Abstract
Method for detecting short-circuits in a coil in an electric machine, includes: a) arranging a coil in an air gap between the rotor and stator; c) recording signal curves generated by the coil; d) determining zero crossings of the curve and storing the times thereof; e) determining zero crossings of the curve corrected by an offset c, identifying a pair of immediately consecutive zero crossings, the time separation of which is longer than the minimum duration; f) in no pair is identified, repeating step e) until identified, wherein the offset c is varied from the zero point to a global extreme value of the curve; g) identifying at least one of the two stored times, which lies between and closest in time to the pair and; h) extracting two half-waves from the curve using times identified in step g), wherein each half-wave corresponds to half a revolution of the rotor.
Claims
1. A method for detecting winding short-circuits in an electric machine comprising: a) arranging a coil in an air gap arranged between the rotor and the stator of the electric machine; b) calculating a minimum duration (t.sub.min)of two immediately sequential zero crossings of a signal curve U(t) generated by means of the coil, taking the rotational frequency and the number of pole pairs of the electric machine into account; c) recording the signal curve U(t) generated by means of the coil during operation of the electric machine, having at least the duration of one revolution of the rotor; d) determining the zero crossings of the signal curve U(t) and storing the times of said zero crossings; e) determining the zero crossings of the signal curve U(t)-c corrected by an offset c, and identifying at least one pair of immediately sequential zero crossings, the time interval of which is longer than the minimum duration (t.sub.min), where c is not equal to zero; f) in the event that a pair is not identified in step e), repeating step e) until a pair is identified, wherein the offset c is varied in the direction from the zero point to a global extreme value of the signal curve U(t); g) identifying at least one of the two stored times which lie between and closest in time to the pair, and; h) extracting two half-waves from the signal curve U(t) using the times identified in step g), wherein each half-wave corresponds to half a revolution of the rotor; i) comparing the two half-waves.
2. The method as claimed in claim 1, further comprising: e1) determining the zero crossings of the signal curve U(t)-d corrected with an offset d, and identifying at least one pair of immediately sequential zero crossings, whose time interval is longer than the minimum duration (t.sub.min), where d is not equal to 0 and has the opposite arithmetic sign to c; f1) in the event that a pair is not identified in step e1), repeating step e1) until a pair is identified, wherein the offset d is varied in the direction from the zero point to the other global extreme value of the signal curve U(t); and wherein in step h one of the two half-waves is extracted making use of the time points identified in step e1).
3. The method as claimed in claim 1, wherein the number of pole pairs of the electric machine is one, and each of the two half-waves is delimited respectively by two of the time points identified in step g) and/or g1) that are immediately sequential.
4. The method as claimed in claim 3, wherein the number of pole pairs of the electric machine is larger than one, and each of the half-waves is formed of a number of partial waves corresponding to the number of pole pairs, wherein each partial wave is delimited in each case by two of the immediately sequential times identified in step g) and/or g1).
5. The method as claimed in claim 2, wherein in steps d), e) and/or e1), the zero crossings are determined by formation of y.sub.0=Ut=*Ut=+1 for all the points of the signal curve U(t) and of the corrected signal curve U(t)-c, where Ut= is a signal value in U(t) or U(t)-c, and Ut=+1 is the immediately sequential signal value.
6. The method as claimed in claim 5, wherein, in the case where y.sub.0 is negative, the two points associated with Ut= and Ut=+1 are linearly interpolated for determination of the zero crossing.
7. The method as claimed in claim 1, wherein the signal curve U(t) exhibits the electrical voltage generated in the coil or the current magnitude generated in the coil.
8. The method as claimed in claim 1, wherein the signal curve U(t) recorded in step c) is smoothed by means of a filter, a Bezier filter, a median filter and/or a gradient filter.
9. The method as claimed in claim 1, wherein the electric machine is a generator, a synchronous machine, and/or an electric motor.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) The invention is explained in more detail below with reference to the schematic drawing attached. Here:
(2) FIG. 1 shows a signal curve with a fundamental harmonic,
(3) FIG. 2 shows the fundamental harmonic U.sub.G(t) with a function f.sub.c(t)=c, where c>0,
(4) FIG. 3 shows the signal curve U(t) with one of the fundamental harmonics U.sub.G(t),
(5) FIG. 4 shows the signal curve with a function f.sub.1(t)=c.sub.1, where c.sub.1>0,
(6) FIG. 5 shows the signal curve with a function f.sub.2(t)=c.sub.2, where c.sub.2>c.sub.1>0,
(7) FIG. 6 shows a detail from FIG. 5,
(8) FIGS. 7, 8 show a schematic illustration of a comparison between two half-waves,
(9) FIG. 9 shows a signal curve with two fault positions,
(10) FIG. 10 shows a cross-section through an electric machine.
