GB2538493A

GB2538493A - A control method and system

Info

Publication number: GB2538493A
Application number: GB1508081.5A
Authority: GB
Inventors: Lopez-Schwarz Roman
Original assignee: INVERTEK DRIVES Ltd
Current assignee: INVERTEK DRIVES Ltd
Priority date: 2015-05-12
Filing date: 2015-05-12
Publication date: 2016-11-23
Anticipated expiration: 2035-05-12
Also published as: GB201508081D0; FR3036238B1; FR3036238A1; GB2538493B; DE102016108471A1

Abstract

A variable speed drive 1 which is configured to drive a motor 9 comprises a DC bus 4 which is operable to carry a DC bus voltage and current. The variable speed drive further comprises a DC-AC inverter 7 to convert the DC bus voltage and current into AC active power flowing in a forward direction from the variable speed drive to the motor along a conduction path 14. A method of controlling the variable speed drive comprises calculating active power flow between the variable speed drive and the motor through a component 16 positioned in the electrical conduction path. The DC-AC inverter within the variable speed drive is controlled in response to the calculated active power flow to limit active power flow in a reverse direction from the motor to the variable speed drive to a pre-determined level, thereby minimising or preventing an increase to the DC bus voltage within the variable speed drive. The motor may be used, for example, in a pump, fan, elevator or conveyor.

Description

Title: A Control Method and System

Description of Invention

The present invention relates to a control method and system, and more particularly relates to a method and system for controlling a variable speed drive.

A variable speed drive (also known as a variable frequency drive) is typically configured to drive and control the speed of rotation of an electric motor, such as a motor that is used in a pump, fan, elevator or conveyor. A variable speed drive is usually configured to control a motor so that any changes in the speed of rotation of the motor occur smoothly. By its nature, a variable speed drive allows motor speed to be varied, allowing a system to be optimised in terms of efficiency. A variable speed drive is therefore usually energy efficient and so it is desirable to use a variable speed drive wherever possible for environmental benefits.

A variable speed drive comprises a switching arrangement which modulates a DC voltage to provide a variable voltage and variable frequency AC signal that is suitable for driving a motor. The variable speed drive controls the speed of the motor by adjusting the frequency and amplitude of the AC voltage output that drives the motor.

Referring to figure 1 of the accompanying drawings, a conventional variable speed drive 1 incorporates a three phase AC power input 2 which is configured to be connected to three phases (A, B, and C) of a three phase AC power supply. The three phase AC power input 2 is connected to a rectifier arrangement 3 which, in this example, is a diode rectifier comprising six diodes D1-D6. The rectifier 3 is connected to a DC bus 4 which comprises a high voltage rail 5 and a low voltage or ground rail 6. In use, the rectifier 3 rectifies the three phase AC power input to provide a DC bus voltage across the DC bus 4.

A capacitor C1 is connected across the DC bus 4 to smooth out ripple in the DC bus voltage. The capacitor C1 is typically a large value capacitor with a capacitance of several thousand microfarads, such as a capacitance in the range 1000pF to 4000pF.

A DC-AC inverter 7 is connected to the DC bus 4. The DC-AC inverter 7 comprises a switching arrangement which incorporates six switches 01-06 which are connected across the DC bus 4. The DC-AC inverter 7 comprises an AC power output 8 which incorporates three power output phases (1-3) which are connected to an AC motor 9.

The variable speed drive 1 comprises a control unit 10 which is configured to control the switches 01-06 in the DC-AC inverter so that the switches 01-06 are switched in a specific sequence to convert the DC voltage across the DC bus 4 into a three phase AC power output signal to drive the motor 9. The control unit 10 is configured to adjust the rate of change of the duty cycle at which the switches 01-06 are switched to adjust the amplitude and frequency of the AC voltage output signal which in turn adjusts the speed at which the motor 9 rotates.

One problem with a conventional variable speed drive 1 is that the operation of the variable speed drive 1 can lead to an increase in the DC bus voltage when the motor 9 enters generator mode. The increase in the DC bus voltage is undesirable since it may require the variable speed drive 1 to be shut down to prevent damage to the DC-AC inverter 7.

A conventional variable speed drive is usually provided with a voltage sensor 11 which is configured to sense the voltage across the DC bus 4 and to provide an indication of the sensed voltage to the control unit 10. The control unit 10 is configured to control the switching of the switches 01-06 within the DC-AC inverter 7 to modify the speed and/or torque of the motor 9 in order to maintain the DC bus voltage at an appropriate level.

In a conventional variable speed drive which incorporates a high capacitance capacitor C1 in the DC bus 4, any voltage transients that occur on the DC bus 4 as a result of the motor 9 entering generator mode are damped by the capacitor C1. This allows sufficient time for the control unit 10 to react to control the switching of the switches Q1-Q6 to control and limit the regeneration energy from the motor 9 in order to maintain the DC bus voltage at an appropriate level, thereby preventing an over-voltage trip in the variable speed drive.

