Patent Application
20100134057
The invention relates to a motor module for an electric motor, in particular a permanent-magnet synchronous motor, and to a control device which has a motor module such as this. The invention furthermore relates to a method of protection of a converter, which is intended to drive an electric motor, against overvoltage.
In the case of an electric motor as is used, by way of example, as a drive for a production machine or machine tool, a rotating-field winding is normally provided on the stator side. The rotating-field winding of the motor has one or more winding sections, generally three winding sections, and—fed with an appropriately single-phase or polyphase drive current, generally an approximately sinusoidal drive current—produces a magnetic field which revolves in the air gap between the stator and the rotor of the motor, and drives the rotor. The winding sections of the rotating-field winding, which are generally connected in star with one another, are also referred to in the following text as motor phases.
The motor phases are normally electronically commutated by means of a so-called converter circuit (referred to for short in the following text as a “converter”). Conventionally, a converter such as this comprises a so-called electrical intermediate circuit, which carries an electrical DC voltage (referred to in the following text as the intermediate circuit voltage). An associated half bridge is in each case connected in the intermediate circuit for each motor phase (in contrast to this, in the case of a single-phase electric motor, the single motor phase is connected between two half bridges). Each half bridge comprises two series-connected power switches, between which a phase connection for the associated motor phase is arranged. The power switches are normally in the form of electronic switching elements, in particular so-called IGBTs or MOSFETs. With regard to their respective arrangement with respect to the phase connection and the voltage drop in the intermediate circuit, the two power switches of a half bridge are referred to in the following text as power switches on the high-potential side and on the low-potential side. A freewheeling diode is in each case connected in parallel with each power switch and is oriented in the reverse-biased direction with respect to the voltage drop in the intermediate circuit.
In addition to the converter, a control device for an electric motor normally has control logic for driving the power switches in the converter. The control device for an electric motor furthermore normally has a regulation component which generates a control signal, which is in turn supplied as an input variable to the control logic, by monitoring an operating variable of the electric motor, normally the motor current or the rotation speed.
In order to achieve a modular design, the converter with the associated control logic on the one hand and the regulation component on the other hand are also produced as mutually separate assemblies. The assembly comprising the converter and the control logic is in this case referred to as a motor module.
During operation of the electric motor, the voltage induced by rotation of the rotor in the stator windings is proportional to the rotation speed of the motor and to the magnetic flux linkages, which represent a measure of the magnitude of the magnetic field in the air gap between the rotor and the stator.
If the strength of the magnetic flux linkages is approximately constant, as is the case in particular in a permanent-magnet rotor, the induced voltage is therefore approximately proportional to the rotation speed of the motor.
Particularly in the case of a motor which is designed for high rotation speeds, the induced voltage may in this case reach high values which, without suitable protective measures, would lead to damage to a conventional converter. In order to prevent the induced voltage from exceeding a maximum permissible value, an electric motor is normally operated in a so-called weak-field mode at high rotation speeds. In this case, current is passed through the motor phases such that the stator produces a magnetic field with a field component which opposes the rotor magnetic field, thus weakening the magnetic field in the air gap between the rotor and the stator.
However, if the motor control fails, the induced voltage in the motor is in general connected to the intermediate circuit via the freewheeling diodes of the converter, without being weakened. Suitable measures must therefore be taken in order to prevent the converter from being damaged or destroyed in this case by the voltage induced in the motor.
In addition to the converter, a so-called voltage protection model (VPM) in the form of an electrical circuit connected between the motor phases is normally provided for this purpose. A voltage protection module such as this, as is known by way of example from DE 298 13 080 U1, is formed essentially by six diodes and a thyristor connected between them, wherein the motor phases can be short-circuited to one another by switching on the thyristor. The thyristor is driven via an evaluation circuit in the voltage protection module, as a function of the voltage that exists in the motor phases.
The invention is based on the object of specifying overvoltage protection, which can be implemented easily and at low cost, is compact and is at the same time effective, for a converter which is intended to supply an electric motor.
