Means for Varying the Speed of Asynchronous Alternate Current Motors.
Year: 1896

page_text: Date of Application, 11th Mar., 1896
Complete Specification Left, 21st Nov., 1896-Accepted, 2nd Jan., 1897
[Communicated from abroad by SIEMENS & HALSKE, of 94 Markgrafenstrasse,
Berlin, in the German Empire.]
Means for Varying the Speed of Asynchronous Alternate Current
Motors.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate,
Westminster, in the County of London, Electrical Engineers, do hereby declare
the nature of this invention to be as follows:-
The induced part or armature of asynchronous alternate current motors is now
generally provided with a winding so arranged that it is possible to introduce
resistances into it, this being effected by providing such induced part with a
complete polyphase winding which, according to requirements terminates in three
or more contact rings or screw terminals according to whether the induced part is
the rotor or the stator. By means of these rings or terminals the currents are led
to the outside where they can be manipulated as may be required whilst the motor
is running.
The present invention relates to means whereby the speed of a motor arranged
as above described can be varied, such means being based upon a hitherto unknown
property of such motors. Fig. 1 of the accompanying drawing is a diagram of the
winding and connections of the induced part of such a three phase motor. The
annular core N has six bobbins, or rather three double bobbins I I II [["
III III. Corresponding ends of these bobbins are connected directly with each
other and the corresponding free ends are led to the outside and are connected
with each other by means of cut-out switches J¹ J³.
In place of a three phase winding there might of course be used a two or
multiple phase winding. At Fig. 2 which shews diagrammatically the entire device
requisite for carrying out the invention, an arrangement for a two phase motor
with four bobbins is shewn for the sake of clearness.
Referring to Fig. 1, it will be seen that by opening one of the switches J, such
as J as shewn one of the 3 branch windings or bobbins will be interrupted, whereby
the other two branches which remain connected through the switch J, are made
no longer to represent a three-phase winding but a single phase winding. In
other words the polyphase winding is, by the interruption of one of its branches,
converted into an ordinary Gramme winding, the passage of the currents then
taking place as indicated by the arrows.
If under these circumstances an alternating current be generated in the windings,
no rotating field will be produced in the annular core Ñ but only an alternating
field which occurs in the direction indicated by the large arrow.
By means of the above described property of the winding the speed of the
motor can be varied during the working. For this purpose an arrangement may
be employed such as is shewn by way of example at Fig. 2. In this figure N, is
a stationary annular core (stator) which is provided with an ordinary two phase
winding.
The four leads I II III IV to which the winding of the stator is connected are
supplied with current from a generator of polyphase alternate currents. The

page_text: sequently one matctor, it is however event that me
induced part might be the stator.
Within the ring N, is the revolving annular core N, (rotor) mounted on a
shaft A which is also provided with a two phase winding divided into four sections.
Corresponding ends of this winding are led to the contact rings C, C, C, C, fixed
on the shaft A.
Each pair of rings, that is C, C, and Ca C, may be connected through
resistances R, and R, and at least one pair, such as C3 C4, is provided with a
cut-out switch J so arranged that the conducting connection between these rings
can be entirely interrupted.
If this machine be supplied with current through the leads I II III IV, the
rotor will, as is known, acquire a speed that will only lag behind the synchronism
due to the frequency of the current by a small percentage. The speed can be
reduced according to the known method by two different means, namely either
by reducing the frequency of the current supplied, or by introducing resistances
into the branches of the induced part, so that a greater reduction of speed is
required for producing the strength of current necessary for generating the
requisite torque. This means may no doubt be applicable in cases where the speed
is only required to be temporarily reduced. But as this arrangement necessarily
causes a reduction of the useful effect, it is not advantageously applicable for
continued working at reduced speed.
The reduction of speed can according to the present invention be attained
without such loss. The action in this case is based upon the following. If, in
Figure 2, during the working the cut out switch J be opened, so that the currents
generated in the induced part can no longer produce a rotary field, the action of
the motor will be as represented by the curve II in Fig. 3. This curve is produced
by marking the speed of the motor as abscisse and the torque as ordinates.
The end point chosen for the curve, which by way of example is marked 100,
corresponds with the synchronism due to the frequency of the current supplied to
the inducing part N. Fig. 2.
As the speed decreases, the torque increases, but only very slowly. It never
attains a considerable amount, and soon arrives at a maximum, from which point it
decreases again, so that when the speed is reduced to one half that of synchronism,
indicated by the point 50, it again becomes zero.
If the speed be still farther reduced, the torque will become negative, that is to
say, the machine will consume mechanical energy and will produce electrical
energy, being converted from a motor into a generator, but this condition is not a
stable one. If on the other hand the variations of torque be observed from zero
speed upward, the curve I will be obtained.
With low speeds the torque is very great; it decreases with increasing speed
and at a speed equal to one half that of synchronism, it will be zero again;
beyond that point it will become negative again, and the motor becomes a
generator.
This peculiar behaviour of a polyphase motor whose induced part has a single
phase winding has been proved by experiment.
In practically applying the invention, the cut out switch J, Fig. 2 is turned into
the open position when it is desired to cause the motor to run at half speed, thereby
converting the winding of the induced part into a single phase winding, while
when the motor is required to run at full speed, the switch J is closed so as to
convert the winding into a polyphase one. The consumption of energy is in both
cases about equal while the useful effect of the motor is also only subject to a slight
variation.
From what has been stated above it will be seen that the appliance may also be
employed usefully in cases where the machine is required at one time to act as a
motor and at another time as a generator, such for instance as in working a lift.
Here the machine may be made firstly to work as a motor at full speed or at
half, according to whether goods or persons are to be raised, or it may be made to

