HEALTH MONITORING IN ELECTRIC MOTOR CONTROL SYSTEMS

Abstract

The present invention relates to a system and method for the monitoring and detection of insulation degradation in electric systems. The system comprises a controller for an electric motor (3), including input circuitry (2a) for connecting the controller to a power supply (1), power conversion circuitry (2b) for providing a power output for the electric motor (3), and sensing circuitry (2c) for monitoring a current inside the controller that is representative of a return leakage current from the electric motor to the motor controller. A condition of the insulation may be determined based on the monitored current.

Claims

1. A controller for an electric motor, the controller comprising: input circuitry for connecting the controller to a power supply; power conversion circuitry for providing a power output for the electric motor, wherein the input circuitry comprises an electromagnetic interference filter for isolating the power conversion circuitry from electromagnetic interference from the power supply; and sensing circuitry for monitoring a current inside the controller that is representative of a return leakage current from the electric motor to the motor controller.

2. The controller of claim 1, wherein the electromagnetic interference filter comprises a common mode choke.

3. The controller of claim 1, wherein the sensing circuitry is configured to monitor a voltage drop between a chassis in which the controller is mounted and the electromagnetic interference filter.

4. The controller of claim 1, wherein the sensing circuitry is configured to monitor a voltage drop between a chassis in which the controller is mounted and the power conversion circuitry.

5. The controller of claim 3, wherein the sensing circuitry comprises a shunt resistor connected between the chassis in which the controller is mounted and the motor controller.

6. The controller of claim 1, wherein the sensing circuitry is configured to monitor a flux generated in the motor controller by the return leakage current.

7. The controller of claim 6, wherein the sensing circuitry comprises at least one of (i) a wire coil such as a Rogowski coil and (ii) a flux concentrator, and wherein the wire coil or flux concentrator is provided downstream of the input circuitry of the controller.

8. The controller of claim 6, wherein the sensing circuitry comprises an additional coil winding within an inductive choke of the EMI filter of the input circuitry of the controller, wherein the sensing circuitry measures a flux through the input circuitry.

9. The system of claim 8, wherein the EMI filter of the input circuitry comprises a first choke and second choke, the first choke connected in series between the second choke and the power supply; and wherein the sensing circuitry comprises an additional winding on the second choke, and wherein the sensing circuitry is operable to measure a flux in a core of the second choke.

10. The controller of claim 1, wherein the power conversion circuitry comprises a first stage for converting an alternating current to a direct current and a second stage for converting a direct current to an alternating current for output as the motor load, and a capacitor arrangement between the first stage and the second stage.

11. The controller of claim 10, wherein a shunt resistor is connected between a chassis in which the controller is mounted and the capacitor arrangement of the power conversion circuitry.

12. The controller of claim 1, further comprising a computing device for processing the monitored current to generate an output indicative of a condition of insulation of the electric motor, and optionally wherein the controller comprises a memory device for storing the monitored current values.

13. An electric motor system comprising a power supply, an electric motor and a controller according to claim 1 connected between the power supply and the electric motor.

14. The electric motor system of claim 13, further comprising a computing device for processing the monitored current to generate an output indicative of a condition of insulation of the electric motor, wherein the sensing circuitry is operable to transmit a signal indicative of the monitored current to the computing device for processing.

15. A method for monitoring a condition of insulation of an electric motor connected to a controller, the controller comprising: input circuitry for connecting the controller to a power supply; power conversion circuitry for providing a power output for the electric motor, wherein the input circuitry comprises an electromagnetic interference filter for isolating the power conversion circuitry from electromagnetic interference from the power supply; and sensing circuitry for monitoring a current inside the controller that is representative of a return leakage current from the electric motor to the motor controller; wherein the method comprises: monitoring, using the sensing circuitry, a current representative of the return leakage current; and determining, based on the monitored current, a condition of the motor insulation.

Description

BRIEF DESCRIPTION

[0034] FIG. 1 is a schematic of an electric motor system.

