WO2013136098A1

WO2013136098A1 - Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils

Info

Publication number: WO2013136098A1
Application number: PCT/HR2013/000002
Authority: WO
Inventors: Ante ELEZ; Stjepan CAR
Original assignee: Končar - Institut Za Elektrotehniku D.D.
Priority date: 2012-03-16
Filing date: 2013-02-26
Publication date: 2013-09-19
Also published as: HRP20120245A2; HRPK20120245B3

Abstract

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils is based on the use of two measuring coils (1) and (2) shown in the drawing 1. The measuring coils (1) and (2) are installed on the stator teeth or the stator wedges that, by absolute value, have the same vector magnetic potential. A mutual distance between the measuring coils (1) and (2) is equal to the pole pitch τρ or to the multiple of the machine pole pitch. The ends of the measuring coils (1) and (2) are connected in series over the executive member (3). By differential measurement of the voltage U that is induced in the measuring coils (1) and (2), connected in series, it is possible to unambiguously detect rotor winding damage in all rotating alternating machines. By connecting the measuring coils (1) and (2) in series, magnetic field present in a properly operating machine is excluded from the measured value without any winding damage so that the measured value depends only on the magnetic field caused by rotor winding damage. An increase of the measured voltage U in relation to a reference voltage, signals the presence of rotor winding damage. The executive member (3) can be used for turning off the machine or for a sound or light damage signalization.

Description

METHOD FOR ROTOR WINDING DAMAGE DETECTION IN ROTATING ALTERNATING MACHINES BY DIFFERENTIAL MEASUREMENT OF MAGNETIC FIELD BY USING TWO MEASURING COILS DESCRIPTION OF THE INVENTION

Technical Field

The invention relates to the field of system diagnostics, fault detection, monitoring and protection of rotating alternating machines.

According to the seventh edition of the International Patent Classification the invention belongs to the following technical fields:

· H02K 1 1/00 - Constructional integration with measuring or protective devices or electrical components, such as resistor, switch, radio interference suppressor,

• H02K19/00 - Synchronous motors or generators,

• H02K 7/00 - Induction motors; Induction generators,

· G0 R31/06 - Testing of electrical windings, e.g. polarity,

• G0 R31/34 - Testing of dynamo electric machines.

Technical Problem

A technical problem solved by the invention is detection of rotor winding inter-coil short circuits of synchronous and squirrel cage induction machines, as well as rupture of one or more rotor winding bars and cage ring of rotor winding in induction squirrel cage machines. Classical protection methods used in rotating alternating machines cannot detect this damage. Therefore, various methods that detect the listed damage, with more or less success, have been developed over the years. This invention falls into the category of new methods that allow the detection of the mentioned defects. Rotor winding damage such as rotor winding inter-coil short circuits or rupture of the squirrel cage can cause moment and current oscillations in the case of motors and voltage deterioration in the case of generators.

Background Art

Rotor winding damage in alternating rotating machines is a result of electrical, mechanical and thermal stress. This phenomenon is common in turbo and hydro generators and induction motors. Continuous monitoring of the magnetic field in the air-gap of a rotating machine as well as outside the machine is a confirmed and proven method that can detect damage of the rotating AC machine winding. By installing the measuring coils or the Hall sensors inside the air gap, and the measuring coils for measurement of leakage magnetic fields outside the machine, it is possible to detect the rotor winding damage. There are numerous methods used for the rotating machines damage detection. The reference literature stated below provides technical solutions that are nowadays used for the rotor winding damage detection of turbo generator:

1 - Albright, D. R.: "Interturn Short-circuit Detector for Turbine-Generator Rotor Windings", IEEE Transactions on Power Apparatus and Systems, Volume-90, Number 2, March/April 1971 ,

2 - Donald R. Albright, David J. Albright and James D. Albright: "Generator Fields Winding Shorted Turn Detection Technology", IRMC, May 1999,

3 - M. P. Jenkins, D. J. Wallis: "Description and Utility Evaluation of a Continuous On-line Monitor", EPRI Predictive Maintenance and Refurbishment Conference, December 1993.

