Abstract:
The invention relates to an electronically commutated electric motor. The electric motor comprises a stator, and a rotor, in particular a permanent-magnetic rotor. The electric motor further comprises a control unit connected to the stator. The control unit is designed to actuate the stator such that the stator can generate a magnetic rotating field for rotationally moving the rotor. According to the invention, the control unit of the electric motor is provided with a power output stage having semiconductor switches. Subject to the low-resistance, or short-circuited, semiconductor switch of the power output stage, in particular as a result of defect, the control unit is designed to actuate the stator for generating the rotating field such that during a complete rotor revolution, the rotor can provide a mechanical output, or in the operational mode, a braking torque of the electric motor caused by the defect is reduced, or completely neutralized, by the low-resistance, or short-circuited, semiconductor switch.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electronically commutated electric motor. The electric motor has a stator and a rotor, which in particular has permanent magnets. The electric motor also has a control unit which is connected to the stator. The control unit is designed to control the stator such that the stator can produce a magnetic rotating field in order to rotate the rotor. 
     SUMMARY OF THE INVENTION 
     According to the invention, the control unit for the electric motor of the type mentioned initially has a power output stage with semiconductor switches. The control unit is designed to control the stator to produce the rotating field as a function of the semiconductor switch, which is shorted or connected with a low impedance in particular as a result of a defect, in the power output stage, such that the rotor can emit mechanical power over a complete rotor revolution, or a braking torque, caused by the defect, on the electric motor in the operating state is reduced or compensated for completely by the semiconductor switch which is shorted or connected with a low impedance. 
     The electric motor is advantageously provided with an emergency running characteristic, such that, in the event of a defective semiconductor switch in the power output stage, the electric motor can still continue to emit torque, and thus mechanical power, or produces at least a small or even no braking torque, such that, in the case of interaction with servo steering in a motor vehicle, the defect at least does not result in steering requiring any more steering force than without servo steering. 
     By way of example, the electric motor can emit the torque in mutually opposite operating directions. By way of example, one operating direction may be directed in the revolution direction—that is to say the positive torque direction—and an operating direction opposite this can be directed in the opposite direction to the revolution direction—as the negative torque direction. 
     By way of example, the semiconductor switch may be a field-effect transistor, in particular an MOS field-effect transistor or an insulated-gate bipolar transistor (IGBT). In another embodiment, the semiconductor switch may also be formed by a thyristor or a triac. 
     The electric motor described above can thus advantageously compensate for a defective switching path in the semiconductor switch by means of appropriate, preferably corrective, control of the power output stage and therefore also of the stator. 
     In one preferred embodiment of the electric motor, the control unit is designed to control the stator such that the rotor can use stored rotation energy to overcome the defect angle range of the rotor revolution which corresponds to the defective semiconductor switch. This type of control in the event of the defective semiconductor switch allows the rotor to be moved over the defect angle range by means of the previously stored rotation energy. By way of example, the defect angle range is characterized in that—because of the shorted semiconductor switch—the stator coil associated with the defective semiconductor switch permanently produces a magnetic field which corresponds to the current-flow direction through the stator coil. Because of this influence, the stator can no longer be sufficiently influenced by the stator coils in the defect angle range. 
     In particular and preferably, the control unit can build up sufficient rotational energy in the rotor before a magnet pole of the rotor enters the defect angle range. The rotor can therefore advantageously be used as an energy store, to overcome the defect angle range. 
     The control unit of the electric motor is preferably connected to, or has, a memory for mutually different control patterns and is designed to select a control pattern and to control the stator using the selected control pattern as a function of the low-impedance connection or the short. 
     By way of example, a control pattern for operation with functional semiconductor switches can be kept available by means of the memory and by keeping mutually different control patterns available, and can be used to control the stator during an operation with functioning semiconductor switches. 
     Preferably, the control patterns are each formed by a time sequence of switching patterns, wherein the switching patterns each represent a voltage state and/or current-flow state of the stator coils of the stator. Also preferably, each switching pattern is formed by a code word, and the code word has one bit for each stator coil, which bit represents a connection state of the stator coil, and thereby the voltage applied to the stator coil. The code words, and/or the connection states or the control patterns, can each be represented by one data record. In another embodiment, a control pattern has at least one or only one switching pattern, with each switching pattern being associated with a predetermined rotor angle in a rotor revolution. For this purpose, each code word may have at least two, three or a plurality of additional rotor position bits, which together code a rotor position of the rotor at which the voltages which correspond to the switching pattern can be applied to the stator coils. 