page_text: work as a generator driven by the descent of the lift cage, thus operating as a
brake.
Lastly the present invention may be employed in combination with the existing
means for varying the speed of motors, thus for example at Fig. 2 resistances R, R
may be interposed between the branches of the winding of the in luced part.
Dated this 11th day of March 1806
ABEL & IMRAY,
Agents for the Applicants.
COMPLETE SPECIFICATION.
Means for Varying the Speed of Asynchronous Alternate Current
Motors.
We, SIEMENS BROTHERS & Co. LIMITED, of 12 Queen Anne's Gate,
Westminster, in the County 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 following statement:-
The induced part or armature of asynchronous alternate current motors is now
generally provided with a winding so arranged that it is possible to introduce
resistances into it whilst in motion this being effected by providing such. induced
part with a complete polyphase winding which, according to requirements terminates
in three or more contact rings or screw terminals according to whether the induced
part is the stator or the rotor.
By means of these rings or terminals the induced currents may be brought to
the exterior of the motor where they can be manipulated as may be required
whilst the motor is running.
The present invention relates to means whereby the speed of the motor arranged
as above described can be varied, such means being based upon a hitherto unknown
property of such motors. Fig. 1 of the drawing accompanying our Provisional
Specification is a diagram of the winding and connections of the induced part of
such a three phase motor. The annular iron core N has six bobbins, or rather
three pairs of bobbins I I II° 11° 111 111. Corresponding ends of these
bobbins are short circuited and the corresponding free ends are brought out and
are connected with each other by means of switches J1 J2.
In place of a three phase winding there might of course be used a two or multiple
phase winding. At Fig. 2 which shews diagrammatically the entire device
requisite for carrying out the invention, an arrangement for a two phase motor with
four bobbins is shewn for the sake of clearness.
Referring to Fig. 1 it will be seen that by opening one of the switches J, such
as J¹ as shewn one of the 3 branches of the winding will be interrupted, whereby
the other two branches which remain connected through the switch J, no longer
represent a three phase winding but a single phase winding. In other
words the polyphase winding is, by the interruption of one of its branches,
converted into an ordinary Gramme winding, the currents then circulating
as indicated by the arrows. If under these circumstances an alternating current
be sent through the windings, no rotating field will be produced in the annular
iron core N but only an alternating field which occurs in the direction indicated
by the large arrow. By means of the above described property of the winding the
speed of the motor can be varied during the working.
For this purpose an arrangement may be employed such as is shewn by way of
example at Fig. 2. In this figure N, is a stationary annular iron core (stator)
which is provided with an ordinary polyphase winding in four parts.

