WO2008106910A1

WO2008106910A1 - Electric machine and method for measuring the potential present on a movable element of an electric machine

Info

Publication number: WO2008106910A1
Application number: PCT/DE2007/000425
Authority: WO
Inventors: Jörg HASSEL; Hans-Richard Kretschmer; Arno Steckenborn; Oliver Theile
Original assignee: Siemens Aktiengesellschaft
Priority date: 2007-03-08
Filing date: 2007-03-08
Publication date: 2008-09-12
Also published as: DE112007003497A5

Abstract

The potential on a movable element (10) of an electric machine is measured in that a capacity to the movable element is formed by means of an additional stationary element (18). The surface of the movable element is configured such that the capacity is modified to the predetermined type thereof during a movement of the movable element (10). A Maxwell displacement current flows by means of this modification of the capacity, the current being proportional to the potential to be measured.

Description

description

Electric machine and method for measuring the potential applied to a movable element of an electrical machine

The invention relates to an electrical machine and to a method for measuring the potential applied to a movable element of an electrical machine.

It is known to use a capacitive element for measuring a static potential, at least part of which is moved in such a way that the capacitance of the capacitive element changes and a Maxwellian displacement current flows. The potential can then be derived from the current intensity of the Maxwell's displacement current. This principle is used for example in the so-called field mill (see, for example, US Pat. No. 5,315,232) and in the measuring device known from WO 2005/121819 A1. The potential to be measured is applied to an immovable element.

From US Pat. No. 4,963,829 there is known a device for measuring the rotational speed of a shaft, in which a non-circular plate is axially coupled to the shaft, and wherein a second plate is provided as a counterpart to the plate and thus forms with it a capacitive element , A voltage is applied externally between the plates. Since the plate coupled to the shaft is not circular, the capacitance of the capacitive element changes during rotation of the shaft, and thereby a Maxwellian displacement current flows. From its temporal behavior, the rotational speed of the shaft can be derived. A potential which forms on the shaft during operation of the shaft plays no role in US Pat. No. 4,963,829. To measure the rotational speed, a voltage must be applied between the two plates. In electrical machines having an electrically conductive movable element, electrical voltages between the movable elements and the other components of the electrical machine, namely the immovable elements (stator and housing) can form. The reasons for this can be manifold: The voltages can be due to capacitive or inductive coupling, atmospheric fields and defects in the machines. The developing potentials can lead to functional impairment. Frequently, flashovers are triggered by high voltages, which can lead to the destruction (by combustion) of the lubricant provided between the movable element of the electric machine and immovable elements, and spark erosion can even lead to the destruction of the bearing. So far, one recognizes the flashovers only by their effects. However, the effect of the flashovers may be a sudden machine failure. As part of the so-called preventive maintenance (known in English as "preventive maintenance") should take place at an aging of the machine regularly replacement of components of the electric machine or the entire electric machine to avoid a surprising defect in the electrical machine. It would be desirable to be able to detect or even avoid flashovers in a timely manner, since it is reasonable to do so by measuring the potential applied to the moving element of the electric machine During the operation of the electrical machine, no method for measuring the potential on such a movable element of an electrical machine is known in the prior art.

It is an object of the invention to enable such a measurement and to provide an improved electrical machine and a suitable measuring method for this purpose. The object is achieved by an electrical machine having the features according to patent claim 1 and a method having the features according to patent claim 10. Thus, the electric machine according to the invention comprises a predetermined manner movable, electrically conductive element and a stationary, electrically conductive element which is coupled to the movable member such that the immovable member and the (electrically conductive) surface of the movable member each side of a form capacitive element. The surface of the movable element is designed such that the capacitance of the capacitive element changes as the movable element moves in the predetermined manner. Finally, the electric machine has means for measuring the amplitude of the current flowing to and away from the immovable element.

The invention is based on the recognition that, with variable capacitance between immovable element and the surface of the movable element, a Maxwellian displacement current flows whose current intensity is proportional to the electric potential difference given between the movable element and the immovable element. It is only necessary to know or determine the proportionality factor. Since the potential measurement on the movable element does not have to be highly accurate, but merely has to provide a clue as to whether arcing can take place, it is sufficient if the proportionality factor is known on the order of magnitude.