It is, however, desirable for the DC bus capacitance C1 to be as low as possible in order to improve power factor and to minimise harmonic distortion in the power supply that is connected to the power input 2. If the DC bus capacitance C1 is reduced to a low capacitance, such as 1% of the DC bus capacitance of a typical conventional variable speed drive, then the above method of monitoring the DC bus voltage and controlling the switching of the switches 01-06 no longer provides a robust control that reliably prevents over-voltage trips.

A conventional variable speed drive may also be provided with a resistor R and a switch S that are connected in series across the DC bus 4. The control unit 10 is configured to close the switch S if the sensed voltage across the DC bus 4 is above a predetermined level. Power flowing into the variable speed drive from the motor 9, when the motor 9 enters generator mode, is dissipated as heat by the resistor R to minimise the risk of damage to the inverter 7.

Although this can be reliably used to prevent over-voltage trips in the variable speed drive, many applications which do not need regenerative braking of the motor (for example pumping or fan applications) do not wish to carry the additional cost of this braking circuit just to handle occasional transient conditions.

There is therefore a need for an improved method and system for controlling a variable speed drive to minimise or prevent shut down of the variable speed drive due to an excessive increase in the DC bus voltage.

The present invention seeks to provide an improved control method and system.

According to one aspect of the present invention, there is provided a method for controlling a variable speed drive which is configured to drive a motor, the variable speed drive comprising a DC bus which is operable to carry a DC bus voltage and current and a DC-AC inverter which, in use, converts the DC bus voltage and current into AC active power flowing in a forward direction from the variable speed drive to the motor along a conduction path that extends between the variable speed drive and the motor, wherein the method comprises: calculating active power flow between the variable speed drive and the motor through a component positioned in the electrical conduction path, and controlling the DC-AC inverter within the variable speed drive in response to the calculated active power flow to limit active power flow in a reverse direction from the motor to the variable speed drive to a predetermined level, thereby minimising or preventing an increase to the DC bus voltage within the variable speed drive.

Preferably, the method further comprises: calculating a voltage value of the voltage output from the DC-AC inverter, and sensing a current value flowing through the component positioned in the electrical conduction path representing AC active power output.

Conveniently, the method further comprises: controlling the DC-AC inverter so vid that i"jg where i"f" is the q-axis current reference in an arbitrary v" reference frame, vd is the d-axis voltage value, id is the d-axis current value, v" is the q-axis voltage value and K corresponds to the predetermined level.

Advantageously, the predetermined level is zero.

Preferably, the method is a computer implemented method.

According to another aspect of the present invention, there is provided a tangible computer-readable medium storing instructions which, when executed by a computer, perform steps of the method of any one of claims 1 to 5 as defined hereinafter.

According to a further aspect of the present invention, there is provided a variable speed drive comprising the tangible computer-readable medium of claim 6 as defined hereinafter.

According to another aspect of the present invention, there is provided a system for controlling a variable speed drive which is configured to drive a motor, the variable speed drive comprising a DC bus which is operable to carry a DC bus voltage and current and a DC-AC inverter which, in use, converts the DC bus voltage and current into AC active power flowing in a forward direction from the variable speed drive to the motor along a conduction path that extends between the variable speed drive and the motor, wherein the system comprises: an active power flow calculator for calculating active power flow between the variable speed drive and the motor through a component positioned in the electrical conduction path, and a controller for controlling the DC-AC inverter within the variable speed drive in response to the calculated active power flow to limit active power flow in a reverse direction from the motor to the variable speed drive to a predetermined level, thereby minimising or preventing an increase to the DC bus voltage within the variable speed drive.

Preferably, the system further comprises: a voltage calculator configured to calculate a voltage value of the voltage output from the DC-AC inverter, and a current sensor for sensing a current value flowing through the component positioned in the electrical conduction path representing AC active power output.

Conveniently, the controller is configured to control the DC-AC inverter so that i > refq -K-vvqd-td where irefq is the q-axis current reference in an arbitrary reference frame, vd is the d-axis voltage value, id is the d-axis current value, vg is the q-axis voltage value and K corresponds to the predetermined level.

Advantageously, the predetermined level is zero.

According to another aspect of the present invention, there is provided a variable speed drive comprising the system as defined in any one of claims 8 to 11 hereinafter.

So that the present invention may be more readily understood, embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of a conventional variable speed drive 30 arrangement, and Figure 2 is a schematic diagram of a variable speed drive arrangement comprising an embodiment of the invention.

