METHOD FOR CHECKING THE OPERATIONAL READINESS OF THE ELECTRICAL SUPPLIES TO A THREE-VOLTAGE MOTOR AND OF THE THREE-VOLTAGE MOTOR ITSELF IN A STATE OF STANDBY

Description

[0001] The present invention relates to the verification and potential monitoring of the operational readiness of a three-phase motor connected to a control circuit during a non-operating state, including in particular a test of the operational readiness of the electrical supply lines leading to the three-phase motor.

[0002] Three-phase motors are used in engineering to perform mechanical movements with high power consumption. While some applications involve more or less continuous operation of these three-phase motors, many others use them only intermittently, switching them off for certain periods but still requiring them to be ready for operation. There are also applications where operating the three-phase motors is an absolute exception, yet the continuous operational readiness of the motors themselves, and especially of the supply lines connecting them to a power source and / or control system, is of utmost importance or even absolutely critical for safety.

[0003] For example, fire gas and supply air fans as well as drives for natural smoke and heat exhaust devices, which are important and legally regulated components in many buildings and which ensure the removal of fire gases and the protection of people and buildings in the event of a fire, are operated with three-phase motors due to the high power required for the supply of air and the rapid removal of smoke and fire gases.

[0004] These fire exhaust and supply air fans are kept in reserve for emergencies and are not used during regular building operation. To ensure their functionality in an emergency, despite the long periods of inactivity, and to guarantee that the three-phase motors start correctly and the fans rotate properly when activated, the wiring from the control unit to the sensors and / or actuators is checked for breaks and short circuits. This check, along with other building safety equipment such as alarm systems, fire detection systems, and the aforementioned smoke and heat exhaust systems, allows for the detection of any faults and the initiation of corrective action. Such testing to enhance operational safety is also largely mandated by building regulations.

[0005] While it is well established practice to monitor three-phase motors and their electrical supply lines during operation, and to deduce the proper condition of the monitored motor and its connecting electrical lines from data acquired by various sensors, the reliable inspection or even monitoring of three-phase motors, and especially their electrical connection lines, in a non-operating state presents a challenge that has not yet been satisfactorily resolved. Security systems in buildings, in particular, but not exclusively, are in standby mode most of the time. Since conventional inspection or monitoring methods are not always feasible, the monitoring of these systems is often in a state of readiness.Since monitoring devices in this state do not check or even monitor three-phase supply lines to three-phase motors, faults, particularly in the electrical supply lines, are typically only detected during routine functional tests performed at longer intervals, during which the systems are put into operation. In the worst-case scenario, such faults, if they only occur after the last functional test, are only discovered when the systems are needed, but then fail due to the lack of operational readiness, especially of the electrical supply lines. This can, in the worst case, endanger human life, or at the very least, endanger the health of people whose lives and health the safety systems are meant to protect in such an emergency.

[0006] Documents WO 2016 / 033576 A2, DE 10 2009 000659 A1, US 2020 / 049769 A1, and US 2011 / 037495 A1 demonstrate the state of the art in wiring the power supply of a three-phase motor in a non-operated state.

[0007] While DE 10 2022 001 334 A1 already describes a procedure for checking three-phase motors and their connections even when not in operation, it does not specifically address the testing of the electrical supply lines. Furthermore, the procedure disclosed therein is undifferentiated and cannot specifically address possible causes of faults. In the method disclosed in this prior art, a voltage is applied between two of the three conductors of the three-phase supply line; the third phase remains disconnected. A current is then set in this circuit, corresponding to a target current at a target voltage determined by calculation from the values ​​for the rated current and rated voltage of the three-phase motor. The voltage in the resulting circuit is then measured and compared with the calculated target voltage.A fault is detected if the measured voltage deviates from the target voltage above a tolerance threshold. As already mentioned, this approach is undifferentiated and does not allow for clear conclusions. Furthermore, it is prone to errors if, due to opposing effects from faults occurring in different elements of the system, the expected deviations balance each other out, and thus no irregularity or failure of the tested system can be detected.

[0008] Against the background of this state of the art, the object of the invention is to demonstrate a safe, reliable and economical procedure for checking whether, in a three-phase motor, in a "standby" operating mode (i.e., in a state in which the three-phase motor is not being operated), the three-phase supply line, the electrical connection and the induction coils of the three-phase motor, such as a drive motor of an air supply or fire gas fan or a drive of a natural smoke and heat exhaust device in a building, are in a proper and functional condition, with particular emphasis on checking the electrical connection lines as well.

[0009] Essentially, the invention relates to the intended verification or monitoring of a three-phase line from a control unit to a three-phase motor, e.g., to a three-phase motor for mechanical and natural smoke extraction, in standby mode, i.e., with the motor stationary.

