Improvements in Starting Devices for Alternating Current Motors.
Year: 1897

page_text: Date of Application, 23rd Nov., 1897-Accepted, 8th Jan., 1898
COMPLETE SPECIFICATION.
Improvements in Starting Devices for Alternating Current Motors.
We, THE BRITISH THOMSON-HOUSTON COMPANY LIMITED, of 83 Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to them from abroad by William J.
Foster, of Schenectady, in the County of Schenectady, State of New York, United
States of America) and in what manner the same is to be performed, to be
particularly described and ascertained in and by the following statement:-
This invention relates to alternating current motors and particularly to those of
the synchronous type. Synchronous alternating current motors are in general
not self starting, even when supplied with multiphase currents, as the polar line of
the inducing member, which is usually the rotor, rotates so rapidly, that, owing to
the inertia of the moving parts and their inability to respond instantaneously to
the magnetic attraction, rapid and violent thrusts in opposite directions are pro-
duced, instead of the unidirectional torque required. In attempting to start such
motors it is usual to open the field circuits, and to rely upon the currents induced
in the body of the pole pieces, acting in much the same manner as the induced
currents in the armature of an induction motor, to furnish the required torque.
But it is usual to build the field magnets of such motors with polar projections
which tend, even when unexcited to lock the rotor in its position, particularly
when it happens to have stopped at a dead point. In addition, the polar faces of
the field, particularly when the machine is built with laminated poles, are not well
adapted to be the seat of induced currents. To avoid these difficulties and provide
a synchronous motor which will start under considerable load with unexcited field
magnets, according to this invention between each of the polar projections of the
field magnet or induced member is placed a magnetic bridge, preferably of such
form as to provide a good path for the induced currents. This bridge acts in two
ways, first to do away with the sharp lines of magnetism emanating from the pole
pieces and avoid dead points, and second to add to the strength and effect of the
induced currents.
The particular device which is employed is illustrated in the accompanying
drawings, in which Fig. 1 is a side elevation, partly broken away, of a synchronous
motor embodying the invention; Fig. 2 is a plan of one of the bridges or magnetiz-
able coils; Fig. 3 is a section on the line 3-3 of the parts shown in Fig. 2.
A is the base of the machine; B is its armature or inducing member shown in
this case as the rotor; CC are the field poles, and D D are the bridges or coils.
The armature B is shown diagrammatically, as consisting of a core wound with
an inducing winding b, connected to rings and brushes in the usual way. Any
preferred type of armature may be employed, as is well known in the art. It is
evident that the armature shown is intended for three phase current, but the in-
vention is not restricted thereto. 4 is the shaft of the machine.
In Fig. 2 the bridges are shown as composed of a bar of iron, conforming
approximately to the curvature of the armature. Through the middle of the bar,
slots E E are provided thus forming with the sides d d and a central portion d¹,

page_text: two short-circuited coils of magnetizable material. End pieces F, F¹, by which
the bar may be conveniently attached to the field poles, are also provided and
secured to the bridge by means of bolts or in any other suitable way. For this,
any other approved construction may be adopted.
The action is practically as follows. Current is admitted to the inducing mem-
ber B, through the rings c, preferably with unexcited field magnets. A rotating
polar line is produced, which induces in the polar faces and in the short circuits
formed by the bridges, currents which react on the inducing member to produce
torque, as in an ordinary induction motor. As the magnetic ring in which the
armature is placed is practically continuous, there are no dead points. The arma-
ture soon attains such speed that the field magnets may be excited, and when this
is done the slip becomes zero, so that the losses due to currents in the bridges
become very small.
The advantages which result from placing the short-circuited coil or bridge D
between the polar projections instead of winding such a coil in slots in the polar
faces, as has been formerly proposed, are two:-First, by placing it between the
polar projections the coil is able to act, if it is formed of magnetic material, both as
a starting coil and as a magnetic bridge, as above set forth, and, Second, that in
this way the necessity of slitting the polar faces is avoided, which tends to add to
the resistance of the magnetic circuit by adding to the average width of the air
gap, and also causes an irregularity in the flux through the inducing member or
armature. In addition, the invention may be readily applied to existing motors,
which is a great advantage.
It is evident that various forms may be given to the bridge, and that it may be
employed in connection with various types of machines. The invention, there-
fore, is not restricted to the particular forms shown and described, but
Having now particularly described and ascertained the nature of the said in-
vention (as communicated to us from abroad) and in what manner the same is to
be performed, we declare that what we claim is:-
1. As a means of increasing the starting torque of a synchronous alternating
current motor, the use of a bridge of magnetic material arranged adjacent to the
polar faces of the induced member, substantially as described.
2. As a means of increasing the starting torque of a synchronous motor, the use
of a short-circuited coil between two of the polar faces of the field magnets, sub-
stantially as described.
3. As a means of increasing the starting torque of a synchronous motor the use
of a short-circuited coil of magnetizable material adjacent to the polar faces, sub-
stantially as described.
4. As a means of increasing the starting torque of a synchronous motor, the use
of a short-circuited coil of magnetizable material, placed between two of the polar
faces of the field magnet, and serving both to preserve the magnetic continuity
of the field ring and to furnish an induced current for starting the motor, sub-
stantially as described.
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