Operating electrical machines from a DC link

Abstract

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 minimised 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.

Description

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:

FIG. 1 is a schematic drawing of a prior art switched reluctance drive;

FIG. 2 is a prior art excitation circuit for the switched reluctance machine of FIG. 1;

FIG. 3( a) is a phase voltage waveform for the circuit shown in FIG. 2;

FIG. 3( b) is the phase current waveform corresponding to FIG. 3(a);

FIG. 3( c) is the supply current waveform corresponding to FIG. 3(a);

FIG. 4( a) shows a prior art motoring current waveform;

FIG. 4( b) shows a prior art generating current waveform;

FIG. 5( a) shows a phase current in the chopping mode;

FIG. 5( b) shows the supply current corresponding to FIG. 5(a);

FIG. 6( a) shows a phase current in the chopping mode with freewheeling;

FIG. 6( b) shows the supply current corresponding to FIG. 6(a);

FIG. 7 shows a generating system supplying a single motor;

FIG. 8 shows a generating system supplying several motors;

FIG. 9( a) shows a waveform of DC link voltage for the operation of the system of FIG. 7;

FIG. 9( b) shows a waveform of motor phase current for the system of FIG. 7;

FIG. 10( a) shows a waveform of DC link voltage where the current demand collapses the link;

FIG. 10( b) shows the corresponding motor phase current;

FIG. 11( a) shows a waveform of DC link voltage according to an embodiment;

FIG. 11( b) shows profiled phase current waveforms corresponding to FIG. 11(a);

FIG. 12 shows phase current according to another embodiment;

FIG. 13 shows a flow chart depicting an embodiment of the invention;

FIG. 14 shows phase current according to a yet further embodiment; and

FIG. 15 shows a flow chart depicting a further embodiment of the invention.

The illustrations below refer to a system configured as shown in FIG. 7 with 3-phase machines. However, machines with any number of phases could be used in accordance with the invention and the phase number of the generator and motor(s) need not be the same. Control of the respective machines 72 and 78 is implemented in software loaded in the memory 81 of controls 75 and 79. According to their respective control algorithms the power converter switches are actuated. In this embodiment, the technique is implemented in the software. FIG. 7 also shows a monitoring device 83 which is arranged to monitor the DC link voltage. This is used for general system monitoring purposes, but is also used in relation to an embodiment disclosed herein. The monitoring device can take a number of different forms that will be known to the person of ordinary skill in the art and will not be elaborated on here.

FIG. 9 shows the operating characteristics of a system as shown in FIG. 7 but which is conventionally controlled. The motor is accelerating from a very low load at low speed to full load at higher speed. This could correspond to a vehicle operating under full acceleration demand, where the demand has a ramp to avoid exciting resonances in the drive train. The DC link voltage is shown as a function of time in FIG. 9(a) and the current waveform for one phase of the motor is shown in FIG. 9(b). The other phase currents are not shown, but will be interleaved with the waveform of FIG. 9(b) at the appropriate spacing. In FIG. 9(a) it will be seen that the voltage is essentially constant until the phase current begins to rise. The large periodic dips in the voltage correspond to the switch-on point for each incoming phase of the motor, indicating that the DC link capacitor cannot supply sufficient energy to maintain the voltage. The smaller perturbations on the waveform are due to a combination of the switching of the phases in the generator (which is at a much higher frequency) and the chopping action in the phases of the motor. In this particular example, the nominal voltage of 600V dips to 250V as the motor moves the load from rest, though the system survives this and recovers to successfully accelerate the load up to speed.

FIG. 10 shows the same system of FIG. 7 operating under slightly different conditions. The DC link voltage is shown as a function of time in FIG. 10(a) and the current waveform for one phase of the motor is shown in FIG. 10(b). In this example, the switching operations in the motor have coincided to produce a demand on the DC link that suddenly drags the voltage below the point where it is unable to supply sufficient excitation to the generator. The output of the generator fails to provide enough output to meet the system demand, so the link “stalls” to zero volts and the motor current collapses. This would represent a failure of the system to start and accelerate to the required speed. To address these stall conditions, the DC link needs to be stabilised against the random disturbances.

