WO2008000264A1

WO2008000264A1 - A method of driving an inductive load

Info

Publication number: WO2008000264A1
Application number: PCT/DK2007/000315
Authority: WO
Inventors: Nils Christensen; Rune Thomsen
Original assignee: Danfoss Compressors Gmbh
Priority date: 2006-06-29
Filing date: 2007-06-28
Publication date: 2008-01-03

Abstract

A method for driving an inductive load (A, B, C) arranged in an electrical circuit (1 ) further comprising at least two switching elements (T1T6) and a DC power source (3) having a set of terminals. The method comprises the steps of closing a switching element, thereby connecting the inductive load to the terminals, opening the switching element, thereby disconnecting the inductive load, closing a second switching element, thereby connecting the inductive load to a current sink in order to allow a demagnetisation current in the inductive load to decay, and opening the second switching element when the demagnetisation current has decayed to a minimum level. Using this method the demagnetisation current does not have to decay via a diode. Thereby energy is not dissipated in a diode, and the efficiency of the circuit is thereby improved. Furthermore, the requirements to the components of the electrical circuit are reduced, and it is thereby possible to use less expensive components.

Description

A METHOD OF DRIVING AN INDUCTIVE LOAD

FIELD OF THE INVENTION

The present invention relates to a method of driving an inductive load in such a manner that demagnetisation currents which are introduced in the inductive load are allowed to decay. More particularly, the present invention relates to a method as defined above in which the energy losses in an electrical circuit having the inductive load inserted therein are reduced as compared to prior art methods. The method of the present invention is very suitable for driving the coils of a multiphase direct current (DC) motor.

BACKGROUND OF THE INVENTION

Driving a DC motor is often done using an inverter comprising switching elements which in turn connect the ends of windings of the motor to positive and negative terminals of a DC power source. Fig. 1 illustrates an electrical circuit 1 with an inverter 2 comprising six switching elements Tr T₆. The inverter 2 is coupled to terminals of a DC power source 3 and to a motor 4. The motor 4 is a three phase motor, i.e. it comprises three motor coils A, B and C. A controller (not shown) controls switching of the switching elements TrT₆ between their open and closed states in order to obtain appropriate commutation of the motor coils A, B, C. Each of the switches TrTe has a diode DrD₆ associated with it.

A typical switching sequence is illustrated in Fig. 2. For each of the switching elements TrT₆ the ON and OFF states are shown as a function of time with time divided into sequence steps of an appropriate duration. It is clear from Fig. 2 that the switching elements T₁-T₆ are sequentially opened and closed. Accordingly, the motor coils A, B, C are sequentially connected to and disconnected from the terminals of the DC power source 3. The resulting switching pattern for the motor coils A, B, C as a function of time is illustrated in Fig. 3. In Fig. 3 '+' indicates that the corresponding motor coil is connected to the positive terminal of the DC power source 3, '0' indicates that the corresponding motor coil is completely disconnected from the DC power source 3, and '-' indicates that the corresponding motor coil is connected to the negative terminal of the DC power source 3. It is clear from Fig. 3 that the switching sequence of Fig. 2 has the consequence that at any given time one motor coil is connected to the positive terminal of the DC power source 3, one motor coil is connected to the negative terminal of the DC power source 3, and one motor coil is disconnected from the DC power source 3.

When the controller switches from one step to the next, the motor coil which was previously excited, but is now disconnected from the DC power source 3, will experience a current which continues to flow in the same direction as before. This is due to the inductive nature of the coil. This current is known as the demagnetisation current, and it will decrease to zero. In the electrical circuit 1 shown in Fig. 1 and using the switching sequence illustrated in Fig. 2 the demagnetisation current will run through one of the diodes D₁-De- Thereby energy is dissipated in the relevant diode, and this leads to an energy loss in the circuit. Furthermore, the components used for the electrical circuit 1 , in particular the diodes Di-D₆, must be able to withstand the relatively large demagnetisation currents which will run through the circuit 1. Thereby it may be necessary to chose relatively expensive components in order to ensure reliable operation of the circuit 1.