The patent document HR20060393 describes a detection method for inter-coil shorted turns of excitation winding in a hydro generator. The method is based on the measurement and frequency analysis of voltage that is induced in the measuring coil with a width of two pole pitches or multiples of two pole pitches. The patent document JP58005682 describes a detection method for the rotor winding damage in rotating electrical machines by using the measuring coil installed on the machine stator in the air gap. Processing of the measured signal includes removal of the influence of stator winding magnetic field by using a low- pass filter. Damage detection of the rotor winding is carried out in such a way that the peak values of the processed filtered measured signal are observed.

The patent document JP54058807 describes a detection method for squirrel cage rotor winding damage in induction motors by using the Hall sensors that are installed on the motor stator in the air gap. This method uses two Hall sensors installed at the distance of the pole pitch or multiples of two pole pitches. Detection of squirrel cage rotor damage is conducted in the following way: The Hall sensor signal is firstly amplified by using an amplifier, than the values of amplified measured signals connected in parallel are compared with each other, using two controlled rectifier circuits. If one of the signals is greater than the second signal, the relay will switch from state "0" to state "1 ". The state "1 " means that the damage is present in the motor windings.

Patent document CN101262153 describes a detection method for induction motor squirrel cage damage by using a measuring coil installed on the motor stator in the air gap. A measuring coil is installed in such a way that its sides are diametrically opposed in relation to the bore of the machine. Based on the measurements and the frequency analysis of the measuring coil, a decision on the presence of cage rotor damage of induction motor is made. The increase of the amplitude of the observed harmonic, which is periodically repeated, leads to the conclusion that the squirrel cage damage is present in the motor.

Essence of the Invention

The technical novelty of this invention is a differential measurement of the magnetic field in a rotating alternating machine with two serial connected measuring coils, installed on the places in the machine which have, by absolute value, equal magnetic vector potential. The distance between the measuring coils is n · Tp , where τ_ρ is a pole pitch, and n=1 , 2, 3, 4, ... is a multiple of the pole pitch. Measuring coils are installed to the stator of the machine in the air gap so that they encompass the whole stator tooth or just part of the tooth. However, it is possible to install the measuring coils on the stator slot wedge, instead of on the stator tooth, wherein measuring coil may encompass the whole wedge or just the part of the stator slot wedge.

The method proposed by the invention in comparison to the method described in the patent document HR20060393, differs for being also applicable to the two-pole machines. Namely, the method described in this patent document is applicable to the machines that have 4 or more poles, which is a limiting factor, taking into account the significant number of rotating AC machines having two poles. Moreover, the patent document HR20060393 stated that the damage detection method is only applicable to synchronous generators with salient poles, i.e. hydro generators. The method from the patent document HR20060393 has significant drawbacks that are manifested as measurement errors caused by unwanted leakage magnetic fields. Namely, the measuring coil from the patent document HR20060393 is installed so that its two sides are laid on the stator tooth or stator slot wedge, whereas it is very important that they are distanced from each other for a double pole pitch. The remaining two sides of the measuring coil must be installed so that they have the same shape. As this is almost impossible to achieve in practice, the influence of leakage magnetic fields is expressed to the extent that it significantly affect the measurement results, as well as the damage detection. Particularly due to this problem, the measuring coils for measuring the magnetic field in order to detect defects are installed around the stator tooth according to the invention, because the area through which the magnetic field is measured is this way strictly defined. Furthermore, the lack of large surface of measuring coil can also be noticed. For the machines with a small number of poles, e.g. a quadrupole machine, the surface of the measuring coil from the patent document HR20060393 comprises half of the area of the bore, which considerably complicates the installation of the measuring coil.

