DE102012002023A1 - Method for operating inverter circuit of permanent magnet synchronous electric machine e.g. electric motor, involves operating electric machine in active short-circuit mode, when rotational speed is above threshold value - Google Patents

Abstract

The method involves detecting the rotational speed (n) of electric machine (3). The electric machine is operated in the active short-circuit mode or in the free-running mode based on the detection of cut-off condition. The electric machine is operated in active short-circuit mode, when the rotational speed is set above the threshold value. The electric machine is operated in free-running mode, when the rotational speed is set below the threshold value. An independent claim is included for device for operating inverter circuit of electric machine.

Description

The invention relates to a method and a device for operating an inverter circuit of an electric machine.

In today's electric and hybrid vehicles mainly permanent-magnet electric machines are used as drives. Often these electric machines are hard connected to the wheel and can not be decoupled mechanically from the wheel during operation. For safety reasons, the electric drive must bring no or only a very small torque to the drive shaft and thus to the wheels in the event of a safety shutdown.

The easiest way to ensure this is to stop the clocking in the pulse-controlled inverter and to operate the power semiconductors either in freewheel mode or in short-circuit mode. In free-running mode, the active semiconductor components of the pulse-controlled inverter are not activated. However, the current can flow freely through the diodes from the electric machine into the traction network. In the short-circuit mode, on the other hand, either the three lower or the three upper semiconductor components are permanently driven, so that the phase connections of the electric machine are short-circuited.

Both modes have their advantages and disadvantages and can not provide optimal results for the entire working range of the electric machine. The main drawback of freewheeling mode is that the voltage generated by the electric machine may exceed the allowable traction network voltage, which may lead to the destruction of the components. This is especially true for the high-power and very field-weakened permanent-magnet electric machines. For this reason, almost exclusively the short circuit mode (also called active short circuit or AKS) is used in these electric machines. Due to internal winding inductances only a very small braking torque is formed when short-circuiting a permanently excited electric machine in the region of high rotational speeds.

This low short-circuit torque can not cause the electric machine and ultimately the vehicle wheels to lock and is generally tolerated. In the lower speed range, however, each permanent-magnet electric machine has a significant increase in the short-circuit torque.

Depending on the design of the electric machine and the vehicle, this torque can lead to locking of the wheels, to strong driving vibrations and to other negative phenomena in the drive train.

• – einem Eingangstor zum Bereitstellen einer elektrischen Spannung;
• – einer dem Eingangstor nachgeschalteten Brückenwechselrichterschaltung zur Erzeugung einer elektrischen Wechselspannung für den Elektromotor auf Basis der elektrischen Spannung, wobei die Brückenwechselrichterschaltung zumindest einen Halbbrückenzweig mit Schaltelementen aufweist und ausgebildet ist, zur Überführung eines Elektromotors in einem Freilaufzustand die Schaltelemente zu öffnen; und
• – einem Entkopplungsschaltelement, welches ausgebildet ist, den zumindest einen Halbbrückenzweig in dem Freilaufzustand des Elektromotors von dem Eingangstor zu entkoppeln, um eine Rückwirkung des freilaufenden Elektromotors auf das Eingangstor zu unterbinden.

From the DE 10 2009 047 616 A1 is an inverter arrangement for operating an electric motor having the following features:

• An input port for providing an electrical voltage;
• - A downstream of the input bridge bridge inverter circuit for generating an electrical AC voltage for the electric motor based on the electrical voltage, wherein the bridge inverter circuit has at least one half-bridge branch with switching elements and is designed to open the switching elements for transferring an electric motor in a freewheeling state; and
• - A decoupling switching element, which is designed to decouple the at least one half-bridge branch in the freewheeling state of the electric motor from the input port to prevent a reaction of the free-running electric motor to the input port.

The invention is based on the technical problem of providing a method and a device for operating an inverter circuit of an electric machine, by means of which a stable working range can be set by simple means.

The solution of the technical problem results from the objects with the features of claims 1 and 6 Further advantageous embodiments of the invention will become apparent from the dependent claims.