[0035] FIG. 2 is an example of an electric motor system with a single-stage EMI filter.

[0036] FIG. 3 is an example of an electric motor system with a two-stage EMI filter.

[0037] FIGS. 4a and b are example systems for monitoring degradation of the insulation using a resistor according to an embodiment.

[0038] FIG. 5 is an example system for monitoring degradation of the insulation using a coil according to an embodiment.

[0039] FIG. 6 is an example system for monitoring degradation of the insulation using an additional choke winding according to an embodiment.

[0040] FIG. 7 is an example system for monitoring degradation of the insulation according to an embodiment.

[0041] FIG. 8 is an example flow diagram for a method according to an embodiment.

[0042] Like reference numerals are used for like components where appropriate in the Figures.

DETAILED DESCRIPTION

[0043] FIG. 1 is a schematic diagram showing an electric motor system. The system comprises a power source 1, an electric motor 3 and a motor controller 2 for the electric motor 3. The power source 1 is typically an AC power supply. The controller comprises input circuitry 2a for connecting the controller 2 to the power supply and a power conversion circuitry 2b for providing a power output to the electric motor 3.

[0044] At least the motor 3 sits within a chassis or motor housing (not shown), and is insulated from the chassis and wider aircraft structure by insulation such as the motor winding insulation. However, as the condition of the insulation deteriorates (for example, due to age), the leakage current travelling through the insulation of the motor winding to the chassis is expected to change, for example to increase.

[0045] The Applicants have realised that at least a part of the leakage current will form a closed loop by travelling from the motor 3 to the controller 2 (via the chassis), and then returning to the motor 3 from the controller 2. For example, in system for an aircraft, the leakage current may flow in a closed loop from the controller 2 to the electric motor 3 via feeder cables. The displacement current may then enter an aircraft grounding or a feeder cable shield and return to the controller 2, via e.g. an EMI filter.

[0046] The controller 2 therefore further comprises sensing circuitry 2c, operable to monitor a current representative of this returning leakage current. The electric motor system further comprises a computing device 4 to determine the returning leakage current from the monitored current, and thereby determine a status of the motor insulation. The computing device 4 can be an external device that receives data from the sensing circuitry, or may otherwise be incorporated into the controller 2 and/or the sensing circuitry 2c. Optionally, the controller 2 and/or computing device 4 may include a memory device (not shown) for storing past values and/or features of the leakage current for further analysis, such as long term drift analysis.

[0047] FIG. 2 shows an example of a typical electric motor system 200, such as an electric motor system suitable for use on an aircraft. The system 200 comprises a power source 1, an electric motor 3 and a motor controller 2a,b. The controller includes input circuitry 2a and power conversion circuitry 2b. As previously discussed, as the condition of the insulation of the motor 3 deteriorates, an increasing leakage current may start to travel through the insulation of the motor winding of the motor 3 to the chassis, as shown by connection 3a. Connection 3a can be provided by various sources. For example, the connection 3a may be e.g. a direct electrical connection between the motor and the chassis, or alternatively may represent a current induced in the chassis by the motor 3.

[0048] The input circuitry 2a comprises an EMI filter for reducing reduce electronic noise that may cause interference with other devices. The EMI filter includes a choke 5 and differential capacitors 6. The choke 5 is connected to the input of the input circuitry 2a and separates the motor controller from unwanted frequencies, such as high frequencies, from external components such as the power source 1. The effect of this is therefore that any common mode current measured by the sensing circuitry 2c will be dominated by the current leaking through the insulation and returning the motor controller through the chassis, such that by including appropriate sensing circuitry 2c within the motor controller it is possible to reliably monitor such return leakage current, and in turn to monitor the condition of the motor insulation. Further, this can be done in a relatively simple manner, by including such sensing circuitry 2c within the motor controller, as described further below, without having to attempt to monitor the current directly in the grounding network of the system.