In a first embodiment, a predetermined profile is set for the phase currents of the motor(s) supplied by the DC link by means of the control regime implemented by the software in the memory 81 of the controllers 75 and 79. The current profile is chosen so that, when the profiled chopping currents from different phases are added, there is a greatly reduced likelihood of them combining to give the sudden demand which will provoke instability in the link voltage. FIG. 11(b) shows a suitably profiled phase current and should be compared with the conventional “square” current shape in FIG. 9(b). The current profiling can be achieved in the control software by any one of a number of known methods, e.g. by imposing angle-variable values for the upper and lower bounds of the hysteresis band, or by applying a pulse width modulation (PWM) function to the supply voltage via the phase switches. In each case, the resultant current waveform has an angle-dependent (and hence time-dependent) envelope which is characterised by a relatively slow rise at the front end and a relatively slow fall at the back end. The same profile can be applied to the other phases in the machine and to other phases of other machines operating on the same bus. The modified chopping profile is implemented in the software loaded in the control memory so that in effect a variable chopping limit is created.

FIG. 11( a) shows the resulting DC link waveform, which shows that the current profiling has the effect of reducing the amount of energy that needs to be drawn from the capacitor when the phase is switched on, and significantly reduces the voltage dip when the switch-on points of two or more phases coincide. FIG. 11(a) should be compared with FIG. 9(a), and it will be seen that, for the same peak value of phase current, the voltage dip has been reduced to less than half, thus providing a greatly enhanced stability of the link and security of system performance.

This embodiment provides a simple but effective method of stabilising the DC link. The reduction in output applies at all times, not just when a current spike would otherwise have been produced. This will be acceptable in many situations. It will be appreciated that a variety of different current profiles could be used to equal effect.

A second embodiment only shapes the currents at the moments when it is actually required. In this embodiment, the control system uses its knowledge of the instantaneous value of the DC link voltage from the monitoring device 83 in FIG. 7 and, in response to a signal from the monitoring device 83, prevents the switch-on of any phase when the voltage is dipping near to a predetermined threshold. Thus, although a rotor angle may have arrived at which a phase would have normally been switched on, the switch-on is delayed until the DC link voltage rises again and the dip, resulting from the delayed switch-on, will not present a hazard to the converter. This delay in switch-on inevitably produces a slight loss of torque, but only on a relatively small number of occasions. The current waveforms are as normal for most conduction periods with a few profiled (narrower) blocks when required. This is illustrated in FIG. 12, where the start of the third block of phase current has been delayed because of a dip in the DC link. The flow chart for implementing this embodiment is illustrated in FIG. 13.

This embodiment has the advantage that it takes no action until the stability of the DC link is threatened, thus allowing full torque to be obtained from the motor until a potentially dangerous switching action is encountered. Naturally, this requires that the bandwidth of the controller is sufficiently high to work out the potential danger and to modify the switching action(s) accordingly, but this is readily achievable with the processors currently used in such controllers.

A third embodiment, implemented in the control software, includes monitoring the DC link voltage using the monitoring device 83 as before, but does not delay the switching of any incoming phase. Instead, a predetermined DC link voltage threshold is set at a level below which operation is considered to be hazardous because the risk of the system stalling is unacceptably high. The control system monitors the DC link voltage, switching the phases as normal. When the predetermined DC link voltage threshold is reached, it forces one or more of the conducting phases into a freewheel condition by opening one of the phase switches in response to a signal from the monitoring device 83. This immediately reduces the current demand from the DC link for that phase to zero (as previously shown in FIG. 6(b)) yet maintains a significant level of current in the phase. The overall effect on the output of the motor is therefore relatively small. The control system continues to monitor the DC link voltage and, when it recovers to a value above the threshold, the phase(s) are put back into full conduction by re-closing the switch in response to a further signal from the monitoring device. Those skilled in the art will realise that a simple hysteresis band can be put around the threshold to avoid switching in and out of the stall prevention mode too frequently (commonly known as “chatter”). This is illustrated in FIG. 14, where the control system has sensed a voltage dip as the first full block of phase conduction has begun, and a period of freewheeling has been inserted in the rising edge of the block to allow the DC link to recover. The flow chart for implementing this embodiment is illustrated in FIG. 15.

This embodiment has the advantage that no action is taken until absolutely necessary, and even then the impact on the motor output is small, while the DC link is held in a stable and safe condition at all times. Effectively, this embodiment makes full use of the capacitance in the DC link by maintaining it at or just above the point of instability during conditions of high demand. Because of this, the capacitance on the link can be reduced to levels below that normally considered safe, so that a more economical system can be produced. The threshold voltage is predetermined, taking account of such factors as the number of machines in the system, the size of the generator relative to the motors, the speeds of the machines and their coupled inertias, the amount of capacitance, the reliability required from the system, etc. In one system operating on a nominal 650V bus, a threshold voltage of 320V is chosen, with a hysteresis band of 30V sitting above the 320V.

