Method for detecting a motor phase fault of a motor arrangement and drive circuit for driving an electronically commutated motor

Abstract

In a method for detecting a motor phase fault of a motor arrangement, the motor phases of which are connected to a drive circuit having a DC voltage intermediate circuit and an inverter. A motor phase voltage at at least one of the motor phases with respect to a reference potential is captured while the inverter is switched off; and a voltage profile of the captured motor phase voltage is used to determine whether there is a motor phase fault on one of the motor phases of the motor arrangement.

Claims

1. A method for detecting a motor phase fault of a motor configuration, motor phases of the motor configuration being connected to a drive circuit having a DC voltage intermediate circuit and an inverter, wherein a first motor phase of the motor phases is connected to a first reference potential via at least one first resistor, and each other one of the motor phases is connected to a second reference potential via at least one further resistor, the method which comprises: while the inverter is switched off, capturing a divided motor phase voltage of the first motor phase by a voltage divider; using a voltage profile of the voltage measurement captured to determine whether there is the motor phase fault on one of the motor phases of the motor configuration; and quantitatively calculating a fault current value of an insulation fault during a fault check and/or a resistance value of the insulation fault and/or a maximum possible fault current value of the insulation fault during operation with the inverter switched on based on the motor phase voltage captured.

2. The method according to claim 1, which further comprises calculating the fault current value of the insulation fault during a fault check and/or the resistance value of the insulation fault solely on a basis of resistance values of the at least one first resistor and the at least one further resistor, a measurement voltage of the divided motor phase voltage, as captured by the voltage divider, a network voltage of a supply network and an intermediate circuit voltage across the DC voltage intermediate circuit.

3. The method according to claim 1, which further comprises calculating a maximum possible fault current value of the insulation fault during operation with the inverter switched on solely on a basis of resistance values of the at least one first resistor and the at least one further resistor, a measurement voltage of the divided motor phase voltage, as captured by means of the voltage divider, a network voltage of a supply network and an intermediate circuit voltage across the DC voltage intermediate circuit.

4. A method for operating a drive circuit having a DC voltage intermediate circuit, a power factor correction filter and an inverter for driving an electronically commutated motor, which further comprises: providing a motor configuration including motor phases connected to the inverter of the drive circuit, wherein the motor configuration includes the electronically commutated motor; checking for a presence of a motor phase fault in the motor configuration by: capturing a motor phase voltage from at least one of the motor phases with respect to a reference potential while the inverter is switched off; and using a voltage profile of the motor phase voltage captured to determine whether there is the motor phase fault on one of the motor phases of the motor configuration; and switching off the power factor correction filter when driving the electronically commutated motor after capturing the motor phase voltage if a maximum possible fault current value for an insulation fault on a motor phase below a predefined limit value has been determined.

5. A drive circuit for driving an electronically commutated motor, the drive circuit comprising: a power correction filter; a DC voltage intermediate circuit; an inverter connected to said DC voltage intermediate circuit, wherein the inverter is connectable to motor phases of a motor configuration including the electronically commutated motor, and wherein at least one of the motor phases of the electronically commutated motor is connected via resistors to a first reference potential and a second reference potential; a detection circuit for capturing a motor phase voltage from at least one of the motor phases with respect to a reference potential; and a controller configured to operate said detection circuit for capturing the motor phase voltage while said inverter is switched off and to determine, on a basis of a voltage profile of the motor phase voltage captured, whether there is a motor phase fault on one of the motor phases of the motor configuration; wherein said controller is configured to prevent switching-on of said inverter and/or of said power factor correction filter after capturing the motor phase voltage if the motor phase fault has been determined.

6. The drive circuit according to claim 5, wherein said detection circuit has at least one first resistor, via which at least one motor phase is connected to the first reference potential, and at least one further resistor, via which the at least one motor phase is connected to the second reference potential.