     If the control unit detects a defect in the semiconductor switch, for example an MOS field-effect transistor, then the control unit can read a control pattern which corresponds to that defect from the memory, and can control the stator using the previously assigned control pattern. It is advantageously still possible to operate the electric motor. During operation with the control pattern while the semiconductor switch is defective, a torque, which in particular is at least small, can at least still be emitted while, in contrast, in the case of electric motors in which the stator is disconnected from the power output stage by means of a relay, torque can no longer be emitted. For example, during an emergency running characteristic which is produced by means of the control unit, it is thus possible to accept the rotor not running at a constant speed or not emitting a constant torque throughout the rotor revolution. 
     By way of example and advantageously, the electric motor may be an electric motor for servo steering in a motor vehicle. In the event of a defective semiconductor switch in the power output stage, the electric motor can thus advantageously still be used to assist steering of the motor vehicle by means of the emergency running characteristic, or at least to reduce a braking torque—caused in particular by the defective semiconductor switch—to a motor shaft such that the vehicle can still be steered. 
     When assisting steering, the electric motor can be controlled, for example, to produce a torque acting in the revolution direction—for example with a positive torque. 
     By way of example, the electric motor can be controlled to produce a braking torque, which acts on the servo steering, in an opposite revolution direction—opposite the revolution direction which makes steering easier—thus producing a negative torque acting in the opposite direction to the revolution direction. Steering of the vehicle can then become more difficult, in particular in the case of correspondingly designed servo steering, with the torque acting in the negative direction. Making steering more difficult may, for example, be used for braking of the assistance effect or as a function of a steering situation which requires this to be more difficult. 
     The control unit is preferably designed to detect a defect, in particular a shorting of the semiconductor switch, as a function of a voltage which is dropped across at least one stator coil of the stator. 
     In another embodiment, the control unit is designed to detect the defect in the semiconductor switch as a function of a star-point voltage or a phase voltage of a stator which is connected in star or delta. Furthermore, the control unit is preferably designed to keep a defect control pattern, corresponding to the semiconductor switch, available for each semiconductor switch, corresponding to the defective semiconductor switch. This allows the control unit to quickly produce control signals corresponding to the control pattern, by means of which the power output stage can be controlled. 
     In one preferred embodiment, the control unit is designed to control the stator by means of the semiconductor switches which are not connected with a low impedance or shortened, in an angle range which follows the defect angle range in the rotor revolution direction, such that the rotor can be moved away from the defect angle range. Locking of the electric motor can thereby advantageously be avoided. If the electric motor locks up, the rotor can, for example, be held fixed in the defect angle range by the magnetic field which is produced continuously in the defect angle range, such that it no longer continues to rotate in the revolution direction. The semiconductor switches which have not been connected with a low impedance are in this case those semiconductor switches which are not defective, that is to say which are still intact. 
     The control unit is preferably designed to use the stator coils to produce the next-possible voltage vector following the defect angle range and to use the next-possible voltage vector to move the rotor out of the defect angle range—in particular in the revolution direction. By way of example, the next-possible voltage vector may be further away from the defect angle range in the rotor revolution direction during normal operation of the electric motor—which is produced, for example, by a control pattern corresponding to normal operation—and may therefore not represent the next-possible voltage vector in the case of a defective semiconductor switch. In the case of a control pattern intended for normal operation, for example, a voltage vector which is intended to be the next for movement of the rotor may, for example, no longer be sufficient to produce a torque in an angle range following the defect angle range in order to move the rotor reliably away from the defect angle range. 
     By way of example, the stator of the electric motor has at least three, or three and only three stator coils. It is also feasible for a stator to have any desired number of stator coils. 