As an illustrative example, Fig. 4 shows the current flow during step 1 as defined by the switching sequence of Fig. 2. In Fig. 4 switching elements Ti and T₅ are closed while the remaining switching elements are open. As a result motor coil A is connected to the positive terminal of the DC power source 3, motor coil B is connected to the negative terminal of the DC power source 3, and motor coil C is disconnected from the DC power source 3. Accordingly, the current will run through motor coils A and B as indicated by the arrows. When the switching sequence is moved on to step 2 switching element T₅ is opened and switching element Te is closed. Thereby motor coil B is disconnected from the DC power source 3, and motor coil C is connected to the negative terminal of DC power source 3.

As motor coil B is disconnected from the DC power source the demagnetisation current induced in that motor coil while it was connected to the DC power source 3 will continue to run. The only available path for the demagnetisation current is through diode D₂. This is illustrated in Fig. 5. Accordingly, energy is dissipated in diode D₂. This is very disadvantageous because the dissipated energy is lost, and this has an adverse effect on the overall efficiency of the system. Furthermore, the components used in the system, in particular the diodes DrD₆ must be able to withstand the heat arising from the dissipated energy, and therefore it may be necessary to employ more expensive components, thereby adding to the manufacturing costs.

SUMMARY OF THE INVENTION

It is, thus, an object of the invention to provide a method for driving an inductive load arranged in an electrical circuit, in such a manner that the total efficiency of the circuit is improved.

It is a further object of the invention to provide a method for driving an inductive load at an improved efficiency without increasing component count. It is an even further object of the invention to provide an electrical circuit with an inductive load, in which the manufacturing costs are reduced as compared to similar prior art electrical circuits.

According to a first aspect of the invention the above and other objects are fulfilled by providing a method for driving an inductive load arranged in an electrical circuit further comprising at least two switching elements and a direct current (DC) power source having a set of terminals, the method comprising the steps of:

- closing at least a first switching element, thereby connecting the inductive load to the terminals of the DC power source,

- opening the first switching element, thereby disconnecting the inductive load from at least one of the terminals of the DC power source,

- closing at least a second switching element, thereby connecting the inductive load to a current sink in order to allow a demagnetisation current induced in an inductive component of the inductive load to decay, and

- opening the second switching element when the demagnetisation current in the inductive load has decayed to a minimum level.

In the present context the term 'inductive load' should be interpreted to mean a component or a number of interconnected components, having an inductive nature, and being capable of being connected across the terminals of the DC power source. Thus, the inductive load may advantageously be or comprise one or more coils or windings. In the case that the inductive load comprises two or more inductive components, such as two or more coils or windings, these may be connected in series and/or in parallel. The inductive load may further comprise one or more components which do not have an inductive nature, such as one or more capacitors and/or one or more resistors.

The electrical circuit comprises at least two switching elements. In the present context the term 'switching element' should be interpreted to mean an element which is adapted to switch between a closed state and an open state. In the closed state the switching element establishes an electrical connection between components arranged on opposite sides of the switching element, and in the open state such a connection is interrupted. The switching elements are preferably ordinary switches, such as Mosfets or IGBTs.

The electrical circuit also comprises a DC power source, i.e. a power source which delivers DC power. The DC power source may advantageously be a battery. Thereby the method of the invention is suitable for use in movable applications, such as in electrical circuits which are located on board vehicles, such as cars, trucks, boats, trains, aeroplanes, etc.

When driving the inductive load in accordance with the method of the first aspect of the invention the following happens.

The inductive load is connected to the terminals of the DC power source. This is done by closing at least a first switching element, and it may optionally involve closing at least one additional switching element. While the inductive load is connected to the terminals of the DC power source a current runs through the inductive load. Due to the inductive nature of the inductive load, a magnetic field is thereby induced in the inductive load.