7. The drive circuit according to claim 5, wherein said detection circuit has at least one first resistor, via which a first motor phase of the motor phases is connected to the first reference potential, and at least one further resistor, via which another motor phase of the motor phases is respectively connected to the second reference potential.

8. The drive circuit according to claim 5, wherein said detection circuit has a voltage divider for capturing the motor phase voltage.

9. The drive circuit according to claim 5, wherein said detection circuit is configured to capture the motor phase voltage when the at least one of the motor phases is connected to the first reference potential and the at least one of the motor phases or at least one other one of the motor phases is connected to a second reference potential.

10. A method for detecting a motor phase fault of a motor configuration, motor phases of the motor configuration being connected to a drive circuit having a DC voltage intermediate circuit and an inverter, which comprises the steps of: connecting at least one motor phase to a plus pole of a DC-bus via at least one first resistor; connecting the at least one motor phase to a minus pole of the DC-bus via at least one further resistor; capturing a divided motor phase voltage from at least one of the motor phases with respect to a reference potential while the inverter is switched off; and using a voltage profile of the motor phase voltage captured to determine whether there is the motor phase fault on one of the motor phases of the motor configuration.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

(1) FIG. 1 is a circuit diagram of a drive circuit with a connected motor according to a first exemplary embodiment of the invention;

(2) FIG. 2 shows graphs of a network voltage, a fault current and motor phase voltages during fault check for the drive circuit from FIG. 1 in a fault-free case without insulation faults;

(3) FIG. 3 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 1 in the case of an insulation fault on one of the motor phases;

(4) FIG. 4 is a circuit diagram of the drive circuit with the connected motor according to a second exemplary embodiment of the invention;

(5) FIG. 5 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 4 in the fault-free case without insulation faults;

(6) FIG. 6 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 4 in the case of an insulation fault on one of the motor phases;

(7) FIG. 7 is a circuit diagram of the drive circuit with the connected motor according to a third exemplary embodiment of the invention;

(8) FIG. 8 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 7 in the fault-free case without insulation faults;

(9) FIG. 9 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 7 in the case of an insulation fault on one of the motor phases;

(10) FIG. 10 is a circuit diagram of the drive circuit with the connected motor according to a fourth exemplary embodiment of the invention;

(11) FIG. 11 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 10 in the fault-free case without insulation faults;

(12) FIG. 12 shows graphs of the network voltage, the fault current and the motor phase voltages during the fault check for the drive circuit from FIG. 10 in the case of an insulation fault on one of the motor phases; and

(13) FIG. 13 is a circuit diagram of an embodiment variant of the detection circuit according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(14) Referring now to the figures of the drawings in detail and first, particularly to FIGS. 1-3 thereof, there is shown the structure and method of operation of a drive circuit 10 for an electronically commutated motor 12 according to a first exemplary embodiment are explained in more detail.

(15) The drive circuit 10 is used to drive an electronically commutated motor 12. In the exemplary embodiment from FIG. 1, the motor is a three-phase brushless motor 12 having three motor phases U, V, W which are connected to one another at a neutral point SP. The motor 12 is fed from a DC voltage intermediate circuit 14 via an inverter 16. The DC voltage intermediate circuit 14 has an intermediate circuit capacitor C1, and the inverter 16 has a three-phase inverter bridge circuit in this exemplary embodiment having a total of six power semiconductor switches M1 to M6 (for example MOSFETs or IGBTs having diodes with an antiparallel connection) in its half-bridges. The three motor windings of the motor 12 are connected, via a motor cable 18, to a motor phase connection 20 which is connected to the three center taps of the half-bridges of the inverter 16. The motor 12 and the motor cable 18 each have three motor phases U, V, W and are parts of the motor arrangement.