With regard to a motor module, this object is achieved by the features of claim 1. With regard to a method of protection of the converter against overvoltage, the object is achieved according to the invention by the features of claim 15.
The invention provides, in the case of a converter of the type described above, for the intermediate circuit voltage to be detected and, in the event of an overvoltage—specifically when the intermediate circuit voltage exceeds a predetermined maximum value—for the power switches on the high-potential side or the power switches on the low-potential side of all the half bridges in the converter to be switched on. Switching on the power switches on the high-potential side and those on the low-potential side results in the motor phase or phases of an electric motor which is connected to the converter being short-circuited, thus decreasing the intermediate circuit voltage.
The operating state of the converter in which the power switches on the high-potential side or those on the low-potential side of all the half bridges are switched on is correspondingly referred to for short in the following text as a “short-circuit”. Thus, in the case of a “short-circuit”, such as this, the motor phases are short-circuited, but not the intermediate circuit. If the short-circuit is formed via the power switches on the high-potential side, then the power switches on the low-potential side are correspondingly switched off at the same time, and vice versa. “Switched on” in this case refers to an operating state of a power switch in which the relevant power switch is electrically conductive. “Switched off” correspondingly refers to an operating state of a power switch in which the relevant power switch does not conduct.
In principle, the invention can be used both for a single-phase electric motor and for a polyphase electric motor. Just for simplicity reasons, the following text refers exclusively to the motor phases in the plural. This also covers the special case of a single motor phase.
The invention achieves effective overvoltage protection by suitably switching on the power switches, which are necessarily present in any case, in the converter, in such a way that the overvoltage protection, at least to a major extent, does not require any additional hardware components. This makes it possible to produce a simple, low-cost and compact control device.
In particular, protection logic is provided in order to carry out the overvoltage protection method. In this case, “logic” refers in particular to a software module which is implemented in an associated hardware component, in particular a controller. However, the protection logic can furthermore also be formed by a logic circuit.
In one advantageous embodiment to the invention, the control logic is integrated in the motor module. This results in particularly high reliability, with a simple design at the same time.
In order to prevent the intermediate circuit voltage from collapsing in the event of a fault—particularly after the external voltage supply to the intermediate circuit has collapsed—in one advantageous embodiment of the method according to the invention, the short-circuit is removed when the intermediate circuit voltage is below a predetermined minimum value. After removing the short-circuit, the intermediate circuit is charged again by the current induced in the motor. This embodiment of the method is particularly advantageous for embodiments of the motor module according to the invention in which the motor module is supplied with voltage—within the module or via an external supply component—from the intermediate circuit. The short-circuit is expediently imposed again when the intermediate circuit voltage once again exceeds the predetermined maximum voltage.
In one preferred embodiment of the invention, the power switches of the converter are configured, that is to say designed, such that they can permanently carry the short-circuit currents which are expected to occur during the short-circuit, without damage.
As an alternative to this, or for safety, one advantageous development of the invention additionally provides, with regard to such a configuration, that the short-circuit current be detected and the short-circuit be interrupted when the measured short-circuit current exceeds a predetermined maximum value. The short-circuit is in this case preferably only temporarily interrupted until the short-circuit current has decreased. The short-circuit is therefore produced intermittently. The short-circuit can be interrupted for all motor phases. In an alternative variant of the invention, the short-circuit current is in contrast detected separately for each motor phase, and the short-circuit is interrupted only for the relevant motor phase or phases in the event of an overcurrent.
Additionally, or as an alternative to this, a further advantageous embodiment of the invention provides for a decision variable to be determined, which is characteristic of the temperature of one or more of the power switches which are switched on during the short-circuit. In this case, according to the method, the short-circuit is interrupted when this decision variable exceeds a predetermined maximum value. In this case, the temperatures of the operated power switches themselves, an average or maximum temperature derived from these temperatures, or a variable which is correlated with this temperature, in particular which is proportional to this temperature, can optionally be used as the decision variable. The temperatures are in this case either measured or modeled in advantageous method variants, that is to say they are calculated on the basis of a predetermined temperature model, in particular on the basis of the time profile of the currents flowing through the switched-on power switches.