The patent document JP58005682 proposes the use of only one measuring coil. The process requires a complicated analysis of the measured voltage in order to allow damage detection, wherein it is necessary to use a complex measured data processing. In contrast, the solution proposed by the invention allows winding rotor damage detection without complex analysis of the measured data, by measuring the value of voltage of two measuring coils connected in series. This is possible because the serial connection of two measuring coils enables the removal of the magnetic field present in a properly operating machine, without any damage, from the measured value, so that the measured value depends exclusively on the magnetic field that causes damage to the rotor windings. The method proposed by the invention considerably simplifies the measuring equipment used in the damage detection of the rotor winding, when compared to the method described in the patent document JP58005682.

The patent document JP54058807 describes the use of the Hall sensors, which are active measuring elements unlike the measuring coils which are passive measuring elements, as described in the solution proposed by the invention. Namely, for the Hall sensors to function, it is necessary to supply them with the control current which requires a special power source. Moreover, the Hall sensor gives voltage on its terminals, which is usually amplified, so that the technical solution that uses measuring coils instead of the Hall sensor represents a more reliable solution due to a smaller number of components. The method that is mentioned in the patent document JP54058807 uses the Hall sensor circuit in parallel, with the damage detection based on mutual comparison of values of amplified measured signals by two controlled rectifier circuits. The drawback of this method, which is based on a voltage comparison of two Hall sensors, is reflected in the reliability of damage detection influenced by: the sensitivity of the Hall sensors and their persistence as well as by the installation position of a Hall sensor on the stator tooth that significantly affects the measurement result. Also, this method is based on a comparison of two measured wave forms of the total magnetic field, which is measured by two sensors. Improvement of the solution proposed by the invention in relation to the method from the patent document JP54058807 is also reflected in the fact that the serial connection of two measuring coils eliminates the magnetic field of the correct machine, in such a way that only rotor winding damage affects the measured value.

In the patent document CN101262153 one measuring coil is applied, which is installed on the stator of the machine in the air gap so that the sides of the measuring coil are diametrically opposite in relation to the bore of the machine. The method described in this patent document has significant shortcomings that are manifested in the measurement error caused by unwanted leakage magnetic fields. Namely, the measuring coil is installed so that its two sides are laid on the stator tooth or stator slot wedge where it is very important that they are set diametrically opposite. The remaining two sides of the measuring coil must be installed so that they have the same shape. As that is in practice almost impossible to achieve, the influence of leakage magnetic fields is expressed to the extent that it significantly affects the results of the measurements, as well as damage detection. Moreover, the lack of large surface of the measuring coil can be noticed.

Descriptions of Drawings

Figure 1 shows a part of the stator of a rotating alternating machine with measuring coils mounted on the stator teeth.

Figure 2 shows a part of the stator of rotating alternating machine with measuring coils mounted on the stator slot wedges.

Figure 3 shows how to connect measuring coils whose mutual distance is an odd multiple of the pole pitch. Figure 4 shows how to connect measuring coils whose mutual distance is an even multiple of the pole pitch.

Figure 5 shows an example of distribution of magnetic field lines in a four pole rotating machine.

Figure 6 shows the diagrams of the voltages measured at the ends of each of the measuring coils separately, with one broken bar of the rotor squirrel cage of the induction motor.

Figure 7 shows the diagram of the voltage measured at the ends of each of two measuring coils connected in series, with one broken bar of the rotor squirrel cage of the induction motor.

Figure 8 shows an executive member that detects damage of the rotor winding, based on voltage of two measuring coils connected in series and the reference voltage, and alerts the user to the damage.

