Improvements in Alternating Current Motors.
Year: 1897

page_text: WALTER LANGDON-DAVIES, 16, Red Lion Street, Clerkenwell, E.C., Engineer,
do hereby declare the nature of this invention to be as follows:-
My invention is for the purpose of increasing the ratio between starting torque
and the current taken at starting.
To attain this end, I wind a coil of many turns having great self-induction, to
act as the running coil at starting, and use the whole or part of the normal run-
ning coil with resistance in series as the starting coil at starting. By this means
the angle of lag between the two currents may be increased. When the motor has
sufficiently run up, I switch over to the normal running coil only.
Sometimes instead of winding a coil of many turns, I wind the running coil
with two or more wires in parallel which I switch into series for starting, and into
parallel for running on single alternating field.
By running coil, I mean the coil which energises the motor when running on
simple alternating field.
By starting coil, I mean a coil which produces a field differing in phaze from
that of the running coil, for obtaining a rotation of the field for starting.
25th March 1897
W. LANGDON-DAVIES.
Improvements in Alternating Current Motors.
I, WALTER LANGDON-DAVIES, of 16, Red Lion Street, Clerkenwell, in the City
of London, 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:
In single phase motors as now ordinarily constructed, the energising current
passes when the motor is running through a single set of coils which may be in
series.
These are called the running coils. They have as low resistance as possible.
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page_text: To start it, another set of coils which may also be in series with each other, are
used in addition to the running coils.
These are called the starting coils and are set at an angle to the running coils;
they are wound with fewer turns than the running, coils, and a resistance or con-
denser is added in series with them to reduce the lag (behind the volts) of the
current flowing in them. They are then joined in parallel to the running coils.
The current therefore lags behind the volts less in the starting coils than in the
running ones, and thus a rotating field is obtained by which the motor is started.
A choking coil is sometimes put in series with the running coils to increase their
angle of lag.
This method of starting has this inconvenience, that the running coil is put
straight on to the mains when the machine is at rest under which circumstances
it takes a large and wasteful amount of current, or if a choking coil is added there
is a loss of efficiency. This is a necessity if the running coils are properly wound
for running.
To avoid this inconvenience I use the whole or part of the running coils when
starting with a resistance in series, and as the starting coils, wind other coils with
numerous turns to produce the required difference of phase.
The running coil now lags less than the starting coil, and the rotating field is
more economically obtained.
When the motor has attained sufficient speed to allow of its being driven
synchronously, current is cut off from the starting coils and the resistance cut out
of the running coil circuit.
Figure 1 shews in end view an example of the field magnet ring of an electric
motor thus wound with starting and running coils and having switch connections
shewn diagrammatically for putting a resistance into the circuit of the running
coils at the time of starting.
The field magnet ring is shewn to be formed as heretofore of numerous thin
rings of sheet iron held together side by side, and each having slots cut through
it for the windings to pass through.
The armature may be of the well known squirrel cage, or of other suitable con-
struction.
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:.
A are the running coils with the ends of the winding carried to terminals A³, B*.
B. are the starting coils of more numerous turns, and usually of thinner wire to
occupy less space, with the ends of the windings carried to the terminals A*, C*.,
The terminal C is coupled to the contact C₁ of the switch mechanism, the ter-
minal B is coupled to the contact B₁ and this contact is coupled through a resist-
ance R with the contact B².
The switch lever D is coupled with one conductor E of a supply main and the
terminal A is coupled to the other conductor E¹.
For starting the motor the switch lever D is turned into the position shown in
the drawing, current then passes through the resistance R and running coils A at
the same time that it also passes through the starting coils B.
When the motor has attained sufficient speed' to allow of its being driven
synchronously the switch lever is moved over into its opposite position, current
then passes direct through the coils A. without passing through the resistance and
no current is passed through the starting coils B.
Or in place of the above arrangement I may wind the running coils with two
or more wires in parallel which may be switched into series for starting. This
arrangement is shewn, at diagram Figure 2.
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:
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1. An alternating current motor in which the starting coils are of numerous
turns and relatively high self induction as compared with the running coils and

page_text: in which a resistance is added at starting to the circuit through the running coils
substantially as described.
2. An alternating current motor in which the running coils have two or more
wires which may be switched into series for starting and put in parallel when the
motor has run up.
Dated this 28th day of December 1807
W. LANGDON-DAVIES.