Method of operating a switched reluctance machine

Abstract

A controller for a switched reluctance machine is operated to close a switch which would otherwise be open so as to connect a phase winding to the DC link to which the winding is connected during other parts of the electrical cycle. This produces a condition which allows the insulation of the system to be monitored by applying a voltage between the DC link and ground.

Claims

1. A method of operating a 2-switch-per-phase power converter for a switched reluctance machine, the method comprising: keeping both switches of a phase closed for a first period within a phase cycle of the phase to connect a phase winding to a DC link; subsequent to the first period, opening both switches of the phase for a second period within the phase cycle and allowing any current in the phase winding to decay to zero; and thereafter closing one of the switches of the phase to connect the winding to the DC link for a third period during which no current flows in the phase winding.

2. A method according to claim 1, comprising keeping the one of the switches closed at the end of the third period and closing the other one of the switches at the start of the first period of the next phase cycle, thereby starting the first period of the next phase cycle immediately as the third period ends.

3. A method according to claim 1, wherein the third period substantially fills the interval between the second period and the first period of the next phase cycle.

4. A method according to claim 1, comprising freewheeling the phase for a fourth period between the first and second period.

5. A method according to claim 1, wherein the periods substantially fill the phase cycle.

6. A method according to claim 1, comprising monitoring insulation integrity of the phase during the periods.

7. A switched reluctance drive system comprising: a switched reluctance machine; a controller; a 2-switch-per-phase power converter coupled to one or more phase windings of the switched reluctance machine, wherein the controller is coupled to the power converter to control power being supplied to the one or more phase windings and wherein the controller is configured to: keep both switches of a phase closed for a first period within a phase cycle of the phase to connect a phase winding to a DC link; subsequent to the first period, open both switches of the phase for a second period within the phase cycle to allow any current in the phase winding to decay to zero; and thereafter close one of the switches of the phase to connect the winding to the DC link for a third period during which no current flows in the phase winding.

8. A switched reluctance drive system according to claim 7, wherein the controller is configured to keep the one of the switches closed at the end of the third period and close the other one of the switches at the start of the first period of the next phase cycle, thereby starting the first period of the next phase cycle immediately as the third period ends.

9. A switched reluctance drive system according to claim 7, wherein the third period substantially fills the interval between the second period and the first period of the next phase cycle.

10. A switched reluctance drive system according to claim 7, wherein the controller is configured to freewheel the phase for a fourth period between the first and second period.

11. A switched reluctance drive system according to claim 7, wherein the periods substantially fill the phase cycle.

12. A switched reluctance drive system according to claim 7, wherein the controller is configured to monitor insulation integrity of the phase during the periods.

13. A switched reluctance drive system according to claim 7, wherein the controller is configured to open and/or close the switches based on parameters of the switched reluctance machine comprising one or more of a rotor speed, a phase current, a phase voltage, or a phase flux, wherein the phase flux comprises a magnetic flux of the phase winding.

14. A switched reluctance drive system according to claim 13, further comprising a current transducer configured to provide electrical signals associated with the phase current to the controller.

15. A switched reluctance drive system according to claim 7, wherein the controller determines a duration of one or more of the first, the second, and/or the third period of the phase cycle based on rated operating conditions comprising one or more of rotor speed, phase current, or phase voltage of the switched reluctance machine.

16. A switched reluctance drive system according to claim 7, further comprising a current transducer, coupled to the phase winding, configured to sense a current flowing in the phase winding and provide an electrical signal to the controller, wherein the controller is configured to open and/or close one or more of the switches at least in part in response to the electrical signal.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) These and other aspects and advantages of the invention will become apparent upon reading the following detailed description of exemplary embodiments of the invention and upon reference to the accompanying drawings, in which:

(2) FIG. 1 shows the principal components of a switched reluctance drive system;

(3) FIG. 2 shows a typical switching circuit for a power converter that controls the energisation of the phase windings of the machine of FIG. 1;

(4) FIG. 3 shows a typical 3-phase switching circuit for a power converter;