     The electric motor preferably has an intermediate-circuit capacitor which is at least indirectly connected to the power output stage, in particular to the semiconductor switches in the power output stage, wherein the electric motor has an isolating switch, in particular a relay or a semiconductor switch, which is designed to be controllable and whose switching path connects the intermediate-circuit capacitor to the power output stage, wherein a control connection of the isolating switch is connected to the control unit, which is designed to produce a control signal for disconnection of the switching path of the isolating switch as a function of a defect of the intermediate-circuit capacitor, in particular through the defect of electrodes of the intermediate-circuit capacitor being connected to one another with a low impedance or being shorted, and to send this control signal to the isolating switch. 
     The invention also relates to a method for production of a rotary movement by means of an electronically commutated electric motor, wherein the electric motor has a stator and a rotor. In the method, a magnetic rotating field for rotating the rotor is produced by controlling a plurality of semiconductor switches which are connected to the stator. Furthermore, in the method, the rotating field is produced as a function of a semiconductor switch which, in particular, is shorted or connected with a low impedance because of a defect, in that the rotor can emit mechanical power over a complete rotor revolution, or a braking torque, caused by the defect, of the electric motor ( 1 ,  160 ) in the operating state is reduced or completely compensated for by the semiconductor switch which is shorted or connected with a low impedance. 
     In one preferred embodiment of the method, the stator is controlled such that the rotor can use stored rotation energy to overcome a defect angle range of the rotor revolution which corresponds to the defective semiconductor switch. 
     Preferably, in the method described above, a control pattern for operation with intact semiconductor switches, and at least one control pattern for at least one defective semiconductor switch, are kept available. Furthermore, a control pattern which corresponds to the defective semiconductor switch is selected as a function of the low-impedance connection or the short, and the stator is controlled using the selected control pattern. 
     The control pattern can advantageously represent a transfer function. By way of example, a transfer function may represent a torque—over the rotor revolution—as an input parameter, and a control voltage for each stator coil as an output parameter. By way of example, the transfer function can be kept available for every possible defective semiconductor switch, in particular MOSFET. By way of example, the transfer function may be kept available for a stator coordinate system, in particular a u-v-w coordinate system, or a rotor coordinate system, in particular a d-q coordinate system. 
     One control pattern for the defective semiconductor switch is preferably kept available for each semiconductor switch. 
     In one preferred embodiment of the method, a control pattern which corresponds to the defective semiconductor switch is produced as a function of the low-impedance connection or the short, and the stator is controlled using the control pattern produced for the defective semiconductor switch. 
     The production, in particular in-vivo production, of the control pattern—in particular as a function of the nature of the defect—advantageously makes it possible to keep a small number of defect control patterns available. For example, a phase shift which corresponds to the defective semiconductor switch in the control pattern that is produced can be taken into account when producing the defect control pattern. In this way, it is then advantageously possible, for example, to keep the same control pattern available for each of a plurality of semiconductor switches—which may possibly become defective at a later time—as a starting point for production of the respectively appropriate control pattern for the defective semiconductor switch. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention will now be described in the following text with reference to figures and further exemplary embodiments. 
         FIG. 1  shows—schematically—one exemplary embodiment of an electronically commutated electric motor, and a method for operation of the electric motor. The electric motor has a power output stage and a control unit, which is designed to control the power output stage by means of a control pattern, which is designed for a faulty output stage, such that a stator, which is connected to the output side of the power output stage, of the electric motor can produce a rotating field in order to rotate a rotor of the electric motor. 
         FIG. 2  shows—schematically—one exemplary embodiment of a torque profile for the electric motor illustrated in  FIG. 1 , with a power output stage in which a transistor in the power output stage has been shorted or connected with a low impedance, in particular because of a defect; 
         FIG. 3  shows—schematically—a diagram illustrating one rotor revolution of an electronically commutated electric motor with three stator coils. 
         FIG. 4  shows a circuit arrangement for an electronically commutated electric motor with an intermediate-circuit capacitor and with an isolating switch, which is designed to be controllable and, in the event of a defect of the intermediate-circuit capacitor, can isolate a power output stage of the electric motor from the intermediate-circuit capacitor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows—schematically—an arrangement  2  having an electronically commutated electric motor  1 . The electronically commutated electric motor  1  has a stator  3 . The stator  3  has a stator coil  5 , a stator coil  7  and a stator coil  9 . The electric motor  1  also has a rotor  10  as well as a Hall sensor  17 . The Hall sensor  17  is arranged such that a rotor rotation speed and/or a rotor position of the rotor  10  can be detected by means of the Hall sensor  17 . The Hall sensor  17  is designed to produce an appropriate Hall voltage as a function of a magnetic field produced by the rotor  10 , and to emit this Hall voltage on the output side. 
