Improvements in Alternate Current Induction Motors.
Year: 1897

page_text: Date of Application, 5th Nov., 1897-Accepted, 22nd Jan.,
. 1898
COMPLETE SPECIFICATION.
Improvements in Alternate Current Induction Motors.
We, THE BRITISH THOMSON-HOUSTON COMPANY LIMITED, and HENRY METCALF
HOBART, all of 83, Cannon Street, in the City of London, Electrical Engineers,
do hereby declare the nature of this invention, and in what manner the same is
to be performed, to be particularly described and ascertained in and by the fol-
lowing statement:
This invention relates to the construction of alternate current induction motors
and particularly to the secondary element thereof and consists in a method of
arranging the windings and connections of the secondaries of induction motors;
the object of the invention is to secure a more satisfactory condition of the motor
at starting.
It is well known that when an induction motor is so designed as to operate
satisfactorily under running conditions, the proportions of the secondary are
seldom such as to permit of starting either with or without load in a thoroughly
satisfactory manner when the secondary is connected in the same way as under
running conditions. Several methods have been suggested to overcome this
difficulty, the most satisfactory of which is, probably, that which makes use of a
rheostat mounted, when the secondary is the rotor (as is generally the case) upon
the spider of the rotor. As usually constructed this rheostat comprises suitably
adjusted resistances, which at starting are connected in series with the secondary
windings but which are cut out when the armature has reached a sufficiently high
speed by means of a suitable device, such as a collar capable of a small longitudinal
sliding movement on the rotor shaft, and adapted to cut out the resistances and
short circuit the secondary windings by being forced by a conveniently arranged
lever, against spring clips or the like connected to the junctions of the secondary
windings and the resistances.
According to the present invention we accomplish the result above mentioned,
and attain our objects in a very simple and efficient manner, and while our improved
method is not limited to any particular number of phases it will for convenience
be herein described as applied to a three-phase secondary winding. Our improved
method consists in constructing a secondary winding made up of two or more
distinct component windings each of said windings forming an independent
insulated conducting system and the said windings being connected up in a
manner such as is hereinafter explained, whereby the degree to which they shall
cooperate may be varied. Of course, as is well understood by those skilled in the
art to which this invention applies, any one point of each of such independently in-
sulated systems might be interconnected without in any way altering or affecting the
electrical operation of such system, and, in fact, it might in some cases be con-
venient to so interconnect them particularly at the points of common connection,
but as a rule we prefer to have the component systems entirely separate and
insulated from one another. Probably the most suitable number of windings
would generally be two, each of which might be a complete three-phase winding
in itself, or one winding might be wound for any given number of phases, the
other being wound for a different number: we will however in explaining

page_text: our invention consider its adaptation to a secondary with two component three-
phase windings.
Referring as we proceed to the annexed figure, the two windings are, for diagram-
matical purposes, represented as simple gramme ring windings, but we wish it
to be understood that we in no way limit our invention to such windings; as a
fact it would, in practice, be generally applied to drum windings, these appearing
better adapted to induction motor design.
The two component windings A and B are wound on the secondary element,
which in this case is the rotor R (though as far as regards the underlying principle
it is matter of indifference whether the two secondary windings be wound upon
the rotor R or the stator S) in a double spiral, so that they may be considered to
be practically superposed, so far as relates to their position in regard to the
primary rotary field P. The two component windings of the secondary are repre-
sented diagrammatically as side by side. The Figure shows the connections for
starting the motor, and for running. The winding A is permanently tapped off
at three points a, a, a, as shown (for two poles, and for larger numbers of poles,
there will be three taps for each pair of poles), and a common connection is made,
so that it forms a permanently short-circuited delta-connected winding. These
connections a, a, a, for A are the same for starting as for running. The winding B
is tapped off at a corresponding set of points b, b, b, as nearly coincidental
with a, a, a, as two adjacent independent windings can be (but shown spaced
away from a, a, a, in the Figure for clearness of illustration). The same
winding B is, however, also tapped off at another similar set of points c, c, c, which
are situated at a certain angular distance from the set of points a, a, a. This
angular distance between the two sets of connections a and c will be determined for
each particular motor in accordance with the starting torque required.
The principle involved, is that when the rotor R is at rest and the current
is switched on to the primary P of the stator S, currents will be induced in the
two component secondary windings A, B of the rotor R. If these two com-
ponent windings A and B were independently short circuited from corresponding
points as a, a, a, and b, b, b, the two sets of current-carrying conductors composing
the two windings A and B would co-operate electromagnetically in reacting on
the system, and in the majority of cases, the magnitude of this reaction would
be greater than that best adapted to secure the desired starting torque; (in other
cases there would be no need to have recourse to this invention) but by making the
common connection from the points c, c, c, of the winding B, the points c, c, c, are
at the proper angular distance from the connections a, a, a, of winding A, and
therefore the resultant secondary reaction (of the two windings A and B) is
decreased to the value most suitable for obtaining the desired electro-magnetic
condition at starting. When sufficient speed has been acquired, the connections
may by any suitable device be rearranged by opening the connection of the
three leads c, c, c, and connecting together the leads b, b, b, and the motor will
then be properly arranged for running.
In some cases it may be advisable for purposes of adjustment and to modify
the performance of the motor for certain purposes to interpose resistances in the
usual manner above referred to thus combining to a certain extent the old principle
with the present invention. Such resistances would be inserted as shown for
example in dotted lines at r in the figure in each or any of the three sets of
leads a b c on the ways respectively from the winding A or B to the point of
short-circuit and they may be inserted either in the leads connecting with A
or in any of the leads connecting with B or in all or any of such sets of leads.
It will be apparent that the above described principle of our invention may
frequently be used with advantage in conjunction with already well known
arrangements such for instance as that in which resistances or condensers are
made use of to obtain the desired electromagnetic condition at starting, as an
example we might use in combination with two windings as described in reference
to A and B a third secondary winding connected with resistances in the ordinary

page_text: manner or with condensers. Moreover it might be of advantage to furnish the
winding B with more than the two sets of leads b, b, b, and c, c, c, thus affording
intermediate steps.
Having now particularly described and ascertained the nature of our said
invention and in what manner the same is to be performed, we declare that what
we claim is: —
1. In an alternate current induction motor, winding the secondary element
(whether rotor or stator) with a winding made up of two or more component
windings such as A and B, each of said windings forming an independent
insulated conducting system of itself (except as hereinabove reserved) the said
windings being connected in such manner that the degree to which they shall
cooperate may be varied, as set forth.
2. An alternate current induction motor having a secondary wound with two
or more component windings, entirely insulated from each other, in one of which
the connections are permanent and never altered in any way, the connections of the
others however being so arranged as to be changed so as to give the most efficient
results both for starting and for running.
3. An alternate current induction motor in which the secondary is made up
of two independent windings A and B one of which is permanently short circuited
and the other B so arranged that it can be short circuited by either one of two
or more sets of leads, said sets of leads being tapped off from the winding Bat
different angular positions and any or all of the leads from A and B being
provided or not with resistances substantially as described for the purposes
specified.
4. An alternate current induction motor having a secondary wound and con-
nected up and adapted for use substantially in the manner and for the purposes
mereinabove set forth.
Dated the on day of November foT.
For the Applicants,
J. C. CHAPMAN,
Chartered Patent Agent, 70, Chancery Lane, London, W.C.