[0010] The problem described above is solved according to the invention by a method for checking the operational readiness of a three-phase motor connected to a control circuit, including the three-phase supply lines (L1, L2, L3) connecting this three-phase motor to the control circuit, in a non-operating state, as defined in claim 1. Advantageous embodiments of the method according to the invention are described in claims 2 to 5.

[0011] In another aspect, the invention proposes a control circuit for controlling a connected three-phase motor, wherein the control circuit is configured for carrying out a method claimed herein. Furthermore, the invention provides a smoke extraction device for a building with at least one air conveyor driven by a three-phase motor, in which a method disclosed herein is used to check or monitor the operational readiness of the connected and non-operated three-phase motor.

[0012] In an inventive method for checking the operational readiness of a three-phase motor connected to a control circuit, including the three-phase supply lines (L1, L2, L3) connecting this three-phase motor to the control circuit, in a non-operating state, two different test procedures are carried out, unlike in the prior art according to DE 10 2022 001 334 A1. Specifically, in a first test procedure for checking for a possible cable break, a first current source in the form of a DC current source or an AC current source of low frequency is connected simultaneously or successively to one of two different pairs of supply lines L1, L2, L3.

[0013] A low-frequency alternating current is defined as an alternating current whose frequency is chosen to be so low that the inductance of the stator winding of the three-phase motor in the respective circuit exhibits no significant reactance for an alternating current of this frequency, in particular an reactance that is at least one order of magnitude lower than the total resistance of the supply line and the conductors within the three-phase motor. The current flowing in the circuit thus connected and / or the voltage applied therein is / are measured, and the measurement result(s) is / are used as an indicator of a first electrical resistance. In particular, the resistance can also be precisely determined from the measured current and voltage values.In this case, a predetermined current or voltage can also be set at the DC source or the AC source with low frequency, and the set value can be used to determine the first resistance, taking into account the value measured for the other quantity not specified at the current source.

[0014] In a second test procedure, to check for a possible short circuit, a second current source in the form of a high-frequency AC source is connected simultaneously or sequentially to at least two different pairs of the supply lines L1, L2, L3. The frequency of the second AC source is chosen to be high enough that the reactance of the inductance of the stator winding of the three-phase motor in the respective circuit is significantly higher than the resistance of the supply line being tested or monitored. The current flowing and / or the voltage present in the circuit thus connected is / are measured, and the measurement result(s) is / are used as an indicator for a second electrical resistance.In this way, the second resistance can also be precisely determined from the known (set) and / or measured values ​​of the current and voltage. Alternatively, a predetermined current or voltage can be set at the high-frequency AC power source, and this set value can be used to determine the second resistance, taking into account the measured value for the other parameter not specified at the power source.

[0015] To assess the operational readiness of the three-phase motor, i.e., the functionality of the motor and supply lines, the measured values ​​are compared with values ​​expected for a proper condition.

[0016] Thus, if at least one of the measured values ​​for current and / or voltage obtained in the first test procedure indicates a first resistance increased by a value above a predefined tolerance threshold compared to a previously defined normal state, and / or if at least one of the measured values ​​for current and / or voltage obtained in the second test procedure indicates a second resistance decreased by more than a predefined tolerance threshold compared to a previously defined normal state, a conclusion is drawn that there is a fault in the electrical system of the three-phase motor and / or the supply lines that prevents operational readiness.

[0017] The two testing procedures can be carried out multiple times and successively for different pairings of the supply lines.

[0018] The measured values ​​expected for the normal operating state can be calculated from known electrical characteristics of the three-phase motor, possibly taking into account the influence of the supply lines. However, they can also be determined empirically by first performing a functional test immediately after the installation of the three-phase motor connected to the control circuit, and then, after confirming its functionality, performing the first and second test procedures and defining the corresponding normal operating state based on the measured values ​​obtained from the three-phase motor and supply lines system.

[0019] It should be emphasized here that for the method according to the invention, it is irrelevant whether the two testing procedures are carried out in a specific order. In this respect, the terms "first testing procedure" and "second testing procedure" should not be understood as defining a chronological sequence for these steps. Rather, they merely serve to distinguish these different components of the method. Similarly, the order in which these two testing procedures are listed in claim 1 is not meant as a chronological requirement. The only important factor is that both testing procedures are carried out to perform the method according to the invention. It is also irrelevant whether both testing procedures are performed exactly once or even the same number of times in a single run of the method.It may well be provided within the scope of the invention that one of the two testing procedures is carried out multiple times and the other testing procedure is carried out, for example, only once or at least in a smaller number of times.

[0020] The method according to the invention allows for the repeated verification of the operational readiness of a three-phase motor and, in particular, its electrical connecting cables, thus enabling monitoring of operational readiness. The test procedures, first test procedure and second test procedure, are performed repeatedly at predetermined time intervals. By defining the duration of these predetermined time intervals, the monitoring intensity or density can be determined. If a particularly short sequence of time intervals is chosen, quasi-continuous monitoring can be performed.