Disclosure of the Invention

The essence of the method for damage detection of rotor winding in rotating alternating machines proposed by the invention is based on two measuring coils 1 and 2 as shown in drawings 1 to 4. The first measuring coil 1 is installed on an arbitrarily selected stator tooth, as shown in drawing 1 , or arbitrarily selected stator slot wedge, as shown in drawing 2. The second measuring coil 2 is installed on the stator tooth, as shown in drawing 1 , which by absolute value has the same magnetic vector potential as the stator tooth on which the first measuring coil 1 is installed at a distance that is equal to the value n * x_p , whereby τ_ρ is a pole pitch and n its multiple, or on the stator slot wedge, as shown in drawing 2, which by absolute value has the same magnetic vector potential as the stator slot wedge on which the first measuring col 1 is installed at a distance that is equal to the value n * Tp , whereby τ_ρ is a pole pitch and n its multiple.

The measuring coils 1 and 2 are installed around the stator tooth or the stator slot wedge, and this way the area through which magnetic field is measured is strictly defined. With this type Of installation, the area on which a magnetic field is measured is equal for both measuring coils, which is crucial for this differential measurement method. The measuring coils 1 and 2 are made of insulated wire, and the ends of each measuring coil, after forming a closed loop, necessarily have to be mutually twisted in order to reduce the influence of leakage magnetic field as shown in drawing 1 and drawing 2. As shown, the measuring coils 1 and 2 can be, along with the executive member 3, easily installed to any rotating alternating machine.

The measuring coils 1 and 2 must be installed to such locations in the machine that have the same magnetic vector potential by absolute value. In order to meet this requirement and realize the measurement method proposed by the invention, it is necessary to connect the measuring coils 1 and 2 with each other in series depending on the parity of the multiple of the pole pitch τ_ρ.

If an odd multiple of n=1 , 3, 5, 1... of the pole pitch τ_ρ is chosen for the distance between the measuring coils, then the measuring coils 1 and 2 should be connected as shown in drawing 3. The inner side 1.1 of the coil 1 connects with the inner side 2.1 of the coil 2. The outer side 1.2 of the coil 1 , through executive member 3, connects with the outer side 2.2 of the coil 2. The executive member 3 measures the total voltage U that is induced in two measuring coils 1 and 2, connected in series. If the measured voltage U exceeds a predefined value, the executive member 3 activates the output relay. The measuring coils 1 and 2 connected in series in this way, whose mutual distance is an odd multiple n=1 , 3,

5, Ί ... of the pole pitch τ_ρ, will have the same absolute value of a magnetic field, but the direction of the magnetic field lines will be different. The measuring coils connected and installed in this way allow mutual subtraction of the voltages that are induced in the measuring coils 1 and 2.

If an even multiple of n=2, 4, 6, 8... of the pole pitch τ_ρ is chosen for the mutual distance of the measuring coils, then the measuring coils 1 and 2 should be connected as shown in drawing 4. The inner side 1.1 of the coil 1 connects with the outer side 2.2 of the coil 2. The outer side 1.2 of the coil 1 , through executive member 3, connects with the inner side 2.1 of the coil 2. The executive member 3 measures the total voltage U that is induced in two measuring coils 1 and 2, connected in series. If the measured voltage U exceeds a predefined value, the executive member 3 activates the output relay. The measuring coils 1 and 2 connected in series in this way, whose mutual distance is an even multiple n=2, 4,

6, 8... of the pole pitch τ_ρ ,will have the same absolute value of a magnetic field and the same direction of the magnetic field lines. The measuring coils connected and installed in this way allow mutual subtraction of the voltages that are induced in the measuring coils 1 and 2.

The drawing 5 shows an example of the distribution of the magnetic field lines in a four pole rotating alternating machine. The examples of installation of measuring coils with odd and even pole pitches are given in the description that follows.

Places for installation of measuring coils in a four pole machine, at a mutual distance of multiple pole pitch π · τ_ρ, where n=1 , 2, 3, 4, 5, 6, 7... , marked by positions A, B, C and D as shown in drawing 5, have by absolute value the equal magnetic potential for the selected observation moment. However, the direction of the magnetic field lines M on the observed positions is not the same, but it differs on the positions A and C in relation to positions B and D. At the positions A and C the magnetic field lines M are in the direction from the periphery of the machine toward the center, whereas at the positions B and D they are in the direction from the center toward the periphery.