[0049] The power conversion circuitry 2b can comprise an input stage 8, such as a rectifier, for converting an alternating current to a direct current, and an output stage 10, such as an inverter, for converting the direct current to an alternating current. Thus, the power conversion circuitry 2b may generally comprise an AC-to-AC converter.

[0050] The power conversion circuitry may also further comprise a DC link capacitor 9, which assists in improving the stability of the DC voltage, acts as a local energy storage and facilitates short communication loops for the safe operation of the input stage 8 an/or the output stage 10. Optionally, the power conversion circuitry 2b may further comprise additional capacitors 11, which are operable to reference the DC link capacitor 9 to the chassis and thereby reduce the common mode currents visible at the input terminals of the DC link capacitor 9.

[0051] FIG. 3 shows a further example of an electric motor system 300. FIG. 3 is largely identical to FIG. 2. However, the input circuitry 2a of electric motor system 300 includes a two-stage EMI filter. The two-stage EMI filter comprises a first choke 5 and a second choke 7. The first choke 5 is connected in the same manner as choke 5 of FIG. 1, while second choke 7 is connected between the differential mode capacitors 6 and power conversion circuitry 2b.

[0052] FIG. 4a shows a system 400a suitable for monitoring deterioration of the motor insulation according to an embodiment. The system 400a comprises sensing circuitry consisting of a shunt resistor 12 connected between the input circuitry and the chassis of the system 400a.

[0053] Specifically, the shunt resistor 12 is connected between the differential mode capacitors 6 and the chassis. By measuring the voltage drop across the shunt resistor 12, the leakage current flowing from the chassis to the input circuitry can be determined accordingly.

[0054] FIG. 4b shows a system 400b which is similar to system 400a, but comprises sensing circuitry between the chassis and the power conversion circuitry. The sensing circuitry again comprises a shunt resistor 13, which is connected between capacitors 11 and the chassis. Shunt resistor 13 provides the same function as shunt resistor 12, in that the voltage drop across it can be measured to provide an indication of the returning leakage current.

[0055] In both systems 400a and 400b the leakage current forms a closed loop, flowing from the motor 3 into the chassis at connection 3a, before returning to the motor 3 via the controller. However, in system 400a the leakage current flows into the controller via the EMI filter of the input circuitry, while in system 400b the leakage current is split between the EMI filter (via capacitors 6) and the power conversion circuitry (via capacitors 11). As a result, analysis of the leakage current in FIG. 4b may primarily focus on features of the leakage current (such as the rate of change of the current or a threshold increase relative to an initial measurement) rather than an absolute measurement when determining a state of the insulation.

[0056] Alternatively, it will be understood that the sensing circuitry may comprise a means for the providing an indication of the returning leakage current at both the EMI filter (i.e. via capacitors 6) and the power conversion circuitry (i.e. via capacitors 11). For example, the system may comprise both shunt resistor 12 and shunt resistor 13.

[0057] It will be further understood that system 400a, and all other systems disclosed herein (including systems 500, 600 and 700) may optionally comprise the additional capacitors 11, and that the analysis methods discussed in relation to FIG. 4b may also be used to determine the state of the insulation based on the measurements of the leakage current returning via capacitors 6.

[0058] In each of systems 400a and 400b, the sensing circuitry may further comprise a resistor and/or a capacitor (not shown) connected in series with shunt resistors 12 or 13. A resistor with a greater resistance than shunt resistor 12, 13 can assist in limiting the unbalance current which may flow due to imbalances in three phase AC supplies. Similarly, the capacitor may provide an additional common mode filter stage without eliminating the common mode current signature

[0059] As the shunt resistors 12 and 13 provide similar methods for monitoring the returning leakage current, they can be incorporated into motor controllers as desired based on the available space in the system.

[0060] While systems 400a and 400b are shown with two-phase EMI filters, it will be understood that shunt resistors 12 and 13 may instead be implemented in systems comprising a single phase EMI filter.