     The electric motor  1  also has a power output stage  12 , whose output side is connected to the stator, and to the stator coils  5 ,  7  and  9  there, and whose input side is connected to a gate driver  14  by means of a connection  24 . The gate driver  14  is connected to an interface  18  via a bidirectional connection  22 . The interface  18  is connected via a bidirectional connection  20  to a processing unit  16  which, for example, is in the form of a microcontroller or microprocessor. 
     The processing unit  16 , or additionally the gate driver  14 , can form the control unit mentioned above. 
     The processing unit  16  is connected to a memory  15  via a bidirectional connection  25 . The memory  15  is designed to keep a plurality of data records available, which each represent one control pattern. By way of example, the control patterns are annotated  70  and  71 . The bidirectional connections  20 ,  22  and  25  and the connection  24  may each be formed, for example, by a data bus in particular a fieldbus, or may be a component of a data bus. The power output stage  12  has a plurality of semiconductor switches, in this exemplary embodiment six MOS field-effect transistors, specifically a transistor  40 , a transistor  44 , a transistor  48 , a transistor  42 , a transistor  46  and a transistor  49 . The transistors mentioned above are connected together in a B6 circuit. The gate connections of the abovementioned transistors are each connected to the gate driver  14  via the connection  24 . The gate driver  14  is designed to control the gate connections of the transistors  40 ,  42 ,  44 ,  46 ,  48  and  49  via the connection  24 . A freewheeling diode for protection of the transistor against overvoltage is connected in parallel with the switching paths of each of the transistors, with the switching path connecting a source connection of the transistor to a sink connection of the transistor. 
     The transistor  40  has a source connection which is connected to a sink connection of the transistor  42  via a connecting node  52 . A source connection of the transistor  44  is connected to a sink connection of the transistor  46  via a connecting node  54 . A source connection of the transistor  48  is connected to a sink connection of the transistor  49  via a connecting node  56 . The source connections of the transistors  42 ,  46  and  49  are each connected to a connecting node  69 . The connecting node  69  is connected to a ground connection  36  via a resistor  34 . The resistor  34 , in particular a shunt resistor, has a low impedance and is designed for current detection. 
     The connecting node  52  is connected to a first connection of the stator coil  7  via a connecting line  72 . The connecting node  54  is connected to a first connection of the stator coil  5  via a connecting line  74 . The connecting node  56  is connected to a first connection of the stator coil  9  via a connecting line  76 . The second connections of the stator coils  5 ,  7  and  9  are each connected to one another via a common star point. The star point is connected to the interface  18  via a connecting line  78 . The output side of the power output stage  12  is connected to the gate driver  14  via, in particular, a multichannel connection  26 . The multichannel connection  26  connects the connecting nodes  52 ,  54  and  56  to the power output stage  14 . The power output stage  14  can therefore receive the potentials of the connecting nodes  52 ,  54  and  56 , and therefore of the corresponding stator coils, via the connection  26 . 
     The sink connections of the transistors  40 ,  44  and  48  are each connected to a connecting node  68 . The connecting node  68  is connected to an on-board power supply system  50  in a motor vehicle via a connecting line  66 . The on-board power supply system  50  is connected to the ground connection  36  and is designed to supply electrical power to the power output stage  12  via the connecting line  66 . The figure also shows a relay  28 —illustrated by dashed lines—which is designed to galvanically isolate the connecting nodes  52 ,  54  and  56  from the stator  3 , in particular as a function of a disconnection signal which is received via the connecting line  62 . As indicated by dashed lines, the relay  28  may be omitted. The input side of the relay  28  is connected via the connecting line  62  to the interface  18 , and can receive the disconnection signal from it. 
