Variable speed motor drive system

Abstract

A variable speed drive motor system 10 comprises a variable speed drive motor 15, and an active front end (AFE) 14 for driving the variable speed motor, in which the AFE is configured to vary a DC lin

Description

The present invention relates to a variable speed motor drive system, in particular to a variable speed motor drive system incorporating an active front end (AFE). Known variable speed motor drive systems require a DC link voltage which is different from the level of the DC, or rectified AC, input voltage. Such known systems use an AFE to permanently boost the DC link voltage to the maximum voltage required (Vdc link high in Figure 1) to run the variable speed motor of the drive system. A problem with such known drive systems is that the variable speed motor will spend extended periods of time at speeds significantly below the maximum rated speed of the system, and therefore during these periods, the required output voltage is small compared to the DC link voltage. As a result, the efficiency of the drive system, due to the inverter and AFE switching losses will be higher compared to a drive system operating with a lower DC link voltage (Vdc link low in Figure 1). However, a drive system with a lower DC link voltage would not be able to operate the variable speed motor over the entire speed range of a drive system with a higher DC link voltage. An object of the present invention is to provide a more efficient variable speed motor drive system which can operate over the entire speed range of the variable speed motor. Thus, according to the present invention, there is provided a variable speed drive motor system comprising a variable speed drive motor, and an active front end for driving the variable speed motor, in which the active front end is configured to vary a DC link voltage automatically and continuously as a speed of the variable speed motor changes _according to an output voltage required to drive the variable speed drive motor at the speed. Advantageously, the active front end allows the DC link voltage to be varied according to the voltage required to drive the variable speed drive motor at the speed required which allows the drive motor system to operate with a higher efficiency over a greater speed range compared to a variable speed drive motor system driven by a fixed DC link voltage. The invention will now be described by way of example only with reference to the accompanying drawings, in which: Figure 1 is a graph showing the DC link voltage according to the required output voltage as a function of time, Figure 2 is a circuit diagram of the variable speed drive motor system according to the present invention, Figure 3 is flow diagram showing the operation of the variable speed drive motor system of Figure 1, Figure 4 is a graph showing the variation of the DC link voltage according to the required output voltage as a function of time, and Figures 5 (a) to (c) are graph showing the variation of the DC link voltage according to the required output voltage and motor speed as a function of time . In Figure 2, a variable speed drive motor system 10 includes a drive system 12 and a variable speed drive motor 15. Examples of motors include a pump, a fan or a hybrid propulsion motor. The drive system 12 includes an EMC filter 16, an active front end (AFE) 14 which comprises an inductor 18, a first switching device 20 and a second switching device 22, a capacitor 24 and an inverter 26 connected in a circuit 13 as shown in the circuit diagram in Figure 2. The AFE 14 includes an AFE controller 21 which varies the DC link voltage Vdc link as will be described below. Each of the switching devices 20,22 comprise a switch and an anti-parallel diode. The circuit 12 of the drive system 12 operates in a known way to selectively vary the DC link voltage Vdc link with respect to the input voltage Vinput by passing current through the inductor 18 and selectively through switching devices 20,22 then capacitor 24. An inverter controller 17 monitors the DC link voltage Vdc link and therefore the modulation index (Ml) of the inverter 26, with the Ml being representative of the output voltage Vout. The modulation index (Ml) relates the peak voltage of the motor supply (VPk) to the DC link voltage Vdc link. For a 3-phase system, VPk = Ml * Vdc link * sqrt(3) / (2). The inverter controller 17 is connected to the AFE controller 21. Typically, the Ml is kept below 1 to avoid inverter output harmonics. For example, the drive system can run with a maximum Ml of 0.9 which allows the inverter 26 to react to a perturbation without the AFE 14 having to immediately react. In Figures 3 and 4, when the drive motor 15 is operating, the DC link voltage Vdc link is controlled by the AFE controller 21 of the AFE 14 using feedback from the inverter controller 17 which is measuring the Ml. When the speed of the variable speed drive motor 15 needs to change, for example via a manual input, the inverter 26 increases the frequency and magnitude of the output voltage Vout to increase the motor speed as is known in the art. As the motor speed increases, the Ml increases. If the Ml is above an upper threshold, for example, 0.9, the AFE raises the DC link voltage Vdc link (between ti and t2). If the Ml drops below a lower threshold, for example, 0.7, the AFE decreases the DC link voltage Vdc link (between t2 and ts). If the Ml remains between the upper and lower thresholds, the DC link voltage Vdciink Is maintained (between 0 and ti and above ts). In the above embodiment, the AFE is configured to continuously vary the DC link voltage Vdciink based on the Ml only and only raises the DC link voltage Vdciink when the Ml is above 0.9. It is also important that a maximum rate of change of the DC link voltage Vdc link is lower than a capacity of the inverter 36 otherwise the DC link voltage Vdciink will change the output voltage Vout. The variable speed drive motor system 10 is configured to ensure that a rate of change of the DC link voltage Vdciink is greater than a rate of change of the required output voltage Vout to drive the variable speed motor 15 at a maximum variable speed motor rate of change. The variable speed drive motor system 10 is configured further to ensure that a maximum rate of change of the DC link voltage Vdciink is sufficiently low to ensure the inverter controller 17 remains stable. This is important as it is the inverter controller 17 which controls the output voltage Vout. If the DC link voltage Vdc link changes faster than the inverter controller 17 is able to control, then the inverter controller 17 loses control of the output voltage Vout of the motor. The variable speed drive motor system 10 is configured to allow the variable speed motor 15 to operate in a clockwise motoring mode, a clockwise regeneration mode, an anticlockwise motoring mode, and an anticlockwise regeneration mode. This is achieved by controlling current flow in either direction through switching device 22 via the shown switches, including while boosting the DC link voltage Vdc link de link. In the case of a DC supply, switching devices 20,22 are operated as a synchronous boost converter as opposed to a conventional boost which only has a diode equivalent to the diode of switching device 22 of the present invention. The switching device 22 is switched on when switching device 20 is off and vice versa. By controlling the on / off ratio of the switches, the level of boost and direction of current flow can be controlled. In Figure 4, the variation of input voltage Vinput, DC link voltage Vdc link, and output voltage Vout, is shown as a function of time t over a cycle of operation of the variable speed motor 15. Between 0 and ti: The required output voltage Vout is increasing and is below the input voltage Vinput and therefore able to be generated using the input voltage Vinput. In this period, the AFE is not operational, and the DC link voltage Vdc link is equal to the input voltage Vinput. Between ti and t2: The required output voltage Vout is increasing and is above the input voltage Vinput and therefore not able to be generated using the input voltage Vinput. In this period, the AFE is operational, and the DC link voltage Vdc link is increased to above the output voltage Vout. Between t2 and ts: The required output voltage Vout is decreasing but is above the input voltage Vinput and therefore not able to be generated using the input voltage Vinput. In this period, the AFE is operational, and the DC link voltage Vdc link is increased to above the output voltage Vout. Above ts: The required output voltage Vout is decreasing but is below the input voltage Vinput and therefore able to be generated using the input voltage Vinput. In this period, the AFE is not operational, and the and the DC link voltage Vdc link is equal to the input voltage Vinput. Figures 5(a) to (c) describe the above operation in more detail, noting the graphs show the trend, with the numerical values being arbitrary. As the motor accelerates, at first there is sufficient link voltage without the need for it be raised or boosted. Between 44 and 45 seconds, the Ml hits the upper threshold and the AFE raises the link voltage (which decreases the Ml). The link voltage is increased until the Ml is in the middle of the range. This repeats each time the Ml reaches the upper threshold. When the motor speed reduces, the link voltage is reduced each time the Ml hits the lower threshold until there is no boosting. In Figure 5(b), it can be seen that there will always be sufficient voltage for the motor as the rate of change of link voltage is higher than the change of voltage required by the motor maximum acceleration rate. It will be understood therefore that by monitoring the Ml of the motor 15, the AFE allows the DC link voltage to be increased, decreased, or not changed according to the voltage required to drive the motor 15 at the speed required which allows the drive motor system to operate with a higher efficiency over a greater speed range compared to a variable speed drive motor system driven by a fixed DC link voltage.