At a desired time the first switching element is opened, and thereby the connection between the inductive load and the terminals of the DC power source is interrupted. Accordingly, the DC power source no longer supplies a current to the inductive load. However, as explained above, a demagnetisation current will continue to flow from the inductive load, or from an inductive component of the inductive load. In order to allow this demagnetisation current to decay, at least a second switching element is closed, thereby connecting the inductive load to a current sink. In the present context the term 'current sink' should be interpreted to mean a point in the electrical circuit to which the demagnetisation current can run with no or only very little energy dissipation in the components of the electrical circuit. The current sink may, e.g., be a terminal of the DC power source. Alternatively, the demagnetisation current may be allowed to circulate in the electrical circuit, or in part of the electrical circuit, until it has decayed to an acceptable level.

When the demagnetisation current has decayed to a minimum level, the second switching element is opened. A minimum level may, e.g., be zero, or it may be a threshold value defining an acceptable level for the demagnetisation current. This will be described in further detail below.

It is an advantage that the demagnetisation current is allowed to decay in the manner described above because it is thereby avoided that energy is dissipated in the components of the electrical circuit, and thereby the total efficiency of the electrical circuit can be improved. When using a prior art method for driving the inductive load, the total efficiency is approximately 94%. Using the method of the present invention the total efficiency can be increased to approximately 98%. Furthermore, the requirements to the components of the electrical circuit are reduced, and it is thereby possible to use less expensive components. Thereby the manufacturing costs can be reduced without compromising the performance of the electrical circuit. Finally, the method may be introduced on existing electrical circuits without increasing the component count. The time interval elapsing from closing the first switching element until opening the first switching element may be substantially longer than the time interval elapsing from closing the second switching element until opening the second switching element. According to this embodiment the time interval during which the second switching element is closed in order to allow the demagnetisation current to decay is very brief as compared to the time interval during which the inductive load is connected to the terminals of the DC power source. It should be noted that during the time interval in which the inductive load is connected to the terminals of the DC power source, the connection may actually be connected and disconnected rapidly. This is, e.g., the case when a pulse width modulation (PWM) control strategy is used. This will be further described below.

The length of the time interval during which the demagnetisation current is allowed to decay will depend upon the load, the specific application and the ambient temperature. In a preferred embodiment in which the method is used for driving a DC motor the time interval will depend on the speed of the motor. This is because at maximum speed the motor is capable of delivering maximum torque. This has the consequence that the magnetisation current in the motor is maximised, and thereby it will take longer for the demagnetisation current to decay.

The method may further comprise the step of monitoring the demagnetisation current, and the second switching element may be opened when it is determined that the demagnetisation current has decayed to a predefined threshold level. According to this embodiment the time interval during which the demagnetisation current is allowed to decay does not have a fixed length. Instead the length of the time is determined by the time it takes for the demagnetisation current to decay to the predefined threshold value. Thus, the second switch is opened when the demagnetisation current has reached an 'acceptable' level. The predefined threshold value may be zero, i.e. the second switch is opened when it has been found that the demagnetisation current has decayed completely. Alternatively, the predefined threshold value may be a level which is larger than zero, but sufficiently small to ensure that no, or only an insignificant amount of, energy is dissipated in the components of the electrical circuit as a result of the decay of the demagnetisation current. The demagnetisation current may, e.g., be monitored by means of a microcontroller.

Alternatively, the time interval during which the demagnetisation current is allowed to decay may have a fixed length, e.g. chosen as a representative duration during which the demagnetisation current will, in the application in question, almost certainly have decayed to zero.

The electric circuit may comprise at least two inductive loads and at least four switching elements, and the method may further comprise the steps of:

- closing at least a third switching element, thereby connecting a second inductive load to the terminals of the DC power source,

- opening the third switching element, thereby disconnecting the second inductive load from at least one of the terminals of the DC power source,

- closing at least a fourth switching element, thereby connecting the second inductive load to a current sink in order to allow a demagnetisation current induced in the inductive load to decay, and - opening the fourth switching element when the demagnetisation current in the second inductive load has decayed to a minimum level.

