WO2021099013A1

WO2021099013A1 - Activating inductive electrical loads in part-load operation with reduced switching losses

Info

Publication number: WO2021099013A1
Application number: PCT/EP2020/076687
Authority: WO
Inventors: Jochen Wessner; Illia Manushyn
Original assignee: Robert Bosch Gmbh
Priority date: 2019-11-18
Filing date: 2020-09-24
Publication date: 2021-05-27
Also published as: DE102019217687A1

Abstract

The invention relates to a method (100) for operating an electrical converter (1), which applies (120) a direct-current voltage (2) presented at the input (1a) to an inductive load (4) in pulse width modulation by actuation of at least one switching element (3a-3f), such that a defined effective current (4a) flows through the inductive load (4), wherein a timer program (10) is used (110) for the actuation and contains at least one interval (11) which is divided into • one or more idle periods (13), in which no pulse width modulation takes place, and • one or more modulation periods (12), in which the amount of energy required over the entire interval (11) is supplied to the inductive load (4) by means of pulse width modulation.

Description

description

Title:

Control of inductive electrical loads in partial load operation with reduced switching losses

The present invention relates to the operation of inductive electrical loads on electrical converters, such as, for example, inverters, the effective current being specified by pulse width modulation.

State of the art

In an electrical drive system that converts electrical energy into mechanical energy, three- or multi-phase AC motors are preferably used, which have a high degree of efficiency and, in contrast to DC motors, do not have brushes that are susceptible to wear. Such drive systems are used, for example, in elevators, in pumps and in drive trains for electrically powered vehicles. However, electrical energy sources that can be carried in the vehicle, such as batteries or fuel cells, generally supply a direct voltage. For industrial applications mostly three-phase alternating current is used, which is rectified with a rectifier.

Inverters are used to convert the direct voltage into a three- or multi-phase alternating voltage.

The demands on torque and speed are constantly changing during ferry operation. A higher speed is required to accelerate and a higher torque is required to maintain the speed on an incline. DE 19608039 Al discloses a control device with which an inverter can respond to the changing Can respond to requirements that the efficiency of the electric motor can be improved, especially in the partial load range.

Disclosure of the invention

In the context of the invention, a method for operating an electrical converter was developed. This converter applies a DC voltage presented at its input to an inductive load by driving at least one switching element in pulse width modulation, so that a specified effective current flows through the inductive load. A current that is constantly changing flows through the pulse width modulation, and the effective current is the direct current that would convert the same electrical power at an ohmic resistor as the pulse width modulated current. The converter can in particular be designed, for example, as a DC-DC converter for an inductive direct current load or as an inverter for an inductive alternating current load.

A time program is used to control the at least one switching element. This time program contains at least one time interval, which is divided into

• one or more rest periods in which no pulse width modulation takes place, and

• One or more modulation times in which the inductive load is supplied with the amount of energy required during the entire time interval by means of pulse width modulation.

It was recognized that for the functioning of current-controlled inductive loads, it is primarily important that the correct effective current flows on average over a sufficiently small time scale (e.g. the period of an alternating current provided for operating the load) and that the load receives its expected amount of energy . The exact timing of the current, however, is much less important. For example, unlike with many other loads, it is not necessary to simulate a particularly smooth direct current or an exactly sinusoidal curve of an alternating current. By dispensing with this requirement, however, it becomes possible to supply the required amount of energy to the load with significantly fewer switching operations of the switching elements. This reduces the switching losses in the electrical converter. It was recognized that the proportion of switching losses in the total losses increases significantly when electric motors are operated at partial load or even idling: If the required effective current is reduced by a factor of 20, the switching losses according to the current state of the art are only reduced by that factor 2. In idle operation, this can reduce the efficiency of the inverter to below 25%. A significant improvement is now brought about at the price that the time course of the current deviates more or less clearly from the smooth direct current or sinusoidal alternating current.

The idle times in the context of this invention are not to be confused with the regular switch-off times, which alternate periodically with switch-on times in the case of pulse width modulation. Rather, the rest periods are significantly longer periods of time compared to the period of the pulse width modulation, in which no switching processes take place and the respective switching element remains in the switched-off state. The introduction of rest times in the control time program therefore has the tendency that the transfer of the total amount of energy required by the load is concentrated on the remaining modulation times.