page_text: The four leads I II III IV to which the winding of the stator is connected are
supplied with current from a generator of polyphase alternate currents.
The stator of the motor is consequently the inductor, it is however evident that
the induced part might be the stator.
Within the stator N, is the revolving annular iron core N, (rotor) mounted on a
shaft A which is also provided with a polyphase winding divided into four
sections.
Corresponding ends of this winding are led to the contact rings C₁ C, C, C, fixed
on the shaft A.
Each pair of rings, that is C₁ C, and C3 C₁ may be connected through
resistances R, and R, and at least one pair, such as C, C, is provided with a
switch J so arranged that the conducting connection between these rings can be
entirely interrupted.
If this machine be supplied with current through the leads I II III IV, the
rotor will, as is known, acquire a speed that will lag behind the synchronism due
to the frequency of the current by only a small percentage.
The speed can be reduced according to the known method by two different
means, namely either by reducing the frequency of the current supplied, or by
introducing resistances into the branches of the induced part, so that a greater
reduction of speed is required for producing the strength of current necessary for
generating the requisite torque:
This means may no doubt be applicable in cases where the speel is only required
to be temporarily reduced.
But as this arrangement necessarily causes a reluction of the useful effect, it is
not advantageously applicable for continued working at reduced speed.
The reduction of speed can according to the present invention be obtained
without such loss. The action in this case is based upon the following.
If, in Figure 2, during the working the switch J be opened, so that the currents
induced in the rotor can no longer produce a rotary field but only an alternating
one the action of the motor will be as represented by the curve II in Fig. 3. This
curve is produced by marking the speed of the motor as abscissæ and the torque as
ordinates.
The end point chosen for the curve, which by way of example is marked 100,
corresponds with the synchronism due to the frequency of the current supplied to
the stator N, Fig. 2.
As the speed decreases, the torque increases, but only very slowly. It never
attains a considerable amount, and soon passes through a maximum, decreasing
again, until when the speed is reduced to one half that of synchronism, indicated
by the point 50, it again becomes zero.
If the speed be still farther reduced, the torque will become negative, that is to
say, the machine will consume mechanical energy and will produce electrical energy,
being converted from a motor into a generator, but this condition is not a stable
one. If on the other hand the variations of torque be observed from zero speed
upward, the curve I will be obtained.
With low speeds the torque is very great, it decreases with increasing speed and
at a speed equal to one half that of synchronism, it will be zero again; beyond
that point it will become negative again, and the motor becomes a generator.
This peculiar behaviour of a polyphase motor whose induced part has a single
phase winding has been proved by experiment.
In practically applying the invention, the switch J, Fig. 2 is turned into the
open position when it is desired to cause the motor to run at half speed, thereby
converting the winding of the induced part into a single-phase winding, while when
the motor is required to run at full speed, the switch J is closed so as to convert
the winding into a polyphase one. The consumption of energy is in both cases
about equal while the useful effect of the motor is also only subject to a slight
variation. With a
single phase alternate current motor having rotary phase
winding for the induced part arranged as shewn by the accompanying drawing

page_text: where J is the cut out switch on the one branch of the three leads, the conditions
are quite similar to what has been stated with reference to polyphase motors; in
this case also, the same movement of the switch serves to work the motor at either
full speed or half speed.
It will be obvious that instead of the switch above described may also be
employed a regulating resistance inserted in the one branch of the induced part of
the motor, by means of which the strength of the current can be reduced to the
desired degree instead of entirely interrupting it.
From what has been stated above it will be seen that the appliance may also
be employed usefully in cases where the machine is required at one time to act as
a motor and at another time as a generator.
If for example a lift be worked by a motor arranged according to the present
invention this can be worked on the single phase system when persons are to be
raised that is to say when the lift is to work slowly, while when goods are to be
raised and the lift is consequently required to run fast, the motor is worked on the
polyphase system. When working in either way, the motor can, in this application,
also be made to act as generator, in which case its braking action is utilised for
causing the lift cage to descend slowly.
If namely, when lowering the cage, the connections of two leads of the outer
winding of the induced part be reversed by a switch the motor will then run in the
contrary direction.
If the winding of the induced part is arranged as a polyphase winding, the speed
of the descending cage will increase until the speed of the motor exceeds that
corresponding to synchronism.
A considerable braking action then ensues, and
the motor then works as generator and supplies curr
The same action will also take place at the half s,
induced part has been converted into a single phase
lead thereof.
speed remains constant, as
to the supply network.
ed, when the winding of the
inding by cutting out the one
Lastly the present invention may be employed in combination with the existing
means for varying the speed of motors, thus for example at Fig. 2 resistances R₁ R,
may be interposed between the branches of the winding of the induced part.
Having now particularly described and ascertained the nature of this inven-
tion and in what manner the same is to be performed we declare that what we
claim is :-
Varying the speed of asynchronous motors for polyphase and single phase
alternating currents while maintaining the torque by converting the polyphase
winding of the induced part into a single phase winding either by cutting out one
or more leads thereof or by reducing the current in such leads by the introduction
of resistances so that the polyphase current assumes the character of a single phase
current substantially as described.
ABEL & IMRAY,
Agents for the Applicants.