In this method variant as well, the short-circuit is expediently interrupted only temporarily, until the relevant power switches have cooled down sufficiently. The short-circuit is therefore once again produced intermittently. In alternative method variants, the short-circuit is once again interrupted either for all the motor phases or for each relevant motor phase separately.
Instead of interrupting the short-circuit completely when there is a threat of the power switches overheating, according to one alternative method variant, a change is made alternately between the power switches on the high-potential side and the power switches on the low-potential side in order to form the short-circuit. This change optionally takes place at predetermined time intervals, wherein the length of these time intervals can optionally vary as a function of further parameters, for example the magnitude of the short-circuit current. As an alternative to this, the temperature of the switched-on power switches or a decision variable which is correlated with it is once again determined, and the change is carried out only when the temperature or decision value exceeds a predetermined maximum value, and there is therefore actually a threat of overheating of the power switches which are switched on at that time.
In one preferred embodiment of the motor model, the protection logic, and therefore the overvoltage protection method carried out by it, can be reversibly activated and deactivated by presetting a switching signal. This characteristic makes it possible to also use the motor module for controlling motors in which the overvoltage protection is not necessary or would even be damaging. The latter relates, for example, to asynchronous motors.
In this case, the control logic is expediently designed to check the switching signal at predetermined, in particular regular, time intervals. In the case of a control device which comprises the motor module and an additional regulation module, this switching signal is preferably made available by the regulation module.
The control logic is expediently designed to store the respective most recent value of the switching signal. The control logic uses this stored value to autonomously decide whether the overvoltage protection method should or should not be carried out during starting and when no switching signal is transmitted.
Exemplary embodiments of the invention will be explained in more detail in the following text with reference to a drawing, in which:
Mutually corresponding parts, variables and structures are always provided with the same reference symbols in all the figures.
The motor 1 comprises a stator 5 (which is indicated only schematically in the illustration), which is wound with a rotating-field winding 6. The rotating-field winding 6 comprises three winding sections, referred to in the following text as motor phases L1, L2 and L3, which are connected together at a star point 7. The physical characteristics of each motor phase L1, L2, L3 are characterized by an inductance LL1, LL2, LL3, resistors RL1, RL2, RL3, and an induced voltage UL1, UL2, UL3. The inductances LL1, LL2, LL3, resistances RL1, RL2, RL3 and voltages UL1, UL2, UL3 are shown in the form of an equivalent circuit in
The motor module 3 comprises a converter 8 and a control unit 9. The converter 8 comprises an electrical intermediate circuit 10 with a high-potential side 11 and a low-potential side 12, between which an intermediate circuit voltage UZ is present during operation of the motor 1.
In the intermediate circuit 10, three half bridges 13a, 13b, 13c are connected in parallel in order to feed a respective motor phase L1, L2, L3. Each half bridge 13a, 13b, 13c has a phase connection 14a, 14b, 14c to which the associated motor phase L1, L2, L3 is connected. The motor phase L1 is therefore connected to the phase connection 14a of the half bridge 13a, the motor phase L2 to the phase connection 14b of the half bridge 13b, and the motor phase L3 to the phase connection 14c of the half bridge 13c.
Between the respective phase connection 14a, 14b, 14c and the high-potential side 11 of the intermediate circuit 10, each half bridge 13a, 13b, 13c has a power switch 15a, 15b, 15c on the high-potential side, in particular in the form of an IGBT. A freewheeling diode 16a, 16b, 16c is respectively connected in parallel with each of these power switches 15a, 15b, 15c. Within each half bridge 13a, 13b, 13c, a respective power switch 17a, 17b, 17c on the low-potential side is connected between the motor connection 14a, 14b, 14c and the low-potential side 12 of the intermediate circuit 10. Each of the power switches 17a, 17b, 17c on the low-potential side is once again in particular in the form of an IGBT, and is flanked by a parallel-connected freewheeling diode 18a, 18b, 18c.