If the pole pitch r_p or odd multiple of pole pitch τ_ρ is chosen as a mutual distance of the measuring coils during their installation in the machine, then the measuring coils 1 and 2 should be installed at the positions A and B or A and D or C and B or C and D and connected according to the drawing 3. The magnetic field at the positions A and B, or A and D, or C and B, or C and D is equal by absolute value, however, the direction of the magnetic field lines M is different. For the measuring coils 1 and 2 installed at the positions that correspond to the above mentioned couples, the voltage that is induced in the measuring coils for a properly operating machine has the same value, but a different direction. The connection according to the drawing 3, for the described installation procedure, allows mutual subtraction of voltages of the measuring coils 1 and 2 and exclusion of the magnetic field present in the properly operating machine from the measurement.

If an even multiple pole pitch x_v is chosen as a mutual distance of the measuring coils during their installation in the machine, then the measuring coils 1 and 2 should be installed at the positions A and C or B and D and connected according to the drawing 4. Magnetic field at the positions A and C, i.e. B and D is equal by absolute value and by direction, hence the voltage that is induced in the measuring coils 1 and 2 for a properly operating machine has the same value and direction. The connection according to the drawing 4, for the described installation procedure, allows mutual subtraction of voltages of the measuring coils 1 and 2, and exclusion of the magnetic field present in the properly operating machine from the measurement.

In the case of a properly operating machine, in the measuring coils 1 and 2 installed in the corresponding places in the machine and connected according to the drawings 3 and 4, the same voltage will be induced due to the changing magnetic field of the machine. Therefore, in the case of a properly operating machine, the total voltage of two measuring coils, connected in series, is approximately equal to zero for each selected time during one turn of the machine. In case of rotor winding damage, and at the time of the arrival of the damaged rotor winding on one of the measuring coils 1 and 2, the voltages that are induced in the measuring coils 1 and 2 will no longer be equal. Thus the total voltage of two measuring coils connected in series will not be equal to zero for any chosen moment during one turn of the machine.

By measuring the total voltage of two measuring coils 1 and 2 connected in series during one turn of the machine, one can unambiguously detect a rotor winding failure in induction and synchronous machines. In addition to winding failure detection, the number of rotor winding failures can be determined as well.

By serial connection of the measuring coils 1 and 2, the magnetic field present in a machine operating without damage is eliminated from the measured value and the measured value depends only on the magnetic field caused by rotor winding damage.

The method of rotor winding damage detection was tested by numerical calculations using Finite Element Method in the software Infolytica MagNet. Furthermore, the laboratory tests were conducted that confirmed the effectiveness of this method through experiments.

The drawing 6 shows waveform diagrams for voltage U that is measured at the end of each measuring coil separately, with one broken rotor squirrel cage bar in the induction machine. The waveform of the voltage induced in the measuring coil has the same shape for the measuring coil 1 and the measuring coil 2. From the drawing 6 it can be seen that the deviation between the curves is negligible for the rotor parts without a rotor winding damage. The voltage waveform shown in the drawing 6 is periodic and repeated for each turn of the machine, which corresponds to the time of 40 ms, i.e. 4 pole pitches, i.e. 4 τ_ρ.

The drawing 7 shows a waveform diagram for voltage U that is measured at the end of two measuring coils connected in series, with one broken rotor squirrel cage bar in the induction machine.