(5) FIG. 4 shows a typical example of an induction machine connected to a 3-phase, symmetrical bridge inverter;

(6) FIGS. 5a, 5b, and 5c show voltage and current waveforms for a phase winding of a switched reluctance machine in single-pulse mode;

(7) FIGS. 6a, 6b show current waveforms for a phase winding of a switched reluctance machine in chopping mode;

(8) FIG. 7 shows the principal components of a switched reluctance drive system in accordance with an embodiment; and

(9) FIG. 8 shows a flowchart describing a method of operating a switched reluctance drive system in accordance with an embodiment.

DETAILED DESCRIPTION

(10) FIGS. 5a-5c show the typical energisation scheme for single-pulse operation of a switched reluctance machine, as described in Stephenson and Blake above. The DC link voltage is applied at a predetermined rotor angle .sub.on, shown in FIG. 5(b) by closing both switches 21, 22. The application continues during the conduction angle .sub.c until the switch-off angle .sub.off, when the switches are opened and the current flows through the diodes 23, 24 back to the DC link. During this period of decaying current, energy is returned to the DC link. While the current in the phase winding is unidirectional, as shown in FIG. 5(b), the current in the DC link is bi-directional. Current continues to decay until the diodes 23, 24 become reverse biased and current flow ceases at .sub.idle.

(11) There is then a period, as shown in FIG. 5(b) where there is no current flowing and the winding is isolated from the DC link.

(12) FIGS. 6a and 6b show corresponding operation in the chopping mode. It is common for the conduction to start at .sub.on in the centre of the minimum inductance region and to end at .sub.off in the centre of the maximum inductance region, as shown in FIG. 6(a). Between these two points, the current is allowed to cycle between upper and lower hysteresis levels I.sub.u, I.sub.l. The period of disconnection from the DC link often approaches 50% of the overall cycle. The DC link current is shown in FIG. 6(b). As in FIGS. 5a and 5b, there is a significant period during which the phase is not connected to the DC link between .sub.idle and .sub.on.

(13) It is well-known, as described above and in Stephenson, to interpose a short period of freewheeling starting at a freewheel angle .sub.fw between .sub.on and .sub.off, that is, between the conduction period and the energy return period, often for reasons of noise or torque ripple control. While effective to ameliorate these problems, this technique does not help with detection of insulation faults, as there are still long periods where the winding is not connected to the DC link.

(14) Inspection of the asymmetrical bridge circuit shown in FIG. 2 shows that, if the phase winding has no current flowing in it, it is possible to close, say, the lower switch 22 (leaving the upper switch 21 open) without provoking any current to flow. This is because the upper switch 21 is open and the diodes are reverse biased. Nevertheless, the winding is now connected to the DC link, so it is possible now to monitor the integrity of the insulation. The same applies, of course, to closing the upper switch 21 while leaving the lower switch 22 open.

(15) In principle, the winding can therefore be connected continuously to the DC link throughout the electrical cycle, either through the switches in the conduction period, the diodes during the energy return period or a single switch during the idle period between the current reaching zero at an angle .sub.idle and the next energisation at .sub.on. In practice, it may be prudent to insert a very short gap after the energy return period to ensure that all the phase current has decayed to zero before closing the switch, but the gap can be small compared to the overall cycle and can be of negligible effect.

(16) It is therefore possible to modify the conventional operation of a switched reluctance machine controller so that, by closing a switch which would otherwise be open, it is possible to allow the integrity of the insulation of the system to be monitored. This monitoring can be done in any of the known ways, for example as described in U.S. Pat. No. 9,069,025.

(17) With reference to FIG. 7, a switched reluctance drive system in accordance with an embodiment comprises a controller 14 having an insulation integrity monitoring module 14a and a switching module 14b adapted to facilitate monitoring of the integrity of the insulation of the system. Otherwise, the system is configured as described above with reference to FIG. 1. In particular, the machine 12 is configured as described above with reference to FIG. 2, with switches 21 and 22 and current return diodes 24 and 23 on either side between a phase winding 16 and voltage rails 26 and 27. As described above, the insulation integrity monitoring module 14a is adapted to monitor the integrity of the insulation of the system by applying a small current (AC or DC) between the DC link 25 and ground.