(16) On the input side, the DC voltage intermediate circuit 14 is connected to an AC connection 24 via a rectifier 22. The drive circuit 10 is connected to a supply network 26 via the AC connection 24. In the exemplary embodiment from FIG. 1, the supply network 26 is a single-phase power supply system, the drive circuit 10 is connected to the phase conductor L1 and to the neutral conductor N of the single-phase power supply system, and the supply network 26 also has protective earthing PE. In this exemplary embodiment, the rectifier 22 has a rectifier bridge circuit with a total of four rectifier diodes D7 to D10.

(17) Optionally, a power factor correction filter (PFC filter) 30 can also be connected between the rectifier 22 and the DC voltage intermediate circuit 14. The PFC filter 30 may be configured in a boost converter topology in this example and contains, in particular, an inductance L8, a switch M8 and a rectifier diode D5. The PFC filter 30 may optionally also be present in the other exemplary embodiments, even though it is not depicted in FIGS. 4, 7 and 10.

(18) The drive circuit 10 also has a non-illustrated control device, for example in the form of a microcontroller, which controls the power semiconductor switches M1 to M6 of the inverter 16.

(19) In the case of such a drive circuit 10, there are various types of motor phase faults which can occur on the side of the connected motor 12. Insulation faults of the motor phases in the motor cable 18 and insulation faults of the neutral point SP of the motor windings of the motor 12 may arise. The various types of insulation faults are illustrated in FIG. 1 as insulation faults with the resistors R7a, R7b, R7c for the motor phases U, V, W of the motor cable 18 and with the resistor R7d for the neutral point SP of the motor windings of the motor 12.

(20) The drive circuit 10 has a detection circuit 28 for the purpose of detecting all of these insulation faults R7a, R7b, R7c, R7d.

(21) In addition, interruptions in the motor phases U, V, W can arise on account of a severed motor cable 18 or a burnt-out motor winding of the motor 12. FIG. 1 indicates, by way of example, an interruption X in the motor phase U in the motor cable 18. In order to detect such interruptions X in motor phases U, V, W, the drive circuit 10 may alternatively have a detection circuit 28′ which is illustrated, by way of example, in FIG. 13 and is described in more detail later.

(22) The detection circuit 28 from FIG. 1 captures the motor phase voltage Uu of the motor phase U (generally at least one of the motor phases U, V, W) with the inverter 16 switched off and preferably also with the PFC filter 30 (if present) switched off. For this purpose, the motor phase U is connected, in a high-impedance manner, to the negative pole of the DC voltage intermediate circuit 14 or to earth as the first reference potential P1 via first resistors R1, R2 and is connected, in a high-impedance manner, to the positive pole of the DC voltage intermediate circuit 14 as the second reference potential P2 via further resistors R8, R9. The first resistors R1, R2 are also used as a voltage divider for capturing the divided motor phase voltage Uu as the measurement voltage Um which can be evaluated by an analogue/digital converter of the control device. In the motor phase fault detection method according to the invention, it suffices to capture one of the motor phase voltages, but motor phase voltages of a plurality of motor phases can optionally also be captured and evaluated.

(23) The inverter 16 is switched off by switching off/opening all power semiconductor switches M1 . . . M6 of the inverter 16, with the result that the motor 12 connected to the motor phase connection 20 is not actively energized by the drive circuit 10. The fault check is preferably carried out by the detection circuit 28 not only when the inverter 16 is switched off, but also when the motor is at a standstill, with the result that no voltages can be induced by a rotor of the motor 12 which is still rotating.

(24) In the exemplary embodiment from FIG. 1, the sum of the resistance values of the first resistors R1, R2 is selected to be equal to the sum of the resistance values of the further resistors R8, R9, wherein these sums of the resistance values are each approximately 1 megohm. As a result, the motor phase voltage Uu assumes half the value of the intermediate circuit voltage U.sub.+HV across the DC voltage intermediate circuit 14 in the fault-free state. Instead of the two further resistors R8, R9, only an individual resistor R8 may also be provided.

