Improvements in Alternating Current Induction Motors.
Year: 1899

page_text: Date of Application, 17th Mar., 1899
PROVISIONAL SPECIFICATION.
Improvements in Alternating Current Induction Motors.
I, WILLIAM GOULD RHODES, Master of Science, Member of the Institution of
Electrical Engineers, Lecturer in Electrical Engineering and Consulting Elec-
trical Engineer, The Royal Technical Institute, Peel Park, Salford, do hereby
declare the nature of this invention to be as follows:-
The objects of my invention are
(I) To increase the starting torque, or initial turning effort of induction motors
whatever be the number of phases, whether monophase, di-phase, or tri-phase,
without inserting resistances in the rotor circuits.
(2) To increase the power factors of induction motors.
An induction motor may be regarded as a transformer in which the primary
winding is stationary while the secondary winding free to rotate. The stationary
part is called the Stator" and the rotating part the "Rotor."
When large starting torque is required it is usual to insert resistance coils in
the rotor circuits, the object being to diminish the currents in the rotor circuits
which by their demagnetising action reduce the torque on the rotor shaft.
I propose to attain the same end by inserting opposing, or counter, electro-
motive forces in the rotor circuits and claim this idea as my invention whatever
may be the method of carrying out the same. My method of attaining this
object is to connect portions of the stator windings, in a suitable manner, across
the terminals of the rotor windings by means of copper brushes making rubbing
contacts on copper rings rigidly attached to the shaft of the rotor but electrically
insulated therefrom, to which copper rings the extremities of the rotor windings
are in electrical connection. By means of a suitable switch with a sufficient
number of contacts to which different points on the stator windings are con-
nected the opposing electromotive forces thus inserted in the rotor circuits can
he gradually diminished as the speed of the rotor increases and (if necessary)
finally cut out altogether when the full speed of the rotor is attained.
That this process will attain the desired object is seen by considering that the
electromotive force induced in the secondary winding of a transformer differs by
180 degrees of phase from the electromotive force applied between the primary
terminals.
When starting resistances are used in connection with the stator windings, all
or portions of those resistances may be used to supply the requisite counter electro-
motive forces for insertion in the rotor circuits and thus avoid tapping the stator
windings themselves.
The same device will increase the power factors of induction motors since the
presence of the large rotor currents diminishes the self-induction of the stator
windings and greater currents become necessary in the stator windings to pro-
duce the necessary magnetic field.
Dated this 16th day of March 1899

page_text: Improvements in Alternating Current Induction Motors.
1, WILLIAM GOULD RHODES, of The Royal Technical Institute, Salford, in the
County of Lancaster, Electrical Engineer, 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 following statement:-
This invention has reference to improvements in alternate current motors of the
induction type, that is to say those having a part to which the alternate current
is supplied and another part provided with a winding or windings (hereinafter
referred to as a winding) in which currents are induced.
In this description it is assumed that the usual arrangement of these parts is
followed, viz. that the former part is stationary and the latter part rotary; these
Sometimes the rotor wind-
parts are respectively called the stator and the rotor.
ing forms a closed circuit or circuits, but sometimes it is provided with contact
rings and brushes for the purpose of closing its circuit or circuits through an
external circuit or circuits.
When starting such an alternate current motor provided with a contact arrange-
ment, it is usual to insert a simple resistance in the rotor circuit in order to
check the current in the rotor winding, to prevent excessive currents being
drawn from the mains, and to increase the torque on the rotor shaft. Some-
times instead of a simple resistance, a choking coil or other arrangement giving
With such
a back electromotive force of self induction has been employed.
arrangements there is a lagging displacement of the currents in the rotor winding
which reduces the magnetising action of the stator currents and also the torque
of the rotor, and allows of the passage of unduly large currents into the stator.
Now this invention has for object to obtain a larger starting torque than
hitherto, whether using single-phase or multi-phase currents, at the same time
keeping the stator currents relatively small.
'
For this purpose instead of using simple resistances as herein before mentioned,
or arrangements giving a back EM F by self induction, for checking the cur-
rents in the rotor winding at starting, the rotor winding is subjected to the
action of a counter electromotive force obtained directly or indirectly from
parts of the main supply circuit and so applied as to reduce, prevent, or counter-
act the lag of the currents in the rotor winding or windings, or even such as to
cause a lead or displacement contrary to that which would be caused by self
induction.
As will be obvious the invention can be carried out in various ways.
Figs. 1, 2, 3, 4 and 5 are diagrams simply showing by way of example, different
ways in which the invention can be carried into practice.
In the arrangement shown in Fig. 1, a is the stator winding, b the 'rotor,
and c, the alternating current supply mains. The rotor winding is connected
by means of the ordinary contact rings e, e brushes f, f and conductors g, g, to
a switch whereby it can be connected to suitable portions of the stator winding
in such a way that the opposing E MF that can be thus applied to the rotor
winding, at starting, can be gradually diminished as the speed of the rotor
increases, and, if required, finally cut out altogether when full speed is attained.
In the example shown, the switch comprises a movable contact or slider i that is
electrically connected to one of the conductors g, and a series of insulated fixed
contacts that are electrically connected to different points of the stator wind-
and one of which is also connected to the other conductor g. With this
ing