DETAILED DESCRIPTION OF INVENTION
(11) FIG. 10 shows a cross-section through an electric machine 24. The electric machine 24 comprises a stator 20 located radially outside, and a rotor 21 located radially inside. The stator 20 comprises a plurality of stator grooves 23 arranged adjacently to one another in the circumferential direction, into which electrical conductors are inserted. Each of the stator grooves 23 is delimited in the circumferential direction by two stator teeth 27 respectively. The rotor 21 comprises a plurality of rotor grooves 22 arranged adjacently to one another in the circumferential direction, into which electrical conductors are inserted to generate a magnetic field. A plurality of electrical partial conductors are inserted into each rotor groove 22, each of which is surrounded by an electrical insulation in order to insulate the partial conductors electrically from one another. Damage to the insulation can lead to a winding short-circuit. Each of the rotor grooves 22 is delimited in the circumferential direction by two rotor teeth 28 respectively. The number of pole pairs of the electric machine 24 in FIG. 10 is one. An air gap 26 is arranged between the stator 20 and the rotor 21. A coil 25 is inserted into the air gap 26 in order to measure a change of the magnetic flux. The coil 25 is attached in FIG. 10 to the surface of a stator tooth 27 located radially inward.
(12) FIG. 1 shows a plot of a signal curve U(t) recorded by means of the coil 25. Time is plotted on the abscissa 4, and the electric voltage, or the current magnitude, is plotted on the ordinate 5. A fundamental harmonic 1 with the form U.sub.G=.Math.*sin(t) is also plotted, where is the angular frequency of the rotation of the rotor 21, and .Math. is the amplitude. Since the number of pole pairs of the electric machine 24 is one, each oscillation period of the fundamental harmonic 1 consists of a first half-wave 2 that is characterized by a positive arithmetic sign for U.sub.G(t), and of a second half-wave 3 that is characterized by a negative arithmetic sign for U.sub.G(t). Each of the two half-waves 2, 3 corresponds to half a rotation of the rotor 21. In the method according to the invention, those sections of the signal curve U(t) that belong to the first partial wave 2 or to the second partial wave 3 are identified. A comparison of the two sections is then made. As can be seen from FIG. 1, not all the zero crossings 6 of the signal curve U(t) correspond to a zero crossing 11 of the fundamental harmonic 1. A zero crossing refers to a point in the signal curve U(t) at which U(t)=0.
(13) The fundamental harmonic 1 is illustrated with its zero crossings 11 in FIG. 2. The zero crossings 11 are illustrated in FIG. 2 as the intersections of the fundamental harmonic 1 with the function f.sub.0=0. If the signal curve U.sub.G(t) is corrected by an offset c such that the corrected signal curve adopts the form U.sub.G(t)-c, the time interval between two immediately sequential zero crossings 7 changes in comparison with the zero crossings 11 of the signal curve U.sub.G(t). The zero crossings 7 of the corrected signal curve U(t)-c are illustrated in FIG. 2 as intersection points with the function f.sub.c(t)=c, where c>0. The time interval here between two immediately sequential zero crossings 7 is alternately shorter and longer than the time interval between two immediately sequential zero crossings 11 of the signal curve U.sub.G(t).
(14) How those of the zero points 6 of the signal curve U(t) that correspond to a zero point 11 of the fundamental harmonic 1 are found is illustrated in FIGS. 3 to 6. All the zero points 6 of the signal curve U(t) are determined for this purpose, as illustrated in FIG. 3. The fundamental harmonic 1 is also shown in FIG. 3, as is a minimum duration t.sub.min, which is the time interval between two immediately sequential zero crossings 11 of the fundamental harmonic 1. The minimum duration t.sub.min is estimated by the equation for the minimum duration t.sub.min=1/(f*2*n), where f is the frequency of rotation of the rotor 21 and n is the number of pole pairs of the electric machine 24.
