WO2010089186A2 - Control circuit for a three-phase asynchronous motor - Google Patents

Abstract

The invention relates to a control circuit (1) for a three-phase asynchronous motor (2), wherein each of three net phases (L1, L2, L3) is connected to at least one of three stator windings (3.1, 3.2, 3.3) of the three-phase asynchronous motor (2) by way of a phase transistor (MP1, MP2, MP3). Each of the stator windings (3.1, 3.2, 3.3) is connected to an anode of a recovery diode (D1, D2, D3), wherein the cathodes of the recovery diodes (D1, D2, D3) are connected to a recovery collection potential (4), wherein each of the net phases (L1, L2, L3) is connected to the recovery collection potential (4) by way of a recovery transistor (MF1, MF2, MF3). The invention further relates to a method for controlling a three-phase asynchronous motor, wherein the recovery transistor (MF1, MF2, MF3) is turned on when the respectively associated net phase (L1, L2, L3) has a higher potential than each of the net phases (L1, L2, L3) of the two other recovery transistors (MF1, MF2, MF3).

Description

description

Control circuit for a three-phase asynchronous motor

The invention relates to a control circuit for a three-phase asynchronous motor.

Three-phase asynchronous motors can be operated directly on a three-phase system via a frequency converter, via a variable transformer or via a phase control.

When operating directly on the three-phase system, high starting currents, high idling reactive power and high currents occur even at low load. A control of the speed is not possible. The power of the three-phase asynchronous motor depends on the level of the mains voltage.

Although the speed can be controlled with a frequency converter, high losses occur in the frequency converter. Due to the large number of components required in the frequency converter, its costs and the risk of failure are high. In addition, the frequency converter has a high harmonic content on the three-phase system due to system feedback.

Variable transformers are heavy components with which the speed can only be controlled to a limited extent. The operation of the variable transformer is hardly automatable.

If the three-phase asynchronous motor is operated with phase control, harmonics also occur as network feedback. The engine develops unwanted noise during operation. The starting current can only be reduced inadequately and the speed can only be controlled to a limited extent.

To prevent the high start-up currents, some special start-up devices are temporarily switched to the mains phases. German Patent No. 1 809 438 discloses a protective circuit for limiting overvoltages for semiconductor circuits, the voltage limiting being effected by means of three zener diodes, which are each combined at the input and output of a three-phase controller to form a star connection.

From DE 34 09 299 C2 a limitation by means of a switching transistor is known, wherein the Induktionssspannungsbe- limiting circuit is connected in parallel with the motor windings.

It is an object of the present invention to provide an improved control circuit for a three-phase asynchronous motor and a method for its control.

The object is achieved by a control circuit with the features of claim 1 and a method having the features of claim 11.

Advantageous developments are the subject of the dependent claims.

A control circuit according to the invention for a Drehstroma- synchronous motor is constructed so that each of three network phases is connected via a phase transistor with at least one of three stator windings of the three-phase asynchronous motor. The three-phase asynchronous motor has three stator windings (also called motor windings or main inductances), each of which is connected to an anode of each freewheeling diode. The cathodes of the freewheeling diodes are connected to a freewheeling collection potential. Each of the network phases is additionally connected to the freewheeling collection potential via a freewheeling transistor.

Such a construction of a control circuit enables the phase transistors to connect the terminals led out of the three-phase asynchronous motor to the three mains phases and the phase transistors can be controlled accordingly. In the case of a current flow (technical current direction) out of the mains phase, that is in the three-phase asynchronous motor, the connection can be taken over by a freewheeling diode integrated in the phase transistor, in particular, when the phase transistor as a metal oxide semiconductor field effect transistor (hereinafter short MOSFET called MOSFET = metal oxide semiconductor field effect transistor) is formed. In this state eliminates the control of the phase transistor. If the phase transistor is turned off while flowing through a current in the direction of the mains phase, a high voltage, which is caused by the inductances of the three-phase asynchronous motor, would occur at the motor-side connection of the phase transistor. In the case of the MOSFET, this is its drain. However, this induction voltage is limited by the motor current at the moment of switching off the phase transistor is applied to the reconnection or until the decay of energy in the respective inductance by another part of the circuit. This function is taken over by the freewheeling transistors, the freewheeling diodes and the freewheeling collection potential.

