Improvements in Controlling Electric Motors.
Year: 1899

page_text: Date of Application, 30th Dec., 1899-Accepted, 3rd Feb., 1900
Improvements in Controlling Electric Motors.
We, THE BRITISHI 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 Maxwell W.
Day, 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, we declare that what we claim is:-
This invention relates to systems for controlling electric motors, the object being
to permit motors of standard types of construction to be operated at variable
speeds. In carrying out our invention we carry through the neighborhood in
which the motors are installed or located, three distributing wires, two of which
connect with the terminals of the generator of the supply system, and the third
of which is a compensating or equalizing wire corresponding in some respects to
the neutral wire of a three-wire supply system. Between this wire and one side
of the system we interpose an auxiliary generator, which acts as a potential
equalizer and balances the load on the two sides of the system, acting either as a
generator or motor, according to the inequality of balance. This auxiliary
generator may in some cases be installed at the same station as the supply
generator, and may be bolted thereto, or it may in some cases be located at a
distant point and be driven by current derived from the system or by an indepen-
dent prime mover. The potential of this auxiliary generator may be adjusted to
equal that of the main generator, or it may in some cases he less than that of the
main generator The motors supplied by the system are installed so that in the
normal condition of load, where all the motors are operating at one speed, the load
will be equally distributed between the two sides of the system. The switch is so
arranged that the motors may be coupled between the compensating wire and one
of the outside wires or between the compensating wire and either of the outside
wires, thereby imposing a different voltage and therefore a different speed on
the motors. The motor connections are so arranged that when all the motors
are operated at the lower speed, the load will be equally divided between the two
sides of the system; that is to say, part of the motors will be between one line
wire and the compensating wire, and a part between the other line wire and the
compensating wire. When however all the motors on one side of the system and
only part of the motors on the other side are operating at the higher speed, the
balance of load will be destroyed and the auxiliary generator will act as a com-
pensator, either by making up for the excess of load on the side on which it is
located by acting as a generator, or by absorbing the deficiency of load by acting
as a motor and helping to drive the main generator. Obviously therefore the
maximum range of inequality in a system of this kind necessary to compensate
will be one-fourth the capacity of the driven motors; and by making an auxiliary
generator of this capacity, all or any number of the motors may be operated at
full or fractional speed as desired.
A further range of variation in speed may be provided for by employing a
rheostat in the field-magnet circuit of each motor.
Our invention therefore comprises a system of control for electric motors, having
a main generator, and two distributing wires, and a third wire of intermediate
potential between which and one of the outside motors is inserted a compensating