page_text: arrangement it will be seen that when the contact i is moved into the position
shown in full lines, the maximum counter électromotive force the arrangement
is capable of producing, can be impressed 'upon the rotor winding at starting,
and that by moving the contact in the direction of the arrow, the counter
electromotive force impressed on the rotor winding can be gradually reduced, and
when the said contact assumes the position shown in dotted lines, will be finally
removed altogether.
When a starting resistance is used in connection with the stator winding, the
required counter electromotive force may be obtained therefrom by using the
difference of potential across portions of the said resistance, as shown for example
in Fig. 2 where the fixed contacts of the switch are connected to different
points n of the stator starting resistance ».
When the pressure of the electric supply used for driving the motor is not
suitable for providing the required counter electromotive force for starting, a
transformer may be used to provide the necessary counter electromotive force.
The transformer may be of the so-called auto-transformer type, or one of the
ordinary kind, and the required variable counter electro-motive force be obtained
by tapping suitable portions thereof, or by varying its pressure in any known or
suitable way, as for example by inserting a resistance in either its primary circuit
or its secondary circuit.
Fig. 3 shows one arrangement of this kind wherein the switch to which the
conductors are connected has its fixed contacts k connected to different points >>
of the winding g of an auto-transformer which is connected up to any suitable
part of the motor or supply circuit or circuits so that by moving the contact the
counter electromotive force impressed on the rotor winding can be varied.
Fig. 4 shows a modified arrangement wherein the fixed switch contacts are
connected to different points of the secondary winding r of an ordinary trans-
former the primary winding of which is connected up to some suitable part
of the motor or supply circuit or circuits. The switch is connected up to
the rotor winding as in the other examples so that a back electromotive force of
variable amount can be impressed upon the rotor winding as required.
Fig. 5 shows a further arrangement in which the conductors are connected
to the secondary winding of an ordinary transformer and the electromotive
force thus obtained is reduced either by gradually inserting a resistance t in the
primary windings, or a resistance t in the secondary winding r, so as to gradually
reduce the counter electromotive force impressed on the rotor winding at starting.
In the diagrams, for simplicity, only one pair of brushes f, f is shown. It is
to be understood however that each arrangement is intended to be applicable to
any kind of induction motor either single phase or multi-phase and with the rotor
winding in two or more circuits.
The switch used in carrying out the invention may be of any suitable form
but one is preferred in which the slider is provided with a resistance to avoid
short circuiting adjacent fixed contact pieces as it passes from one fixed contact
piece to another. This construction of switch is well known.
With each of the arrangements described, or other arrangements embodying
the same principle, there is impressed upon the rotor winding a counter electro-
motive force such that the tendency of the current in the rotor winding to lag
is prevented or reduced or counteracted, and, in consequence, the magnetising
action of the stator currents and the torque of the rotor are increased.
Having now particularly described and ascertained the nature of the said
invention and in what manner the same is to be performed I declare that what
I claim is:-
1. The combination with an alternate current motor of the induction type, of
means whereby there can be impressed upon the rotor winding at starting, a
counter electromotive force derived from the main supply circuit and which will

page_text: reduce, prevent or counteract lagging displacement of phase of the currents
-induced in the rotor winding by the stator, substantially as described.
2. In an alternate current motor of the induction type, the combination with
.the stator and rotor windings of variable contact devices whereby the said rotor
winding can be connected with different points of the stator winding between
which points there is a dierence of potential, substantially as described.
3. In an alternate current motor of the induction type, the combination with
the motor circuit and rotor winding, of a switch and electrical connections
whereby portions of the motor circuit can be successively tapped so as to impress
a gradually decreasing counter electromotive force on the rotor winding, when
starting the motor, substantially as described.
4. Alternate current induction motors constructed, arranged and operating
substantially as herein before described with reference to Fig. 1 and to Fig. 2 of
the drawings annexed.
Dated this 13th day of January 1900.
For the Applicant,
W. LLOYD WISE,
46, Lincoln's Inn Fields, London, W.C., Chartered Patent Agent.
TOI The Applicant,
W. LLOYD WISE,
46, Lincoln's Inn Fields, London, W.C., Chartered Patent Agent.