(25) It is now described, on the basis of FIGS. 1 to 3, how the presence of an insulation fault R7a, R7b, R7c, R7d can be detected with the aid of this detection circuit 28. In this respect, the current arrows of the fault current I.sub.R7 and of the capacitor current ΔI from the DC voltage intermediate circuit 14 are depicted, by way of example, in FIG. 1 in the positive half-wave of the network voltage U.sub.Netz of the supply network 26 with the inverter 16 switched off and with power semiconductor switches M1 . . . 6 switched off/open.

(26) The graphs from FIG. 2 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the fault-free case without an insulation fault, and the graphs from FIG. 3 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the case of an insulation fault, in each case during the fault check. The curve profiles from FIGS. 2 and 3 show the case of a fault check with the PFC filter 30 switched off; however, the fault check can also be carried out in a similar manner with the PFC filter 30 switched on.

(27) Since the insulation fault resistance is infinite in the fault-free case, the motor phase voltage Uu (for the case of R1+R2=R8+R9) assumes half the intermediate circuit voltage U.sub.+HV/2. As a comparison of the graphs in FIGS. 2 and 3 shows, a voltage profile of the motor phase voltage Uu can be used to easily detect whether or not there is an insulation fault on the side of the motor arrangement 12, 18.

(28) The fault current I.sub.R7 flows only when the diode D7 of the rectifier 22 is conductive in the positive half-wave of the network voltage U.sub.Netz and the diode D9 of the rectifier 22 is conductive in the negative have-wave of the network voltage U.sub.Netz. When the inverter 16 is switched off, this state occurs only when the network voltage U.sub.Netz exceeds half the value of the intermediate circuit voltage U.sub.+HV/2. As illustrated in FIG. 3, a fault current I.sub.R7 for this reason flows only when the magnitude of the network voltage U.sub.Netz is greater than half the intermediate circuit voltage U.sub.+HV/2.

(29) The motor phase voltage Uu can be expressed as a function of the measured divided voltage Um by means of a simple voltage divider formula:

(30) $\mathrm{Uu}=\frac{R1+R2}{R2}*\mathrm{Um}.$

(31) In the positive half-wave of the network voltage U.sub.Netz, the insulation fault resistance R7 can be calculated by means of the following expression:

(32) $R7=\frac{\left(R8+R9\right)*\left(U_{\mathrm{Netz}}-U_{+\mathrm{HV}}+\frac{\mathrm{Um}*\left(R1+R2\right)}{R2}\right)}{2*\mathrm{Uu}-U_{+\mathrm{HV}}}$

(33) and in the negative half-wave of the network voltage U.sub.Netz, the insulation fault resistance R7 can be calculated by means of the following expression:

(34) $R7=-\frac{U_{\mathrm{Netz}}-\frac{R1+R2}{R2}*\mathrm{Um}}{\frac{\mathrm{Um}}{R2}-\left(U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}\right)/\left(R8+R9\right)}$

(35) In the positive half-wave of the network voltage U.sub.Netz, the fault current I.sub.R7 during the fault check can be calculated by means of the following expression:

(36) $I_{R7}=-\frac{U_{+\mathrm{HV}}-\frac{R8+R9+R1+R2}{R2}*\mathrm{Um}}{R8+R9}$

(37) and in the negative half-wave of the network voltage U.sub.Netz, the fault current I.sub.R7 during the fault check can be calculated by means of the following expression:

(38) $I_{R7}=\frac{\mathrm{Um}}{R2}-\frac{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}}{R8+R9}$

(39) The maximum possible fault current I.sub.R7,max during operation with the inverter 16 switched on can be determined, on the basis of the quotient of the intermediate circuit voltage and the determined resistance value R7 of the insulation fault, during the positive half-wave of the network voltage U.sub.Netz by means of the following expression:

(40) $I_{R7,\max }=\frac{U_{+\mathrm{HV}}}{R7}=-\frac{U_{+\mathrm{HV}}*\left(2*\mathrm{Uu}-U_{+\mathrm{HV}}\right)}{\left(R8+R9\right)*\left(U_{\mathrm{Netz}}-U_{+\mathrm{HV}}+\frac{\mathrm{Um}*\left(R1+R2\right)}{R2}\right)}$