The converter 8 furthermore has a capacitor 19, which is connected in parallel with the half bridges 13a, 13b, 13c in the intermediate circuit 10, in order to compensate for voltage ripple during operation of the motor 1.
The control unit 9 is formed by a microcontroller, or has at least one such microcontroller. The control unit 9 is supplied via a module-internal voltage supply unit 20 with a supply voltage UV of typically 24 volts. In this case the voltage supply unit 20 is itself fed from the intermediate circuit 10.
Control logic 21 and protection logic 22 are implemented in the form of software modules in the control unit 9. The control unit 9 switches the power switches 15a, 15b, 15c on or off during operation of the motor 1 in accordance with a control method that is predetermined by the control logic 21, by emitting respectively associated control signals C, in order to produce phase currents IL1, IL2, IL3, which produce a rotating field, in the motor phases L1, L2 and L3. The phase currents IL1, IL2, IL3 are tapped off by current measurement devices 23a, 23b, 23c, with measured values of these phase currents (for simplicity reasons likewise referred to as IL1, IL2, IL3) being supplied as an input variable to the control unit 9. The control unit 9 is also supplied with the intermediate circuit voltage UZ or a measured value that is proportional to it, as an input variable.
The regulation module 4 contains regulation logic (which is not illustrated in any more detail), which regulates the rotation speed and/or power of the motor 1 on the basis of a predetermined regulation variable. In this case, the motor current, in particular, is used as the regulation variable. In this case, the control unit 9 uses the measured phase currents IL1, IL2, IL3 to calculate a current actual value I, and supplies this as an input variable to the regulation module 4. On the basis of a comparison of the current actual value I with a stored current nominal value, the regulation module 4 produces a voltage nominal value US as an output variable, and feeds this back to the control unit 9. The control logic 21 produces the control signals C on the basis of this voltage nominal value US and the measured motor currents IL1, IL2, IL3.
During operation of the motor 1, the protection logic 22 monitors the intermediate circuit voltage UZ and, in the event of an overvoltage, short-circuits the motor phases L1, L2, L3 via the intermediate circuit 10 by selectively switching on either all the power switches 15a, 15b, 15c on the high-potential side or all the power switches 17a, 17b, 17c on the low-potential side. In order to allow the motor module 3 to also be used, in contrast to the described embodiment, with motors in which there is no need for such overvoltage protection or this would even be damaging—for example with an asynchronous motor instead of the synchronous motor—the method carried out by the protection logic 21 can be reversibly activated and deactivated by presetting an appropriate switching signal S. This switching signal S is made available as an input variable to the control unit 9, and therefore to the protection logic 22, by the regulation module 4.
The method carried out by the protection logic 22 will be described in more detail using a first exemplary implementation, on the basis of
The first program part, as shown in
The second program part, which is illustrated in
In a first step 34 of the program part as shown in
As a consequence of the short-circuit, the intermediate circuit voltage UZ decreases successively. In this case, subsequent steps 38 to 42 ensure that the intermediate circuit voltage UZ, and therefore the supply voltage UV to the control unit 9 as well, do not collapse as a result of the short-circuit, and that the power switches 15a, 15b, 15c which have been switched on are not overloaded.
For this purpose, the intermediate circuit voltage UZ is once again detected first of all in step 38. Then, in step 38, a value is determined for the short-circuit current IK flowing through the switched-on power switches 15a, 15b, 15c, and a decision variable T is determined for the temperature of the switched-on power switches 15a, 15b, 15c. The protection logic 22 in this case determines the short-circuit current IK on the basis of the measured phase currents IL1, IL2, IL3. In particular, the maximum value of the phase currents IL1, IL2, IL3 is used as the short-circuit current IK, while the decision variable T is determined on the basis of a stored temperature model of the power switches 15a, 15b, 15c by detection with respect to time, in particular integration, of the phase currents IL1, IL2, IL3.