     In  FIG. 1 , the electric motor  1  is a component of an arrangement  2 . The arrangement  2  comprises the electric motor  1 , the on-board power supply system  50  and servo steering  32 . The servo steering  32  is rotationally connected to the rotor  10  by means of a motor shaft  30 . The rotor  10  can thus emit a torque  31  to the servo steering  32  via the motor shaft  30 . 
     The method of operation of the arrangement  2  will now be described in the following text: 
     The processing unit  16  can read the control pattern  70  from the memory  15  via the connection  25 . By way of example, the control pattern  70  represents a control pattern for normal operation of the electric motor  1 . By way of example, none of the transistors in the power output stage  12  is defective during normal operation of the electric motor  1 . The processing unit  16  can control the gate driver  14  via the connection  20  and the interface  18 , and then via the connection  22 , such that the gate driver  14  produces control signals in order to control the transistors in the power output stage  12  such that the power output stage  12  drives the stator  3 , and the stator coils  5 ,  7  and  9  there, in order to produce a rotating field. The rotor  10  can be made to rotate along the rotor revolution by means of the rotating field. The gate driver  14  can send the control signals for controlling the transistors in the power output stage  12  via the connection  24  to the power output stage  12 , and to the gate connections of the transistors there. 
     The rotation speed of the rotor  10  can be detected by means of the Hall sensor  17 , which can produce a corresponding Hall signal and can send this via the connecting line  60  to the interface  18 . The interface  18  can send the Hall signal to the processing unit  16  via the connection  20 . The processing unit  16  can receive the Hall signal and can produce or change control signals—which together represent the control pattern—such that the torque of the rotor  10 , which can be emitted to the servo steering  32  via the motor shaft  30 , corresponds to a required torque. For this purpose, the processing unit can receive a control signal, which represents the required torque, via a control input  19 . 
     For example, if the transistor  49  is defective, then the source connection of the transistor  49  can be connected with a low impedance to the sink connection of the transistor  49 . The figure also shows a connection  38 , which represents the low-impedance connection or the short. The low-impedance connection  38  thus bridges the switching path of the transistor  49 , and represents the defective transistor  49 . 
     The processing unit  16  can detect the low-impedance connection  38  via the resistor  34 , and in particular a voltage which is dropped across the resistor  34 , and the processing unit  16  can receive this voltage via the connecting node  69  and then—indicated partially by dashed lines—via the connecting line  64 , the power output stage  14 , the connection  22 , the interface  18 , and the connection  20 . 
     The processing unit  16  can detect the low-impedance connection  38  for example—in addition to or independently of the resistor  34 —via the star-point potential of the stator  3 , via the connecting line  78 , the interface  18  and the connection  20 . 
     The processing unit can detect—in addition to or independently of the resistor  34  or the star-point potential—the low-impedance connection  38  via the potentials at the connecting nodes  52 ,  54  or  56 , or at all the connecting nodes. In this case, the potentials at each of the connecting nodes correspond to a potential of a stator coil. 
     Furthermore, the processing unit  16  can detect the defect in the transistor  49  as a function of the low-impedance connection  38  and, furthermore, can read an appropriate control pattern  71  from the memory  15  as a function of the defect, and can produce appropriate control signals for operating the gate driver  14  and the power output stage  12 . The rotating field produced in this way makes it possible to compensate at least partially for the defect in the transistor  49 , and thus at least partially to heal it. In this case, the rotating field is represented by the control pattern  71  which corresponds to the defective transistor  49 . 
     By way of example when a plurality of transistors are defective—if it is no longer possible to produce a rotating field for production of a positive torque  31  by means of a corresponding control pattern which at least partially compensates for the defect—the processing unit  16  can disconnect the stator  3  from the power output stage  12  by means of the relay  28 . For this purpose, the processing unit  16  can produce a corresponding disconnection signal, and can send this via the connection  20 , the interface  18  and the connecting line  62  to the relay  28 . 
       FIG. 2  shows one exemplary embodiment for a diagram  80 . The diagram  80  has an abscissa  82  and an ordinate  84 . 
     The abscissa  82  represents an angle of the rotor revolution of the rotor  10  illustrated in  FIG. 1 . 