The invention can be used both in rotary drives and in linear drives. Thus, the movable member may be a rotatably movable cylindrical member, and then a portion of the cylinder surface of the movable member provides a part of the capacitive element. The movable element may also be a translationally movable plate-shaped component, and a portion of a planar surface of the movable element then provides a part of the capacitive element. Separate evaluation means are preferably provided in the electrical machine which receive measurement signals from the means for measuring the current intensity and are designed to derive a value for the potential applied to the movable element from the measurement signals on the basis of a mathematical or an empirically determined relationship. As already mentioned, this value need not be highly accurate, but may be subject to an error such that the evaluation means indicate an interval for the value of the electric potential.

Since the formation of the surface of the movable element predetermines the Maxwellian displacement flow, the more accurately the time characteristic of the measured Maxwell's displacement current can be assigned to the surface, the more accurate the measurement of the electrical potential. This interference can affect the accuracy of the evaluation. To avoid this, the evaluation is preferably coupled to a measurement of the respective rotational position. This can be done, for example, by providing optical sensors for sensing the movement of the movable element, which is possible when the surface of the movable element is designed so that the capacitance of the capacitive element changes as it moves their appearance changes so that the phase is derivable.

The optical sensors can then send measurement signals to the evaluation means, and the evaluation means only have to incorporate the measurement signals of the optical sensors in the derivation of the electrical potential from the measurements of the means for measuring the amplitude of the current amplitude. For example, the optical signals can be used to specify clock rates for the interrogation of the current amplitude signals.

The capacitance for a plate capacitor is a function of the area of the plates, the spacing of the plates, and the dielectric constant of the inter-plate dielectric, which may be air. Accordingly, the formation of the surface of the movable member for For purposes of changing the capacitance of the capacitive element in the movement of the movable member so that the electrically conductive surface of the movable member is structured so that when moving the movable member, the distance between immovable member and the surface of the movable member changes. It is also possible to apply electrically non-conductive material to a part of the electrically conductive surface of the movable element, and both measures can be taken simultaneously. (A change in the area of the facing sides of the capacitive element is difficult to constructively realize.)

The simplest form of capacitive element is a plate capacitor. Thus, the immovable element is preferably plate-shaped, wherein the plate shape may be flat or curved. For example, in an electric machine having a rotatable movable element, the curvature radius may be selected such that the center of the circle defining the curvature radius coincides with the center point of the cylindrical shape of the movable component.

The immovable element is preferably mechanically connected to other immovable components of the electrical machine, ie with the stator or at least the housing in which the stator is arranged. The electrical connection, however, is not immediate, but z. B. via a rectifier or z. B. via an analog-to-digital converter with subsequent digital evaluation, then continue on the downstream means for measuring the current amplitude and possibly the evaluation.

The method according to the invention for measuring the potential applied to a movable element of an electrical machine comprises the steps:

Providing an immovable element that is capacitively coupled to the movable element, Providing the surface of the movable element with such structures or material properties that the capacitance between the movable element and the immovable element changes during a movement of the movable element,

Detecting measured values of the current amplitude amplitude of the electric current flowing away from the stationary element or from it,

Deriving the potential at the movable element from the measured values on the basis of a mathematical or an empirically determined relationship.

The evaluation is preferably coupled to the actual movement of the movable element. Thus, in the event that the moveable element moves in rotation, its rotation speed is measured and included in deriving the potential from the measured values of the current amplitude amplitude. In the case that the movable element moves in translation, the translation speed of the movable element is measured and included in deriving the potential from the measured values of the current amplitude.

Hereinafter, preferred embodiments of the invention will be described with reference to the drawings, in which:

1 shows in cross-section the capacity-forming parts of the electric machine according to a first embodiment of the invention,

FIG 2 shows in perspective the complete arrangement of the electric machine in this embodiment,

3 shows in cross-section the capacity-forming parts of the electric machine according to a second embodiment of the invention, 4 shows in cross section the capacitance-forming parts of the electric machine according to a third embodiment of the invention, and

FIG 5 illustrates in cross section to the second embodiment of the invention according to FIG 3, an optical scanning.