(41) and in the negative half-wave of the network voltage U.sub.Netz, the maximum possible fault current I.sub.R7,max can be calculated by means of the following expression:

(42) $I_{R7,\max }=\frac{U_{+\mathrm{HV}}}{R7}=-\frac{U_{+\mathrm{HV}}*\left(\frac{\mathrm{Um}}{R2}-\left(U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}\right)/\left(R8+R9\right)\right)}{U_{\mathrm{Netz}}-\frac{R1+R2}{R2}*\mathrm{Um}}$

(43) That is to say, the insulation fault resistances R7, the fault current values I.sub.R7 of the insulation fault and the maximum possible fault current values I.sub.R7,max during operation with the inverter 16 switched on can be calculated solely on the basis of the resistance values of the first and further resistors R1, R2, R8, R9 of the detection circuit 28, the measurement voltage Um of the divided motor phase voltage Uu, as captured by the voltage divider R1, R2 of the detection circuit 28, the intermediate circuit voltage U.sub.+HV across the DC voltage intermediate circuit 14 and the network voltage U.sub.Netz.

(44) In addition to the qualitative detection of an insulation fault, a quantitative detection of an insulation fault can also be carried out with the aid of the last two expressions. The maximum possible fault current values I.sub.R7,max calculated in this manner can be compared with predefined limit values, for example. An insulation fault is detected, for example, if a fault current value I.sub.R7 exceeds 6 mA. If, in contrast, only a small maximum possible fault current value I.sub.R7,max below a predefined limit value of 6 mA or 10 mA, for example, is determined, the motor 12 can then be operated at possibly low power with the PFC filter 30 switched off and a fault signal can also be transmitted to the user or customer service.

(45) The drive circuit 10 having the detection circuit 28 according to the invention for detecting an insulation fault on the side of the motor arrangement 12, 18 can also be used in combination with other supply networks 26 and accordingly adapted rectifiers 22.

(46) Referring to FIGS. 4 to 6, the structure and method of operation of a drive circuit for an electronically commutated motor according to a second exemplary embodiment are explained in more detail. In this case, identical or corresponding components and parameters are provided with the same reference signs as in the first exemplary embodiment.

(47) In the exemplary embodiment from FIG. 4, the supply network 26 is a three-phase power supply system, the drive circuit 10 is connected to the phase conductors L1, L2, L3 of the power supply system, and the supply network 26 also has protective earthing PE. In this exemplary embodiment, the rectifier 22 has a rectifier bridge circuit with a total of six rectifier diodes D7 to D12. For the rest, the drive circuit 10 corresponds to that in the first exemplary embodiment from FIG. 1.

(48) The graphs from FIG. 5 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the fault-free case without an insulation fault, and the graphs from FIG. 6 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the case of an insulation fault, in each case during the fault check. The curve profiles from FIGS. 5 and 6 show the case of a fault check for a drive circuit 10 without a PFC filter 30 or with the PFC filter 30 switched off; however, the fault check can also be carried out in a similar manner with the PFC filter 30 switched on.

(49) Since the insulation fault resistance is infinite in the fault-free case, the motor phase voltage Uu (for the case of R1+R2=R8+R9) assumes half the intermediate circuit voltage U.sub.+HV/2. As a comparison of the graphs in FIGS. 5 and 6 shows, a voltage profile of the motor phase voltages Uu, Uv, Uw can be used to easily detect whether or not there is an insulation fault on the side of the motor arrangement 12, 18.

(50) In the case of the three-phase power supply system, a fault current I.sub.R7 continuously flows in the event of a fault because at least one diode D7 . . . 12 of the bridge rectifier 22 is conductive at any time.

(51) The resistance values and fault currents of the insulation fault are calculated in the case of the three-phase power supply system using the same formulas as for the single-phase power supply system in the first exemplary embodiment.