     The diagram  80  shows a rotation speed curve  86 , which represents a rotation speed of the rotor  10  illustrated in  FIG. 1 , in particular as a function of the rotor revolution angle, and this is plotted on the abscissa  82 . 
     The diagram  80  also shows a curve  87  and a curve  88 . The curve  87  represents a relative torque, which can be emitted from the rotor  10  in  FIG. 1 . The relative torque describes the torque of the rotor with respect to a maximum torque to be emitted by the rotor  10 . The maximum relative torque therefore assumes the nondimensional value=1. 
     The diagram  80  also shows a section  93 , a section  94 , a section  95 , a section  96  and a section  97 . The section  93  corresponds to an area of the rotor revolution between 0 and 120°, the section  94  corresponds to an area of the rotor revolution between 120 and 210°, the area  95  corresponds to an area of the rotor revolution between 210 and 270°, the area  96  corresponds to an area of the rotary revolution between 270 and 360°, and the area  97  corresponds to a further rotor revolution beyond 0°. 
     A section  93  of the curve  87  represents a maximum torque of the rotor  10 , in which case the torque of the rotor  10  can be seen to decrease in the area  94 . 
     The rotor  10  cannot emit any torque in the area  95  which corresponds to the defect angle range. The defect angle range, represented by the area  95 , is followed by the area  96  in which a torque can once again be emitted—described by the curve  88 . The torque which can possibly be emitted in this case increases as the rotor revolution angle increases. 
     The diagram  80  also shows a section  90  which denotes the rotor revolution angle between 0° and 210°. The torque  31  emitted by the rotor  10  can still be controlled in the section  90 . 
     In the area  91 , which extends between the rotor revolution angles 210° and 360°, the rotor is influenced by a magnetic field produced by the stator coil through which current flows permanently because of the defective transistor. In the area  91 , the rotor can continue to move in the revolution direction, or in the opposite direction to the revolution direction, virtually or completely without any power. In the area  96  from the rotor revolution angle 270° to a greater rotor revolution angle, the rotor can be moved out of the area  95  by means of a voltage vector which is the next that can possibly be produced. In the area  93  and/or  94 , the rotor  10  can be supplied with rotational energy by means of the control pattern produced for the situation in which the transistor is defective, and which is adequate to move the rotor beyond the area  95 , specifically the defect angle range. The figure also shows an ordinate  85 , which represents a torque which can be emitted. 
       FIG. 3  shows a diagram in which the rotor revolution angle of the rotor  10 , as already illustrated in  FIG. 1 , is illustrated with respect to the stator coils  5 ,  7  and  9 . The diagram shows a magnet vector  106 , which represents an alignment of the magnetic field produced by the rotor  10 , which has permanent magnets. The diagram also shows an axis  120 , which represents a 0° position of the rotor revolution, an axis  122  which runs at right angles to the axis  120  and represents a 90° position of the rotor revolution, furthermore an axis  128  which runs coaxially with respect to the axis  120 , and represents a 180° position of the rotor revolution, as well as an axis  132  which represents a 270° position of the rotor revolution. 
     Furthermore,  FIG. 3  shows the stator coils  5 ,  7  and  9 , which have already been illustrated in  FIG. 1 . The stator coil  5  is located on an axis  134  which represents a 330° position of the rotor revolution. The stator coil  7  is located on an axis  122  which represents the 90° position of the rotor revolution. The stator coils  9  are located on an axis  130  which represents a 210° position of the rotor revolution. The figure also shows an axis  124 , and an axis  126 , wherein the axis  124  represents a 120° position, and the axis  126  represents a 150° position, of the rotor revolution. 
     The illustration also shows an angle range  150  which extends between the axes  120  and  124 . The angle range  150  corresponds to the area  93  illustrated in  FIG. 2 . An angle range  152  extends between the axis  124  and the axis  130 , and corresponds to the area  94  illustrated in  FIG. 2 . The rotor  10  can still emit a torque in the angle range  152 , decreasing along the rotor revolution towards the axis  130 . The rotor revolution is denoted by a rotation arrow  140 . The angle range  154  extends between the axes  130  and  132  and corresponds to the defect angle range  95  in  FIG. 2 . The rotor  10  cannot emit any torque, or only a small amount of torque, in the area of the angle range  154 . The area  156  in which the rotor  10  can still increasingly emit a torque—as the revolution angle increases—extends between the axis  132  and the axis  120 . The figure also shows voltage vectors  108 ,  110  and  112 . 