For this purpose, the device comprises an electric machine, an inverter circuit, a control device for controlling the inverter circuit and a device for detecting a rotational speed of the electric machine, wherein upon detection of a shutdown condition, the electric machine is operated in the active short circuit mode or freewheeling mode, wherein the speed is associated with at least one threshold , wherein at speeds above the limit, the electric machine is operated in the active short-circuit mode and at speeds below the limit, the electric machine is operated in the freewheeling mode. Thus, the advantages of both modes are intelligently combined and their respective disadvantages avoided. The electric machine is preferably a permanent-magnet synchronous electric machine. A preferred application of the electric machine is the use as a prime mover in an electric or hybrid vehicle.

It is further preferably provided that in case of failure of the speed detection the Electric machine is operated in active short circuit mode.

In one embodiment, the rotational speed is assigned at least two limit values, wherein at speeds above the first limit value, the electric machine is operated in the active short circuit mode and at speeds below the second limit value, the electric machine is operated in freewheeling mode. The first limit value is greater than the second limit value. Preferably, the switching between the modes takes place by means of a hysteresis function. Thus, switching oscillations between the short-circuit mode and the free-wheeling mode are avoided, since in particular on substrates with a low coefficient of friction (eg ice), speed fluctuations can occur. The hysteresis now prevents unexpected switching to the active short-circuit mode and thus blocks the wheels at low speeds.

In a further embodiment, the distance between the first and the second distance is therefore chosen such that it is greater than the speed fluctuations due to low coefficients of friction of the substrate.

In a further embodiment, a critical speed is determined at which a maximum allowable braking torque occurs, wherein the limit value for the freewheeling mode is selected to be greater than the critical speed. This ensures that no uncontrollable braking torque can occur.

The invention will be explained in more detail below with reference to a preferred embodiment. The figures show:

1 1 is a schematic block diagram of an apparatus for operating an inverter circuit of an electric machine,

2 a schematic representation of a short-circuit torque on the speed of a permanent-magnet electric machine (prior art),

3 a temporal course of the short-circuit torque and the speed after initiation of an active short circuit (prior art) and

4 a schematic representation of a short-circuit torque on the speed of an electric machine with limits for switching.

In the 1 is a schematic block diagram of a device 1 for operating an inverter circuit 2 an electric machine 3 shown. The device 1 includes an inverter circuit 2 , an electric machine 3 , a control device 4 for controlling the inverter circuit 2 as well as a facility 5 for detecting a rotational speed n of the electric machine 3 , The inverter circuit 2 comprises six semiconductor switching elements T1-T6, to each of which a freewheeling diode D1-D6 is connected in parallel. The semiconductor switching elements T1-T6 are connected in pairs as a half-bridge, wherein in each case a center tap of the half-bridge with a phase connection of the electric machine 3 connected is. Parallel to the semiconductor bridges an intermediate circuit capacitor C is arranged. About a traction network 6 is the inverter circuit 2 with a traction battery 7 connected via a battery contactor 8th with the traction network 6 connected is. The control device 4 generates control signals S1-S6 for the semiconductor switching elements T1-T6, wherein the control means for calculating the control signals S1-S6 to different input signals E and the rotational speed n of the electric machine 3 recourse.

Before the invention is explained in more detail, first the problem with reference to 2 and 3 be briefly shown, in 2 the short-circuit torque M _{K is shown} above the rotational speed n, wherein the maximum short-circuit torque M _{tilt is achieved} at a relatively low rotational speed n _tilt . If an active short circuit is then initiated at a time t ₀ (see 3 ), the speed n decreases with time. If the speed n then reaches the range of the speed n _tilt at time t ₁ , then strong vibrations with high short-circuit values M _{K occur} .