(15) As can be seen from FIG. 4, following the determination of the zero crossings 6 of the signal curve U(t), zero crossings 8 are determined in a signal curve U(t)-c.sub.1 that has been corrected by an offset c.sub.1. The zero crossings 8 in the signal curve U(t)-c.sub.1 are illustrated in FIG. 4 as the intersections of the signal curve U(t) with the function f.sub.1(t)=c.sub.1. To determine the offset c.sub.1, the global maximum in the signal curve U(t) is first determined, and c.sub.1 is then chosen to be positive and to be a fraction of the global maximum, for example to be one tenth of the global maximum. Those immediately sequential zero crossings 8 in the signal curve U(t)-c.sub.1 whose time interval is greater than the minimum duration t.sub.min are now searched for. As can be seen in FIG. 4, such a pair of immediately sequential zero crossings 8 cannot be found in the signal curve U(t)-c.sub.1 with the offset c.sub.1.
(16) For this reason, zero crossings 9 are then determined in a corrected signal curve U(t)-c.sub.2 with an offset c.sub.2. The zero crossings 9 in the signal curve U(t)-c.sub.2 are illustrated in FIG. 5 as the intersections of the signal curve U(t) with the function f.sub.2(t)=c.sub.2. The offset c.sub.2 is here increased over c.sub.1 by the fraction. Those immediately sequential zero crossings 9 in the signal curve U(t)-c.sub.2 whose time interval is greater than the minimum duration t.sub.min are now searched for. As can be seen in FIG. 5, two such pairs of immediately sequential zero crossings 9 can be found in the signal curve U(t)-c.sub.2. Each of the two pairs comprises a first zero crossing 12 and a second zero crossing 13, wherein the first zero crossing 12 is located earlier in time than the second zero crossing 13. Each of the zero crossings 6 of the signal curve U(t) that correspond to the zero crossings 11 of the fundamental harmonic 1 are identified as the zero crossings that are located in time between the first zero crossing 12 and the second zero crossing 13, and are closest in time to the first zero crossing 12 and to the second zero crossing 13. FIG. 5 shows a detail of FIG. 4, namely the second zero crossing 13, together with the zero crossing 11 of the fundamental harmonic 1.
(17) FIGS. 7 and 8 show schematically how the first half-wave 2 is compared with the second half wave 3. The respective start points and end points for the first half-wave 2 and the second half-wave 3 are extracted for this purpose from the signal curve U(t) with reference to the zero crossings 11. This can, for example, be done in that, with reference to a pair of immediately sequential zero crossings 11 of the fundamental harmonic 1 that have been found, one of the two half-waves 2, 3 is first extracted as the section of the signal curve U(t) that is delimited by the pair. The second of the two half-waves 2, 3 can, for example, be extracted as the section of the signal curve U(t) that is located earlier than the first zero crossing 12 of the pair, or after the second zero crossing 13 of the pair, by a duration that corresponds to the time interval of the pair. It is also conceivable that further zero crossings 11 of the fundamental harmonic 1 are found, in that a negative offset d is varied in the direction towards the global minimum of the signal curve U(t). Altogether three immediately sequential zero crossings 11 of the fundamental harmonic 1 can be found by varying the positive offset c and the negative offset d, wherein the two of the three zero crossings 11 that are earliest in time delimit the first half-wave 2, and the second and third in time of the three zero crossings delimit the second half-wave 3.
(18) As can be seen from FIGS. 7 and 8, the two half-waves 2, 3 are brought into alignment by shifting one of the two half-waves 2, 3 in the direction of the abscissa 4, as is suggested by the arrow 14. The two half-waves 2, 3 are added together, as is suggested by the arrow 15 in FIG. 8. In the absence of a fault, the signal curve over time of the half-waves 2, 3 that have been added together is zero. If the signal curve of the half-waves 2, 3 that have been added together is not equal to zero, then this signal curve must be analyzed as to whether a winding short-circuit is truly present, or whether external influences have caused a corruption of the signal curve U(t).
(19) A typical fault case is illustrated in FIG. 9. A first fault signal 16 is present in the first half-wave 2, and a second fault signal 17 is present in the second half-wave 3. The time interval between the fault signals 16, 17 and the zero point 11 that separates the two half-waves 2, 3 from each other is identical. The fault signals 16, 17 can be associated with one of the rotor grooves 23, since a local minimum 18 in the signal curve U(t) corresponds to a rotor tooth 28, and a local maximum 19 in the signal curve U(t) corresponds to a rotor groove 23.
(20) Although the invention has been more closely illustrated and described in more detail through the preferred exemplary embodiment, the invention is not restricted by the disclosed examples, and other variations can be derived from this by the expert without leaving the scope of protection of the invention.