According to this embodiment the electrical circuit comprises at least two inductive loads which may be connected to and disconnected from the terminals of the DC power source as described above. Each of the inductive loads may further be connected to the current sink in order to allow the induced demagnetisation current to decay.

The first switching element and the third switching element may be operated sequentially, thereby providing a switching pattern. Thus, the two or more inductive loads may be connected to/disconnected from the terminals of the DC power source in a sequential manner. This is, e.g., relevant when the method is used for driving a multiphase DC motor, such as a three-phase DC motor. It should be noted that the method of the present invention may be extended to driving three, four, five, etc. inductive loads in the electrical circuit in the manner described above.

The switching pattern may be provided by a pulse amplitude modulation (PAM) control strategy. Alternatively, the switching pattern may be provided by a pulse width modulation (PWM) control strategy. PAM as well as PWM are well known per se in the art, and will therefore not be described in further detail here.

According to one embodiment, each of the switching elements may be used for connecting an inductive load to the terminals of the DC power source as well as for connecting an inductive load to the current sink. In this embodiment the switching elements have multiple functions, and the method may thereby be realised without increasing the component count of the electrical circuit, and thereby without increasing the manufacturing costs. In a preferred embodiment the method may be used for driving a DC motor, e.g. a brushless DC motor. The motor may preferably be a multiphase motor, such as a three phase motor. The motor may suitably be used for driving a compressor, e.g. a compressor for use in a refrigeration system.

The step of closing the second switching element may be performed immediately after the step of opening the first switching element. According to this embodiment the inductive load is connected to the current sink immediately after it has been disconnected from the terminals of the DC power source. Thereby the demagnetisation current is immediately allowed to decay via this path, and dissipation of energy in the components of the electrical circuit due to the decay of the demagnetisation current is thereby avoided to the greatest extent possible.

According to a second aspect of the invention the above and other objects are fulfilled by providing an electrical circuit comprising:

- an inductive load,

- at least two switching elements,

- a direct current (DC) power source having a set of terminals, and

- a control unit adapted to operating the switching elements in order to switch them between an open state and a closed state,

wherein the inductive load and the switching elements are arranged relatively to each other in such a manner that closing a first switching element causes the inductive load to be connected to the terminals of the DC power source, and in such a manner that closing a second switching element causes the inductive load to be connected to a current sink, thereby allowing a demagnetisation current induced in an inductive component of the inductive load to decay.

It should be noted that a skilled person would readily recognise that any feature described in combination with the first aspect of the invention can equally be combined with the second aspect of the invention, and vice versa.

The electrical circuit is very suitable for performing the method according to the first aspect of the invention. Thus, the control unit may be adapted to perform the method according to the first aspect of the invention, Accordingly, the remarks set forth above are equally applicable here.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in further detail with reference to the accompanying drawings in which

Fig. 6 shows an electrical circuit during decay of a demagnetisation current in accordance with the method of the invention, and

Fig. 7 shows a switching sequence for the electrical circuit illustrated in Figs. 1 and 4-6, and in accordance with the method of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 6 illustrates the step of the method of the invention, in which a demagnetisation current induced in motor coil B is allowed to decay via a current sink. Comparing Fig. 6 to Fig. 5 it is clear that switching element J₂ has been closed. Thereby the demagnetisation current which was induced in motor coil B while it was connected to the terminals of the DC power source 3, as illustrated in Fig. 4 and described above, is allowed to decay via the connection established by switching element T₂ as illustrated by the arrows. Since this path represents a much smaller resistance than the alternative path via diode D₂, the current will run this way rather than via diode D₂. Thereby no energy is dissipated in diode D₂ during the decay of the demagnetisation current.