Rotor winding damage detection from the waveform of the voltage U that is induced in the measuring coil as shown in the drawing 6, requires a complex signal processing if only one measuring coil is used. However, by observing the waveform of the voltage U that is induced in two measuring coils connected in series as shown in the drawing 7, one can conclude that a rotor winding damage is easy to detect. Serial connection of two measuring coils cancels out the magnetic field of a properly operating machine so that the two measuring coils connected in series measure the magnetic field caused by rotor winding damage. In the drawing 7, winding damage is marked with positions F, which repeat two times during a full turn in the case of one rotor winding failure. The damaged winding part, i.e. terminated rotor squirrel cage bar, comes across the measuring coil 1 and measuring coil 2 during one machine turn. Therefore, also in the measured waveform during one machine turn there are two extremely high voltage levels U, marked by positions F on the drawing 7. The drawing 7 shows four high voltage levels U marked with the positions F because there are two full machine turns in the drawing. Accordingly, the number of such measured overshoots of the measured voltage U during one full machine turn, divided by two, gives the number of damaged rotor windings. In the drawing 7 it can be seen that the overshoot value of the voltage U of two measuring coils connected in series with only one bend, caused by damage on one rotor cage bar, moves from 1 V to 1.25 V, which is a sufficient value for reliable damage detection and there is no need for amplification of the measured signal. This method, besides for rotor winding failure detection, can be used for determination of the number of rotor winding failures, but also for determination of their mutual positions.

In the drawing 7 it can be seen that the voltage U of two measuring coils connected in series, for the rotor part without any winding damage, is not equal to zero. This occurs due to allowed tolerances in the machine manufacturing and assembly and imperfections of measuring coils installation. However, one can clearly see the change in the value of the measured voltage U in the case of rotor winding damage, which is almost 150% in the described case. The method has been tested for the entire operating range of the machine and it successfully detects rotor winding damage in the entire operating range. The method has been tested and verified in the case when machine is powered from the frequency converter.

The measuring coils 1 and 2, made of standard insulated wire, are fixed by gluing on the stator in the machine air gap. The measuring coils 1 and 2 are placed on a stator tooth or wedge, and can encompass an entire tooth or a wedge or just a part of a tooth or a wedge. The measuring coils can have one or more bends, but only one is sufficient.

The executive member 3 shown in the drawings 3 and 4, whose schematic depiction is shown in the drawing 8, uses comparator K, which compares a currently measured voltage value Ui with the reference voltage U₂. The reference value of voltage U₂ is adjustable, and should be adjusted above the error voltage, which depends on tolerances in the machine manufacturing and assembly and imperfections of measuring coils installation. When a currently measured voltage value Ui of two measuring coils, connected in series, exceeds the reference value of U₂, the output of the comparator K is activated, i.e. the output voltage U3 increases from the value zero to the value that represents the change in the output. That value is usually 5 or 24 V, but it is possible to use any other voltage level.

By simple application of this measurement method one should take into account the time of voltage U increase caused by rotor winding damage, which is 1/28 of full turn duration for the analyzed machine. Thus for each machine turn, digital output of the executive member would be turned on two times in duration of 0.5 ms. The duration of one machine turn is 20 ms.

In order to avoid periodical turn on and turn off of the executive member 3 for the damaged rotor winding, the output of the executive member 3 can be a bistabil, i.e. remain in a state that indicates the damage until the executive member 3 is not set to default settings.

In this way, two measuring coils 1 and 2 connected in series in combination with the executive member 3, whose function is to measure the voltage U and compare it to an adjustable voltage reference, can attain independent rotor winding damage detection without the need for the measured signal analysis. Increase of the measured voltage U in relation to the reference voltage signals the presence of rotor winding damage using a digital output. The user can utilize the output of the executive member 3 to turn off the machine or as a sound or light signalization. The Application of the Invention

This method can be applied in monitoring and diagnostics of rotating machines and besides the rotor winding damage detection it can be applied to detect the number of damaged parts of the rotor winding. By using an analog input and standard measuring equipment, voltage sampling of two measuring coils connected in series is performed. The analog input must meet the following requirements: sampling frequency 10 kHz or more, voltage level ±10 V for measuring coils with one bend. By measuring the voltage value and by counting the exceeding of the measured voltage in relation to the preset reference value during one machine turn and by dividing that number by two, it is also possible to determine the number of damaged rotor windings.

