Improvements in Safety Devices for Dynamo Electric Machines.
Year: 1899

page_text: Date of Application, 30th Dec., 1899-Accepted, 10th Feb., 1900
COMPLETE SPECIFICATION.
Improvements in Safety Devices for Dynamo Electric Machines.
We, THE BRITISH THOMSON-HOUSTON COMPANY, LIMITED, of 83, Cannon Street,
in the City of London, Electrical Engineers, do hereby declare the nature of our
said invention (which has been communicated to us from abroad by Harold
W. Buck, residing at Schenectady, in the County of Schenectady, State of New
York, United States of America, Electrical Engineers), and in what manner
the same is to be performed, to be particularly described and ascertained in and
by the following statement:-
65
Under certain conditions existing in practice rotary converters or similar
machines are liable to increase abnormally in speed or to run away." The most
serious danger in this respect exists when the rotary converter is inverted in
function, and is fed with direct current to be converted into alternating current.
Under such conditions the character of the load on the alternating-current mains
has a marked effect upon the speed of the machine. Thus, if the load be induc-
tive, the lagging currents which are produced, react upon the field of the machine
and weaken the same. The rotary converter with its field thus weakened speeds
up in the same manner as a direct-current motor with weakened field, and if
the amount of lagging current be sufficient, the speed of the machine may get
beyond control so as to seriously endanger the machine from the action of centri-
fugal force.
It also sometimes happens that the alternating supply mains of a rotary
converter are interrupted. If the direct current end of the rotary converter is
connected to an independent circuit no harm will result. If, however, the direct
current end of the rotary converter be connected with the direct current end of
other rotary converters or indeed with any other source of direct current, the
immediate result is a reversal of current in the direct current side of the rotary
converter which thereupon runs as a direct current motor. If the machine is
provided with a series field winding as usual, the field strength is reduced by
reversal of current through this winding and the machine therefore speeds up.
There are other causes which affect, or may affect, the speed of a rotary con-
verter, but we have mentioned some of the most serious ones, merely by way of
illustration of the difficulties met with in practice.
pur-
It has therefore been found necessary to provide some automatically-actuated
means to interrupt the supply of energy to the rotary converter or other machine,
when its speed rises a predetermined amount above the normal. For this
pose we have provided a device responsive to the frequency of current in the
alternating mains, this device acting through suitable co-operating means to
cut the converter out of circuit or otherwise diminish its speed.
Our invention will be more readily understood in its general nature and details
by reference to the following description, taken in connection with the accom-
panying drawings, while its scope will be particularly pointed out in the
appended claims."
Figs. 1 and 2 represent, respectively, different embodiments of our invention.

page_text: In Fig. 1 we have indicated a rotary converter at R. The machine is supplied
with collector rings 1, 2, 3, which are connected through suitable brushes with
alternating mains 4, 5, 6. Direct-current mains 7, 71 are in a similar manner
connected with the commutator of the converter. We have shown the machine
as supplied with a shunt field winding 8 in the usual manner, and this winding
alone may be used if desired. It will be obvious of course that a series winding
may be added if automatic regulation is desired.
In series with the main 71 of the direct current side of the converter, we have
indicated a circuit-breaker 9, of any suitable form, the tripping lever 10 of the
circuit-breaker being operated by current passing through the coil 1 in a local
circuit. The means for controlling the flow of current in this local circuit con-
sists primarily of two solenoids, one of which, indicated at 12, is connected
across two of the alternating mains, and the other, indicated at 13, connected
across the direct current mains 7, 71. Within the influence of these two solenoids
is located, either a single core, or two cores 14 and 15 rigidly connected together
by a bar 16 as shown. The contact-making lever 17 is suitably pivoted at 18
to a supporting structure, one end of the lever being provided with an elongated
slot the walls of which engage a pin 19 on the bar 16, while its other end carries
a bridging contact 20, arranged to operate in conjunction with the fixed con-
tacts 21 and 22, which form the terminals of the local circuit above referred to.
This local circuit includes a battery 23, or some other suitable source of electro-
motive force, and this battery is in series with the coil 11, which by reason of the
flow of current therein, operates upon the tripping lever 10 of the circuit-breaker 9
and thus opens the direct-current mains 7 and 7 at the fixed contacts 24 and 25,
as will readily be understood. It is of course obvious that the local circuit may
be supplied with current from the direct-current mains 7 and 71 instead of from
the battery 11.
The two solenoids 12 and 13 are so constructed that at normal frequency of
the current in the alternating current mains their effect upon their respective
cores 14 and 15 exactly balance each other, irrespective of fluctuations in voltage,
thus producing no movement of the contact making lever. If from any cause
however, the speed of the machine increases the inductance of the solenoid 12
connected across the alternating mains increases, thus cutting down the current
therein and reducing the pull on its core 14. Since the current in the other
solenoid 13 connected across the direct-current mains remains approximately
constant, the contact-making lever 17 is, when a certain speed has been reached,
caused to move to one side of its central position, and the bridging contact 20
bridges the fixed contacts 21 and 22. The parts are so proportioned that the
bridging contact 20 closes the local circuit when the frequency of current in
the alternating mains reaches a predetermined percentage increase in value, as
for instance 20 per cent. Upon the local circuit being closed, current flows
through the tripping coil 11, thus actuating the circuit-breaker or cut-out 9, and
so opening the direct-current mains, thus shutting off the supply of energy to
the machine and decreasing its speed. If desired, suitable cut-outs or circuit-
breakers may be included in the alternating mains and operated from the local
circuit simultaneously with the operation of the circuit-breaker 9. This latter
construction is desirable in cases where the converter is fed from the alternating
current side, but we have not considered it necessary to illustrate this obvious
change.
In Fig. 2 we have shown a somewhat different arrangement. As before, R
indicates diagrammatically a rotary converter having collector rings 25, 26
and 27, and brushes 28 and 29. The collector rings are connected to alternating
mains 30, 31, 32, while the brushes are connected, one with the direct-current
main 33, and the other through the circuit-breaker or cut-out S with the other
direct current main 34. Both shunt and series field windings may be employed
on the converter if desired, but we have in the drawing shown only the shunt
field 35.