This means that higher instantaneous currents flow during these modulation times. There are practical limits here, as the converter, the inductive load and the leads to this load are usually designed for certain maximum instantaneous voltages and instantaneous currents. In particular, the switching element can also be loaded with high instantaneous voltages when it is switched off. These voltages arise due to the self-induction of the inductive load.

It is therefore not necessarily sensible to want to save switching operations in full load operation of the inductive load by concentrating the amount of energy transferred. In contrast, the efficiency of the electrical converter can be significantly improved in partial load operation. Especially in operational phases, in where the requested effective current is very low compared to the nominal current at full load, the instantaneous currents and instantaneous voltages are still well below the specified maximum values despite the concentration. An operating phase of the inductive load is therefore advantageously selected in which the inductive load is operated with a maximum of 20%, preferably with a maximum of 10% and very particularly preferably with a maximum of 5% of its nominal power.

In road traffic, vehicles are typically only driven at top speed for a very small proportion of their operating time. Accordingly, part-load and idle operation together make up the largest proportion of the operating time of the electric drive train. Therefore, by reducing the switching losses in the drive train of vehicles, significant amounts of energy are saved. Since vehicles usually have a limited supply of energy with them, the energy saved is also reflected in an increased range. The same applies to on-board units other than traction motors, which are also supplied with alternating current via an inverter. For example, the compressor in a vehicle's air conditioning system will also run for the greater part of its operating time in partial load or idle mode.

Therefore, an electric motor which is provided for operation with a single-phase or multiphase alternating current, or, quite generally, any load intended for operation with such an alternating current, is advantageously selected.

The time interval, which now contains both idle times and modulation times, corresponds to a half-wave of the alternating current. According to the previous state of the art, pulse width modulation took place throughout the entire half-wave of the alternating current.

As explained above, the electrical converter is particularly advantageously designed as an inverter that connects one or more alternating current phases of the inductive load by activating switching elements either with the positive pole or the negative pole of the direct voltage provided at the input of the inverter. With such an inverter, alternating currents can be generated in a wide frequency range, so that an electric motor can be operated in a correspondingly wide speed range. In a particularly advantageous embodiment, the time interval begins with a modulation time that comprises at least two pulses. The rest of the time interval is rest time. In this way, the instantaneous value of the current can be brought to a maximum value very quickly, for example with a suitable sequence of pulses in an essentially linear increase. The amount of energy required by the inductive load during the time interval is therefore only supplied to these pulses and then gradually consumed by the load. With the load remaining the same, the current drops again essentially linearly until it reaches the value zero and remains there for the remainder of the half-cycle. This is a significant departure from the usual sinusoidal shape of AC current, but the effective current is still the desired one. Therefore, for example, an electric motor in the partial load range can be operated without any problems with the alternating current modified in this way. The number and the spacing of the pulses can depend in particular, for example, on what current load the switching element can carry for what time and what cooling time is then required. In particular in the case of switching elements based on semiconductors, excessive heating can reduce the service life.

In a further particularly advantageous embodiment, the time interval begins with a first rest period. This first rest period is followed by at least one modulation period. The time interval ends with a second rest period. This is based on the knowledge that, according to the usual sinusoidal shape of the alternating current, comparatively little energy is supplied to the load at the beginning and at the end of the half-wave compared to the maximum of the half-wave. These amounts of energy can therefore be shifted to the center of the half-wave, so that switching operations can be saved at the beginning and at the end of the half-wave.

In a further advantageous embodiment, the modulation times include a first secondary modulation time, a main modulation time that follows later and a second secondary modulation time that follows even later. These modulation times can merge seamlessly into one another, but they can also be separated from one another by idle times. During the main modulation time the inductive load is supplied with at least 50% of the amount of energy required during the entire time interval. The additional modulation during the secondary modulation times approximates the time course of the current flowing through the inductive load caused by the modulation during the main modulation time to a sinusoidal shape. In this way, for example, harmonics that are generated by a time curve that deviates greatly from the sinusoidal shape and that can cause undesired electromagnetic emissions, for example, can be suppressed.

In a further particularly advantageous embodiment, the inductive load is supplied with the energy required during the entire time interval in a single pulse. The saving of switching processes is then maximal, because only one switch-on process and one switch-off process have to be carried out in the entire time interval. At the same time, however, the instantaneous value of the current is maximally concentrated in this one pulse. This type of control is therefore particularly suitable for no-load operation, in which, on the one hand, very little energy has to be transmitted to the inductive load and, on the other hand, the switching losses would be particularly significant if switching more frequently.