A shaft 10 shown in FIG. 1 in cross-section and in FIG. 2 in a perspective illustration is a movable element

Part of an electric machine 12 shown as a whole in FIG. 2. The shaft 10 is mounted in a bearing 14, which in turn is located in a stator 16. An electrical voltage U can form between the shaft 10 and the stator 16. This should be measured in the present case. To measure the potential, a variable capacitance is to be used so that a Maxwellian displacement current proportional to the applied voltage U flows. To form a capacitance, a capacitor plate 18 is provided as a stationary element. 2 and dashed line in FIG. 1, or it may be curved, compare solid line in FIG. 1. The plate 18 is provided with a measuring transducer 20 (eg current-voltage converter). connected. The transducer 20 is connected at its output to means 22 for measuring the level of the signals emanating from the transducer 20, and the means 22 for measuring this signal level are followed by evaluation means 24 which receive and evaluate the measured values.

The mode of operation of the electric machine according to the invention can be approximately explained with reference to the equation for a plate capacitor. For the capacitance C of a plate capacitor, the following applies:

where A is the area of the individual plates of the plate capacitor, d is their spacing, so the dielectric constant ^' and ε the dielectric constant of the between the two plates located material, where for vacuum ε = 1 applies. At the same time, for the capacity C:

(2, C-§, where Q is the charge on the capacitor plates and U is the voltage between the capacitor plates.) The charge Q is now the product of the current t times t. From equations (1) and (2 ) you get the new equation:

In fact, known electrometers measure the charge to return to the voltage U. In the present case, this is not readily possible with the electric machine 12. If, however, one of the quantities that define the capacitance according to equation (1) changes, the charge changes, so that a current flows. While the area A is difficult to change, it is possible to change the distance d of the plates of the plate capacitor in the short term or to change the dielectric located between the plates and thus to change the dielectric constant ε. In an extension d and ε can be changed simultaneously. The remaining determination variables of the capacitance C can be known or the constant factors can be determined by calibration, so that it is possible to deduce the voltage U due to the flowing current (ie the time derivative of Q), which in equation (3) constant factor is to be considered.

1 shows an embodiment in which approximately a plate capacitor is formed between the shaft 10 and the plate 18, wherein the distance between the plates changes periodically. For this purpose, 10 teeth 26 are formed on the shaft 10 via an axial portion of the shaft, which are also electrically conductive. To form such a structure, the shaft 10 can be suitably milled from a metal part. If a tooth 26 passes by the plate 18, the distance between the two sides of the capacitive element is small. If a gap 28 between two teeth 26 arrives the plate 18 over, this distance is greater. By changing between teeth 26 and gap 28 flows a Maxwell shear displacement current which is proportional to the voltage U. This Maxwell's displacement current is rectified by the transducer 20 and its amplitude is measured by the means 22 for measuring the amplitude. The measured values are supplied to the evaluation means 24. In the evaluation means 24 is either a mathematical relationship, for example according to equation (3) with numerical values for the constant factors, stored, or an empirical relationship with table values. The evaluation means 24 can then react on the voltage U. The voltage U should be measured in order to be able to foresee damaging flashovers in good time for the electric machine 12. Either the numerical value of the voltage U is displayed or indicated on a display 30 that the voltage U has exceeded a limit from which the electrical machine 12 could suffer damage due to currents and flashovers laying down on the voltage U, or it is sent to a warning lamp 32 a current signal is output so that it indicates when the voltage U has exceeded a predetermined limit.

In the embodiment according to FIG. 3, in contrast to the embodiment of FIG. 1, the interspaces between the teeth 26 are filled with a dielectric 34. In the case of rotation of the shaft 10 ', as in the first embodiment, the distance d changes, but not so that ε is constant, but over a part of the distance the dielectric constant ε of the dielectric 34 is to be used. It should be noted that in the case of the embodiment according to FIG. 3 the Maxwellian displacement flow is lower, because an increase of ε counteracts an increase of the distance d, compare the plate capacitor formula (3).

In the shaft 10 '' in FIG 4, a third embodiment is realized. Here, the electrically conductive material of the shaft is continuously circular in cross-section, and over an axial portion of a dielectric 36 is applied. The dielectric 36 can be provided, for example, by simple adhesive tape ("tape"). In this case, only the dielectric is changed upon rotation of the shaft 10 ", ie ε changes, while the distance d between the electrically conductive ones changes Components is defined, remains constant.