[0061] FIG. 5 shows a system 500 according to a further embodiment. As in system 400a, in system 500 the leakage current flows from motor 3 into the chassis via contact 3a, and then forms a closed loop by flowing into the controller via the EMI filter of the input circuitry.

[0062] In system 500, the sensing circuitry of the controller comprises a flux concentrator 14, such as a wire coil (for example a Rogowski coil) or a magnetic flux concentrator. Optionally, a Rogowski coil may be provided without an integrator in order to assist in recording the magnetic flux as an instantaneous (di/dt) value. The flux concentrator 14 can detect a flux flowing through the controller power cables. The returning leakage current results in a magnetic flux, and as a result the flux total through the controller provides an indication of the magnitude and/or features of the returning leakage current (which may include, for example, the insulation displacement current). Flux concentrator 14 therefore provides a means for measuring the returning leakage current based on these variations.

[0063] The flux concentrator 14 maybe positioned within the input circuitry (for example, wrapped around the input phases in position 14a as shown in FIG. 5), or may alternatively be positioned in the power conversion circuitry (for example, wrapped around the DC link capacitor terminals in positions 14b or 14c).

[0064] While system 500 is shown with a two-stage EMI filter, it will be understood that wire coil 10 may instead be implemented in a system comprising a single-stage EMI filter, such as system 300 of FIG. 3.

[0065] FIG. 6 shows a system 600 according to a further embodiment. As in systems 400a and 500, in system 600 the leakage current flows from motor 3 into the chassis via contact 3a, and then forms a closed loop by flowing into the controller via the EMI filter of the input circuitry.

[0066] In system 600, the sensing circuitry of the controller comprises an additional winding 15 on the second choke 7. The returning leakage current results in a variation in the flux in the common mode core of the second choke 7. Therefore, similarly to the wire coil of system 500, the additional winding 15 provides a means for measuring the returning leakage current based on these variations in the measured flux.

[0067] It will be understood that the sensing circuitry of the controller is not limited merely to the above examples, and may instead comprise any combination of the above described components for monitoring the returning leakage current. Providing multiple means for monitoring the returning leakage current ensures that there is redundancy within the system and improves the reliability of the measurements. For example, FIG. 7 shows an example system 700 including a sensing circuitry that comprises each of shunt resistor 12, flux concentrator 14 and additional winding 15. Equally, it will be understood that any other combinations of the above described sensing circuits may be employed, for example utilising shunt resistor 13 in place of or in addition to the shunt resistor 12 shown in FIG. 7. Similarly, flux concentrator 14 may be provided at any one or more of the locations 14a, b and c shown in FIG. 5, and the system may optionally be provided with multiple flux concentrators at different locations and/or the same location. For example, the system may include a Rogowski coil at location 14a and a magnetic flux concentrator at location 14b, or vice versa.

[0068] FIG. 8 is a flow diagram 800 for showing an example method according to an embodiment. The method comprises determining a returning leakage current of the system in step 802, and then, based on this returning leakage current, determining a status of the motor insulation in step 804.

[0069] Step 802 may comprise monitoring a current indicative of the returning leakage current by any one or more of the methods described above, and then, based on the data provided by the sensing circuit, determining the returning leakage current. For example, if the sensing circuitry comprises a shunt resistor with a known resistance (as discussed in e.g. FIGS. 4a and 4b), the returning leakage current may be determined by measuring a voltage drop across the shunt resistor and applying Ohm's law to find the current through the shunt resistor.

[0070] In step 804, the status of the insulation can be determined using any suitable relationship between the leakage current and the insulation status, as desired. For example, the status of the insulation may be determined using a threshold leakage current and/or a threshold increase from an initial leakage current, or, in another example, by monitoring the leakage current for a characteristics such as a spike that indicates a sudden deterioration in the condition of the insulation or a characteristic rate of change of the leakage current.

[0071] Variations on the examples described above fall within the scope of the claims.