Improvements in Electric Motors.
Year: 1897

page_text: Date of Application, 18th Sept., 1897
Complete Specification Left, 17th June, 1898-Accepted, 3rd Sept.,
1898
Improvements in Electric Motors.
I, WILLIAM ROBERT LAKE, of the Firm of Haseltine, Lake & Co., Patent Agents,
45, Southampton Buildings, in the County of Middlesex, do hereby declare the
nature of this invention to be as follows:.
The asynchronous monophase alternate current motors, as heretofore known, are
started by superimposing upon the alternating field an oblique field differing in
phase.
Their field magnet or what is termed the stator (see Polyphase Electric Currents
by S. P. Thompson 1895 page 112) consisting of two groups of coils one of them
.e. the main coils being fed by the alternate monophase current, which is to keep
the motor in motion, whilst it will perform useful work; the other group of
coils say the auxiliary coils receiving a current, which only flows through them,
while the motor is being started and which is out of step with the current in the
main coils.
I have now found out that by inserting an additional resistance into the closed-
coil armature (the rotor, according to the nomenclature introduced by Profr. S. P.
Thompson) and by imparting to the said rotor, or armature, an initial motion
at a very low speed such as may be obtained by turning it by hand, asynchronous
monophase motors can be started, without resorting to the employment of a
rotating magnetic field, only one group of coils, say the main coils, being fed
with current and the motor acting as an asynchronous monophase motor during the
period of starting, just as the motors, fitted with the starting-gears heretofore.in
use, do when they have got up speed and the auxiliary starting coils have been
switched out.
This remark has led me to construct a new improved asynchronous monophase
alternating current motor, which has only one group of coils on the stator, any
auxiliary winding on the stator, intended for the purpose of starting being dis-
pensed with. The monophase motor so constructed, started by means of an
additional resistance to be inserted into the closed-coil armature (rotor) and of
an impulse given by hand, goes on running in the same. direction, in which it
received the first impulse. The said additional resistance must be capable of
admitting gradual reduction so that it may be completely switched out gradually
and without shocks a moment before the motor has got up speed. Under these
conditions the strength of current, during the whole period of starting will at no
moment exceed the value it takes just when the motor begins to turn. I have also
ascertained that the additional resistance, that is to be inserted in the armature
just at the beginning, to cause the motor to start without auxiliary windings on
the stator when a very low speed is imparted to the armature, has a value, which

page_text: nearly corresponds to an effective current double than that absorbed during the
normal running of the motor under the same load. Those conditions do not
materially differ from those which take place during the starting of the
asynchronous monophase motors, fitted with auxiliary coils on the stator as here-
tofore in use.
I, WILLIAM ROBERT LAKE, of the Firm of Haseltine, Lake & Co., Patent Agents,
45, Southampton Buildings, in the County of Middlesex, 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 asynchronous monophase alternate current motors, as heretofore known, are
started by superimposing upon the alternating field an oblique field differing in
phase.
Their field magnet or what is termed the stator (see Polyphase Electric Currents
by Prof. S. P. Thompson 1895 page 112) consists of two groups of coils one of
them .e. the main coils being fed by the alternate monophase current which is
to keep the motor in motion, whilst it will perform useful work; the other group
of coils, say the auxiliary coils, receiving a current, which only flows through them,
while the motor is being started and which is out of step with the current in the
main coils.
I have now found that by inserting an additional resistance, the value of which
can be theoretically ascertained, into the closed coil armature (the rotor, according
to the nomenclature introduced by Profr. S. P. Thompson) and by imparting to
the said rotor, or armature, an initial impulse such as may be obtained by pulling
the belt by hand, asynchronous monophase motors can he started, without resorting
to the employment of a rotating magnetic field, only one group of coils, say the main
coils, being fed with current and the motor acting as an asynchronous monophase
motor during the period of starting, just as the motors, fitted with the starting-
gears heretofore in use, do, when they have got up speed and the auxiliary starting
coils have been switched out..
This remark has led me to construct a new improved asynchronous monophase
alternating current motor, which has only one group of coils on the stator, any
auxiliary winding on the stator, intended for the purpose of starting, being dis-
pensed with. The monophase motor so constructed, started by means of a certain
additional resistance to be inserted into the closed-coil armature or rotor, and of an
impulse given by hand, continues running. in the same direction, in which it
received the first impulse. The said .additional resistance must be capable of
admitting gradual reduction, so that it may be completely switched out gradually
and without shocks a moment before the motor has attained the desired, speed.
Under these conditions the strength of current, during the whole period of starting,
will not at any time exceed the value it takes just when the motor begins to turn.
Trials carried out on large motors, of say up to 100 H.P. and more, running
light, that is to say, without any load except that due to the friction of the belt and
loose pulley, prove that the additional resistance, that is to be inserted in the
armature to cause the motor to start without auxiliary windings on the stator,

