Apparatus for measuring a winding temperature of electric machines

An apparatus for measuring the winding temperature of electric machines uses an a.c. reference voltage source generating an a.c. reference voltage with a predetermined, non-line frequency, preferably below 10 Hz., a current detector detecting the current generated from the a.c. reference voltage and/or the a.c. reference voltage directly, and a measuring and evaluating device measuring the current to determine the winding temperature. The a.c. reference voltage is added geometrically to the line voltage and generates a current proportional to the winding conductance which is temperature dependent.

BACKGROUND OF THE INVENTION 
The present invention relates generally to an apparatus for measuring the 
winding temperature of electric machines, and more particularly to 
apparatus for measuring the winding temperature by using an a.c. reference 
voltage source, a current detector and a measuring and evaluating device. 
An apparatus which uses one or more sensors in the machine to measure 
temperature is disclosed in G. Muller, "Elektrische Maschinen" [Electric 
Machines], Verlag Technik, Berlin 1974. Furthermore, apparatus which have 
a reference voltage source and a current generated by the reference 
voltage, whereby a direct measurement of resistance is performed using 
d.c. current are disclosed in IEC Publication 279, "Measurement of the 
Winding Resistance of an a.c. Machine During Operation at Alternating 
Voltage," Geneva, 1969. The former disclosed apparatus require temperature 
sensors and the requisite supply lines, i.e., increased equipment. The 
latter disclosed apparatus can deliver erroneous measurements. D.C. 
voltage potentials on the mains, caused for example, by concurrently 
operating frequency converters or by thermoelectromotive forces caused by 
soldered junctions, welded points or clamping points in the mains, which 
have different temperatures, can greatly falsify the measurement. The 
possibility of blocking d.c. voltage potentials from the mains by means of 
capacitors is only useful in small machines. In large machines, the cost 
and space requirements for the capacitors are disproportionately great. 
A method is also disclosed in E & I, Anno 105, Issue 7/8, pp 315 to 318, 
which makes possible a measurement of the rotor temperature of 
squirrel-cage induction machines without using thermal sensors. In this 
case, the rotor temperature is calculated from the variation of the 
voltage across terminals after switching off a machine that operating 
without a load and from the time constants of the rotor. 
The present invention is directed to the problem of further developing an 
apparatus for measuring the winding temperature across the line feed of an 
operating electric machine while dispensing with the additional sensors, 
and the capacitors which are connected in series to the machine, through 
which the entire power received by the machine would have to flow. 
SUMMARY OF THE INVENTION 
The present invention solves this problem by connecting an a.c. reference 
voltage generating one or more a.c. reference voltages in series with one 
or more phases of the mains which supply the electric machine such that 
one or more predetermined, non-line frequency, voltage components are 
added to the line voltage. This generates a current with the predetermined 
frequencies that pass through the winding of the machine. The current is 
detected by a current detector and then measured by a measuring and 
evaluating device. By knowing the voltage and measuring the current the 
conductance of the winding can be calculated, which then leads to a 
determination of its temperature, since the winding conductance is 
temperature dependent. 
The present invention is capable of detecting the actual winding 
conductance while the machine is operating. In the case of known 
temperature coefficients of the winding material (generally copper: 0.004 
1/K) and in the case of a known resistance at a reference temperature, the 
actual, average winding temperature can be derived from this variable. 
This winding temperature is an important parameter for operating a 
machine, since the permissible maximum temperature should not be exceeded 
for long period of time for reasons of machine longevity, yet for reasons 
of economy, operation should take place at the margin of the nominal 
values. 
It has proved particularly advantageous when the frequencies of the current 
are below 10 Hz. By properly selecting the amplitude of the a.c. reference 
voltage regarding the additional heating caused by the a.c. reference 
voltage in relation to the permissable machine temperature, one can avoid 
damaging influences on the machine temperature. In the case of the present 
invention, it is advantageous if the amplitude amounts to 1-2% of the 
amplitude of the a.c. supply voltage. 
