Improvements in Devices for Changing the Frequency of Alternating Currents in Systems of Electrical Distribution.
Year: 1899

page_text: Date of Application, 14th Mar., 1899-Accepted, 22nd Apr., 1899
Improvements in Devices for Changing the Frequency of Alternating
Currents in Systems of Electrical Distribution.
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 Charles P.
Steinmetz, residing at Schenectady, in the County of Schenectady, State of New
York, United States of America, Electrical Engineer; and in what manner the
same is to be performed to be particularly described and ascertained in and by
the following statement:-
―
The present invention relates to a certain novel arrangement for changing
the frequency of alternating electric currents of one or a plurality of phases,
the object being to take currents from any distributing system, or branch of a
system on which the currents have a given frequency, and to efficiently convert
these currents into other currents having a frequency different from that of the
currents supplied.
A system of distribution is illustrated in the accompanying drawing, showing
a typical embodiment of the invention, the system shown being a three-phase
system, though, as above stated, the invention is equally applicable to currents
of any other phase relation. a, b, c, represent distributing mains connected with
a source of three-phase currents in any usual manner. An induction or trans-
former motor M, has its primary coils P, connected directly or through trans-
formers, with the distributing mains. These coils are shown in the drawing
connected in delta, but this, of course, is an immaterial matter, the Y connection
of the coils being equally good. The secondary of the motor, S, is connected
to collector rings or brushes, as indicated, and to a circuit a¹, b, c, and it is also
geared so as to run in unison with a second machine of the synchronous type, G.
This may be accomplished by mounting the rotating members of the two machines
on the same shaft, or by any desired form of intermediate gearing.
A belt B, is indicated in the drawing, passing it around pulleys p, p, on the
armature shafts of the machines. The synchronous machine, G, is a three-phase
machine having coils connected in Y or delta and with their terminals coupled
to the circuit a, b, c, in multiple with the secondary of the motor M. The
field circuit of the machine G, is supplied with current from a direct current
exciter E, shown shunt wound, and provided with a rheostat R, for regulating
the exciter, and consequently the field strength of the main machine. The
circuit a, b, c, is shown supplying group of lamps L, through trans-
formers T, T, T. This, however, is merely to represent that the secondary
circuit may feed translating devices of any desired description. In practical
installations where the frequency is reduced to a comparatively small number
of cycles, the circuits a, b, c, will be better adapted for operating motors, while
the lamps, where lighting is to be done, would be supplied from the high fre-
quency mains a, b, c. An induction motor of any ordinary or desired
construction, is shown having its primary coils connected to mains a¹, b, c¹

page_text: With this arrangement, the motor M will consume energy which is expended
in driving the machine as a generator, or, in other words, the machine Gacts
as a mechanical load or brake upon the induction motor maintaining the speed
of rotation of the secondary member of the motor below that corresponding with
the frequency, of the primary currents. Hence, as is well known, the frequency
of the currents induced in the secondary member of the motor will be equal to
the difference between the frequency of the primary currents and the frequency
with which the secondary circuit is rotated. Since generator G, however, is
connected in multiple with the secondary, of the motor, the mechanical energy
expended in driving the generator is reconverted into electrical energy in the
form of alternating currents having the same frequency as those induced in the
secondary member of the motor and supplied to the same circuit.
It will be seen that there is a transfer of mechanical energy from the motor M
to the generator G, along the belt B, and that this energy appears as current
of the lower frequency at the terminals of the generator G. Though this transfer
of energy is shown as taking place along a belt, it is evident that it could be
accomplished in a variety of ways.
Hence, it will be understood, that all the energy supplied to the motor (dis-
regarding the losses in the machines themselves) is supplied to the secondary
circuit a, b, c, at the changed frequency. The number of poles of the gener-
ator G will, of course, be so proportioned to the speed at which it is to rotate
that the frequency of the currents generated corresponds with the frequency of
the induced currents in the secondary of the motor. Assume, for example, that
the gearing between the machines is such that both rotate at the same speed,
and that it is desired to reduce the frequency of the initial currents by one half.
Then the motor would be rotated at one half the speed corresponding with the
initial frequency, and the poles of the generator G would be such that when
driven at a corresponding speed the generator also delivers to the circuit a¹, b', c¹,
currents of half the frequency maintained by the system. a, b, c. The motor M¹
would then receive currents of half frequency and would tend to rotate at the
same speed as motor M when geared to generator G. The presence of the syn-
chronizer not only reconverts the mechanical energy consumed by the motor M
into electrical energy, but it also makes the motor stable. Were the synchronizer,
or some other equivalent means, not used for controlling the speed of the motor
it would be impracticable to maintain it at half speed, but when coupled in
multiple with the synchronizer, as explained, the induction motor will run rigidly
at half frequency, for with increasing speed the frequency of the currents induced
in the secondary of the motor would decrease while the frequency of the cur-
rents generated by the generator would tend to increase, hence the synchronizer
will get into step at half frequency and will control the speed. The synchronous
machine, while described as a generator, will not always work as such, but may
occasionally work as a motor, as for example, when no external energy is
consumed in the low frequency circuit.
We do not limit ourselves to the specific forms and connections shown and
described, as it is evident that various changes may be made without departing
from the scope of the invention.
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 transformer or induction motor, and a synchronous
generator geared to run in unison therewith, with a source of alternating cur-
rents of given frequency supplied to the primary of the motor, and a circuit,
for supplying current to translating devices, connected in 'multiple with the
generator terminals, and with the secondary of the motor.
2. The combination of a transformer motor, a synchronous dynamo electric

page_text: machine geared to run in unison with the motor, and reconverting the mechan-
ical energy expended in driving the machine into the secondary circuit of the
motor in the form of alternating currents of the same frequency as the currents
induced in the secondary of the motor, and one or more induction motors con-
nected, to the circuit leading from the synchronous machine or motor, as set
forth.
3. The combination of an induction or transformer motor having its primary
and secondary winding connected to circuits of different frequencies, a syn-
chronous machine geared to and receiving power from the induction motor and
having its terminals connected to one of the first named circuits, so as to return
the energy to the electrical system expended in driving the synchronous machine.
4. The combination of a system of electrical distribution, of a frequency
changer connected thereto, consisting of two dynamo-electric machines, both
receiving energy from said system, one of said machines being of the induction
type and provided with an inducing and induced member, means from creating
a rotary field in the inducing member, leads from the induced member to a
distribution system, and means whereby the speed of the said machine is so
modified by the action of the other machine that the frequency of the current
in the secondary leads is different from that which would normally be produced
therein.
.
5. A frequency changer composed of two dynamo-electric machines, one being
of the induction type, so connected as to be supplied with energy from a system
of distribution, connections from one of the machines to a secondary system,
and means wherely one of the machines is so modified by the action of the
other, that the current induced in its secondary member, and fed therefrom to
the secondary system, is of a frequency different from the frequency of the
current which would normally be induced therein.
6. A frequency changer composed of two dynamo-electric machines, one being
of the rotary field induction type, so connected as to supply energy from a
system of distribution, connections from one of the machines to a secondary
system, and means whereby one of the machines is so modified by the action of
the other, that the current induced in its secondary member, and fed therefrom
to the secondary system, is of a frequency different from the frequency of the
current which would normally be induced therein.
Tata L141
1000