     The axes  120 ,  122 ,  124 ,  146 ,  128 ,  130 ,  132  and  134  relate to the revolution of the rotor field. The voltage vector  108  corresponds to a phase voltage of the stator coil  7 , the voltage vector  110  corresponds to a phase voltage of the stator coil  5 , and the voltage vector  112  corresponds to a phase voltage of the stator coil  9 . The rotor movement of the rotor  10  can be controlled by means of the voltage vectors  108 ,  110  and  112 , for example by means of the processing unit  16  illustrated in  FIG. 1 . 
       FIG. 4  shows an exemplary embodiment of an electronically commutated electric motor  160 . The electric motor  160  has a stator  162 . The stator  162  has five stator coils, namely a stator coil  170 , a stator coil  172 , a stator coil  174 , a stator coil  176  and a stator coil  178 . The stator  162  is designed to produce a magnetic rotating field when current is passed through it, in order to rotate a rotor  163  of the electric motor  160 . By way of example, the rotor  163  has permanent magnets. 
     The electric motor  160  also has a power output stage  164 . The power output stage  164  is connected on the output side via a current sensor  180  to the stator  162 . By way of example, the power output stage  164  has five transistor half-bridges, with the output side of each half-bridge being connected via the current sensor  180  to the stator coil of the stator  162 . The output side of the power output stage  164  is connected via a connecting line  195  to the stator coil  174 . A connecting line  196  connects the power output stage  164  to the stator coil  172 , a connecting line  197  connects the output side of the power output stage  164  to the stator coil  170 , a connecting line  198  connects the power output stage  164  to the stator coil  178 , and a connecting line  199  connects the power output stage  164  to the stator coil  176 . The power output stage  164  is designed to pass current through the stator  162 , in particular through the stator coils of the stator  162 , in order to produce a magnetic rotating field, in order to rotate the rotor  163 . To this end, the input side of the power output stage  164  is connected to an intermediate circuit, comprising a voltage source  186 , in particular a DC voltage source, and an intermediate-circuit capacitor  184 , which is connected in parallel with the voltage source  186 . 
     One connection of the voltage source  186  is connected to the power output stage  164  via a connecting line  185 . The connecting line  185  is a ground line in this exemplary embodiment. The power output stage  164  is connected via an isolating switch  182  to another connection of the voltage source  186 , and therefore also to another connection of the intermediate-circuit capacitor  184 . By way of example, the isolating switch  182  is a relay or a semiconductor switch. By way of example, the semiconductor switch is a switching transistor, in particular a field-effect transistor or a thyristor. The isolating switch  182  has a control input  187 , which is connected to a control unit  168  via a connecting line  194 . 
     The control unit  168  is connected to one input  192  of the electric motor  160  and is designed to control the power output stage  164  via a gate driver  166  as a function of a control signal, which, for example, represents a current nominal value of a stator coil current of the stator  162 , such that the stator  162  can produce a magnetic rotating field in order to rotate the rotor  163 . 
     The output side of the control unit  168  is connected via a connection  190  to the gate driver  166 . The output side of the gate driver  166  is connected via a connection  188  to the power output stage  164 . The input side of the control unit  168  is also connected via a connection  181  to the current sensor  180  and can receive current signals, which each represent a stator coil current in one of the stator coils of the rotor  162 , via the connection  181  from the current sensor  180 . For this purpose, for example, the current sensor  180  may have a shunt resistor for each stator coil of the stator  162 . The method of operation of the electric motor  160  will now be described in the following text: 
     The control unit  168  is designed to produce control signals in order to rotate the rotor  163 , as a function of a control signal which is received at the input  192 , and to send these control signals via the connection  190  to the gate driver  166 . By way of example, the control signals may each be coded in accordance with a binary coding, in which case the binary coding has one bit for each stator coil. The control signals produced by the control unit  168  for rotating the rotor  163  may, for example, have a time sequence of code words which each have one bit for each stator coil: in this exemplary embodiment, the code words are five-bit words, and each have five bits. Each bit in the bit words in this case represents a connection state of the stator  162 , in particular a switching state of the power output stage and therefore also a voltage state and/or a current-flow state for each stator coil in the stator  162 , also referred to in the following text as a switching pattern. For this purpose, the control unit  168  is connected via a connecting line  189  to a memory  200 , in which data records are stored, which represent respectively mutually different switching patterns. A time sequence of switching patterns together form one of the control patterns mentioned above. 