If the a.c. reference voltage source is designed as a voltage generator 
supplying a resonance transformer, then the voltage from the generator can 
be decoupled from the mains. 
If the a.c. reference voltage source is designed as a voltage generator 
supplying a broadband transformer, then the voltage from the generator can 
again be decoupled from the mains. However, the capacitor, which is 
required for the resonance voltage transformer, is no longer necessary, 
and the available bandwidth for the a.c. reference signal becomes 
relatively large. 
If the a.c. reference voltage source is integrated in a pulse-controlled 
a.c. converter that is modulated with an additional signal having the 
predetermined frequency, then the transformer can be dispensed with so 
that the lower frequency can be freely selected. 
If the a.c. reference voltage source is designed as a d.c. voltage source 
which is modulated by the predetermined frequency, then again, a 
transformer is not required and the lower frequency limit can be freely 
selected. This design is preferred when a pulse-controlled a.c. converter 
is not present. 
If the a.c. reference voltage source is designed as a mains voltage 
modulator, then the a.c. reference signal can be derived directly from the 
supplying main. For this purpose, apparatus consisting of controlled 
equivalent conductances or susceptances as well as switches can be used as 
are disclosed in "Modualtionsverfahren in der Nachrichtentechnik" 
[Modulating Methods in Telecommunications], R. Mausl, UTB Huthig Verlag, 
Heidelberg, 1976, Chapter 1.3, pp. 35-55, the disclosure of which is 
hereby incorporated by reference. Since in this case the current and 
voltage are simultaneously influenced, it is necessary to measure both 
variables as well as to form their quotients in order to calculate the 
temperature. 
If a.c. reference signals consisting of at least three different frequency 
ranges, which are evaluated separately, are generated by the mains voltage 
modulator, then the use of signals with multiple frequencies, such as 
noise, is possible. 
In addition to the indicated possible designs for the a.c. reference 
voltage source, the simultaneous evaluation of a.c. reference signals of 
different frequencies or frequency ranges has proved advantageous for a 
distinct possibility of identifying deterministic interferences. Since 
these interferences are narrow-banded and have known frequency 
differentials, they can be identified by simply comparing the amplitudes 
of several a.c. reference signals of different frequencies. For example, 
if two of at least three a.c. reference signals are the same and are 
therefore able to be evaluated within the limits of measuring uncertainty, 
the third can be discarded. 
The are also several possible designs for the specific embodiment of the 
current detector. If the current detector is designed as a resonance 
transformer, then in addition to the voltaic separation from the mains, 
the line-frequency signals are effectively damped. However, an additional 
capacitor is required. If the current detector is designed as a broadband 
transformer, then also, as in the case of the resonance transformer, there 
is a voltaic separation from the mains. The capacitor in this case is 
superfluous and the available bandwidth becomes large. 
The most economical possibility of detecting the current signal occurs when 
the current detector is designed as a shunt. In this case, the current 
transformer can be dispensed with; however, the voltaic separation must 
also be dispensed with. 
A simple design for the measuring and evaluating device is by means of 
selective amplitude measurements. 
If the measuring and evaluating device is designed as a spectral analytical 
device, preferably one using the Fast-Fourier transformation, parasitic 
frequencies can be identified and the frequency resolution can be high. 
If the measuring and evaluating device is designed as a synchronous 
demodulator which is controlled by the a.c. reference voltage source, then 
selectivity and noise reduction are attainable with regard to stochastic 
noise. 
If the a.c. reference voltage source is modulated by means of a generator 
with a band spreading function, and the broadband a.c. reference signal is 
supplied to a synchronous modulator functioning as the current detector, 
then a reduction of noise is attainable with regard to deterministic 
interferences. 
Additional machine errors can be identified without much further effort 
when the a.c. reference voltage source and the current detector are 
designed as several devices present in each phase in the polyphase mains, 
to which devices for monitoring the symmetry of the signals are connected 
in order to identify a short circuit to a winding and/or an exposed 
conductive part. 