A method and system of an embodiment of the invention will now be described with reference to figure 2 of the accompanying drawings. Figure 2 shows a three phase variable speed drive which comprises the same components as the variable speed drive 1 shown in figure 1. The same reference numerals will be used for the variable speed drive in figure 2 as the reference numerals used for the variable speed drive in figure 1.

The system of an embodiment the invention comprises a control unit 12 which is configured to control the switching of the switches Q1-Q6. In this embodiment, the control unit 12 comprises a memory 13 which is a tangible computer-readable medium. The memory 13 stores instructions which, when executed by a processor within the control unit 12, perform steps of a method of an embodiment of the invention. The method is therefore preferably a computer-implemented method.

A conduction path 14 extends between the variable speed drive 1 and the motor 9. The system comprises a current sensor 15 which is configured to measure the current flowing through a component 16 positioned in the electrical conduction path 14. The current sensor 15 is coupled to the control unit 12 to provide a current value to the control unit 12 which is indicative of sensed current flowing through the component 16. The control unit 12 is configured to calculate a voltage value from the voltage output from the DC-AC inverter 7. The control unit 12 is configured to use the sensed current value and the calculated voltage value to calculate the active power flow between the variable speed drive 1 and the motor 9 through the component 16.

When the variable speed drive 1 is operating to drive the motor 9, active power flows in a forward direction from the variable speed drive 1 to the motor 9. If the motor 9 enters generator mode and generates active power which flows in a reverse direction from the motor 9 back to the variable speed drive 1, then the control unit 12 controls the switching of the switches Q1-06 to limit active power flow in the reverse direction.

The control unit 12 is configured to respond to the sensed reverse active power flow by modifying the torque current set point (which directly relates to the level of power flow) used within the control unit 12 to control the switching of the switches 01-06. The control unit 12 adjusts the switching of the switches Q1-06 to limit the reverse active power flow from the motor 9 to the variable speed drive 1 to a predetermined level. The predetermined level is zero in one embodiment of the invention but may be another level greater than zero in other embodiments of the invention. The reduction in the reverse active power flow minimises or prevents an increase to the DC bus voltage across the DC bus 4 within the variable speed drive 1.

An embodiment of the invention reacts instantaneously or substantially instantaneously to any reverse active power flow to minimise or prevent an increase to the DC bus voltage across the DC bus 4. An embodiment of the invention is well suited to a variable speed drive with a low DC bus capacitance C1 because an embodiment of the invention provides a more robust control than a conventional arrangement that relies on monitoring the DC bus voltage. An embodiment of the invention therefore provides robust control which prevents an over-voltage trip within the variable speed drive, whilst the low DC bus capacitance C1 minimises harmonic distortion in the power supply connected to the variable speed drive.

In a preferred embodiment of the invention, a current sensor is provided for each of the phases of the AC output 8 so that the current sensors can sense a current value of each of the phases. The control unit 12 is configured to calculate a voltage value for each of the phases. The control unit 12 is also configured to derive control parameters from the sensed current values and calculated voltage values which the control unit 12 uses to control the torque current set point of the variable speed drive. The following mathematical derivation describes aspects of the control method that are implemented in the control unit 12.

Starting from the instantaneous active power expression in the a-p stator reference frame: S = eafl tafl = (v, ia vfl * ifl) 4-j * (vfl * -v, * ifl) (1) p = (vala+ vfl*ifl) (la) q = (vfl * is -11a * ) (lb) Where: eas,i,fl: stator voltage and current in the stator reference frame S: instantaneous complex power.

p: instantaneous active power.

q: instantaneous reactive power.

For convenience we express S, p, and q in the d-q rotating reference frame.

For that we re-formulate eas, iag: eafl = eaq* ej6 -Lag * e" (2a, 2b) With: ethvidg stator voltage and current in the rotating reference frame 8: angle between the a-p. stator and the d-q rotating reference frames And then replace (2a, 2b) in the instantaneous power expression (1, 1 a, 1 b): S = eap * = (edq * e") * (idq * e") = edq * -l5 = edq * 1dg (3) P = (vd +vq * 14 (3a) q = (vg * id -vd * io) (3b) S, p and q are now expressed in the rotating reference frame.

In a low capacitor value (C1) drive it is imperative for the instantaneous active power consumption (3a) to be equal or higher than zero to prevent an increase in the DC bus voltage level.

P = id ± Vg * lo > 0 (4) If we solve for the q-axis component stator current, we obtain the expression: lq > 0 -vd * id vq (5) If we then replace the stator current by the q-axis current reference, we obtain the expression: ire fq 0 -Vd * id (6) Vq We can use (6) to impose a minimum limit to the actual torque current demand (the torque current set point, i"fq) based on the instantaneous active power consumption, stopping the flow of active energy back to the drive even before the DC bus voltage starts to increase. This lack of delay in modifying the torque current reference before the DC bus voltage achieves dangerous levels is key for low capacitor value drives, especially under no load or generator mode operating conditions.