     By way of example, a bit word for a time at which the stator coil  170  is activated, but the remaining stator coils are intended to be deactivated, may have a bit with the logic value “one” for the stator coil  170 , and may have a bit with the logic value “zero” for each of the remaining stator coils. 
     In order to pass current through the stator  162 , the control unit  168  can, for example, produce a time sequence of control signals which each represent a code word, with each code word representing a switching pattern of the rotor  162 . 
     The gate driver  166  can control the power output stage  164 , in particular the gate connections of the transistor half-bridges in the power output stage  164 , as a function of the control signals received on the input side via the connection  190 , in this exemplary embodiment, the code word received on the input side, in order to produce the switching pattern which corresponds to the control signal, in particular to the code word. 
     Furthermore, the power output stage  164  can apply a voltage to the stator  162  corresponding to the switching patterns, as a function of the control signals produced by the gate driver  166 , and can thus also pass current through it, corresponding to the control signals received from the gate driver  166 . 
     If for example—in the event of a fault—one semiconductor switch in a transistor half-bridge in the power output stage  164  is defective, in particular with the switching path of the semiconductor switch being shorted or connected with a low impedance, then the corresponding stator coil of the stator  162  is continuously connected to a corresponding potential of the voltage source  186 , and current correspondingly flows through it all the time. The current sensor  180  can send the current signal, which represents the defect in the transistor half-bridge, via the connection  181  to the control unit  168 . The control unit  168  can detect the current signal which represents the defect, and can produce the time sequence of control signals, which each represent a switching pattern, in a corresponding manner to that already described above, in particular in  FIG. 2 , as a function of the current signal which represents the defect. Despite the defective transistor half-bridge, the stator  162  can thus still produce a magnetic rotating field such that a braking torque which acts on the rotor  163  is reduced in comparison to a time sequence of switching patterns which are provided for a power output stage  164  which is not defective. By way of example, the control unit  168  can produce the time sequence of control signals, which each represent a switching pattern, such that the braking torque is reduced or disappears, or, as already described with reference to the electric motor in  FIG. 1 , a positive drive torque can be produced by the electric motor  160 . 
     A different fault situation may occur, for example, as a result of a defective intermediate-circuit capacitor. Specifically, if the intermediate-circuit capacitor  184  is defective during operation of the electric motor  160 , in particular with electrodes of the intermediate-circuit capacitor  184  being connected to one another with a low impedance or being shorted to one another, current still flows from the power output stage  164  through the stator  162 , in particular a lesser current than if the intermediate-circuit capacitor  184  were intact. A current change in at least one stator coil current can be detected by the current sensor  180 , and a corresponding current signal can be sent from the current sensor  180  via the connection  181  to the control unit  168 . By way of example, the current signal is a voltage dropped across a shunt resistor which forms the current sensor. The control unit  168  is designed to produce a control signal for disconnection of the switch  182  as a function of the current signal which represents the current drop, and to send this via the connecting line  194  to the control connection  187  of the switch  182 . The isolating switch  182  can disconnect the power output stage  164  from the voltage source  186 , and from the defective intermediate-circuit capacitor  184  as well, as a function of the control signal received at the input  187 . If current can then no longer be passed through the stator  162  of the electric motor  160 , because the power output stage  164  has been disconnected from a supply voltage, in this exemplary embodiment the voltage source  168 , the stator  162  can also no longer produce any braking torque by means of the rotor  163 . 
     Servo steering, connected to the electric motor  160 , in a motor vehicle can then also not experience an additional braking torque as a result of the defective switching transistor in the electric motor  160 . A vehicle with servo steering can then be steered with a steering force which corresponds to the vehicle being steered without servo steering.