Note: Descriptions are shown in the official language in which they were submitted.
7~3
Arrangsme~t ~or ~e~uring th0
, 81i~ of Eleotri~ Induation Notors
3 l¦ Field of the Invention
4 ~ The present invention relates to an arrangement ~or
j measuring the slip o~ single or multiple phase electric
6 ¦ induction motors, and more particularly, to a measuring and
7 ¦ processing arrangement which directly indicates the slip of
8 induction motors in their operating range or makes it
9 accessible for further processiny. .
1~
11 Background o~ the Invention
12 Slip is a relative measure of the difference between
13 rotary field speed and rotor speed and hPnce is an important
14 operational parameter for induction motors.
Arrangements are known which meaeure the rotational
~6 speed (Trenkler, G. I'Die elektrische Messung ~on ~rehzahl und
17 Winkelgeschwindigkeit", Yerlag E. Geyer, Bad Worishofen, 1974,
18 in particular pages 36 and 37) so that the slip can be
19 calculated. For an inverter supply o~ a motor, it is known to
calculate the slip by means o~ a slip computer from -the motor
21 current, motor flux, and ignition angle (German Published
22 Unexamined Patent Application 35 23 619). Also ~nown are
23 computer-assist~d methods o~ estimating slip (Hillenbrand, F.,
24 "Identification of linear time-invariant systems and their
application to induction motors", Dissertation, TU Berlin
26 1982, pages 149-157). Another ~nown method for measuring slip
27 is the proYision o~ a eeparate measuring motor ~Swiss Patent
28 449,770) having an air gap in which Hall probes are disposed.
29
1~`12~7~
20365-2849
Further, to determine the rotational speed in unlversal motors
utilizing effective harmonics, application of the Fast Fourier
Transform is known ~U.S. Patent 4,527,101).
The above known arrangements and methods require speed
sensors or are suitable only for certain motor types and circuit
variants, or else they require precise prior knowledge concerning
the measured object. The cost involved in the evaluation
electronics and so~tware is usually considerable.
To pxovide rotational speed determination, a measuring
arrangement was proposed (in Canadian Patent Application Ser. No.
563,457 filed April 7, 1988), which detexmines the time responses
of the voltayes at the electrical connections and~or the currents
through the electrical connections of the motor.
The problem presented by the prior art is to provide an
arrangement which permits in a universal and simple manner ~he
measurement of the slip of electric induction motors.
Summary o~ ~he Inven~ion
The above and other problems are solved by the present
invention which provides an arrangement for measuring the slip of
an electric induction motor having at least one phase, the
arrangement comprising a level-adapted preliminary filter stage
coupled to the motor. This stage is supplied with ~he current
signals and/or the voltage signals which are generated by the
motor, these current signals and voltage signals having a
freguency proportional to the slip. A means for signal
convolution o~ a frequency range at either a power supply
frequency or its harmonics
79 20365-28~9
is coupled to an output of the level-adapted preliminary filter
stage. A filter is coupled to an output of the means for signal
convolution, while a display and evaluating unit is coupled to
an output of the filter.
In another embodiment of the invention, a converter is
coupled between the motor and the level adapted preliminary filter
stage. This converter senses from at least one phase of the motor
the slip-proportional current signals that are generated ~y the
motor.
In still another embodiment of the invention~ the neu-
tral points of the power supply and the motor are not connected,
and the multi-phase motor is in star connection. The level-adapted
preliminary filter stage is coupled to both neutral points so as
to pick up~he slip-proportional voltage signals.
The ambodiments of the invention described above pro-
vide an arrangement for a simple measurement of the slip of
electric induction motors.
Brief Description of t e DraWings
~igure 1 is a block diagram o~ an embodiment of the
present invention which uses a current signal.
Figure 2 is a block diagram of another embodiment of the
present invention which uses a voltage signal.
De-tai-led Descri~'tion
Before describing the embodiments of the invention,
the following is an explanation of the principle on which the
invention is based.