(52) Referring to FIGS. 7 to 9, the structure and method of operation of a drive circuit for an electronically commutated motor according to a third exemplary embodiment are explained in more detail. In this case, identical or corresponding components and parameters are provided with the same reference signs as in the preceding exemplary embodiments.

(53) In the exemplary embodiment from FIG. 7, the supply network 26 is a single-phase three-wire network (USA), the drive circuit 10 is connected to the phase conductors L1, L2 of the three-wire network, and the supply network 26 also has a neutral conductor connected to protective earthing PE. In this exemplary embodiment, the rectifier 22 has a rectifier bridge circuit with a total of four rectifier diodes D7 to D10. For the rest, the structure and method of operation of the drive circuit 10 correspond to those of the first exemplary embodiment from FIG. 1. In contrast to the first exemplary embodiment with a single-phase power supply system, the sums of the first resistances R1, R2 and of the further resistances R8, R9 of the detection circuit 28 must not be equal, however, in the case of the single-phase three-wire network since otherwise a fault current would not flow through the diodes of the rectifier 22. In this case too, however, the one motor phase U is connected, in a high-impedance manner, to the negative pole of the DC voltage intermediate circuit 14.

(54) The graphs from FIG. 8 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the fault-free case without an insulation fault, and the graphs from FIG. 9 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the case of an insulation fault, in each case during the fault check. The curve profiles from FIGS. 8 and 9 show the case of a fault check for a drive circuit 10 without a PFC filter 30 or with the PFC filter 30 switched off; however, the fault check can also be carried out in a similar manner with the PFC filter 30 switched on.

(55) The resistance values and fault currents of the insulation fault are calculated in the case of the single-phase three-wire network using the same formulas as for the single-phase power supply system in the first exemplary embodiment.

(56) In the fault-free case, the motor phase voltage Uu assumes the following value:

(57) $\mathrm{Uu}=\frac{R1+R2}{R1+R2+R8+R9}*U_{+\mathrm{HV}}$

(58) In the case of the single-phase three-wire network, a fault current I.sub.R7 flows only when the network voltage between the two outer conductors L1, L2 exceeds the following voltage value:

(59) $U_{\mathrm{Netz},L1,L2}=2*U_{+\mathrm{HV}}*\left(1-\frac{R1+R2}{R1+R2+R8+R9}\right)$

(60) Referring to FIGS. 10 to 12, the structure and method of operation of a drive circuit for an electronically commutated motor according to a fourth exemplary embodiment are explained in more detail. In this case, identical or corresponding components and parameters are provided with the same reference signs as in the preceding exemplary embodiments.

(61) In the exemplary embodiment from FIG. 10, the supply network 26 is a single-phase three-wire network (USA), like in the third exemplary embodiment, the drive circuit 10 is connected to the phase conductors L1, L2 of the three-wire network, and the supply network 26 also has a neutral conductor connected to protective earthing PE. In contrast to the third exemplary embodiment, the rectifier 22 in this exemplary embodiment is in the form of a push-pull rectifier having two rectifier diodes D1 and D2, and the DC voltage intermediate circuit 14 has two intermediate circuit capacitors C1 and C2. For the rest, the structure and method of operation of the drive circuit 10 correspond to those of the first exemplary embodiment from FIG. 1. In contrast to the first exemplary embodiment with a single-phase power supply system, the sums of the first resistances R1, R2 and of the further resistances R8, R9 of the detection circuit 28 must not be equal, however, in the case of the single-phase three-wire network since otherwise a fault current would not flow through the diodes of the rectifier 22. In this case too, however, the one motor phase U is connected, in a high-impedance manner, to the negative pole of the DC voltage intermediate circuit 14.

