Patent Description:
Large generators and motors are routinely examined for laminate shorts and stability when stationary. Various methods are available for this purpose.

One of the methods to determine laminate shorts comprises the magnetization of the entire laminated body by means of an auxiliary coil at the mains frequency, and the measurement of stray fields on the inner surface of the stator bore. The magnetization is carried out to relatively low values of the magnetic induction, typically to about <NUM>% of the normal operating induction. This method of measurement is also known by the name "low-induction laminate short measurement" or by the name "ELCID" (electromagnetic core imperfection detector).

By way of example, <CIT> describes one method of this type. The prior art is therefore for the laminated stator core to be magnetized by means of an auxiliary coil and a sinusoidal auxiliary voltage applied thereto at the mains frequency, to about one tenth of the operating induction. This auxiliary voltage is normally derived directly from the mains voltage. An electrical recording coil is then moved away from the surface of the stator bore, with the recording coil being located close to the surface of the laminated core.

The currents which flow as a result of the interlaminar short circuits in the laminated core now induce voltages with a characteristic phase angle and amplitude magnitude in the recording coil. The characteristic phase angles and amplitudes make it possible to distinguish between points where there are laminate-short currents and points where there are no laminate-short currents. It is therefore possible to locate laminate shorts, and to assess the magnitude of the short-circuit currents, by means of this stray-field recording coil.

<CIT> describes a method and apparatus for detection of interlaminar laminate shorts in the core of electrical machines, in which the laminated stator core is subjected to an externally applied field current and the short-circuit magnetic field induced by the short-circuit currents is measured with the assistance of a sensor coil. A field current and a field voltage are applied which have a waveform which differs significantly from a sinusoidal shape.

"<NPL> discloses a method with low yoke induction, wherein an interlamination short-circuit is detected with a measuring coil. The use of infrared detection for stator core inspection is also mentioned (section III) for methods comprising excitation at rated yoke induction with a high power source in a time-consuming method. This document further describes magnetization by the low voltage net with a variable transformer from the low voltage socket being used to perform the magnetization. Compensation of associated inductive reactive current by means of parallel connected capacitors is performed.

<CIT> describes a method for detecting core faults that includes positioning a magnetic yoke near at least one tooth of the core, the magnetic yoke being wound by a winding; supplying current to the winding to inject magnetic flux into the at least one tooth of the core; measuring a signal resulting from the injected magnetic flux; and using the measured signal to detect core faults.

The invention provides alternative solutions to determine laminate shorts and the stability of a stator core.

The present invention relates to a measuring device and a measuring method according to the independent claims.

Further preferred embodiments of the invention are described in the dependent claims.

The invention will be explained in more detail in the following text with reference to exemplary embodiments and in conjunction with the drawings, in which:.

<FIG> shows a schematic circuit diagram of an example of an excitation device <NUM> as part of a measuring device <NUM> according to an example of the invention. At the left of <FIG> a power supply <NUM> is provided which provides the electric power necessary to charge an excitation coil <NUM>. The power supply <NUM> can be fed by the public grid. The charging voltage is between 50V and 1000V. The charging voltage is adjusted at the power supply <NUM> to reach typically <NUM>%-<NUM>% of the nominal interlaminar voltage. The maximum permanent power of the power supply <NUM> is typically 50kW. The power supply <NUM> is electrically connected via a resistor <NUM> and a capacitor <NUM> to the circuit between the power supply <NUM> and the excitation coil <NUM> as shown. The capacitor <NUM> with a high capacity is charged by the power supply <NUM> and de-charged via a filter <NUM> to charge the excitation coil <NUM> in a specific way. A typical value of the capacitor <NUM> is 1mF, a typical value for the excitation coil <NUM> is 1mH. The filter <NUM> is commonly an inductivity which is saturated during current rise. During charge of the capacitor <NUM> the power supply <NUM> operates as a current source limiting the current output, during de-charge of the capacitor <NUM> the power supply <NUM> operates as a voltage source. In the example according to <FIG> the capacitor <NUM> is connected in parallel with a circuit containing four IGFETs <NUM>, where each two of the IGFETs <NUM> are steered by a controller <NUM>, <NUM>'. The voltage for the excitation coil <NUM> is tapped from the circuit containing the IGFETs <NUM> and the assigned controller <NUM>, <NUM>'. The voltage at the excitation coil <NUM> can properly be controlled by these means.