In this way, the phase transistors can be turned on and off arbitrarily, without being destroyed by an induction voltage that is above the dielectric strength of the phase transistors. The control circuit has a comparatively small number of components and can therefore be realized inexpensively. It is further characterized by a low starting current, a low mains feedback by harmonics, a good controllability of the current flow and the reactive power, a relatively low power loss (compared to a frequency converter), a controllability of the rotational speed of the three-phase asynchronous motor and low noise development, especially when the control circuit is operated with frequencies outside the listening area. The functionality of the control circuit is similar to that of a variable transformer, but is much lighter than this and requires considerably less space. In addition, the control circuit is easier to automate.

The freewheeling transistor is preferably switched on precisely when its respectively associated mains phase has a higher potential than each of the grid phases of the two other freewheeling transistors. This means that, except for a short overlap, only one free-wheeling transistor is switched on at any one time. The freewheeling transistors alternate at mains frequency (for example 50 Hz) in turn into their ON state. This process creates the so-called freewheel collection potential to which all freewheeling transistors are connected in the case of using MOSFETs with their respective drain. This freewheeling collection potential is maintained even under current load. It corresponds to the highest potential, that is the highest phase voltage of the three mains phases. The freewheeling collection potential now adopts the motor current of the phase transistors during their turn-off process, in that the motor current is introduced into the freewheeling collection potential via the additional free-wheeling diodes.

Further components can be provided in the control circuit, in particular drive drivers for the transistors and components for ensuring the electromagnetic compatibility (EMC), for example, in each case a capacitor connected between every two mains phases.

The phase transistor is preferably formed as a field effect transistor (MOSFET). Alternatively, the use of an insulated gate bipolar transistor (hereinafter referred to as IGBT, IGBT = insulated gate bipolar transistor) is possible. Preferably, the drain of the field effect transistor is connected to the stator winding.

Likewise, the freewheeling transistor may be formed as a field effect transistor. Alternatively, the use of an IGBT is possible. The drain of the freewheeling transistor is preferably connected to the freewheeling collection potential. The stator windings of the three-phase asynchronous motor can be connected, for example, in delta connection or in star connection.

Particularly preferably, the phase transistor is designed as a MOSFET, in particular a so-called superjunction MOSFET, for example as a CoolMOS ™ from Infineon. Superjunction MOSFETs have lower switching and throughput losses in power ranges of about 10 kW than an IGBT. The known, high reverse recovery of the body diodes of this superjunction MOSFET does not affect the function of the control circuit.

Preferably, an instantaneous voltage sensor and / or an instantaneous current sensor is provided for each of the mains phases in order to switch the freewheeling transistors on and off accordingly.

The control of the phase transistors and the freewheeling transistors can be done for example by means of a microcontroller.

The control preferably takes place in such a way that the phase transistors are switched on and off in such a way that at least approximately a sinusoidal three-phase current is produced by the three-phase asynchronous motor.

The phase transistors can be driven, for example, pulse-width modulated, in particular with a clock frequency in the range of 20 kHz to 100 kHz. This allows the

Reduce the current flow through the three-phase asynchronous motor to any desired level. The average motor voltage remains as a result of a three-phase system with mains frequency, for example, 50 Hz or 60 Hz, but with a variable voltage amplitude. This results in a

Controllability similar to that of a variable transformer. In contrast to the variable transformer, however, results in a slightly different current curve, which is essentially a Current flow angle of about 30 ° corresponds. This is caused by the fact that in each case over 60 ° of the phase angle, a current flows through the integrated freewheeling diodes (also called body diodes) of two of the phase transistors. However, this effect is masked by the reactive current, which may be required anyway by the three-phase asynchronous motor.

In the following an embodiment of the invention will be explained in more detail with reference to drawings. Show:

1 shows a circuit with a three-phase asynchronous motor and three phase transistors,

2 shows the circuit from FIG. 1, expanded with additional freewheeling transistors and freewheeling diodes,

3 shows the circuit of Figure 2 with the phase transistors, and

FIG. 4 shows the circuit from FIG. 2 with two switched-off phase transistors at the moment of the highest phase potential on the network phase L1.

Corresponding parts are provided in all drawings with the same reference numerals.