Claims

1. A variable speed drive motor system (10) comprising a variable speed drive motor (15), and an active front end (AFE) (14) for driving the variable speed motor (15), in which the AFE (14) is configured to vary a DC link voltage (Vdc link) automatically and continuously as a speed of the variable speed motor (15) changes .according to a required output voltage (Vout) required to drive the variable speed drive motor (10) at the speed.

2. A variable speed drive motor system (10) according to claim 1 in which the AFE (14) is enabled when the required output voltage (Vout) is above the input voltage (Vinput) to increase the DC link voltage (Vdc link) above the required output voltage Vout.

3. A variable speed drive motor system (10) according to claim 1 or 2 in which the AFE (14) is disabled when the required output voltage (Vout) is below the input voltage (Vinput).

4. A variable speed drive motor system (10) according to any preceding claim further comprising an inverter controller (17) configured to measure the required output voltage (Vout).

5. A variable speed drive motor system (10) according to claim 4 in which the inverter controller (17) measures a modulation index (Ml) of an inverter (26).

6. A variable speed drive motor system (10) according to claim 5 in which the AFE (14) is configured to raise the DC link voltage (Vdclink) if the Ml is above an upper threshold, preferably the upper threshold is 0.9.

7. A variable speed drive motor system (10) according to claim 5 or 6 in which the AFE (14) is configured to decrease the DC link voltage (Vdclink) if the Ml is below a lower threshold, preferably the lower threshold is 0.7.

8. A variable speed drive motor system (10) according to any one of claims 5 to 7 in which the AFE (14) is configured to maintain the DC link voltage (Vdc link) if the Ml is between the upper threshold and the lower threshold.

9. A variable speed drive motor system (10) according to any one of claims 5 to 8 in which the Ml is below 1.

10. A variable speed drive motor system (10) according to any one of claims 5 to 9 in which the AFE (14) is configured to continuously vary the DC link voltage (Vdciink) based on the Ml only.

11. A variable speed drive motor system (10) according to any one of claims 5 to 10 in which the AFE (14) is configured to raise the DC link voltage (Vdc link) only when the Ml is above 0.9.

12. A variable speed drive motor system (10) according to any one of claims 5 to 11 further configured to ensure that a maximum rate of change of the DC link voltage (Vdciink) is sufficiently low to ensure an inverter controller (17) remains stable.

13. A variable speed drive motor system (10) according to any preceding claim further configured to ensure that a rate of change of the DC link voltage (Vdciink) is greater than a rate of change of the required output voltage (Vout) to drive the variable speed motor (15) at a maximum variable speed motor rate of change.

14. A variable speed drive motor system (10) according to any preceding claim in which the AFE (14) is configured to allow the variable speed motor (15) to operate in a clockwise motoring mode, a clockwise regeneration mode, an anticlockwise motoring mode, and an anticlockwise regeneration mode.

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