With a little more effort, the rotary speed of the machine can also be 
detected if a device is switched onto the a.c. reference voltage source. 
The device derives a signal which is proportional to the rotary speed of 
the machine from an a.c. voltage which is fed into an induction machine, 
which can appear on the terminals of the machine as a voltage and/or 
current signal.

DETAILED DESCRIPTION 
Before describing the possible designs at the operating levels, the 
following describes the common principle in light of the example of an 
electric induction machine. From viewing the known equivalent circuit 
diagram of this machine (T. Bodefeld, H. Sequenz: "Electric Machines" 
Springer Verlag 1949, 4th edition, p. 159, the disclosure of which is 
hereby incorporated by reference), one can see that only at the frequency 
zero can the stator resistance be measured precisely. At line frequency, 
there is a real component of input impedance which has a great dependence 
on rotor resistance and on slip, i.e. on the load of the machine. However, 
there is the possibility of undertaking the measurement at frequencies 
which are so low that scatter effects and transformations of the rotor 
side can be virtually negligible in the stator circuit. 
The upper frequency limit at which a nearly load-free measurement is 
possible depends on the type of machine. It lies advantageously in the 
range below 10 Hz. 
In the embodiment according to FIG. 1, a suitable a.c. reference voltage 
source 2 is connected in series to the mains 1 supplying the machine 5 
such that this predetermined, non-line, low frequency voltage is added 
geometrically to the line voltage and a current of this frequency is 
driven through the winding of the machine 5 and the mains 1. This current 
is detected by a current detector 3 and is supplied to a measuring and 
evaluating device 4. The amplitude of the a.c. reference voltage is to be 
selected such that no significant additional heating of the machine 5 
takes place, i.e., for example, the amplitude of the a.c. reference 
voltage does not exceed 1-2% of the line voltage. In the case of rotating 
electric machines, no interfering instants arise. In the case of a 
predetermined, constant a.c. reference voltage, the current is in 
proportion to the conductance to be measured and can be directly 
evaluated. If the demand for a constant a.c. reference voltage is not 
satisfied, then this voltage shall also be measured. By forming the 
quotient by means of analog or digital modules, the conductance or the 
resistance can then be determined. These types of modules are known e.g. 
from Tietze, Schenk, "Halbleiterschaltungstechnik" [Semiconductor Switch 
Engineering], Springer Verlag Berlin, 1986, 8th edition, Page 344, the 
disclosure of which is hereby incorporated by reference. The 
aforementioned resistance is the series connection of the winding and the 
mains resistance. The latter can be ignored, however, if it is less than 
the winding resistance by some orders of magnitude. This requirement is 
satisfied in customary mains. If the mains resistance is to be included in 
the measurement in order to increase accuracy, then it is to be measured 
and taken into account in the evaluation by means of simple subtraction, 
since it is constant in practice. 
Several possible designs are given for the circuits of blocks 2, 3, and 4 
which are shown in FIG. 1. The sophistication and power vary so that a 
combination can be selected which is best adapted to the measuring task. 
Possible designs for are: 
1. Design with a resonant voltage transformer 
The a.c. reference voltage source 2, has, e.g., a simple function generator 
feeding the primary of a resonance transformer, i.e., a transformer which 
is operated by means of a capacitor preferably in series resonance, and 
the secondary of the resonance transformer is coupled to the mains 1 
supplying the machine 5. In this manner, line-frequency reactions to the 
function generator are effectively avoided. The demands on the function 
generator are few; its source resistance should not considerably impair 
the quality of the resonant circuit. The decoupling of the generator 
voltage form the mains 1 is advantageous. 
The resonance transformer is to be adjusted to the stable a.c. reference 
frequency which must lie safely above its lower frequency limit which is 
determined by the transformer. The demands on the frequency stability are 
high, since otherwise amplitude errors and phase faults can appear. 
