Operating electrical machines from a DC link
A drive system has a number of electrical machines. One machine operates as a generator supplying an output to a DC link, which in turn supplies one or more machines operating as motors to drive loads. The motors are supplied through power converters. The total capacitance of the DC link is minimized and the link kept in a stable condition by preventing it falling to a hazardous level. One embodiment prevents the phase currents adding in such a manner as to reduce the DC link voltage. Another embodiment employs freewheeling in the phase currents to prevent the DC link voltage falling below a predetermined threshold.
1. A method of operating a plurality of electrical machines from a DC link, each machine having at least one phase winding, the method comprising monitoring the DC link voltage and, if the DC link voltage decreases to a predetermined threshold, controlling the current in at least one phase winding in at least one of the electrical machines to modify the current in the DC link so as to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines, wherein the controlling the current includes making the profile of the current in the at least one phase winding dependent on time.
2. A method of operating a plurality of electrical machines from a DC link, each machine having at least one phase winding, the method comprising monitoring the DC link voltage and, if the DC link voltage decreases to a predetermined threshold, controlling the current in at least one phase winding in at least one of the electrical machines to modify the current in the DC link so as to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines, wherein the controlling the current includes making the profile of the current in the at least one phase winding dependent on angle.
3. A method as claimed in claim 1 or 2 in which the current in the at least one phase winding is controlled by chopping.
4. A method of operating a plurality of electrical machines from a DC link, each machine having at least one phase winding, the method comprising monitoring the DC link voltage and, if the DC link voltage decreases to a predetermined threshold, controlling the current in at least one phase winding in at least one of the electrical machines to modify the current in the DC link so as to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines, wherein at least one of the plurality of electrical machines is a switched reluctance machine, and wherein the controlling the current includes freewheeling the current in at least one of the phase windings of the switched reluctance machine.
5. A method as claimed in claim 4 , including returning current from the at least one phase winding to the DC link if the DC link voltage has not recovered after the freewheeling has been carried out for a predetermined period.
6. A method of operating a plurality of electrical machines from a DC link, each machine having at least one phase winding, the method comprising monitoring the DC link voltage and, if the DC link voltage decreases to a predetermined threshold, controlling the current in at least one phase winding in at least one of the electrical machines to modify the current in the DC link so as to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines, wherein at least one of the plurality of electrical machines is a switched reluctance machine, and wherein the controlling the current includes returning current from the at least one phase winding of the switched reluctance machine to the DC link.
7. A method as claimed in claim 6 , wherein the controlling the current includes freewheeling the current in the at least one phase winding, and thereafter carrying out the returning current if the DC link voltage has not recovered after a predetermined period of the freewheeling.
8. A method as claimed in claim 4 or 6 in which the DC link is supplied by a further electrical machine running as a generator.
9. A method as claimed in claim 8 in which the further electrical machine is a reluctance machine.
10. A system for operating a plurality of electrical machines comprising: a power converter including a DC link to which the electrical machines are connected, each machine having at least one phase winding;
means for monitoring the DC link voltage; and means for operating the power converter, in response to a signal from the means for monitoring that indicates the DC link voltage has decreased to a predetermined threshold, so as to control the current in the at least one phase winding to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines; wherein the means for operating the power converter includes means for controlling the profile of the current in the at least one phase winding of the at least one electrical machine.
11. A system for operating a plurality of electrical machines comprising: a power converter including a DC link to which the electrical machines are connected, each machine having at least one phase winding: means for monitoring the DC link voltage; and means for operating the power converter, in response to a signal from the means for monitoring that indicates the DC link voltage has decreased to a predetermined threshold, so as to control the current in the at least one phase winding to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines; wherein at least one of the plurality of electrical machines is a switched reluctance machine; and wherein the means for operating the power converter includes means for freewheeling the current in the at least one phase winding in response to the signal.
12. A system as claimed in claim 11 , wherein the means for operating the power converter includes means for returning current from the at least one phase winding to the DC link if the DC link voltage has not recovered after the freewheeling has been carried out for a predetermined period.
13. A system for operating a plurality of electrical machines comprising: a power converter including a DC link to which the electrical machines are connected, each machine having at least one phase winding; means for monitoring the DC link voltage; and means for operating the power converter, in response to a signal from the means for monitoring that indicates the DC link voltage has decreased to a predetermined threshold, so as to control the current in the at least one phase winding to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines; wherein at least one of the plurality of electrical machines is a switched reluctance machine; and wherein the means for operating the power converter includes means for returning current from at least one of the phase windings of the switched reluctance machine to the DC link in response to the signal.
14. A system as claimed in claim 13 , wherein the means for operating the power converter includes means for freewheeling the current in the at least one phase winding, and thereafter carries out the returning current if the DC link voltage has not recovered after a predetermined period of the freewheeling.
15. A system as claimed in claim 11 or 13 including a further electrical machine connected to the DC link and arranged to be run as a generator.
16. A system as claimed in claim 15 in which the further electrical machine is a reluctance machine.