All three embodiments (shaft 10 of FIG. 1, shaft 10 'of FIG. 3, shaft 10 "of FIG. 4) are such that the surface also changes optically upon rotation of the respective shaft. Thus, by means of a suitable optical scan 38 (eg LED light barrier), the surface can be permanently scanned. The LED light barrier 38 thus determines the rhythm of the changes on the surface with the rotation of the shaft and the LED light barrier 38 is coupled to the evaluation means 24. The latter can set the measurement signals output by the current amplitude amplitude measuring means 22 in relation to the measurement signals of the LED light barrier 38, so that a phase relationship is determined. The measurement signals from the measuring means 22 can thus be interrogated synchronously. As a result, the influence of interference signals, which naturally can occur with the size of the electric machine 12, is suppressed.

The principles of the invention can also be applied to an electric machine with a translationally movable element (linear motor). For example, the translationally movable member may be plate-shaped, and the surface of this plate may be made uneven, corresponding to the teeth 26 of the shaft 10 or 10 '. The dielectric material can also be applied to the plate. For example, strips of dielectric adhesive tape of the type of strips 36 may be applied to shaft 10 'at fixed intervals.

Claims

claims

Electric machine (12) comprising a predetermined electrically movable, electrically conductive element (10, 10 ', 10' '), and with a stationary, electrically conductive element (18) which in such a way to the movable member (10, 10 ', 10' '), that the immovable element (18) and the surface of the movable element (10, 10', 10 '') each form one side of a capacitive element, wherein the surface of the movable element (10, 10 ', 10 ") is formed such that upon movement of the movable member (10, 10', 10") in the predetermined manner, the capacitance of the capacitive element changes, and wherein the electric machine (12) further comprises means (20, 22) for measuring the amperage of the current flowing to the immovable element and away from it electric current.

2. An electric machine (12) according to claim 1, wherein the movable member (10, 10 ', 10' ') is a rotatably movable cylindrical member and a portion of the cylinder surface of the movable member provides a part of the capacitive element.

3. The electric machine (12) of claim 1, wherein the moveable element is a translationally movable plate-shaped member and a portion of a planar surface of the moveable element provides a portion of the capacitive element.

4. Electrical machine (12) according to one of the preceding

Claims, with evaluation means (24) which receive measurement signals from the means (22) for measuring the current amplitude amplitude and which are designed to derive a value for the potential applied to the movable element from the measurement signals on the basis of a mathematical or an empirically determined relationship ,

5. Electrical machine (12) according to claim 4, with optical sensors (38) for scanning the movement of the movable element (10 ') _/ wherein the optical sensors (38) send measurement signals to the evaluation means (24), and wherein the evaluation means ( 24) are adapted to include the measurement signals of the optical sensors (38) in the derivation of the electrical potential from the measurement signals of the means (22) for measuring the current amplitude.

6. Electrical machine (12) according to one of the preceding

Claims in which the electrically conductive surface of the movable element (10, 10 ') is structured in such a way that, as the movable element (10, 10') moves, the distance between the stationary element (18) and the surface of the movable element (10) 10, 10 ') changes.

.

7. Electrical machine (12) according to any one of the preceding claims, wherein on a part of the electrically conductive surface of the movable member (10 ', 10' ') electrically non-conductive material (34, 36) is applied.

8. Electrical machine (12) according to one of the preceding claims, wherein the immovable element (18) is plate-shaped.

9. Electrical machine (12) according to any one of the preceding claims, wherein the immovable element (18) with a housing (16) of the electric motor of the electric machine (12) or other immovable components of the electric machine (12) is mechanically connected.

10. A method of measuring the potential applied to a movable element (10, 10 ', 10'') of an electrical machine (12), comprising the steps of: - providing a fixed element (18) capacitively connected to the movable element (10 Coupled, 10 ', 10''), - Providing the surface of the movable member (10, 10 ', 10'') with such structures or material properties that the capacitance between the movable member (10, 10', 10 '') and the immovable member (18) at ei a movement of the movable element (10, 10 ', 10'') changes,

Detecting measured values of the current intensity amplitude of the electric current flowing away from the stationary element or from it, deriving the potential at the movable element from the measured values on the basis of a mathematical or an empirically determined relationship.

11. The method of claim 10, wherein the movable member (10, 10 ', 10' ') rotates, and in which the rotational speed of the movable member is measured and included in deriving the potential from the measured values of the current amplitude.

12. The method of claim 10, wherein the movable element moves in translation, and in which the translation speed of the movable element is measured and included in deriving the potential from the measured values of the current amplitude.