Improved Means for Driving the Speed Indicators of Electric Motors.
Year: 1898

page_text: Date of Application, 6th Oct., 1898-Accepted, 19th Nov., 1898
COMPLETE SPECIFICATION.
Improved Means for Driving the Speed Indicators of Electric 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 Ernst J. Berg,
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:
In the operation of induction motors it is well known that the speed drops as
the load increases. This drop in speed may be considerable in some instances,
without being seriously detrimental. In some cases, however, a drop in speed as
great as two or three per cent. is not permissible, and this is particularly true in
cases where induction motors are used for running cotton or other textile mach-
inery, where even a slight speed variation may have a very destructive influence.
In order to fulfil the requirement of close speed regulation, thus called for, it
is necessary to make the motors very large, in which case they become very
expensive in proportion to the amount of power developed. Instead of providing
large motors with close regulation, the same object may be accomplished by using
smaller or less expensive motors and preventing too great a drop in speed by cor-
respondingly speeding up the generators in the power station. To do this it is
necessary to have in the power station a device for indicating either the speed or
drop in speed of the motor in the receiving station. A device of this character
is described and claimed in this application.
The uses to which the device may be put are many and various, so that, for
the purposes of illustration, only one relation is described in which the invention
is of value, although it will be evident to those skilled in the art that the device
is in no way limited in its application thereto, but may be used for many other
purposes.
In the drawings Fig. 1 is a diagrammatical representation of the invention,
and Fig. 2 is a view of one form of apparatus which is employed.
In carrying out the invention for indicating at a distance the speed of an
induction motor, use is made of the fact that currents are developed in the induced
member of the motor of a frequency corresponding to the slip, or, in other words,
to the difference between the actual speed of the motor and that which it would
have at absolute synchronism. By tapping the windings of the induced member
at suitable intervals, currents may be taken off of a frequency corresponding to
the slip. If these currents be conducted to any desired point and the frequency
of the currents there measured, a result is attained indicative of the speed of the
induction motor.
In the drawings, I represents the induction motor the speed of which is to be
measured. The induced member 1 of the motor is tapped at points separated
from each other by an angle equal to one third of the polar pitch. Three-phase
currents are taken off through collector rings 2 and brushes 3. Owing to the low
frequency at which these currents are generated, their voltage may be too low

page_text: to economically run the frequency indicator F. To remedy this objection, a
step-up transformer T, of any suitable character, may be employed. The
secondary mains 4 lead to the frequency indicator F which, as before stated, may
be located at any desired point.
The frequency indicator may assume many forms without departing from the
spirit of the invention, but the form which is preferably employed, and which
is illustrated in the drawings, consists of a motor of the rotary-field type, in which
one member 5 is fed with poly-phase current of the same frequency as that fed
to the motor whose speed is to be measured while the other member 6 is fed
with current from the step-up transformer T; the latter current, as before stated,
having a frequency dependent upon the slip of the motor. It is within this
invention, however, to construct the frequency indicator in the form of a synch-
ronous motor in which the field is fed by direct current from any suitable source.
In this case, the speed of the frequency indicator corresponds to the slip of the
induction motor.
In the normal operation of the preferred form of frequency indicator F, in
which both members are supplied with energy from the same mains, the polar
lines produced in its two relatively rotatable members will coincide and the speed
of the revolving member will be such as to accomplish the result. Thus, if the
polar lines have the same direction of rotation in space, then the revolving
member will have a speed less than that of synchronism by an amount equal to
the slip of the motor I. In other words, the speed of the frequency indicator F
is the same as that of the motor whose speed is to be measured, so that all that is
necessary to determine the speed of the motor I is to measure the speed of rota-
tion of the revolving member of the frequency indicator F. This may be done
by a tachometer 7, of any suitable character, or by any other known means for
measuring the rate of rotation of a shaft. The reading of the tachometer gives
the speed of the motor I. If the number of poles on the frequency indicator F
and the motor I differ, this reading will require to be increased or decreased
accordingly, but, in any case, it will be proportional to the speed to be measured.
If the polar lines of the two relatively rotatable elements of the frequency
indicator Frevolve in opposite directions then it is evident that the revolving
member will rotate at a rate equal to the sum of the rates of rotation of the polar
lines, that is, at a speed equal to the speed of synchronism plus that corresponding
to the slip of the motor I. The reading of the tachometer minus a constant cor-
responding to the speed of synchronism, gives the slip of the motor I. Either
method of speed measurement may be employed, as convenience may dictate.
If it be desired to measure the variation in speed of the machine from the
speed corresponding to synchronism, it is necessary to obtain two readings from
the frequency indicator, one with the two polar lines revolving in the same
direction, the other with the two polar lines revolving in the opposite direction.
One reading represents the speed of synchronism minus the slip of the machine
whose speed is to be measured, while the other reading represents the speed of
synchronism plus the slip.
One half of the difference between these readings obviously represents the
variation in speed from synchronism of the machine whose speed is being mea-
sured. The mean of the two readings gives of course, the speed of synchronism.
In order to reverse the direction of rotation of the polar lines in either of the two
members of the frequency indicator, in order to obtain these two readings it
is only necessary to transpose the connections of two of the leads supplying current
to that member. Although methods of measuring the speed of an induction motor
have been described, one of which gives directly either the exact speed of the
rotating member or a multiple thereof, while the other gives a result correspond
ing to the speed of synchronism plus a quantity equal to the slip or drop in
speed of the motor, ordinarily it is preferred to employ that method of speed mea-
surement which gives directly the speed of the motor, rather than resort to a
method which presupposes a knowledge of the speed of synchronism, since this