In particular, the method can be implemented entirely or partially by computer. The invention therefore also relates to a computer program with machine-readable instructions which, when they are executed on one or more computers, cause the computer or computers to carry out the method. In this sense, control devices for vehicles and embedded systems for technical devices, which are also able to execute machine-readable instructions, are to be regarded as computers.

The invention also relates to a machine-readable data carrier and / or to a download product with the computer program. A download product is a digital product that can be transmitted via a data network, ie can be downloaded by a user of the data network and that can be offered for sale for immediate download in an online shop, for example. Furthermore, a computer can be equipped with the computer program, with the machine-readable data carrier or with the download product.

Further measures improving the invention are illustrated in more detail below together with the description of the preferred exemplary embodiments of the invention with reference to figures.

Embodiments

It shows:

Figure 1 embodiment of the method 100;

FIG. 2 exemplary control with modulation time (12) at the beginning of the half-wave of an alternating current (4a);

FIG. 3 exemplary control with modulation time in the middle of the period of an alternating current;

FIG. 4 exemplary control with main modulation time and secondary modulation time;

FIG. 5 Exemplary arrangement of converter 1 and inductive load 4 for the use of the method.

FIG. 1 is a flowchart of an exemplary embodiment of the method 100 for operating an electrical converter 1. In step 105, an inductive load 4 is selected which is intended for operation with single or multi-phase alternating current, this being an electric motor, for example, according to block 105a can. In step 106, a partial load operating phase 4 ′ of the inductive load 4 is selected. In step 110, a time program 10 is determined for the control of switching elements 3a-3f of the converter 1, the modulation times 12 with pulse width modulation and rest times 13 without Contains pulse width modulation. In step 120, the switching elements 3a-3f are controlled with the time program 10, so that the specified effective current 4a flows through the inductive load 4, but a considerable amount of switching operations of the switching elements 3a-3f is saved compared to constant pulse width modulation without idle times 13.

Within the box 110, some possibilities are given as examples of how the time program 10 can be put together.

According to block 111, a time interval 11 can begin with a modulation time 12 which comprises at least two pulses 14a-14c of the pulse width modulation. According to block 112, the remainder of the time interval 11 can then be idle time.

According to block 113, the time interval 11 can begin with a first rest time 13a. According to block 114, at least one modulation time 12 can follow this first idle time 13a. According to block 115, the time interval 11 can end with a second idle time 13b.

According to block 116, the modulation times 12 can include a first secondary modulation time 12a, a subsequent main modulation time 12b and a still later subsequent second secondary modulation time 12c. During the main modulation time 12b, according to block 117, at least 50% of the amount of energy required during the entire time interval 11 is supplied to the inductive load 4. The additional modulation during the secondary modulation times 12a and 12c serves, according to block 118, to approximate the time curve of the current flowing through the inductive load 4 caused by the modulation during the main modulation time 12b to a sinusoidal shape.

According to block 119, the energy required during the entire time interval 11 can be supplied to the inductive load 4 in a single pulse 14.

FIG. 2 shows examples of time programs 10, according to which the switching elements 3a-3f of the electrical converter 1 can be activated, as well as the ones derived therefrom resulting time curve I, in each case, of the current flowing through the inductive load 4.

The first time program 10 for the time interval 11 comprises a modulation time 12 with three pulses 14a-14c and a subsequent rest time 13. During each of the pulses 14a-14c, the current I rises steeply. In the switch-off times between the pulses 14a-14c, the current I falls back somewhat. When the last pulse 14 has ended, no more energy is supplied, but only consumed by the inductive load 4. The current I therefore drops continuously. To classify the course of time, a sinusoidal half-wave of a conventional alternating voltage Uo is shown for comparison.

The second time program 10 for the time interval 11 comprises the shortened modulation time 12 ', which only contains the first pulse 14a. This is followed directly by the extended rest period 13 '. Here, the entire amount of energy that the inductive load 4 requires during the time interval 11 must be transported into this inductive load 4 much more quickly. The rise in the current is therefore much steeper here than the rise in the current I fed from three pulses 14a-14c.

FIG. 3 shows an example of a time program 10 in which a modulation time 12 in the middle of the half-wave of a conventional alternating voltage Uo is framed by two rest periods 13a and 13b. The modulation time 12 here contains four pulses 14a-14d of the pulse width modulation. The current I rises steeply during each pulse 14a-14d, falls back somewhat in the switch-off times between the pulses 14a-14d, and after the end of the modulation time 12 falls steadily back to zero.

