Improvements in the Method of and Means for Braking Alternating Current Induction Motors.
Year: 1897

page_text: Date of Application, 14th Dec., 1897-Accepted, 29th Jan., 1898
COMPLETE SPECIFICATION.
Improvements in the Method of and Means for Braking Alternating
Current Induction Motors.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of this
invention (which has been communicated to us from abroad by Edwin W. Rice,
Junior, of Schenectady, in the County of Schenectady, State of New York, United
States of America) and in what manner the same is to be performed, to be particu-
larly described and ascertained in and by the following statement:-
The present invention comprises a novel method of and means for braking an
electric car or other mechanism which is normally driven by one or more alternat-
ing current induction motors.
In continuous current systems of electric traction the cars are now often braked
by cutting off the power circuit from the motors, reversing the motors, and con-
necting them in a closed local circuit in which the motors are driven as generators
sending current through a resistance or through a resistance and brake magnet,
as engineers who have followed the art will understand without further explanation.
Efforts are now being made to adopt alternating current induction motors to
electric traction service and one of the objections sometimes raised is that these
motors cannot be used for braking the cars as can direct current motors in a con-
tinuous current system. The object of the invention is to avoid this objection and
to provide means whereby the cars can be braked by driving the induction machines
as generators from the car axles. To accomplish this, a switch or controller is
provided for cutting off the power circuit from the motor or motors and for con-
necting them in a local circuit, so that the induction machines, no longer receiving
power from the line, will be driven from the car axles. In order that the induction
machines may work properly as generators our correspondent also connects to
the local circuit what he terms a phase setter," consisting ordinarily (though
not in all cases necessarily) of a synchronous machine driven by the car and having
a continuous current field excitation.
"
Alternating current induction motors will work as generators when driven above
synchronism and when the proper phase relation is maintained in the circuit.
connecting the phase setter and motors. For example, if an induction motor be
coupled to an ordinary alternating generator of given frequency, and the current
in the connecting circuit is leading and the induction motor be driven at a speed
above synchronism it will act as a generator and deliver energy to the circuit
instead of absorbing it therefrom. Accordingly in a braking system arranged in
accordance with this invention the synchronous and induction machines will be
so designed or geared that the induction machine will be driven above synchronism
at all times no matter what the speed of the car may be. The synchronous
machine, of course, will set the phases of the circuit and the induction machine
will be driven above synchronism either when driven at the same speed as the
synchronous machine but given a larger number of poles, or if the synchronous
and induction machines have the same number of poles and the gearing be such
that the induction machine travels faster. The synchronous machine, since it

page_text: simply furnishes magnetizing current,, needs have no great power, but will be
ordinarily designed to give a voltage considerably higher than that on the power
circuit, so that the induction machines when driven as generators may consume
a large amount of energy quickly...
In the accompanying drawings Fig. 1 illustrates, so far as is necessary, a car
with its electrical equipment; Figs. 2 and 3 are diagrams of circuits; Figs. 4
and 5 illustrate certain features of the induction and synchronous machines, and
Fig. 6 is a detail view in plan hereinafter referred to.
In the drawings a three-phase alternating system is shown, though as engineers
will readily understand, alternating current motors of any other number of phases
may be operated in a similar manner. In Fig. 1, the mains of the three-phase
system are represented at A B C, from which current is carried to the car by
trolleys or collectors T T T2, of any desired description. Controllers D D' are
mounted at opposite ends of the car and will be constructed so as to make the
necessary circuit changes described hereinafter. Engineers will readily understand
how to design the controllers, and no attempt is made herein to indicate more than
the circuit changes which should be made. Alternating current induction motors
are mounted on the truck and geared to the axles.. Also, on the car or truck is
mounted a synchronous machine S, which may resemble in construction ordinary
synchronous. generators, having a field winding excited from any convenient
source of continuous current. A battery E is shown for this purpose having in its
circuit a regulating rheostat e.
The particular manner in which the synchronous machine is mounted and geared
is unessential, though positive gearing is preferable. Fig. 6 shows this part of
the construction more clearly. In this figure P is one of the car axles to which one
of the induction motors Mi is geared by a pinion and gear wheel. Our corres-
pondent prefers, though it is not essential, to connect the armature shaft of the
synchronous machine to the gear wheel t¹ by a clutch 2, or to employ some
equivalent arrangement. The object is to allow the synchronous machine to
remain at rest at all times, except. when it is required to furnish an exciting current.
In this arrangement the induction and synchronous machines may be given the
same number of poles and the synchronous machine geared so as to run slower than
the inductive machine, as is indicated by the relative diameters of gear wheels t t.
In Fig. 2 the mains A B C and trolleys T T T are represented as before.. The
trolleys are connected with cables I I¹ I" and two induction motors M M have their
primary coils connected to the cables while the secondaries of the motors are closed
through resistances R R1 R2 in any ordinary manner. A switch J is shown for
varying the resistances in the secondary circuits of both motors simultaneously.
In this diagram the motors are shown coupled to the power circuit
for normally propelling the car. Of course, in practice it is
at all essential that the different motors (in an equipment where a
number of motors arc used) be always coupled in multiple. They
may be coupled in tandem or preferably for speed regulation, be coupled at times
in tandem and at other times in multiple by what is now known in the art as
tandem-multiple regulation.
not
In Fig. 3 the mains and trolleys are represented as before, but the trolley circuit
is cut off from the induction machine by the switch K. The synchronous
machine S, also a' three phase machine, is connected with the cables I I¹ I² and
resistances N N1 N2 preferably non-inductive, have also been brought into circuit
by the switch 0; so that the induction machines M M¹ are coupled in a local
circuit with the resistances and the synchronous machine. The motors and
synchronous machine are now all driven from the axles and the synchronous
machine acts as a phase-setter for the induction-machine, so that the latter will
generate current and expend energy in the local circuit through the resistance.
As already pointed out it is necessary that the induction machine for braking
purposes be driven above synchronous speed, as fixed by the phase-setting machine..
This result may be secured either by properly adjusting the relative speeds of the