page_text: The circuit-breaker S may be of any suitable construction but as here shown it
consists of a switch-arm 36 suitably pivoted at 37 and making connection with
spring fingers or some other form of fixed contact 38. A spring 39 tends to open
the switch, while a spring actuated latch 40 engages the end of the switch-arm 36
and maintains the circuit closed. The latch 40 is actuated by a magnet 41
included in a normally open circuit, which in this case is shown as connected
across the direct-current mains 33, 34. The terminals of this circuit are brought
to and connected with the fixed contacts 42, 43, which are adapted to be closed
by the bridging contact 44, the latter being controlled or carried by or actuated
from the core 45 of the solenoid 46 connected in circuit across two of the alter-
nating mains, or supplied with current therefrom in any desired manner. The
parts are so arranged that when any amount of current below a predetermined
minimum, flows in the circuit of the solenoid 46 the bridging contact 44 will con-
nect the fixed contacts 42 and 43 and so close the circuit through the magnet 41.
One manner of accomplishing this purpose consists in arranging the core 45
vertically and opposing the attractive force due to current in the solenoid, to
the attraction of gravitation. It is obvious that many other modifications might
be used, such for example as the substitution of the force of a spring for that
of gravity.
Under normal operation of the rotary converter, the current in the solenoid 46
is sufficient to maintain the bridging contact 44 away from its co-operating
fixed contacts. If for any cause, however, the frequency of the current in the
alternating mains were to increase, the inductance of the winding of the sole-
noid would correspondingly increase, thus weakening the current flowing therein
and allowing the bridging contact 44 to descend. Such a proportioning is chosen
that when a predetermined rise in frequency has taken place, the bridging con-
tact 44 will close the circuit through the magnet 41, thus attracting the latch 40
and releasing the end of the switch lever 36 which by means of the actuating
spring 39, operates to open the direct-current connection of the rotary converter,
thus cutting off its supply of energy and reducing its speed. It will of course be
obvious that the circuit controlled by the solenoid 46 may be used to actuate
circuit-breakers or cut-outs placed in the alternating mains, either in addition to
circuit breaker in the direct current system or to the exclusion of the latter, as
may be found desirable.
Having now particularly described and ascertained the nature of our 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 combination of a dynamo electric machine, mains connected thereto
which in normal operation carry alternating currents, a circuit connected across
the alternating mains and responsive to changes of frequency therein and acting
through suitable means to interrupt the supply of energy to the dynamo electric
machine when the electro-motive force in the alternating mains reaches a pre-
determined frequency.
2 The combination of a dynamo electric machine provided with collector rings
and a commutator, alternating current mains connected to the collector rings
and direct current mains to the commutator, a cut-out in one of the mains and
means for controlling said cut-out responsive to changes of frequency of the
alternating electro-motive force but not to changes in value of said electro-
motive force.
3-The combination of a rotary converter, alternating and direct current mains
connected thereto, a cut-out in one of the mains, two solenoids connected across
the alternating and direct current mains respectively, and operating differentially
to control the said cut-out.
4 The combination of a rotary converter, alternating and direct current mains
connected thereto, a cut-out in one of the mains and means for controlling said

page_text: cut-out responsive to changes of frequency of the alternating electro-motive force
but irresponsive to changes in value of said electro-motive force.
5 The combination of a dynamo electric machine provided with collector rings
and a commutator, alternating current mains connected to the collector rings,
direct current mains to the commutator, a cut-out in one of the mains and a
device responsive to frequency of the alternating electro-motive force, for con-
trolling said cut-out.
6-The combination of a dynamo electric machine, alternating current and
direct current mains connected thereto, a cut-out in one of the mains and a device
responsive to frequency of current in the alternating mains, for controlling said
cut-out.
7-The combination of a rotary converter, alternating current and direct cur-
rent mains, circuit interrupting devices for said mains, and means responsive to
the frequency of the electro-motive force impressed on the alternating mains for
controlling the circuit interrupting devices."
8 The combination with a rotary converter, of alternating current and direct
current mains, circuit interrupting devices for said mains, a local circuit for
controlling the circuit interrupting devices, and a circuit responsive to the
frequency of the electro-motive forces on the alternating current mains and
acting to close the local circuit when the frequency of said alternating electro-
motive forces reaches a predetermined value."