Based on 4 Let the invention be further explained. First, the critical range for impermissible torques is defined. The limit results _crit from the maximum permissible braking torque m at the wheels, in which the vehicle remains controllable and no wheel locking can take place at defined road conditions. Based on the short-circuit torque line, the critical speed n _crit can now be determined. The lower switch-off limit for release from the active short circuit and to change to freewheel mode is set at n _FL > n _krit . This limit (second limit) is operated when the speed drops. For increasing speeds (change from freewheel to active short circuit), a switch-on limit n _{AKS is} defined. This limit (first limit value) is to be defined in such a way that on the one hand a sufficient distance to n _{FL is} ensured, on the other hand the generated voltage of the electric machine produced at n _AKS is still within the range tolerable for the traction network. The described hysteresis serves primarily to eliminate switching oscillations between the short-circuit mode and the free-wheeling mode, which otherwise otherwise occur at the low-drive value (eg black ice) could. It should be noted that in case of failure of the device 5 is always switched to the active short-circuit mode as a fallback level to detect the speed n.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 102009047616 A1 [0007]

Claims (10)

Method for operating an inverter circuit ( 2 ) an electric machine ( 3 ) with a safety shutdown, by means of a device ( 5 ) for detecting a rotational speed (n) of the electric machine ( 3 ), wherein upon detection of a shutdown condition, the electric machine ( 3 ) is operated in the active short-circuit mode or in freewheeling mode, characterized in that the rotational speed (n) is assigned at least one limit value, wherein at speeds (n) above the limit value the electric machine ( 3 ) is operated in the active short circuit mode and at speeds (n) below the limit value, the electric machine is operated in freewheel mode. A method according to claim 1, characterized in that the rotational speed at least two limit values (n _AKS , n _FL ) are assigned, wherein at speeds (n) above the first limit value (n _AKS ) the electric machine ( 3 ) is operated in the active short circuit mode and at speeds (n) below the second limit value (n _FL ) the electric machine ( 3 ) is operated in freewheel mode. Method according to Claim 2, characterized in that the switching between the modes takes place between the two limit values (n _AKS , n _FL ) by means of a hysteresis function. Method according to one of claims 2 or 3, characterized in that the distance between the first limit value (n _AKS ) and the second limit value (n _FL ) is selected such that it is greater than the speed fluctuations during installation and operation of the electric machine ( 3 ) can occur in an electric or hybrid vehicle due to low friction coefficients of the substrate. Method according to one of the preceding claims, characterized in that a critical speed (n _crit ) is determined at which a maximum permissible braking torque (M _crit ) occurs, wherein the limit value (n _FL ) for the freewheeling mode greater than the critical speed (n _crit ) is selected. Contraption ( 1 ) for operating an inverter circuit ( 2 ) of an electric machine, comprising an electric machine ( 3 ), an inverter circuit ( 2 ), a control device ( 4 ) for controlling the inverter circuit ( 2 ) and a facility ( 5 ) for detecting a rotational speed (n) of the electric machine ( 3 ), wherein upon detection of a shutdown condition, the electric machine ( 3 ) is operated in the active short-circuit mode or in the freewheeling mode, characterized in that the rotational speed (n) is assigned at least one limit value, the control device ( 4 ) is designed such that at speeds (n) above the limit value, the electric machine ( 3 ) is operated in the active short-circuit mode and at speeds (n) below the limit value the electric machine ( 3 ) is operated in freewheel mode. Device according to Claim 6, characterized in that the rotational speed (n) is assigned at least two limit values (n _AKS , n _FL ), the control device ( 4 ) is designed such that at speeds (n) above the first limit value (n _AKS ) the electric machine ( 3 ) is operated in the active short circuit mode and at speeds (n) below the second limit value (n _FL ) the electric machine ( 3 ) is operated in freewheel mode. Apparatus according to claim 7, characterized in that between the two limit values (n _AKS , n _FL ), the switching between the modes by means of a hysteresis function. Device according to one of claims 7 or 8, characterized in that the distance between the first limit value (n _AKS ) and the second limit value (n _FL ) is selected such that it is greater than the speed fluctuations during installation and operation of the electric machine ( 3 ) can occur in an electric or hybrid vehicle due to low friction coefficients of the substrate. Device according to one of claims 6 to 9, characterized in that the limit value (n _FL ) for the freewheeling mode is greater than a critical speed (n _crit ) is selected.

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