2. Design with a broad band voltage transformer 
Here, too, the decoupling of the generator voltage from the mains 1 is 
advantageous. A capacitor is unnecessary. The available bandwidth for the 
a.c. reference signal is large. The source resistance of the feeding 
generator must be very low and the transmission ratio of the transformer 
is not permitted to be too low, since otherwise the line-frequency 
voltages which are transferred to the generator side do not drop 
sufficiently at the source resistance of the generator and can endanger 
it. 
3. The use of a pulse-controlled a.c. converter 
If the electric machine 5 is supplied by a pulse-controlled a.c. converter, 
then the existing modulator can be used to generate the desired a.c. 
reference voltage by additionally modulating it with the desired a.c. 
reference frequency. Thus, no transformer is required; the lower frequency 
limit can be selected freely. 
However, care must be taken that in particular in the case of low operating 
frequencies, possible interference voltages are present in the frequency 
range of the a.c. reference voltage. In this case, a sufficient signal to 
noise ratio is to be ensured. Possibilities for this purpose are indicated 
in the following description of the measuring and evaluating device 4. 
4. Design as a modulated d.c. voltage source 8 
In FIG. 2 a principle design regarding this is shown. A switch 7 is 
controlled by a pulse generator 6 and switches the d.c. voltage source 8 
temporarily in series to the mains 1. The switch 7 is preferably a known 
configuration of semiconductor switches. The pulse generator 6 actuates 
this switch 7 at the a.c. reference frequency. In this manner, a 
modulation of the d.c. voltage which is delivered by the d.c. voltage 
source 8 takes place with the a.c. reference frequency and simultaneously 
an addition of the product of modulation to the line voltage takes place. 
The effective value of the a.c. reference voltage can be determined by way 
of the d.c. voltage and/or by way of the pulse width of the control signal 
of the pulse generator 6. In the output of the d.c. voltage source 8 there 
is usually a filter capacitor present which is temporarily connected in 
series to the machine 5 and to the mains 1 for the duration of the pulse. 
Current, and thus only a negligible part of the machine power, flows 
through it only for the duration of the control signal of the pulse 
generator 6. By appropriately selecting the control times, the effort for 
this capacitor can be minimized. The lower frequency limit of the a.c. 
reference signal is also able to be freely selected in the case of this 
configuration. However, when semiconductor switches are used for the 
switch 7, there is no decoupling of the voltage from the mains 1. Also, 
the d.c. voltage source 8 lies at the mains potential. It is therefore 
necessary to design the supply lines for the pulse generator 6 and the 
d.c. voltage source 8 such that the requisite electrical isolation is 
guaranteed. 
The current detector 3 which is used in FIG. 1 can be designed as a 
resonant current transformer. For this purpose, a current transformer is 
operated in parallel resonance at the stable a.c. reference frequency by 
means of a capacitor. Therefore, the line-frequency signals in its output 
signal are effectively damped so that the further processing is 
simplified. Of further advantage is the separation of the voltage from the 
mains 1. The a.c. reference frequency must lie safely above the lower 
frequency limit of the transformer; the demands on frequency stability are 
great, since otherwise amplitude errors and phase faults can appear. 
In a design for the current detector 3 as a broadband current transformer, 
a simple current transformer is used whose lower frequency limit lies 
below the a.c. reference frequency. In this case, a capacitor can be 
dispensed with. The available bandwidth becomes large. Of further 
advantage is the voltaic separation of the output signal from the mains 
potential. Line-frequency output signals must be sufficiently damped in 
the measuring and evaluating device 4 by means of conventional low pass 
filters. 
The most economical possibility of a design for the current detector 3 is 
by using a simple shunt. A current transformer can be dispensed with; 
there is no lower frequency limit. The shunt is included in the measuring 
result. Therefore, it must be dimensioned such that its influence is 
negligible. In case this is not possible, it can be taken into account in 
the evaluation by means of a simple subtraction since its value is known. 