FIG. 4 shows a further example of a time program 10 in which a main modulation time 12b is framed by two secondary modulation times 12a and 12c. In this simple example, each of these modulation times 12a-12c contains only one pulse 14a-14c. The other times are idle times 13. As indicated in FIG. 4, the time curve of the current I is at least somewhat approximated to a sinusoidal shape. A feature of this In the exemplary embodiment, there are phases between the pulses 14a-14c in which the current I is completely interrupted (ie, it drops back to zero).

FIG. 5 shows an exemplary arrangement of an electrical converter 1 designed as an inverter and an inductive load 4. The converter 1 connects the three phases U, V, W of its output lb either with the positive pole or with the rapid pulsing of the switching elements 3a-3f Negative pole of its input la, to which a direct voltage 2 is applied. As a result, a current 4 a is driven through the inductive load 4. The current 4a has a predetermined effective value. The inductive load 4 is a in this example

Electric motor with three windings, which are each supplied by a phase U, V, W of the converter 1 and have a common non-earthed star point. Each winding is to be regarded as a series connection of an inductance L and a resistor R, because the wire used for the windings also has an ohmic resistance and this cannot be neglected.

Claims

Expectations

1. Method (100) for operating an electrical converter (1) which applies a DC voltage (2) presented at its input (la) to an inductive load (4) by controlling at least one switching element (3a-3f) in pulse width modulation ( 120), so that a predetermined effective current (4a) flows through the inductive load (4), a time program (10) being used for the control (110) which contains at least one time interval (11) which is divided into

• one or more rest periods (13) in which no pulse width modulation takes place, and

• one or more modulation times (12) in which the inductive load (4) is supplied with the amount of energy required during the entire time interval (11) by means of pulse width modulation.

2. The method (100) according to claim 1, wherein an inductive load (4) is selected (105) which is provided for operation with a single or multi-phase alternating current (4a), the time interval (11) of a half-wave (4b ) corresponds to this alternating current (4a).

3. The method (100) according to claim 2, wherein an electric motor is selected as the inductive load (4) (105a).

4. The method (100) according to any one of claims 2 to 3, wherein the electrical converter (1) is designed as an inverter, the one or more alternating current phases (U, V, W) of the inductive load (4) by controlling switching elements ( 3a-3f) each optionally with the positive pole or the negative pole of the DC voltage (2) presented at the input (la) of the inverter (1).

5. The method (100) according to any one of claims 1 to 4, wherein the time interval (11) begins (111) with a modulation time (12) which comprises at least two pulses (14a-14c), and wherein the remainder of the time interval (11 )

Idle time (13) is (112).

6. The method (100) according to any one of claims 1 to 4, wherein the time interval (11) begins with a first rest time (13, 13a) (113), at least one modulation time (12) is based on this first rest time (13, 13a) then (114) and the time interval (11) with a second rest period (13, 13b) ends (115).

7. The method (100) according to claim 6, wherein the modulation times (12) comprise a first secondary modulation time (12a), a later following main modulation time (12b) and a still later following second secondary modulation time (12c) (116), wherein the inductive load ( 4) during the main modulation time (12b) at least 50% of the amount of energy required during the entire time interval (11) is supplied (117) and the additional modulation during the secondary modulation times (12a, 12c) is caused by the modulation during the main modulation time (12b) Time course of the current flowing through the inductive load (4) approximates a sinusoidal shape (118).

8. The method (100) according to any one of claims 1 to 4, wherein the inductive load (4) is supplied (119) with the energy required during the entire time interval (11) in a single pulse (14).

9. The method (100) according to any one of claims 1 to 8, wherein an operating phase (4 ') of the inductive load (4) is selected (106) in which the inductive load (4) with a maximum of 20%, preferably with a maximum of 10 % and very particularly preferably with a maximum of 5%, their nominal power is operated.

10. Computer program containing machine-readable instructions which, when executed on one or more computers, cause the computer or computers to carry out the method (100) according to any one of claims 1 to 9.

11. Machine-readable data carrier and / or download product with the computer program according to claim 10.

12. Computer equipped with the computer program according to claim 10 and / or with the machine-readable data carrier and / or download product according to claim 11.