(18) In some particular embodiments, the module 14a is configured to implement a monitoring method as described in U.S. Pat. No. 9,069,025, which is hereby incorporated by reference in its entirety. In other embodiments, module 14a can be located remotely from the other components of the controller 14.

(19) With reference to FIG. 8, the switching module 14b is now described in further detail. The switching module 14b is configured to operate the switches 21, 22 as follows below.

(20) At step 81, the switching module 14b causes both switches of a phase to be energised at the switch-on angle .sub.on for that phase. The phase is then energised either in chopping or single pulse mode as described above. Subsequently, in some embodiments, at an optional step 82, the switching module 14b causes one of the switches to open to freewheel the phase starting at the freewheel angle .sub.fw. Subsequently (or immediately after the conduction period starting at .sub.on in embodiments where there is no freewheeling) a switching module 14b causes both switches to open at the switch-off angle .sub.off to return current (step 83). Both switches remain open until the current has effectively decayed to zero. Subsequently, at .sub.idle, one of the two switches is closed again, which results in the phase in question being connected again to the voltage supply without any current flow being caused (step 85). Finally the switching module 14b loops back to step 81 and both switches are closed (that is, the switch left open at step 85 is also closed) at .sub.on of the next phase cycle (step 81).

(21) In some embodiments, .sub.idle, the angle at which one switch is closed subsequent to energy return, is fixed for a given operating condition in a similar manner as .sub.on, and .sub.off (as well as .sub.fw in embodiments which use freewheeling), as a function of the drive's operating condition, such that current will have effectively decayed to zero at .sub.idle. The parameters that may be used in this determination may include one or more of rotor speed, phase current, phase voltage, phase flux, current or torque command, etc. In other embodiments, a fixed value of .sub.idle is used, chosen such that current will have decayed to zero in the phase at that point for the range of rated operating conditions of the drive. In yet further embodiments, an additional, optional step 84 is inserted between steps 83 and 85, at which a phase current is monitored to determine the point in the phase cycle when current has actually decayed to zero (or substantially to zero) and to close one of the switches in response to the detector 14c output, thus in effect setting .sub.idle dynamically.

(22) It will be appreciated that, in accordance with the above embodiments, the phase winding will be connected to the DC link substantially during the entire period from when current decays to zero and the diodes 23 and 24 become reversed biased until the switches are closed again at angle .sub.on, that is substantially the entire time between the energy return period starting at .sub.off and the next conduction period starting at .sub.on. Thus, the phase will be connected to the DC link substantially over the entire phase cycle during the phase conduction period, freewheeling period (where used), energy return period and idle period starting at .sub.idle, thereby enabling the insulation integrity monitoring module 14a to monitor the integrity of the insulation over substantially the entire phase cycle. Similarly, the idle period starts when the current has decayed substantially to zero such that there are no undesirable effects on machine performance from any residual current there may be, such as noticeable torque ripple or noise associated with any corresponding residual torques.

(23) While the above description has been made in the context of monitoring the integrity of the insulation of phase windings, it will be appreciated that there may be other contexts in which it may be useful to maintain a connection between the DC link and phase winding over substantially the entire phase cycle. Likewise, it will be appreciated that the benefits of the disclosed invention also accrue where the idle period with one switch open and one switch closed subsequent to energy return extends the period of connection of the phase winding to the DC link over prior art approaches without extending this over the entire phase cycle, for example where both switches are opened again prior to the next switch-on angle .sub.on.

(24) The skilled person will appreciate that variation of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of embodiments is made by way of example and not for the purposes of limitation. It will be clear to the skilled person that minor modifications can be made to the arrangements without significant changes to the operation described above. The present invention is intended to be limited only by the scope of the following claims.