page_text: device, to maintain the balance on the two sides of the system. It comprises
also a system of the character just noted, in which the motors are connected by
switches so that they may be put in circuit with the high-potential or low-
potential distributing wires and operated at different speeds. It comprises
also such a system of distribution, in which with certain speed adjustments, the
motors will be distributed between the two sides of the system so as to maintain a
balance, and when the balance is destroyed, the auxiliary generator will act as
a compensator to restore the balance.
Referring now to the drawings, Fig. 1 is a diagram showing a system embody-
ng our improvements. Figs. 2, 3, 4 and 5 are diagrams showing different condi-
tions of the motor distribution circuit.
1 represents a supply generator located at the central station or any other
suitable point from which distribution is desired. 2 and 3 are distribution wires
'leading from such generator through the neighborhood in which motors or other
translating devices are to be installed. We prefer to operate with shunt-wound
generators and motors, though our improvements are not necessarily limited
thereto, and may be employed in connection with motors of other types.
Between one of the main distributing wires and the third wire, 4, we interpose
an auxiliary generator 5; this forms of the system, at least at the point at which
the motor distribution is installed, a three-wire system. In the diagram we have
shown the auxiliary generator as located at the central station and as belted to
the main generator; this however is not at all necessary, as will readily be
appreciated by those familiar with the art, since the auxiliary generator may be
independently driven and may be located elsewhere than at the generating station.
The function of the auxiliary generator is to maintain an equality of balance on
the two sides of the system. The driven motors 6, 6ª, 6b, 6 have their service
wires connected with the system 2, 3, 4, so that half the load is between the
wires 2 and 4, and half between the wires 3 and 4. Thus, as shown in Fig. 1,
each motor is provided with a three-blade switch adapted to engage the ter-
minals 7, 7, 7 or 8, 8ª, Sh. In the former condition, the motor is connected to
the high-potential mains 2 and 3. It will be evident however that when the switch
is thrown so as to engage the contacts 8, 8ª, 8b, the voltage will be reduced to that
between the mains 3 and 4, and the motor will therefore be driven at fractional
speed. We prefer to operate the system so that the voltage between the mains
and will be half that between the mains 2 and 3, though this is not absolutely
essential, as there may be a greater or less difference. In the former case but two
motor speeds would be available, namely, half and full speed, besides the small
range of difference afforded by the field-regulating rheostat 9; whereas in the
latter a three-step speed would be possible, provided of course a suitable switch
capable of connecting the motor alternately between the mains 2 and 3, 2 and 4,
and 3 and 4, be provided. As shown in Fig. 1, the switches are thrown so that all
the motors are connected between the mains 2 and 3, and are therefore in position
to operate at full speed. If however all the switches were thrown to the open sets
of contacts, it will be evident that motors 6, 6ª will be served by the mains 3
and 4, whereas the motors 6b, 6 will be served by the mains 2 and 4. Whatever 4
be the number of motors the distribution of their contacts should be such as to
maintain this relation, so that the load will be as nearly as possible divided between
the two sets of supply wires.
The different conditions of service are exemplified in Figs. 2, 3, 4 and 5. In
Fig. 2 all four motors (5, 6, 6, 6° are operating at full speed, and with their 5
terminals connected across the high potential mains 2, 3. The auxiliary generator
is on open circuit, and is therefore doing no work. In Fig. 3 all four motors are
shown as operating at half speed, and according to the principle of distribution
herein before described, the load is equally divided on the two sides of the system
and the auxiliary generator 5 is again doing no work. In Fig. 4 the motor C is
operating at full speed, and all the others at half speed. The load here is unequally
divided on the two sides of the system, and the equalizing or compensating agent 5

page_text: acts as a generator to supply one motor. In Fig. 5 the load is again equally
divided, the excess of load being on the side of the system in which the com-
pensating agent is not located, the motors 6, 6ª, 6b operating at half speed and 6°
at full speed. In this case the auxiliary generator acts as a motor and leads the
side of the system in which it is connected sufficiently to compensate for the
inequality of balance and in an organization such as shown in Fig. 1 help to drive
the main generator.
Supposing the potential of the auxiliary generator to be half that of the main
generator, its capacity need be only one-fourth that of the motors installed. Thus,
if each of the four motors installed have a capacity of 10 kilowatts, it would be
sufficient to give the auxiliary generator a capacity of 10 kilowatts. In this case
however where more than two steps in the speed are provided for, as for example,
in the case where the potential of the generator 5 would be fixed at 100 volts and
that of the generator 1 at 250 volts, the auxiliary generator should be slightly
larger in capacity, in the particular case instanced having four-tenths of the
total capacity of the motors.
In an organization of the character described herein, motors of the standard
type and of cheap construction may he installed and operated at variable speeds,
either on a circuit supplied from a central station, or from a local generating
circuit. A system of the kind herein described is particularly useful for operating
machine tools where a system is already installed at some standard voltage, and
where it may be desired to run the machines at a variable speed without incurring
the comparatively great expense of variable-speed motors."
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. A system of control for electric motors, comprising a three-wire system, one
wire being of lower potential than the others, motors distributed between the two
sides of the system, a potential-equalizing device between two of the wires of the
system and means for connecting the motors between the wires having the greater
potential difference.
2. A system of electric motor control, comprising a three-wire distributing
system, means for connecting the motors between the high-potential and low-
potential sides of the system, and an auxiliary, generator between one of the high-
potential wires and the neutral or equalizing wire.
3. A system of electric motor control, comprising a three-wire distributing
system, means for connecting the motors between the high-potential and low-
potential sides of the system, an auxiliary generator between one of the high-
potential wires and the neutral or equalizing wire, and switches controlling the
motors and connecting them in one speed adjustment to equally divide their load
between the two sides of the system.
4. A system of motor control comprising a three-wire supply circuit, motors
distributed between the two sides thereof, to divide the load, a generator connected
to one of the wires, a compensating device to absorb or deliver energy to preserve a
balance of load connected between the balance wire and an outside wire, and
switches for the several motors to connect them between the high-potential or
low-potential wires.
Th