This expression can be generalised as: i > K -vd* id (7) rei q -v q with K being an arbitrary power reference in cases where we want to limit the amount of instantaneous active power flow (permitted reverse power flow) from the motor to the variable speed drive to any non-zero predetermined level.

The expression on (7) is valid for any arbitrary reference frame.

While the embodiments described above comprise a variable speed drive which provides a three phase AC power output, it is to be appreciated that other embodiments of the invention incorporate a variable speed drive which provide an AC power output with two phases or an AC power output with more than three phases.

The embodiment described above comprises a control unit 12 which incorporates a memory 13 which stores instructions for execution by a processor within the control unit 12. However, in other embodiments, a memory is provided separately from the control unit 12 in the form of a tangible computer-readable medium which is configured to be coupled directly or via a network communication arrangement to the control unit 12.

While the embodiments described above comprise a variable speed drive 5 which incorporates a DC-AC inverter, other embodiments incorporate a variable speed drive which comprises a different modulation arrangement to provide an AC output power signal.

The variable speed drive of an embodiment of the invention is configured to drive a motor selected from a group of motor types including induction machines, permanent magnet, synchronous machines, brushless DC machines and synchronous reluctance machines.

When used in this specification and claims, the terms "comprises" and "comprising" and variations thereof mean that the specified features, steps or integers are included. The terms are not to be interpreted to exclude the presence of other features, steps or components.

The features disclosed in the foregoing description, or the following claims, or the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for attaining the disclosed result, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof.

Claims

CLAIMS: 1. A method for controlling a variable speed drive which is configured to drive a motor, the variable speed drive comprising a DC bus which is operable to carry a DC bus voltage and current and a DC-AC inverter which, in use, converts the DC bus voltage and current into AC active power flowing in a forward direction from the variable speed drive to the motor along a conduction path that extends between the variable speed drive and the motor, wherein the method comprises: calculating active power flow between the variable speed drive and the motor through a component positioned in the electrical conduction path, and controlling the DC-AC inverter within the variable speed drive in response to the calculated active power flow to limit active power flow in a reverse direction from the motor to the variable speed drive to a predetermined level, thereby minimising or preventing an increase to the DC bus voltage within the variable speed drive.
2. The method of claim 1, wherein the method further comprises: calculating a voltage value of the voltage output from the DC-AC inverter, and sensing a current value flowing through the component positioned in the electrical conduction path representing AC active power output.
3. The method of claim 2, wherein the method further comprises: x-v controlling the DC-AC inverter so that i" d*id fq > , where i"fq is the vq q-axis current reference in an arbitrary reference frame, vd is the d-axis voltage value, id is the d-axis current value, vq is the q-axis voltage value and K corresponds to the predetermined level.
4. The method of any one of the preceding claims, wherein the predetermined level is zero.
5. The method of any one of the preceding claims, wherein the method is a computer implemented method.
6. A tangible computer-readable medium storing instructions which, when executed by a computer, perform steps of the method of any one of the preceding claims.
7. A variable speed drive comprising the tangible computer-readable medium of claim 6.
8. A system for controlling a variable speed drive which is configured to drive a motor, the variable speed drive comprising a DC bus which is operable to carry a DC bus voltage and current and a DC-AC inverter which, in use, converts the DC bus voltage and current into AC active power flowing in a forward direction from the variable speed drive to the motor along a conduction path that extends between the variable speed drive and the motor, wherein the system comprises: an active power flow calculator for calculating active power flow between the variable speed drive and the motor through a component positioned in the electrical conduction path, and a controller for controlling the DC-AC inverter within the variable speed drive in response to the calculated active power flow to limit active power flow in a reverse direction from the motor to the variable speed drive to a predetermined level, thereby minimising or preventing an increase to the DC bus voltage within the variable speed drive.
9. The system of claim 8, wherein the system further comprises: a voltage calculator configured to calculate a voltage value of the voltage output from the DC-AC inverter, and a current sensor for sensing a current value flowing through the component positioned in the electrical conduction path representing AC active power output.
10. The system of claim 9, wherein the controller is configured to control the DC-AC inverter so that irefq K-vcvie where ire f q is the q-axis current v" reference in an arbitrary reference frame, vd is the d-axis voltage value, id is the d-axis current value, v" is the q-axis voltage value and K corresponds to the predetermined level.
11. The system of any one of claims 8 to 10, wherein the predetermined level is zero.
12. A variable speed drive comprising the system of any one of claims 8 to 11.
13. A method substantially as hereinbefore described with reference to 20 figure 2 of the accompanying drawings.
14. A system substantially as hereinbefore described with reference to and as shown in figure 2 of the accompanying drawings.
15. Any novel feature or combination of features disclosed herein.