why and wherefore of everything they do,
instead of being satisfied, with a rule-of-thumb method
of working.

For instance, some of the questions which inevitably
crop up in the mind of the student are :—What causes
the rotor of an induction motor to rotate ? How is the
rotating field produced, and why does the rotor reverse
its direction of rotation when two leads are changed
over? How can one find the magnetic poles of the
stator, or distinguish one from the other? These and
many other questions are continually cropping up, and
it is the object of this article to try, in some measure,
to deal simply with a number of these questions, so that
they may be intelligently grasped by both students and
practical colliery electricians.

Now, the many text books which deal with the induc-
tion motor fall into two distinct groups. On the one
hand, when writers are dealing exclusively with the
induction motor, a knowledge of vector diagrams and
some trigonometry is assumed, and the student is
plunged into a maze of diagrams and curves which
may be useful to the designer, but are often worthless
to the practical man.

On the other hand, reference to the induction motor
is, in the ordinary elementary handbooks on electrical
engineering, usually confined to a very brief description,
giving one the impression of a desire to get away from
the subject as soon as possible. In the present article
the writer proposes to strike a course between these two
extremes.

In explaining the action of the induction motor,
vector diagrams cannot be wholly dispensed with; but
with the simple explanation of their application, they
will be found to be very helpful.

Before the induction motor can be properly under-
stood, it is essential that the following principles be
grasped:—

(1)	When a conductor is made to cut the lines of a
magnetic field, an E.M.F. is induced, the value of which
depends upon the rate at which the lines of force are
cut.

(2)	Whether the conductor moves across the face of
the electro-magnet from which the lines of force
proceed, or whether the magnetic flux is caused to move
by a movement of the electro-magnet while the. con-

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B

A	C

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employed, and their relative movements during succes-
sive intervals of time.

The currents generated by a three-phase generator pass
through their respective maximum values in succession,
separated by intervals of 120 degs. with reference to a
circle. That is to say, if a complete period covered one
second, the currents in the respective phases would
pass through their maximum values at intervals of
one-third of a second.

In fig. 1, OA, OB, and OC, are three vectors, the
whole being supposed to rotate, in a clockwise direction,
about the centre O on the horizontal line running



paratively smooth surface, save for a slight opening,
above each slot, which may in some cases be left suffi-
ciently wide to enable former wound coils to be used.
If necessary several slots may be employed to carry the
total ampere turns of a given coil, but for the moment, /
our attention will be confined to coils having only one
slot containing the ampere turns.	<

Fig. 4 shows two complete coils, each occupying one
slot, i.e. one on either side of the coil. Two half coils
are- also shown, one at each end of the figure, which is a
plan and represents a stator laid out flat, but unwound
except for the coils shown. A current is assumed to