Electric machine fault detection scheme

Abstract

A short circuit detection system and method that identifies a short circuit between turns of a winding of a permanent magnet machine having a three-phase winding in response to detection of imbalances between the three motor phases at an instant in time. The imbalances are identified by monitoring motor terminal voltages and currents.

Claims

1. A method of detecting an inter-turn short-circuit in a winding of a multiple phase motor, being a multiple winding permanent magnet machine, the method comprising: detecting an imbalance between current and/or voltage of winding of a multiple phase motor, being a multiple winding permanent magnet machine; and determining a short-circuit in a winding of the machine based on detection of the imbalance; wherein at zero to medium motor speeds, the imbalance is detected by injecting a high frequency signal into a signal provided to drive the motor and determining a short-circuit is based on an imbalance in a feedback signal from the motor; and wherein at higher motor speeds determining a short-circuit based on the signal to drive the motor.

2. The method of claim 1, wherein the imbalance is detected in drive signals provided to the permanent magnet machine terminals at higher motor speeds.

3. The method of claim 1, wherein the imbalance is detected in feedback signals from the permanent magnet machine at zero to medium motor speeds.

4. The method of claim 1, wherein the high frequency signal is injected into command voltages in an alpha-beta frame.

5. The method of claim 1, wherein the high frequency signal is injected into voltages in a dq frame.

6. The method of claim 1, wherein the high frequency signal is injected directly into phase voltages at the motor terminals.

7. The method of claim 1, wherein a short-circuit is determined using a processing function comprising an absolute function, a low pass filter and a window comparator to determine whether all phase signals are within a given band, wherein if signals are outside the band, a short-circuit is determined.

8. The method of claim 1, wherein on determination of a short circuit in a winding, the winding is deactivated from operation of the permanent magnet machine.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic circuit diagram showing an example of the short circuit detection scheme according to an embodiment of the disclosure at lower speeds.

(2) FIG. 2 is a schematic circuit diagram showing an example of the short circuit detection scheme according to an embodiment of the disclosure at higher speeds.

DETAILED DESCRIPTION

(3) Referring to the figures, an embodiment of the detection methodology will be described. This description is given by way of example only and variations may fall within the scope of the invention as defined by the claims.

(4) FIG. 1 shows the reference signals idref and iqref provided by a control unit (1) and the controller 2 that includes current loops PI of the PMSM which provide signals to drive the load (6). In a typical system, the signals are transformed from the dq frame to the alpha-beta (αβ) frame (3) and then into the abc frame. The abc frame signals are modulated (4) to control the inverter (5) before being supplied to the load (6). The speed and/or position of the motor load (6) is fed back (8), with measured motor currents (7) converted to dq reference frame (9) to control the PMSM in the known manner.

(5) When the motor is at standstill or operating at low to medium speed, a small high frequency signal is injected into the alpha-beta command voltages (18). Alternatively, if correctly conditioned, the signal could be injected to the dq frame (17) or directly into the phase voltages (19). It is important that the size of the signal is kept small compared to the main motor excitation signals to prevent interference with the main motor control loop.

(6) The frequency of the injected signal is selected to be greater than the bandwidth of the current loops of the controller (2). This prevents the controller 2 from seeing the injection as a disturbance and trying to counteract its effects.

(7) Any inter-turn short circuit fault will result in an unbalanced high frequency current which can be detected in the d-q current feedback using a dedicated processing function (11). The detection processing function (11) may consist of a high-pass filter (12), an inverse Clark and Park transform that converts components in the orthogonal rotating reference frame (dq) to components in an orthogonal stationary frame (αβ) and then into components of a three-phase system (in abc frame) (13), an absolute function that removes the sign of the signal (14), a low pass filter (15) and a window comparator (16) that checks if all of the three-phase signals are within a certain band. In the event of an imbalance between the three phases where the three phase signals are not all within the band, an inter-turn short circuit is detected.

(8) At medium to high motor speeds, an inter-turn short circuit, and a corresponding reduction in phase inductance, causes an imbalance to occur on the motor driving voltages. The closed loop current feedback controller (2) creates an imbalanced voltage since it counteracts for the imbalanced phase inductance caused by the short circuit. The imbalanced voltages are measured in the abc frame (10a) and are then processed using a detection algorithm (11a). The algorithm preferably consists of an absolute function that removes the sign of the signal (14a), a low pass filter (15a) and a window comparator (16a) which, as above, checks if all three phase signals are within a certain band. If not, the imbalance is an indication of a short circuit.

(9) Because the movement of the rotor excites change in the motor voltages, there is no need, at higher speeds, to inject a signal, but this is needed at lower speeds.

(10) Once a short-circuit is detected according to this disclosure, the fault tolerant mechanism described in the Background can be triggered—i.e. the faulty winding can be short-circuited in a known manner. Using the methodology of this disclosure allows faults to be detected sooner than previously.

(11) The inter-turn detection algorithm of this disclosure is an enabling technology for existing fault-tolerant PMSM systems. The methodology makes use of hardware that is already available in typical motor controllers and no additional sensors are required. The scheme operates across the entire motor speed range from stall to high speed. At zero speed, the scheme can also be used as a diagnostic method for checking the health of the generator in the motoring mode.

(12) The method can be implemented within e.g. a programmable logic or a microprocessor-type embedded controller.

(13) The described embodiments are by way of example only. The scope of this disclosure is limited only by the claims.