FIG. 1 shows part of a control circuit 1 with a three-phase asynchronous motor 2, of which three stator windings 3.1, 3.2, 3.3 are shown. The stator windings 3.1 to 3.3 are connected in delta connection. Each of the resulting connections of the three-phase asynchronous motor 2 is assigned a phase transistor MPl, MP2, MP3 for connection to a respective network phase Ll, L2, L3. The phase transistors MPl, MP2, MP3 are formed as a MOSFET or each comprise a MOSFET. In case of a current flow

(Technical current direction) out of the mains phase Ll, L2, L3, that is in the three-phase asynchronous motor 2 in, the connection through a in the phase transistor MPl, MP2, MP3 integrated freewheeling diode (also called body diode) can be adopted. In this state, it is therefore irrelevant whether the phase transistor MPl, MP2, MP3 is turned on or off. If the phase transistor MP1, MP2, MP3 is turned off while flowing through a current from the three-phase asynchronous motor 2 in the direction of the line phase L1, L2, L3, this would lead to a high voltage at the motor-side connection due to the inductances of the three-phase asynchronous motor 2 (the drain) of the phase transistor MPl, MP2, MP3 come.

In order to prevent voltages occurring beyond the dielectric strength of the phase transistors MP1, MP2, MP3, the control circuit 1 according to FIG. 1 is expanded, as shown in FIG.

The stator windings 3.1, 3.2, 3.3 are each connected to an anode of an additional freewheeling diode Dl, D2, D3. The cathodes of the free-wheeling diodes D 1, D 2, D 3 are connected to one another and form a freewheeling collecting potential 4. Each of the mains phases L 1, L 2, L 3 is additionally connected to the freewheeling collecting potential 4 via a freewheeling transistor MF 1, MF 2, MF 3. The resulting when turning off the phase transistors MPl, MP2, MP3 during a current flow from the Drehstromasynchronmo- tor 2 in the direction of the mains phase induction voltage is limited by the motor current at the moment of switching off the phase transistor MPl, MP2, MP3 until the reconnection or until the decay of the Energy in the respective inductance is taken over by the freewheeling collection potential 4.

When the phase transistors MP1, MP2, MP3 are switched on, a current flow Ii results, as shown in FIG. The illustration of the current flow Ii serves only to illustrate at which points of the control circuit 1 in the described

Situation electricity flows. In particular, should not be set with the presentation of the current direction. Likewise should not can be expressed that the current in all marked with Ii sections of the control circuit 1 is equal.

In Figure 4, the generation of the freewheeling collection potential 4 is shown with switched-off phase transistors MP2, MP3 (freewheeling). For this purpose, the freewheeling transistor MF1 is switched on exactly when its associated network phase Ll has a higher potential than each of the other two network phases L2, L3. This means that, except for a short overlap, only one free-wheeling transistor MF1, MF2, MF3 is switched on at any one time. The freewheeling transistors MF1, MF2, MF3 change at line frequency (for example 50 Hz) in order to their on state. This process provides the free-wheeling collection potential 4, to which all free-wheeling transistors MF1, MF2, MF3 are connected, in the present case the use of MOSFETS with their respective drain. This freewheeling collection potential 4 is maintained even under current load. It corresponds to the highest potential, that is to say the highest phase voltage of the three network phases L1, L2, L3.

The freewheeling collection potential 4 now adopts the motor current of the phase transistors MP1, MP2, MP3 in their turn-off process by the motor current via the additional freewheeling D2, D3 is introduced into the freewheeling collection potential 4. This results in the current flow I2.

The representation shown in FIG. 4 is exemplary for the case where the network phase L 1 has the highest potential. In the event that the highest potential is applied to the network phase L2 or L3, the respective associated freewheeling transistor MF2 or MF3 is turned on.

The illustration of the current flow I2 serves only to illustrate where current flows in the control circuit 1 in the situation described. In particular, should not be set with the presentation of the current direction. Likewise it should not be expressed that the current in all with I2 marked portions of the control circuit 1 is the same size.

Further components may be provided in the control circuit, in particular drive drivers for the transistors MF1, MF2, MF3, MP1, MP2, MP3 and components for ensuring the electromagnetic compatibility (EMC), for example, in each case a capacitor C1, C2 connected between two mains phases in each case. C3.