In step 39, the protection logic 22 uses the following decision rule:
UZ<UZ,max OR IK>IK,max OR T>Tmax,
to check whether the intermediate circuit voltage UZ, the short-circuit current IK or the decision variable T is or are less than or greater than stored threshold values UZ,max, IK,max, Tmax. As long as this is not the case (N), steps 38 and 39 are repeated.
Otherwise (J), the short-circuit is removed in step 40, by switching off the power switches 15a, 15b, 15c.
Removal of the short-circuit has the effect that the short-circuit current IK decreases and the power switches 15a, 15b, 15c, which are switched on for the short-circuit, cool down. The removal of the short-circuit also has the consequence that—provided that the motor 1 is still rotating—the intermediate circuit voltage UZ increases again, as a result of the induction effect in the motor 1.
In step 41, the intermediate circuit voltage UZ, the short-circuit current IK (which now flows via the freewheeling diodes 16a, 16b, 16c and 18a, 18b, 18c) and the decision variable T are determined again and, in step 42, they are once again compared with stored threshold values, using the decision rule:
UZ>UZ,max AND IK<IK,max AND T<Tmax
As long as this decision rule is not satisfied (N), steps 41 and 42 are repeated. As soon as the opposite situation (J) is found, in which the decision variable T is less than the associated threshold value Tmax and the short-circuit current IK is less than the associated maximum current IK,max, the short-circuit is created again by jumping back to step 37—provided that the intermediate circuit voltage UZ is greater than the associated maximum value UZ,max again.
In particular when the motor 1 is externally rotated over a relatively long time, in such a way that the induced voltages UL1, UL2, UL3 are sufficient to keep the intermediate circuit voltage UZ at a value which permanently exceeds the maximum value UZ,max, steps 37 to 42 are carried out repeatedly. The short-circuit will therefore operate intermittently in order on the one hand to permanently force the intermediate circuit voltage UZ below the maximum value UZ,max, while at the same time preventing overloading of the power switches 15a, 15b, 15c which are switched on for the short-circuit.
In contrast to the embodiment shown in
UZ<UZ,max OR IK>IK,max
In the case of an undervoltage (UZ<UZ,max) or an overcurrent (IK>IK,max), the short-circuit is interrupted in step 40, analogously to the method described in conjunction with
UZ>UZ,max AND IK<IK,max
is satisfied.
The decision variable T for the temperature of the switched-on power switches 15a, 15b, 15c and 17a, 17b, 17c is determined in step 43 only in the situation in which the threshold-value comparison carried out in step 39 has a negative result (N). In this case, a check is carried out in a subsequent step 44 to determine whether the decision variable T is greater than the stored threshold value Tmax (T>Tmax). If not (N), then the monitoring of the intermediate circuit voltage UZ, of the short-circuit current IK and of the decision variable T is continued by jumping back to step 38. Otherwise, the program procedure returns to step 37, as a result of which the short-circuit is produced once again via the power switches 15a, 15b, 15c and 17a, 17b, 17c which were respectively previously switched off.
On the basis of the characteristic of the program variant illustrated in
In one preferred embodiment, the protection logic 22 identifies when the critical operating range has been left, and in this case reverts to normal operation. The identification is flanked in that the regulation module 4 guarantees or confirms maintenance of the non-critical state (for example by pulse extinguishing). If, for example, the rotation speed of the motor 1 has been decreased to such an extent that the induced voltage UL1, UL2, UL3 is less than the intermediate circuit voltage, the regulation module 4 ensures that the rotation speed is kept in this non-critical range, until the protection logic 22 is ready again.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2007 016 637.2 | Apr 2007 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP08/54083 | 4/4/2008 | WO | 00 | 2/11/2010 |