An alternative version of the above embodiment is to detect when the voltage threshold has been reached and then to put the phase(s) into full energy return by opening both phase switches. While this will force the flux and current down at a much faster rate and hence reduce the torque of the machine more, it has the benefit that the current being returned to the DC link will assist in recovering from the voltage dip. The system designer can therefore choose between the freewheel method giving smoother torque or the energy return method giving faster recovery. It is possible to combine these two variants to firstly put the phase(s) into freewheeling in the expectation that the voltage will recover, but secondly put the phase(s) into energy return if the recovery does not take place within a predetermined time. This predetermined time will be set principally in relation to the generator phase conduction period, which, for a machine with 12 rotor poles operating at 2000 rev/min, will typically be of the order of 2.5 msec.

The above embodiments have been illustrated with switched reluctance machines, but the invention is not restricted to them. It can be used with any electrical machine where there is control over the point at which voltage is applied to the phase windings, for example, electronically controlled synchronous or induction machines, brushless DC machines, etc. Also, the method and system of the invention can be applied to a mixture of electrical machine types, all, some or only one of which may be susceptible to the operating techniques disclosed herein. The operating technique of controlling the phase current can be applied to at least one phase of at least one of the electrical machines contributing to the combined load on the DC link. It will be clear to the skilled person having the benefit of this disclosure that the invention could be applied also to systems which are supplied not from a generator but from an alternating or direct supply which has high source impedance. Such a supply will exhibit characteristics similar to those described in detail above for a generator, so will benefit equally from use of the invention.

Embodiments include the computer program product stored on a computer readable medium as used in the system controllers. The medium may be solid state memory or other storage device enabling processing for controlling the machine to implement the control regime according to the disclosed embodiments. The controller may be a general purpose processor or other computer means running under the command of the program. Equally well, the embodiments can use a dedicated device, such as an application specific integrated circuit (ASIC).

The skilled person will appreciate that variation of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of several 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.

Claims

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 controlling the current in at least one phase winding in at least one of the electrical machines to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines.

2. A method as claimed in claim 1 in which the DC link is supplied by a further electrical machine running as a generator.

3. A method as claimed in claim 2 in which the further electrical machine is a reluctance machine.

4. A method as claimed in claim 1 in which the DC link voltage is monitored and the current in the at least one phase winding is controlled to modify the current in the DC link, thereby to control the magnitude of the said disturbances, in the event that the DC link voltage decreases to a predetermined threshold.

5. A method as claimed in claim 4 in which the current in the DC link is modified by delaying energisation of the at least one phase winding.

6. A method as claimed in claim 4 in which the current in the DC link is modified by making the profile of the current in the at least one phase winding dependent on time.

7. A method as claimed in claim 4 in which the current in the DC link is modified by making the profile of the current in the at least one phase winding dependent on angle.

8. A method as claimed in claim 6 or 7 in which the current in the at least one phase winding is controlled by chopping.

9. A method as claimed in claim 1 in which at least one of the plurality of electrical machines is a switched reluctance machine.

10. A method as claimed in claim 9 in which the current in the DC link is modified by freewheeling the current in at least one of the phase windings of the switched reluctance machine.

11. A method as claimed in claim 9 in which the current in the DC link is modified by returning current from the at least one phase winding of the switched reluctance machine to the DC link.

12. A computer program product stored on a computer readable medium which when installed on a processing device executes the method as claimed in claim 1.

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; and means for operating the power converter to control the magnitude of disturbances in the DC link voltage caused by the combined load of the plurality of electrical machines.

14. A system as claimed in claim 13 including a further electrical machine connected to the DC link and arranged to be run as a generator.

15. A system as claimed in claim 14 in which the further electrical machine is a reluctance machine.

16. A system as claimed in claim 13, including means for delaying energisation of the at least one phase winding of the at least one electrical machine.

17. A system as claimed in claim 13, including means for modifying the current in the DC link by controlling the profile of the current in the at least one phase winding of the at least one electrical machine.

18. A system as claimed in claim 13, including means for monitoring the DC link voltage, and means for controlling the current in the at least one phase winding in response to a signal from the means for monitoring when the DC link voltage decreases to a predetermined threshold.

19. A system as claimed in claim 13, in which at least one of the plurality of electrical machines is a switched reluctance machine.

20. A system as claimed in claim 19, including means for controlling the current in the at least one phase winding by freewheeling the said current.

21. A system as claimed in claim 19 including means for controlling the current in the at least one phase winding by returning current from at least one of the phase windings of the switched reluctance machine to the DC link.