This method for rotor winding damage detection is applicable to all types of rotating alternating machines, which is confirmed by numerical calculation method with the use of Finite Element Method. The method is applicable to synchronous generators and motors, as well as to both slip rings and squirrel cage induction generators and motors.

By measuring voltage of two measuring coils connected in series, in the time domain, without additional processing and conditioning of the measured signal, one can unambiguously detect rotor winding damage in induction and synchronous machines.

Claims

1.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils, characterized in that the differential measurement of the magnetic field in rotating alternating machine is performed by two measuring coils (1 ) and (2) connected in series, which are installed on the machine stator, in that the positions (A, B, C, D) on the machine stator on which the measuring coils (1 ) and (2) are installed have, by absolute value, the equal vector magnetic potential and in that the measuring coils (1 ) and (2) are at the mutual distance that is equal to the value n · τ_ρ , where T_p is a pole pitch, and n = 1 , 2, 3, 4, ... a multiple of the pole pitch.

2.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to the claim 1 , characterized in that the first measuring coil (1 ) is installed on an arbitrarily selected stator tooth, and the second measuring coil (2) is installed on the stator tooth that, by absolute value, has the same vector magnetic potential as the stator tooth on which the first measuring coil is installed, at the distance that is equal to the value n^■ τ_ρ , where τ_ρ is a pole pitch, and n its multiple.

3.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to the claim 2, characterized in that the measuring coils (1 ) and (2), made of insulated wire, are installed on the stator teeth so that they encompass the whole teeth or parts of the teeth, in that they have one or more bends and in that the ends of each of the measuring coil (1 ) and (2) are twisted together.

4.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to the claim 1 , characterized in that the first measuring coil (1 ) is installed on an arbitrarily selected stator slot wedge, and the second measuring coil (2) is installed on the stator slot wedge that, by absolute value, has the same vector magnetic potential as the stator slot wedge on which the first measuring coil is installed, at the distance that is equal to the value n · τ_ρ, where τ_ρ is a pole pitch, and n its multiple.

5.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to the claim 4, characterized in that the measuring coils (1 ) and (2), made of insulated wire, are installed on the stator slot wedge so that they encompass the whole wedges or parts of the wedges, in that they have one or more bends and in that the ends of each of the measuring coils (1 ) and (2) are twisted together.

6.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to any of the claims 1 to 5, characterized in that the measuring coils (1 ) and (2), at a mutual distance that is equal to an odd multiple n=1 , 3, 5, 7... of the pole pitch Tp , are connected in series so that the inner side (1.1 ) of the coil (1 ) is connected with the inner side (2.1) of the coil (2), and the outer side (1.2) of the coil (1 ), through the executive member (3), is connected with the outer side (2.2) of the coil (2).

7.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to any of the claims 1 to 5, characterized in that the measuring coils (1 ) and (2), at a mutual distance that is equal to an even multiple n=2, 4, 6, 8... of the pole pitch Tp , are connected in series so that the inner side (1.1 ) of the coil (1 ) is connected with the outer side (2.2) of the coil (2), and the outer side (1.2) of the coil (1 ), through the executive member (3), is connected with the inner side (2.1 ) of the coil (2).

8.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to the claims 6 or 7, characterized in that the executive member (3), which consists of comparator (K), measures the total voltage U that is induced in two measuring coils (1 ) and (2), connected in series, so that the comparator (K) compares a currently measured value of the voltage Ui to an adjustable reference voltage U₂ and so that the output of the comparator K is activated in the moment when the currently measured voltage value Ui exceeds the reference value U₂, so that the output voltage U₃ increases from the value zero to the value that represents a change in the state of the output that activates the output relay.

9.

Method for rotor winding damage detection in rotating alternating machines by differential measurement of magnetic field by using two measuring coils according to any of the claims 1 to 8, characterized in that it is applied in monitoring and diagnostics areas, rotor winding damage detection, detection of the number of the damaged parts of the rotor winding in synchronous generators and motors and slip rings and squirrel cage induction motors and generators.