A voltaic separation from the mains potential is not present in this 
design. 
As a design for the measuring and evaluating device 4 there are e.g. the 
following possibilities: 
1. Selective Amplitude Measuring 
The a.c. reference voltage and/or reference current is derived by means of 
selective filters which are adjusted to the a.c. reference frequency and 
are supplied to a known averaging unit. The average is calculated in the 
customary manner, and, as described above, is converted into the winding 
temperature. For this simple configuration, a sufficient signal to noise 
ratio between the useful and interference signals is necessary. It can be 
improved by the high quality of the selective filter and/or great time 
constants of the averaging unit. The rate of detection of temperature 
changes increases in this case. 
2. Spectral Analysis 
The a.c. reference voltage and/or the reference current are evaluated with 
a conventional spectrum analyzer. Parasitic frequencies can be identified 
and the frequency resolution can be very high, if needed. Known analog 
analyzers or preferably digital Fast-Fourier transform analyzers can be 
used. 
3. Synchronous Demodulation 
The frequency and phase of the supplied voltage are given: therefore, there 
is the possibility of measuring the current signal by means of synchronous 
power rectification. With this method, selectivity and noise reduction are 
attainable with regard to stochastic interferences. Synchronous 
demodulations are known e.g. from Tietze, Schenk, 
"Halbleiterschaltungstechnik" [Semiconductor Switch Engineering], Springer 
Verlag, Berlin, 1986, 8th edition, page 797 fol, the disclosure of which 
is hereby incorporated by reference. 
4. Band Spreading 
If broadband transformers are used, then the voltage source can be 
modulated by a suitable band spreading operation. For example, modulating 
rapid changes in frequency or phases can be considered as modulating 
methods. The modulated signal is used as an a.c. reference to the 
synchronous demodulation of the current signal. With this type of a 
configuration, a reduction of noise is attainable with regard to 
deterministic interferences. These types of methods are known from Baier, 
Grunberger, Pandit, "Storunterdruckende Funkubertragungstechnik" [Noise 
Suppressing Radio Transmission Technology], R. Oldenburg Verlag, Munich, 
1984, Page 90 fol, the disclosure of which is hereby incorporated by 
reference. 
If voltage feeding and current measuring are performed in each phase in 
polyphase mains, then addition information on the operating condition of 
the electric machine 5 can be derived by monitoring the symmetry of the 
signals. Interferences which appear during operation, e.g. short circuits 
to the winding and/or an exposed conductive part, can be detected in this 
manner. 
In the configuration according to the invention, moreover, frequency 
components can be generated which are based on the modulation of the a.c. 
reference voltage. Since they are in proportion to rotary speed, they can 
be used to measure rotary speed. By appropriately selecting the frequency 
of the a.c. reference voltage, the products of modulation can be converted 
into a noise-free frequency range so that a simple evaluation is possible. 
In the embodiment according to FIG. 3, an a.c. reference voltage source 2 
is connected in series to the mains 1 supplying the machine 5. The a.c. 
reference voltage source 2 can be switched to deliver an additional 
non-line frequency voltage which is advantageously different from the 
temperature a.c. reference frequency. The current detector 3 delivers a 
current- and/or voltage signal in which frequency components are contained 
which are in proportion to the rotary speed. The a.c. reference voltage 
source 2 and the current detector 3 are not to be designed as resonance 
transformers in the case of differing a.c. reference frequencies. Modules 
for the measuring means 9 are known. 
Methods and configurations for detecting the rotary speed of electric 
induction machines have already been proposed (P 37 11 976.1; P 37 34 
071.9 the disclosure of which is hereby incorporated by reference), which 
evaluate frequency components which are proportional to rotary speed and 
are able to be represented by a current and/or voltage. These methods 
evaluate the resulting signals from modulating the line-frequency which 
depend on rotary speed.