The phase transistor MP1, MP2, MP3 may be formed as an IGBT or a MOSFET, in particular a superjunction MOSFET, for example a CoolMOS ™.

The freewheeling transistor MF1, MF2, MF3 may be formed as an IGBT.

The stator windings 3.1, 3.2, 3.3 of the three-phase asynchronous motor 2 may alternatively be connected in star connection.

Preferably, an instantaneous voltage sensor and / or an instantaneous current sensor (not shown) is provided for each of the mains phases L1, L2, L3.

The control of the phase transistors MPl, MP2, MP3 and the freewheeling transistors MFl, MF2, MF3 can be done for example by means of a microcontroller (not shown).

The control is preferably carried out in such a way that the phase transistors MP1, MP2, MP3 are switched on and off in such a way that at least approximately a sinusoidal three-phase current through the three-phase asynchronous motor 2 results.

The phase transistors MPl, MP2, MP3 can be driven, for example, pulse width modulated, in particular with a clock frequency in the range of 20 kHz to 100 kHz.

Claims

claims First control circuit (1) for a three-phase asynchronous motor (2), wherein each of three network phases (Ll, L2, L3) via a phase transistor (MPl, MP2, MP3) with at least one of three stator windings (3.1, 3.2, 3.3) of the Three-phase asynchronous motor (2) is connected, wherein each of the stator windings (3.1, 3.2, 3.3) is connected to an anode of each freewheeling diode (Dl, D2, D3), wherein the cathodes of the freewheeling diodes (Dl, D2, D3) with a Freewheeling collecting potential (4) are connected, wherein each of the network phases (Ll, L2, L3) via a freewheeling transistor (MFl, MF2, MF3) is connected to the freewheeling recording potential (4). 2. Control circuit (1) according to claim 1, characterized in that in each case a capacitor (Cl, C2, C3) is connected between each two mains phases (Ll, L2, L3). 3. Control circuit (1) according to any one of claims 1 or 2, characterized in that the phase transistor (MPl, MP2, MP3) is formed as a field effect transistor or as an insulated gate bipolar transistor (= IGBT) is formed, whose drain or collector with the stator winding (3.1, 3.2, 3.3) is connected. 4. Control circuit (1) according to any one of the preceding claims, characterized in that the freewheeling transistor (MFl, MF2, MF3) is formed as a field effect transistor or as an insulated gate bipolar transistor (= IGBT) is formed, whose drain or collector is connected to the freewheeling collection potential (4). 5. Control circuit (1) according to one of the preceding claims, characterized in that the stator windings (3.1, 3.2, 3.3) are connected in delta connection. 6. Control circuit (1) according to one of claims 1 to 4, characterized in that the stator windings (3.1, 3.2, 3.3) are connected in a star connection. 7. Control circuit (1) according to one of claims 3 to 6, characterized in that the phase transistor (MPl, MP2, MP3) is formed as a metal oxide semiconductor field effect transistor (= MOSFET). 8. Control circuit (1) according to one of the preceding claims, characterized in that a momentary voltage sensor and / or a momentary current sensor for each of the mains phases (Ll, L2, L3) is provided. 9. Control circuit (1) according to one of the preceding claims, characterized in that a microcontroller for controlling the phase transistors (MPl, MP2, MP3) and the freewheeling transistors (MFl, MF2, MF3) is provided. 10. A method for controlling a three-phase asynchronous motor (2) having a control circuit (1) according to one of claims 1 to 10, wherein the mains phases (Ll, L2, L3) are supplied with a voltage, wherein the phase transistors (MPl, MP2, MP3) for supplying power to the three-phase asynchronous motor (2) switched on and / or off, wherein the freewheeling transistor (MFl , MF2, MF3) is turned on when its respective associated phase phase (Ll, L2, L3) has a higher potential than each of the network phases (Ll, L2, L3) of the two other freewheeling transistors (MFl, MF2, MF3). 11. The method according to claim 10, characterized in that the phase transistors (MPl, MP2, MP3) are controlled pulse width modulated. 12. The method according to any one of claims 10 or 11, characterized in that the phase transistors (MPl, MP2, MP3) are so switched on and off that at least anna a sinusoidal three-phase current through the three-phase asynchronous motor (2) results.