MONITORING THE LITTEN CONDITION

Description

TECHNICAL FIELD

[0001] The present disclosure is concerned with monitoring the health of Litz wires and especially, but not exclusively, Litz wires used in electric machines.BACKGROUND

[0002] Litz wires are wires used in various electrical applications for carrying AC current. Litz wires comprise several thin wire strands which are then twisted or woven together to form a multistrand conductor. Because of the way the strands of the Litz wire are twisted together, each strand is on the outside of the conductor for an equal proportion of the overall length and so the current is equally distributed between the strands which reduces resistance of the strands. Litz wires have reduced losses, due to proximity and skin effect characteristics, particularly at high frequencies.

[0003] More recently, designers have begun to use Litz wires to form the coils of electric machines. Litz wires with many strands and small diameter are easy to bend and form into motor windings.

[0004] In recent years, electrical machines have been used more in aircraft as there has been a desire for more electrical aircraft (MEA). As this demand for more electrical aircraft and for more power in aircraft has increased, the electrical motors have started to become the primary drive systems in aircraft rather than providing a backup function. It is, therefore, vital that such machines are robust and reliable.

[0005] Many of these motors now use Litz wires as their windings, for improved efficiency and conductivity. A problem with Litz wires is that because they include many very thin strands, the individual strands can break. The stress placed on the Litz wires during and after assembly can weaken the strands. In applications such as electrical machines in aircraft loss of a strand can have serious effects. In such assemblies, failure is not predictable and can cause unplanned delays to carry out repair or replacement.

[0006] The document EP 1 593 981 A2 discloses a system and a method for detecting damages in wires or cables.

[0007] There is, therefore, a need for a way to monitor the health of Litz wires in a reliable manner, such that the state of the wires can be diagnosed and / or failure can be prognosed.

[0008] The present disclosure provides an arrangement and method for monitoring the health of Litz wires used in electric machines.SUMMARY

[0009] In one aspect, the disclosure provides a method of monitoring a Litz wire as defined by claim 1.

[0010] Also provided is a Litz wire diagnostic assembly as defined by claim 3.BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 shows a first arrangement according to the disclosure. Figure 2 shows an alternative arrangement according to the disclosure. DETAILED DESCRIPTION

[0012] The described embodiments are by way of example only. The scope of this disclosure is limited only by the claims.

[0013] As described above Litz wires (2) are made up of multiple strands. The figures show Litz wires 2 used as windings in an electrical motor. The principle of the disclosure can also apply to classical power distribution cables. The Litz wires are used as stator tooth windings terminated at each end with terminals 3a, 3b. Figs. 1 and 2 show the stator tooth windings when unfolded.

[0014] According to the disclosure, to be able to check the health of the wire strands forming the Litz wire, the windings are formed to effectively represent a transmission line and are formed such that a homogenous medium is created for electromagnetic waves to propagate and create transmission line effects which can be measured. The measurement will indicate any faults in the Litz wire.

[0015] Fig. 1 shows an embodiment using current injection and voltage monitoring. The homogenous medium is created by splitting the strands of the Litz wire into at least two groups 2a, 2b. A voltage detection and excitation unit 6 is connected across the two groups, optionally by means of terminals 4a, 4b which may be e.g. clips or electrically conductive terminals to thus create an electrical loop. Whilst the example shows the Litz wire split into two bundles of strands, the Litz wire can be split into more bundles according to the desired accuracy of detection. In some applications, it may be necessary to detect even a slight deterioration in one single strand, in which case the bundle should have fewer (even only two) strands, whereas in other cases, bigger bundles will still be sufficiently reliable.

[0016] In this embodiment, current pulses are injected into the strands by e.g. a magnetic coupling 5. This may be a transformer 5, e.g. an air cored transformer such as a Rogowski coil arrangement, or a magnetic based transformer, but other means of pulse injection may also be used.

[0017] The injection coupling 5 is energised by inducing a current pulse in the loop. The pulse then enters the Litz wire 2 and induces an electromagnetic wave which travels in the direction of terminal 3b. When the wave reaches terminal 3b, it is reflected back towards terminal 3a. When the wave reaches terminal 3a, it is sensed by the coupling 5 or by measuring the voltage across terminals 4a, 4b. The time for the wave to travel to terminal 3b and then reflect back to terminal 3a, for a healthy Litz wire, will be known and any deviation from this will be picked up by the measurement at coupling 5 or terminals 4a, 4b.