3 --
~ 2~7~ 20365-2849
In induction motors, ferromagnetic materials are used
that have magnetization characteristics which are nonlinear in the
normal operating range, that is, outside saturation. A multipli-
cative linkage of flux components of slip and power supply fre-
quency occurs, resulting in combination frequencies which are
detectable by induction at the terminals of the motor as current
and/or voltage signals. In the signal spectrum thes~ components
appear as frequency groups at once or twice the distance of the
slip frequency about the power supply ~requency as well as their
odd-numbered and even-numbered harmonics. The amplitude of these
components is dependent, among other things, on the magnet mater-
ial and degree of saturation. The amplitude of the components
is normally very small (about -60 to -90 dB), compared to the
power supply parameters. Since the flux components are very close
to the power supply frequency or its harmonics, especially when
the slip is small, direct selection and evaluation of these com-
ponents is not possible.
What is possible, however, is the selection of one
or more suitable frequency groups. Undesirable signal components,
2Q such as rotor slots harmonics, are sufficiently suppressed by this
selection. When the slip-proportional components are shifted by
convolution in the frequency range into an undisturbed frequency
band, e.g. the base band, they can be obtained ~y simple filter-
ing due to the large relative frequency distance from interference
components existing in this undisturbed band. The slip-
propor-tional components can then be displayed directly and/or
~urther processed.
_ ~ _
~ 20365-2849
Thus, for example, a frequency group ~ith components at
the distance of twice the slip frequency 2xfs can be selected.
After convolution and filtering, a frequency counter can then be
used for displayin~ the slipo For a 50 Hz power supply, the slip s
is then indicated directly as a percentage, independently of the
number of pole pairs of the motor, since 2xfs = 2xsx50= 100 s = s (%~.
In the embodiment of the invention illustrated in
Figure 1, the raw signal, comprising the ~eometric sum o~ all com-
ponents delivered by the power supply and generated by the motor,
is obtained from a single or multi-phase induction motor l by at
least one converter 2. This raw signal is supplied to a level-
adapted preliminary filter 3, preferably with a band pass char-
acteristic. ~ subsequent convolution 4 (see E. Oran Brigham,
"FFT Fast Fourier-Transformation", pages 68 to 94, 2nd edition,
R. Oldenbourg Verlag Munich Vienna 1985) is done, for example, by
simple squaring or synchronous rectification with the power supply
frequency. For this convolution, known aevices such as parabolic
function networks, multipliers or ring modulators may be used.
Undesire~ frequency components of the convolved signal are sup-
~0 pressed by a filter 5. As the amplitude o~ the useful signal in-
creases with increasing slip because of the also increasing
reaction of the rotor, an almost constant output signal is generat-
ed by suitable selection of the properties of the filter 5. Such
filters are known for example fro~ the publication by Stearns,
"Adaptive Signal Processing" Prentice Hall, Inc., Englewood
Cliffs, N~ 07632, pages 316 et seq. The selection of the proper-
ties of the filter 5 is a simple matter ~or one of ordinar~ skill
in the art.
-- 5 --
1 If the filter 5 is, .Eor example, a low-pass filter,
2 in most cases level control and limitiny devices are
dispensable if a proper selection of the cut-off frequency and
4 edge steepness is made. Thus, the output signal is able to be
supplied to a display and/or evaluat.ing unit 6 directly.
6 I t Figure ~ illustrates another embodiment for
7 1 obtaining the raw signal. If a three-phase motor 1 i5
8 ,l operated with a star connection, the converter 2 is not needed
9 li if the neutral point of the motor is not connected with the
1I neutral point of the power supply. Since the useful signal
~ components do not cancel each other out because of a lack of
12 ¦~ symmetry, they are available directly as voltage signals
13 1 against the neutral point of the power supply. The further
lÇ - processing for ~he embodiment is the same as that shown in
Figure 1.
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