(62) The graphs from FIG. 11 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the fault-free case without an insulation fault, and the graphs from FIG. 12 show the temporal profiles of the network voltage U.sub.Netz, the fault current I.sub.R7 and the motor phase voltages Uu, Uv, Uw in the case of an insulation fault, in each case during the fault check. The curve profiles from FIGS. 11 and 12 show the case of a fault check for a drive circuit 10 without a PFC filter 30 or with the PFC filter 30 switched off; however, the fault check can also be carried out in a similar manner with the PFC filter 30 switched on.

(63) In the fault-free case, the motor phase voltage Uu assumes the following value:

(64) 0$\mathrm{Uu}=\frac{R1+R2}{R8+R9+R1+R2}*U_{+\mathrm{HV}}$

(65) In the case of the single-phase three-wire network, a fault current I.sub.R7 flows only when the network voltage between the two outer conductors L1, L2 exceeds the following voltage value:

(66) $U_{\mathrm{Netz},L1,L2}=2*U_{+\mathrm{HV}}*\left(1-\frac{R1+R2}{R8+R9+R1+R2}\right)$

(67) In the positive half-wave of the network voltage U.sub.Netz, the insulation fault resistance R7 can be calculated by means of the following expression:

(68) $R7=\frac{\left(U_{\mathrm{Netz}}+\frac{U_{+\mathrm{HV}}}{2}-\frac{R1+R2}{R2}*\mathrm{Um}\right)*\left(R1+R2\right)}{\frac{R1+R2}{R1}*\mathrm{Um}-\frac{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}}{R8+R9}*\left(R1+R2\right)}$

(69) and in the negative half-wave of the network voltage U.sub.Netz, the insulation fault resistance R7 can be calculated by means of the following expression:

(70) $R7=\frac{\left(R8+R9\right)*\left(U_{\mathrm{Netz}}+\frac{R1+R2}{R2}*\mathrm{Um}-\frac{U_{+\mathrm{HV}}}{2}\right)}{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}-\left(R8+R9\right)*\frac{\mathrm{Um}}{R2}}$

(71) In the positive half-wave of the network voltage U.sub.Netz, the fault current I.sub.R7 during the fault check can be calculated by means of the following expression:

(72) $I_{R7}=\frac{\mathrm{Um}}{R2}-\frac{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}}{R8+R9}$

(73) and in the negative half-wave of the network voltage U.sub.Netz, the fault current I.sub.R7 during the fault check can be calculated by means of the following expression:

(74) $I_{R7}=\frac{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}-\left(R8+R9\right)*\frac{\mathrm{Um}}{R2}}{R8+R9}$

(75) Accordingly, the maximum possible fault current I.sub.R7,max during operation with the inverter 16 switched on during the positive half-wave of the network voltage U.sub.Netz can be determined by means of the following expression:

(76) $I_{R7,\max }=\frac{U_{+\mathrm{HV}}}{R7}=-\frac{U_{+\mathrm{HV}}*\left(\frac{R1+R2}{R2}*\mathrm{Um}-\frac{U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}}{R8+R9}*\left(R1+R2\right)\right)}{\left(U_{\mathrm{Netz}}+\frac{U_{+\mathrm{HV}}}{2}-\frac{R1+R2}{R2}*\mathrm{Um}\right)*\left(R1+R2\right)}$
and in the negative half-wave of the network voltage U.sub.Netz, the maximum possible fault current I.sub.R7,max can be calculated by means of the following expression:

(77) $I_{R7,\max }=\frac{U_{+\mathrm{HV}}}{R7}=-\frac{U_{+\mathrm{HV}}*\left(U_{+\mathrm{HV}}-\frac{R1+R2}{R2}*\mathrm{Um}-\left(R8+R9\right)*\frac{\mathrm{Um}}{R2}\right)}{\left(R8+R9\right)*\left(U_{\mathrm{Netz}}+\frac{R1+R2}{R2}*\mathrm{Um}-\frac{U_{+\mathrm{HV}}}{2}\right)}$

(78) Referring to FIG. 13, an alternative embodiment of the detection circuit 28′ is now explained in more detail, which detection circuit can be used, for example, instead of the above-described detection circuit 28 in combination with the drive circuits 10 from FIGS. 1, 4, 7 and 10 in order to also be able to detect the presence of an interruption X in a motor phase U, V, W in addition to the presence of an insulation fault R7a . . . d on a motor phase U, V, W. FIG. 13 shows, by way of example, the motor phase faults R7a, R7b, R7c and X which can be detected with the aid of this detection circuit 28′.