[0018] If there is a fault in one or more of the wire strands of the Litz wire, causing an open circuit or a short circuit, this will cause a discontinuity in the homogenous medium and will cause transmission line effects in that the wave will be partially reflected at the discontinuity before it would normally have been reflected by terminal 3b. This effect will be detected as is known in TDR, and be reported as a fault or deterioration. The more strands that are lost, the greater the discontinuity effect and so the greater the reported fault.

[0019] The voltage detection and excitation unit 6 can then evaluate and / or report the fault e.g. via an external device or computer. The voltage detection and excitation unit 6 could be connected to the external device by wire or wirelessly.

[0020] The arrangement can include a single voltage detection and excitation unit for all teeth / windings or each tooth could have its own unit located in close proximity. The unit 6 can receive power and / or communication signals by wire or wirelessly.

[0021] In an alternative embodiment, as shown in Fig. 2, voltage injection and voltage monitoring is used. Rather than operating by inducing an electromagnetic wave by a current loop, or current injection as in Fig. 1, the electromagnetic wave could be induced by means of voltage excitation. Here a voltage is created by the voltage detection and excitation unit 6 across terminals 4a, 4b which induces the electromagnetic wave. This travels along the wire as described above and the returned signal is measured at the terminals 4a, 4b.

[0022] The speed of propagation of the wave is also linked to the dielectric constant of the Litz wire and so TDR measurements can also give an indication of faults in the winding insulation.

[0023] Whilst the use of TDR is preferred, it is also possible to determine faults in the Litz wire represented as a transmission line by impedance measurement - increased impedance will be indicative of a strand fault.

[0024] The present disclosure thus teaches a system and method that allows reliable diagnosis of Litz wires as used in electric motors. Loss of strands causing an open circuit and / or strand-to-strand short circuiting can be detected and identified.

Claims

1. A method of monitoring a Litz wire forming a winding of an electrical motor, the Litz wire comprising a plurality of strands, the method comprising: splitting the plurality of strands into a first bundle (2a) of strands and a second bundle (2b) of strands, terminating the Litz wire winding at a first end with a first termination terminal (3a) and a second end with a second termination terminal (3b) so that the wire creates transmission line effects; connecting a first terminal (4a) of a time domain reflectometry, TDR, device to one of the first and second bundles and a second terminal (4b) of the TDR device to the other of the first and second bundles; and characterised by: injecting an electromagnetic pulse into the first bundle with a transformer that is coupled to the first bundle; measuring via the TDR device a voltage across the first and second terminals (4a, 4b) and monitoring the state of the strands of the Litz wire based on the measured voltage; wherein the electromagnetic pulse induces an electromagnetic wave that travels along the wire and the voltage measured across the terminals (4a, 4b) is due to the electromagnetic pulse returning along the wire, the measured voltage providing an indication of the health of the wire; wherein the electromagnetic pulse travels to the first termination terminal (3a) at the first end and returns from the first termination terminal at the first end towards the second termination terminal (3b) at the second end.

2. The method of claim 1, wherein the wire is represented as a transmission wire by being represented as a stranded conductor terminated at each end.

3. A Litz wire diagnostic assembly in which a Litz wire, forming a winding of an electric motor, to be tested, comprising multiple strands split into a first bundle (2a) of strands and a second bundle (2b) of strands, the assembly comprising: a first termination terminal (3a) connected at a first end of the wire and a second terminal terminal (3b) connected at a second end of the wire such that the Litz wire represents a transmission line; a time delay reflectometry, TDR, device having a first terminal (4a) to be connected to the first bundle and a second terminal (4b) to be connected to the second bundle, the TDR device configured to excite an electromagnetic pulse into the first bundle that travels to the first termination terminal and returns towards the second termination terminal, the TDR device also being configured to measure a TDR voltage across the first and second terminals (4a, 4b) wherein the voltage is used to determine the health of strands of the wire, the TDR device including a magnetic coupling in the form of a transformer connected to the first bundle that excites the electromagnetic pulse into the first bundle; wherein the electromagnetic pulse induces an electromagnetic wave that travels along the wire and the voltage measured across the terminals (4a, 4b) is due to the electromagnetic pulse returning along the wire; wherein the electromagnetic pulse travels to the first termination terminal and returns from the first termination terminal towards the second termination terminal.

4. The assembly of claim 3, wherein the TDR device excites the electromagnetic pulse by injecting a current pulse into the first bundle.

5. The assembly of claim 3, wherein the TDR device excites the electromagnetic pulse by injecting a voltage into the first bundle.