Improvements relating to the Synchronising of Alternating Current Motors for Electrical Power Transmission.
Year: 1896

page_text: Date of Application, 14th Jan., 1896-Accepted, 2nd May, 1896
COMPLETE SPECIFICATION.
Improvements relating to the Synchronising of Alternating Current
Motors for Electrical Power Transmission.
I, HORACE FIELD PARSHALL, of 38, Parliament Street, Westminster, in the
County of London, Electrical Engineer, do hereby declare the nature of this inven-
tion and in what manner the same is to be performed, to be particularly described
and ascertained in and by the following statement:-.
The object of my invention is to maintain synchronism between the generators
and motors in a system for the transmission of electrical power by means of
alternating currents, and has special reference to traction systems or other power
systems in which the load fluctuates.
Systems of electric traction are subject to many fluctuations in load conditions,
and short or approximately short circuits are liable to occur when a number of cars
are started simultaneously when the power output far exceeds the normal capacity
of the machines. The present improvement is intended for application in such
systems of power transmission and distribution as that described in British Patent
No. 7668, dated April 16th, 1895, granted to James Devonshire.
This
The disadvantage experienced in the operation of such a system is that in cases
of short circuiting or excessive overload the synchronous motor is not sufficiently
excited to maintain proper phase relation with the alternating current generator
with which it is electrically connected; the consequence is that the machine falls
out of synchronism, being unable to meet the demand of power, and consequently
the generator is short-circuited and the system rendered powerless until the
synchronous motor is again brought to synchronism by some suitable means.
requires considerable time and occasions interruptions in the service which render
such a system inefficient and objectionable. In practice it is found desirable to
excite the shunt coil of the synchronous motor from the generator to which it is
furnishing mechanical power, and inasmuch as the voltage of the dynamo falls off in
the case of excessive load, and likewise the excitation of the synchronous motor;
the objection from excessive loading is greatly increased, since to maintain the
proper phase relation between motor and generator for increasing load the
excitation of the motor should be increased with the increasing load. This could
be accomplished by hand were not the fluctuations of load so rapid and frequent.
The proper phase relation for large loads can only be maintained by exciting the
fields of the synchronous motor to a degree which is excessive as compared with
the average excitation required for the average work. It is doubtful, however,
whether the excitation maintained in this way can be made sufficient to compensate
for the maximum variations that occur in the ordinary working of tramway
systems; further this method is inefficient since the loss of energy in the field
magnets (average working conditions considered) is so excessive that the magnets
have to be excessively large and are therefore wasteful in the use of material.
There is a further objection to maintaining the excitation excessive, as compared
with the average load, and this is that at average work the currents in the system
are displaced in phase so that they exceed their proper amount, and hence occurs
an excessive loss in the armature circuits and in the core of the motor and generator
and in the transmitting mains.
Recognising fully the disadvantages in the system as heretofore carried out, it is
the object of the present invention to automatically increase the excitation of the
synchronous motor to correspond exactly and at all times with the load put upon it

page_text: by the dynamo it is driving. When the excitation of the synchronous motor is so
varied, the power transmitted whether great or small is transmitted at a maximum
efficiency, and the ability of the system to transmit energy is also a maximum, that
is, the system is adjusted to transmit the greatest possible amount of power..
To accomplish the above according to my present invention I use two windings
on the field magnets of the synchronous motor. The current of one of the
windings will remain more or less constant and not proportional to the output of
the commutating dynamo, and in the other winding I take a current equal to or a
fraction of the current output of the commutating dynamo; this second winding is
in series with the commutating dynamo armature, i.e., placed in the working circuit
of the dynamo armature.
Referring to the accompanying drawing in further explanation of my invention,
Fig. 1 is a diagram illustrating a commutating dynamo driven by a synchronous.
motor, the whole of which may be termed a complete unit, the function of which is
to receive energy from the alternating current circuits and transform and distribute
same by means of the commutating machine into the continuous current circuits.
Fig. 2 illustrates diagrammatically a traction system in which my invention is
embodied.
Referring to Fig. 1, B is the armature of the synchronous motor and shews the
transmitting circuits from the dynamos connected therewith. A is the armature of
the dynamo driven by the synchronous motor B. D and S are the usual windings
applied to continuous-current dynamos used for traction purposes. The arrange-
ment of these windings is unimportant as affecting the functions of the coils
involved in the present invention and now to be explained.
According to my invention upon the magnets of the synchronous motor I wind a
set of magnet coils F which are in series with the driven armature A and dependent
for the current upon the current output of the said armature A of the commutating
dynamo. The series coils S of the dynamo are shown connected through the
coils F of the synchronous motor, but this method of connecting so far as the series
coils of the driven dynamo are concerned is irrelative to the present invention, since
these coils may be connected in parallel relation to the series coils of the synchronous
motor or may be omitted altogether, this feature being dependent upon the design
of the commutating dynamo employed and the conditions of supply required in the
circuits to which it is connected.
The coils F1 of the synchronous motor are for the purpose of maintaining what
I term the initial excitation of the synchronous motor, the value of the amount of
this excitation being determined with reference to the voltage in the alternating
current line C, and not with reference to the current output of the dynamo A. This
circuit F is the usual and more or less constant exciting circuit for synchronizing
motors, and is the one mentioned when speaking of the excessive waste in the field
magnet circuits.
Referring to Fig. 2, G is the prime generating station, and G and G' two
sub-stations. T, Ts are two continuous current generators supplying current to
the consumers' circuits L'. R1, R2 are two alternating current generators suitably
excited. C represents the transmitting mains, A1, A2, A3, A are the distributing
or commutating dynamos previously described, and B', B2, B3, B4 the synchronous
motors driving same.
The alternating currents are transmitted through the mains C to the synchronous
motors B', B2, B3, B4, the connections of each unit being made as in Fig. 1. The
field circuit is arranged as before described, and the dynamo of each unit is connected
to the consumers' circuits L, L' as shewn, the consumers' circuits in this case being
the trolley wire L and the earth return L.
Assuming overload to have occurred in the consumers' circuits, the phase relation
between the synchronous motors B1, B2, B3, B4 with respect to the alternating
current generators R1, R2 supplying energy to them through the line C is properly
maintained by means of the coils F on the synchronous motor magnets the current
in which is. dependent upon the current output of the dynamos A¹, A³, A³, A

page_text: respectively, driven by these synchronous motors. This truly maintains the phase
relation with the advantages above described.
It will be obvious that the above described system of synchronising may be
utilised to excite one synchronous motor from one or more driven commutating
machines, or alternatively to excite two synchronous motors driving one commutating
machine.
Having now particularly described and ascertained the nature of this inven
tion and in what manner the same is to be performed, I declare that what I
claim is :-
1. The method of automatically exciting a synchronous motor from the commu-
tating machine or machines driven thereby, substantially as for the purpose
described.
2. For the purpose of maintaining synchronism between synchronous motors and
their source of alternating current supply the use of two windings on the field
magnets of the synchronous motor, the current in one winding being more or less
constant, and that in the other winding varying proportionately with the output of
the driven machine, substantially as above set forth.