<FIG> shows a schematic circuit diagram of an example of an excitation device <NUM> similar to <FIG> as part of the measuring device <NUM>. Here, the main part of the circuit at the right is identical to the circuit of <FIG>. The capacitor <NUM> however is fed by two power supplies <NUM>, <NUM>' instead of one. The two power supplies <NUM>, <NUM>' are connected via diodes <NUM>, <NUM>' in parallel to the capacitor <NUM>. The power output is enhanced compared to the example of <FIG> as the second power supply <NUM>' adds additional power to the capacitor <NUM>. The maximum pulse frequency is in this example according to <FIG> doubled compared to the example of <FIG>. An arrangement adding further power supplies <NUM>, <NUM>' to the excitation device <NUM> is conceivable.

<FIG> shows a signal diagram of one example of excitation of a stator core <NUM>. The time is plotted at the horizontal axis and the voltage V is plotted at the vertical axis. The signal shown is the voltage generated by the excitation devices <NUM> to excite the excitation coil <NUM> as described above. The power supply <NUM>, <NUM>' charges the capacitor <NUM> which is de-charged in a controlled manner by the controllers <NUM>, <NUM>' switching the IGFETs <NUM> of the excitation device <NUM>. The shown excitation voltage is applied to the excitation coil <NUM>. As can be seen in <FIG> the voltage signal is discontinuous, the times t a voltage is applied are different to the times t the voltage is zero. Here, during the time tON an excitation voltage is applied consisting of two opposite rectangular pulses, one positive pulse followed by one negative pulse. A typical pulse time is <NUM>- <NUM> with <NUM>- <NUM> pulses per second. During the time tOFF the controllers <NUM>, <NUM>' switch the IGFETs to apply no voltage to the excitation device <NUM>. The time tON is unequal to the time tOFF. The control of the excitation device <NUM> in the way described ensures a low real power and a low reactive power from the feeding grid while assuring high voltages to excite the stator core <NUM>. In the signal shown it equals tON < tOFF.

<FIG> shows a signal diagram of another example of excitation of a stator core <NUM> provided by the excitation device <NUM> as described above. The time t is plotted at the horizontal axis and the voltage V is plotted at the vertical axis. In this example the voltage signal is discontinuous again. The signal shape of the voltage applied to the excitation coil <NUM> is the shape of bursts as shown in <FIG>. First, a steep nearly vertical edge is applied to a maximum power peak which immediately after reaching the peak decays in a steep curve to zero voltage. The time t with a voltage unequal to zero characterized by the voltage burst is referred to as tON. A typical pulse time is <NUM>- <NUM> with <NUM>- <NUM> pulses per second. During the time tOFF the controllers <NUM>, <NUM>' switch the IGFETs to apply no voltage to the excitation device <NUM>. The time tON is unequal to the time tOFF. In the signal shown it equals tON < tOFF.

<FIG> shows a signal diagram of another example of excitation of a stator core <NUM> provided by the excitation device <NUM> with the time t plotted at the horizontal axis and the voltage V plotted at the vertical axis. In this example the stator core <NUM> is again excited by a discontinuous voltage, here with a sinusoid shape. The time t with a voltage unequal to zero characterized by the sinusoid voltage is referred to as tON. A typical pulse time is <NUM>- <NUM> with <NUM>- <NUM> pulses per second. During the time tOFF the controllers <NUM>, <NUM>' switch the IGFETs to apply no voltage to the excitation device <NUM>. The sinusoid voltage is applied with three cycles at the time tON after which end of the last cycle the excitation voltage is set to zero. In the signal shown it equals tON < tOFF.