Fig. 7 shows a switching sequence for the electrical circuit illustrated in Figs. 1 and 4-6, and in accordance with the method of the present invention. Fig. 7 shows the switching sequence as a function of time, the numbers at the bottom indicating step number. Comparing the switching sequence illustrated in Fig. 2 with the switching sequence illustrated in Fig. 7 it is clear that the original switching sequence is maintained, i.e. the motor coils A, B and C are sequentially connected to the terminals of the DC power source 3 as described above with reference to Fig. 2. However, in addition to this switching sequence, the switching elements are briefly switched to their ON states, i.e. they are briefly closed. For instance, when moving from step 1 to step 2, switching element T₅ is opened and switching element T₆ is closed. Thereby motor coil B is disconnected from the negative terminal of the DC power source 3, and motor coil C is connected to the negative terminal of the DC power source 3. Simultaneously, switching element T₂ is closed, thereby allowing the demagnetisation current previously induced in motor coil B to decay. Switching element T₂ is only maintained in the closed state for as long as necessary in order to allow the demagnetisation current to decay to zero or to a predefined threshold value as described above.

Claims

1. A method for driving an inductive load (A, B, C) arranged in an electrical circuit (1) further comprising at least two switching elements (TVTe) and a direct current (DC) power source (3) having a set of terminals, the method comprising the steps of:

- closing at least a first switching element, thereby connecting the inductive load to the terminals of the DC power source (3),

- opening the first switching element, thereby disconnecting the inductive load from at least one of the terminals of the DC power source (3),

2. A method according to claim 1 , wherein the time interval elapsing from closing the first switching element until opening the first switching element is substantially longer than the time interval elapsing from closing the second switching element until opening the second switching element.

3. A method according to claim 1 or 2, further comprising the step of monitoring the demagnetisation current, and wherein the second switching element is opened when it is determined that the demagnetisation current has decayed to a predefined threshold level.

4. A method according to any of the preceding claims, wherein the electric circuit (1 ) comprises at least two inductive loads (A, B, C) and at least four switching elements (Ti-T₆), the method further comprising the steps of:

- closing at least a third switching element, thereby connecting a second inductive load to the terminals of the DC power source (3),

- opening the third switching element, thereby disconnecting the second inductive load from at least one of the terminals of the DC power source (3),

- closing at least a fourth switching element, thereby connecting the second inductive load to a current sink in order to allow a demagnetisation current induced in the inductive load to decay, and

- opening the fourth switching element when the demagnetisation current in the second inductive load has decayed to a minimum level.

5. A method according to claim 4, wherein the first switching element and the third switching element are operated sequentially, thereby providing a switching pattern.

6. A method according to claim 5, wherein the switching pattern is provided by a pulse amplitude modulation (PAM) control strategy.

7. A method according to claim 5, wherein the switching pattern is provided by a pulse width modulation (PWM) control strategy.

8. A method according to any of claims 4-7, wherein each of the switching elements (Ti-T₆) may be used for connecting an inductive load (A, B, C) to the terminals of the DC power source (3) as well as for connecting an inductive load (A, B, C) to the current sink.

9. A method according to any of the preceding claims, wherein the method is used for driving a DC motor (4).

10. A method according to any of the preceding claims, wherein the step of closing the second switching element is performed immediately after the step of opening the first switching element.

11. An electrical circuit (1 ) comprising:

- an inductive load (A, B, C),

- at least two switching elements (T-i-Tβ),

- a direct current (DC) power source (3) having a set of terminals, and

- a control unit adapted to operating the switching elements OVTe) in order to switch them between an open state and a closed state,

wherein the inductive load (A, B, C) and the switching elements (Ti-T₆) are arranged relatively to each other in such a manner that closing a first switching element causes the inductive load to be connected to the terminals of the DC power source (3), and in such a manner that closing a second switching element causes the inductive load to be connected to a current sink, thereby allowing a demagnetisation current induced in an inductive component of the inductive load to decay.

12. An electrical circuit (1 ) according to claim 11 , wherein the control unit is adapted to perform the method according to any of claims 1-10.