page_text: to the effective current absorbed during the normal running of the motor under
the same load. This condition is much more favorable than that which takes place
during the starting of the asynchronous monophase motors, fitted with auxiliary
coils on the stator and acting as polyphase motors while they are being started.
It is shown by theory that said additional resistance has a value given by
the equation:
=
r = 0.414. 2 πn L
wherein n indicates the frequency of the alternate magnetic field, and L the
resistance and the induction of each of the elementary armature coils respectively.
As the value of r 2n L corresponds to the best possible conditions for the starting
of polyphase motors, I may say (as ascertained by the trials alluded to above) that
the additional resistance to be inserted into the armature coils, in order to start
monophase motors, according to my new and improved method, is somewhat lower
than that which would be required, if such motors had to be started under the
influence of a magnetic rotating field.
As on the other hand 2 L is the greatest value that can receive in order
that positive values of the torque correspond to determined values of the revolution
speed, a practical rule for finding the additional resistance to be inserted into the
armature, to start monophase motors according to my improved method, may be
stated as follows::
The value of such resistance must be about half the greatest value (theoretic-
ally 0.414 of the value) which, for given values of the revolution speed, still
admits of the machine acting as a motor instead of as a generating dynamo,
which would be the case if a higher resistance were inserted. This rule fully
agrees with the results of my trials above alluded to.
Instead of starting the rotor or armature by means of an impulse, imparted
thereto by hand, I may obtain the desired result by rendering, at the moment when
the motor is inserted into the circuit the windings of the armature coils un-
symmetrical, this being sufficient to start the armature, provided the same is not
at the beginning, in a position corresponding to a dead point, which inconvenience
can always be avoided.
Such electrical unsymmetry may be obtained by keeping the circuit of one of
the elementary coils of the armature, which is to be closed immediately afterwards,
open at the beginning of the starting period.
In the accompanying drawing,
Figure 1 shows a bipolar motor, whose armature is fitted with a three-phase
winding, also showing how the circuit of one of the coils of the armature may be
kept open at the beginning of the starting period, and
Figure 2 shows the arrangement of the connections of the armature coils.
Suppose S S to represent the field (the stator) windings and S₁, S, Sa (Figure 2)
to be the three elementary coils, forming the winding of the rotor. They unite
at a point at one end, while their free ends are respectively connected to three
rings 1, 2, 3 on which three brushes . y, z, are arranged to bear. The brushes may
be connected with each other either directly or through the resistances R₁. R. Ra
by means of a switch C, each of said resistances being for that purpose connected
at one of its ends with one of the rings 1, 2, 3 and subdivided into several sections
connected with the contacts a, b, c, d, on which the arms of switch C are caused
to slide.
Suppose now one of the three coils (S, for instance which is connected with
ring 2, brush y and additional resistance R) to be kept open by means of cut-
outh just at the moment when by means of cut-out k the motor is inserted into
the circuit P Q of the current, which is to feed the same. The arrangement of
parts may be such (as shown in Figure 1) that, by acting on switch K the motor
circuit P Q is closed at first, whereupon the circuit of the armature coil S,, which
had been momentarily opened, is also closed after a very short time.
2:

page_text: It follows therefrom that when switch K and cut out k are acted upon at first,
the rotor is caused to turn through a part of one revolution at least, owing to the
interruption of the circuit of one of the elementary armature coils, as shown by the
well known experimental researches of Elihu Thomson; if switch K and cut-
outh are further acted upon, so as to close the circuit of coil S₂, the motor begins
to start owing to the effect of the critical resistance of the elementary armature
coils (that is to say of the resistance whose value, I theoretically ascertained to
be 0.414. 2 75 n L) just as if a small impulse had been imparted to the
rotor by hand.
The effect set forth above may be greatly enhanced by causing the resistance of
the armature coils S, and S, to be relatively small at the beginning of the starting
and to attain the critical value (0.414, 2 L) as soon as the switch K has been
acted upon and the circuit of coil S, has been closed by operating cut-out h.
I have stated that the above method is only applicable, under the condition that
the rotor be not in a position corresponding to a dead point at the beginning of the
starting and also that this condition is easily fulfilled. A practical and simple
method of effecting this consists in breaking, by means of cut-out i, (Figure 1) the
circuits of either of the two coils S₁, S, (S, for instance) before starting the motor
and then inserting the motor into the circuit by means of switch K. The circuits.
of the coils S, and S, being then closed and that of coil S, being open, the rotor
will turn through such an angle as to come in a position corresponding to a dead
point with reference to the active coils S, and S. The circuit of coil S₁ being
thereafter closed, the motor will be ready for being started according to the
method set forth above, in as much as the position of the rotor relatively to the
stator will in no case correspond to a dead point with reference to the coils S,
and So, which are the acting ones at the beginning of the starting period, that is to
Bay, when the circuit P Q has been closed by means of switch K and the circuit
coil S has been left momentarily open, as set forth above.
This process also affords the means of reversing the motor as it will be sufficient
for that purpose to break the circuit of coil S3, instead of coil S₁, by means of
cut-out i.
Having now particularly described and ascertained the nature of my said
invention and in what manner the same is to be performed, I declare that what
I claim is:-
-
1. An asynchronous monophase alternate current motor, which can be started
without the employment either of a rotating magnetic field or of any other starting
gear, its stator (the inductor) being fitted with one coil or one set of coils only
(according as the motor is a bipolar or a multipolar one) through which a single
alternate monophase current flows, the same being sufficient not only to keep the
motor in motion, when running, but also to start the same when at rest, if a con-
venient additional resistance be inserted into the closed-coil armature (rotor) and a
small impulse imparted thereto.
2. A method of imparting the initial impulse necessary to start motors con-
structed according to Claim 1, by rendering momentarily unsymmetrical the
arrangement of the rotor coils instead of by hand.
Dated this 17th day of June 1898
HASELTINE, LAKE & Co.,
45, Southampton Buildings, London, W.C.,
Agents for the Applicant.