page_text: synchronous, and induction machine, or by varying the number of poles which the
different machines have, or by both methods. As shown in Figs. 1 and 6 the
gearing is such that the synchronous machine driven at a slower rate, of speed
than the induction machines, and therefore, the induction machine will rotate
above synchronism, though both types of machine have the same number., of poles.
In other cases where it is more convenient to have all the machines driven at the
same speed induction machines will be employed of a certain number of poles and
synchronous machines of a smaller number of poles. This is indicated in Figs., 4
and 5, where there is shown an induction machine M³, having ten poles and a
synchronous machine M, having eight poles. Of course, this ratio may be widely
varied in different installations to suit the requirements in each case.
The syn-
chronous machine, will ordinarily be built of a voltage sufliciently greater than the
line voltage to enable the induction machines to consume the energy necessary
for promptly braking the car; but the synchronous machine need not be a large
one, since its function primarily is to supply magnetizing current to the induction
machine or machines.
Having now particularly described and ascertained the nature of the said
invention (as communicated to us from abroad) and in what manner the same is
to be performed, we declare that what we claim is:-
1. The method of braking the load connected to an induction motor, which
consists in connecting the motor with a circuit capable of absorbing energy, and
passing through its inducing member an alternating magnetizing current, of a
frequency such that the motor will act as an induction generator.
2. The combination of one or more induction machines normally supplied with
power and working as motors with a phase-setter and means for cutting off the
power circuit and connecting a circuit capable of absorbing energy and the phase-
setter to the terminals of the induction machine, whereby the induction machine
will be driven as a generator, and brake the mechanism to which it is connected,
as described.
3. The combination of one or more induction motors for propelling a car or
vehicle, with a phase-setting machine and means for cutting off the power supply,
and connecting a circuit capable of absorbing energy, and the phase setting
machine, to the primaries of said motor, the whole being so arranged that the
induction motor will be driven above synchronism and act as a generator, for the
purpose set forth.
4. The combination of one or more induction motors, a driven mechanism, a
synchronous machine, means for cutting off the power supply and connecting the
induction and synchronous machines in a local circuit, for braking purposes, as
set forth.
5. The combination in an electric traction system, of a car, a multiphase induc-
tion motor for normally driving the car, a synchronous machine carried on the car,
means for cutting off the power circuit from the induction machine, and for con-
necting the synchronous machine, the induction machine and a non-inductive
resistance in multiple in a closed local circuit, the whole being so arranged that
upon braking the car the synchronous and induction machines are both driven by
the car axles, and the induction machine driven above synchronism, for the purpose
set forth.
6. The combination in an electric traction system, of an induction motor for
propelling the car, a synchronous machine of comparatively small power but of
high voltage relatively to the voltage of the power circuit, means for cutting off
the power circuit, and for coupling the synchronous and induction machines in a
closed local circuit, the synchronous and induction machines being so designed
and arranged that the induction machine is driven above synchronism as a gener-
ator delivering energy to the local circuit, as set forth.
7. The combination in an electric traction system of means for braking an

page_text: electrically driven car comprising an alternating current motor geared to the axle
of the car, and having its primary winding closed through a non-inductive resist-
ance, and means for causing the motor to act as an induction generator driven
above synchronism.
8. The combination in an alternating current traction system of an alternating
current induction motor, having its primary winding connected in a local circuit
with a phase-setting machine and with a non-inductive resistance, the said motor
being so arranged as to be driven by the car above synchronism, as set forth.
Dated the 14th day of December 1897.
J. C. CHAPMAN,
Chartered Patent Agent, 70, Chancery Lane, London, W.C.