<FIG> shows a schematic top view of a stator core <NUM> of an electric machine. A rotor inside the stator core <NUM> is removed which is commonly done in measurement mode. In a schematic way the winding of the excitation coil <NUM> around the stator core <NUM> is shown which is the excitation coil <NUM> described under <FIG> and <FIG>. The excitation coil <NUM> is divided into four connected parts in this example. As described above an excitation voltage u(t) is applied to the excitation coil <NUM>. According to the electro-magnetic theory the current flow i(t) in the excitation coil <NUM> induces a magnetic flux density B in the stator core <NUM> in the direction indicated by the arrow. The electric exposure of the stator core <NUM> is reduced with all three exemplary signal curves with discontinuous voltages. The magnetic flux density B in the stator core <NUM> is detectable by different measures from which two are described below as two different embodiments of the invention.

<FIG> shows a first embodiment of the invention. Shown is a schematic perspective view of a measuring device <NUM> which comprises an excitation device <NUM> connected via a power transformer <NUM> to excitation windings <NUM>, 10a, 10b wound around the stator core <NUM> constituting the excitation coil <NUM>. The stator core <NUM> is illustrated in a perspective view with partly cut faces and an axis <NUM>. The stator core <NUM> has a weight of 53t and a length of <NUM> for example. At the inside the stator core <NUM> commonly has notches <NUM> to house stator bars (not shown). The excitation coil <NUM> has ten turns wound around the stator core <NUM> in this example, shown are only two turns. Switching the excitation voltage by the excitation device <NUM> and thus magnetizing the stator core <NUM> leads to a rise in temperature at the surface of the stator core <NUM>. To the end of measuring temperatures and especially temperature differences an optical detection device <NUM> is provided next to the stator core <NUM>. The quantity to be measured is the temperature in this embodiment. The optical detection device <NUM> is in this first embodiment an Infrared (IR) camera. The IR camera is suitable to measure the temperatures at the stator core <NUM> with a high sensitivity. The optical detection device <NUM> is moved along the surface of the stator core <NUM> by an operator and records temperature data. The temperature data is compared to stored data such that deviations between the gathered data and stored data can be determined. The detection device <NUM> comprises a calculation unit <NUM> and an electronic memory to this end. When the temperature measured at the stator core <NUM> with magnetic excitation takes an extraordinary high value it can be deduced that at the corresponding spot an interlamination short exists. In particular, when a large temperature difference between specific measured spots occurs the calculation unit <NUM> identifies an interlamination short at the spot at which the temperature is higher. Interlamination shorts are electric shorts between the insulations of the separate laminated sheets building the stator core <NUM> impairing the power efficiency of the electric machine and reducing the operation safety. The temperature rise at these spots are mainly caused by undesirable eddy currents flowing there. The temperature rise at these spots was found to have a linear relation to the averaged power brought into the spot.

Claim 1:
A method for measuring a stator core (<NUM>) of an electrical machine, with the steps of
a. winding at least one excitation coil (<NUM>) around the stator core (<NUM>),
b. applying via an excitation device with at least a capacitor (<NUM>), a discontinuous voltage by discharge of said capacitor (<NUM>) to the at least one excitation coil (<NUM>) to magnetically excite the stator core (<NUM>),
c. measuring with at least one detection device (<NUM>, <NUM>') a temperature at or vibration of the magnetically excited stator core (<NUM>), and
d. deducing that when the temperature measured at the stator core (<NUM>) with magnetic excitation takes an extraordinary high value, an interlamination short exists at a corresponding spot, or that when extraordinary values of vibrations are detected, a stability of the stator core (<NUM>) diverges at a specific spot.