page_text: speed cannot be definitely known unless specially measured. It is obvious, never-
theless, that the methods of speed measurement, which are described but other
than that which is preferred, are equally within the scope of this invention.
It will be evident to those skilled in the art that this invention is not limited
in its application to the measurement of the speed of induction motors, but may
be employed to measure the speed of any device in which alternating currents are
developed of a frequency dependent upon the speed. Thus, for example the arma-
ture of a direct current motor may be tapped at suitable intervals and the resulting
alternating current fed to one member of a synchronous motor, the field of which
is excited by direct current. The speed of the synchronous motor will correspond
to the speed of the armature of the direct current machine. In the same manner
a synchronous motor might, as before stated, be used in place of that form of
frequency indicator illustrated in the drawings. Under such circumstances the
synchronous motor would indicate simply the slip of the induction motor rather
than its actual speed. Many other modifications will suggest themselves, with-
out departing from the spirit of this 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;-
-
-The method of measuring the drop in speed of an induction motor, which
consists in measuring the frequency of the currents developed in its induced
member.
2-The method of measuring the variation from synchronous speed of a rotary-
field dynamo-electric machine, which consists in developing in one member of
the machine an alternating current, the frequency of which represents the differ-
ence between the actual speed of the machine and that which it would have at
synchronism, and measuring the frequency of the current so developed.
3-The method which consists in supplying polyphase current to one member
of a dynamo electric machine of the rotary-field type, developing in the other
member of the machine a current of a frequency dependent on the variation in
speed of said machine from synchronism, raising the potential of the current
thus developed, transmitting the same, and supplying it to one member of a
motor, supplying current to the other member of the motor and measuring the
speed of its revolving member.
4-The method which consists in supplying polyphase current to one member
of a dynamo electric machine of the rotary-field type developing in the other
member of the machine a current of a frequency dependent on the variation in
speed of said machine from synchronism, transmitting said current and supply-
ing it to one member of a frequency measuring device, the other member of which
is supplied with current of the same frequency as that supplied to said dynamo
electric machine.
5 The combination of an alternating current dynamo-electric machine, having
an inducing and an induced member, means for developing in the induced mem-
ber an alternating current of a frequency dependent on the variation in speed of
said machine from synchronism, a step-up transformer, conductors for passing
said current to the primary of said transformer and a frequency measuring
device in connection with the secondary of the transformer.
G-The combination of an induction motor, conductors connected at suitable
intervals to the winding on the induced member of the motor, and a frequency
indicator in electrical connection with said conductors.
7 The combination of a dynamo electric machine, means for supplying poly-
phase currents thereto, means for developing therein polyphase currents of a fre-
quency different from those of the current supplied to said machine, and a frequency
measuring device for measuring the frequency of the currents developed.
8 The combination of polyphase mains, an induction motor having its indue-
ing member connected thereto, conductors connected in polyphase relation to the

page_text: induced member of said motor and leading therefrom to one of the two rela-
tively revolving members of a frequency indicator, and connections from the other
member of said frequency indicator to said polyphase mains.
9 The combination of a rotary field induction motor, conductors connected in
polyphase relation to the induced member of said motor and leading therefrom
to one or two relatively, rotating members of a frequency-measuring device, means
for developing in the other member a rotary field of the same frequency as that
of the current supplied to the induction motor, and means for measuring the speed
of the rotating member of said frequency-measuring device.
Dated this 4th day of October 1898.
THE BRITISH THOMSON-HOUSTON CO., LTD.,
W. MARTINDALE,
A. R. MONKS,
Directors.
ALFRED CLEMENTS,
Assistant Secretary.