(79) In a similar manner to the above embodiment variant of the detection circuit 28, the detection circuit 28′ has a voltage divider comprising two first resistors R1 and R2, via which a first motor phase (here: W) is connected, in a high-impedance manner, to earth as the first reference potential P1 in order to capture and evaluate the corresponding motor phase voltage Uw as the measurement voltage Um of the voltage divider R1, R2. In contrast to the above embodiment variant of the detection circuit 28, however, this first motor phase W is also not connected to the positive pole of the DC voltage intermediate circuit 14. Instead, the other motor phases U and V are each connected, in a high-impedance manner, to the positive pole of the DC voltage intermediate circuit 14 as the second reference potential P2 via two further resistors R3, R4 and R5, R6, respectively. A divided motor phase voltage Uw is therefore captured as the measurement voltage Um via the voltage divider R1, R2.

(80) If the drive circuit 10 is connected to single-phase or multi-phase power supply systems 26 and in the case of a total of three motor phases U, V, W, the sums of the resistance values R3+R4 and R5+R6 of the further resistors (for example approximately 2 megohms) are preferably twice as large as the sum of the resistance values R1+R2 of the first resistors (for example approximately 1 megohm), with the result that the measured motor phase voltage Um in the fault-free case again assumes half the intermediate circuit voltage, like in the preceding exemplary embodiments from FIGS. 1 to 12.

(81) The detection of an insulation fault R7a . . . c on a motor phase U, V, W and the calculations of the insulation fault resistance R7, the fault current I.sub.R7 during the fault check and the maximum possible fault current I.sub.R7,max during operation with the inverter 16 switched on are carried out in a similar manner to the fault checks described on the basis of FIGS. 1 to 12 using a detection circuit 28. However, if the detection circuit 28′ from FIG. 13 is used, the motor phase voltage Uw also changes in the event of an interruption X in a motor phase since all motor phases U, V, W are then no longer connected to one another and the value of the measured motor phase voltage Um, even without an insulation fault R7a . . . c, does not assume half the intermediate circuit voltage U.sub.+HV/2.

(82) The following is a summary list of reference numerals and the corresponding structure used in the above description of the invention: 10 Drive circuit 12 Motor 14 DC voltage intermediate circuit 16 Inverter 18 Motor cable 20 Motor phase connection 22 Rectifier 24 AC connection 26 Supply network 28, 28′ Detection circuit 30 Power factor correction filter (PFC filter) C1, C2 Intermediate circuit capacitor of 14 D1, D2 Rectifier diodes of 22 D5 Rectifier diode of 30 D7-D12 Rectifier diodes of 22 L1, L2, L3 Phase conductor of 26 L8 Inductance of 30 M1-M6 power semiconductor switches of 16 M8 Switch of 30 N Neutral conductor of 26 PE Protective earthing P1 First reference potential P2 Second reference potential R1, R2 First resistors of 28, 28′ R3, R4 Further resistors of 28′ R5, R6 Further resistors of 28′ R7 Insulation fault resistance R7a . . . d Insulation fault resistances R8, R9 Further resistors of 28 SP Neutral point of 12 U, V, W Motor phases of 12, 16, 18 X Interruption in a motor phase ΔI Capacitor current I.sub.R7 Fault current U.sub.+HV Intermediate circuit voltage U.sub.Lx Voltage captured by 28, 28′ U.sub.Netz Network voltage Uu,Uv,Uw Motor phase voltages