[0021] The basic inspection procedure described above can generally detect faults such as broken wires and short circuits between individual wires of a three-phase AC motor cable. However, this procedure cannot necessarily differentiate precisely between various possible faults, and therefore cannot yet be used for a more specific fault analysis.

[0022] Within the framework of the inventive method and procedure, if a more specific evaluation of the detected resistances is undertaken, it is also possible to carry out a more precisely defined error determination.

[0023] It is not necessarily required to individually measure each conductor with a connected DC power source, a low-frequency AC power source, and a high-frequency AC power source, respectively, thus avoiding a large number of individual measurements. The procedure described below, in which most measurements can be performed simultaneously on supply lines L2 and L3, allows for a more precise differentiation of faults. This approach may also require fewer individual measurements. Tests for line breaks:

[0024] The method disclosed herein can be used to conclude that there is a break in the supply line L1 or in several of the supply lines L1, L2 and L3 if, during a simultaneous measurement with a low-frequency DC or AC source connected between the supply line L2 and the supply line L1, and also with a low-frequency DC or AC source connected between the supply line L3 and the supply line L1, the resistances determined on the supply lines L2 and L3 each exceed a previously defined threshold, a disturbance threshold, i.e., an excessive value is detected for both resistances.

[0025] A break in line L2 can be concluded if, during a simultaneous measurement with one low-frequency DC or AC source connected between supply line L2 and supply line L1, and another low-frequency DC or AC source connected between supply line L3 and supply line L1, only the resistance measured on supply line L2 exceeds a defined disturbance threshold.

[0026] A break in line L3 can be concluded if, during a simultaneous measurement with one low-frequency DC or AC source connected between supply line L2 and supply line L1, and another low-frequency DC or AC source connected between supply line L3 and supply line L1, only the resistance measured on supply line L3 exceeds a defined disturbance threshold. Short circuit tests:

[0027] The method disclosed herein allows a conclusion to be drawn that there is a short circuit between the supply lines L1 and L2 if, during a simultaneous measurement with a high-frequency alternating current source connected between supply line L2 and supply line L1, and furthermore with a high-frequency alternating current source connected between supply line L3 and supply line L1, the resistance measured at supply line L2 falls below a defined threshold, a short-circuit threshold, i.e., a particularly low resistance is determined, and if, at the same time, the resistance measured at supply line L3 exceeds a defined threshold, an intact threshold, i.e., a sufficiently high resistance is determined.

[0028] A short circuit between the supply lines L1 and L3 can be concluded if, during simultaneous measurements with one high-frequency AC source connected between supply line L2 and supply line L1, and another high-frequency AC source connected between supply line L3 and supply line L1, the resistance measured at supply line L3 falls below a defined short-circuit threshold and, at the same time, the resistance measured at supply line L2 exceeds a defined intact threshold.

[0029] A short circuit between the supply lines L2 and L3 can be concluded if the following conditions are met simultaneously: In simultaneous measurements using, firstly, a high-frequency AC source connected between supply line L2 and supply line L1, and secondly, a high-frequency AC source connected between supply line L3 and supply line L1, both the resistance measured at supply line L2 and the resistance measured at supply line L3 exceed a defined integrity threshold; thus, sufficiently high resistance values ​​are determined for both of the aforementioned resistances, AND the difference between the voltages measured in supply lines L2 and L3, in separate measurements using, firstly, a high-frequency AC source connected between supply line L2 and supply line L1, and secondly, a high-frequency AC source connected between supply line L3 and supply line L1, lies below a predetermined threshold, a difference threshold.

[0030] A short circuit between the supply lines L1, L2 and L3 can be concluded if, during simultaneous measurements with one high-frequency AC source connected between supply line L2 and supply line L1, and another high-frequency AC source connected between supply line L3 and supply line L1, the resistances determined on the supply lines L2 and L3 are each below a defined short-circuit threshold.

[0031] A further aspect of the invention lies in a control circuit for controlling a connected three-phase motor, which control circuit is configured for carrying out a method as described above. In particular, the control circuit comprises the current sources required for the two test procedures, i.e., the first and the second current source, via switching devices for connecting different pairs of the supply lines L1, L2, and L3 to the respective connected current source to form a circuit, and via appropriate measuring sensors for measuring current and / or voltage in the respective switched circuit.

[0032] Another embodiment of the invention consists of a smoke extraction device for a building with at least one air conveyor, which has a three-phase motor for driving an air conveying element, wherein the three-phase motor is connected to a control circuit comprising a control circuit as mentioned above, via which the air conveyor is switched on to operate the air conveying element when required.

[0033] Another embodiment of the invention consists of a building opening, such as a ventilation flap or window, equipped with a motor-driven closing element, in which a flap element of the ventilation flap or a window sash of the window can be moved by means of a three-phase motor to open and / or close the building opening, and wherein the three-phase motor is connected to a control circuit comprising a control circuit as mentioned above, via which the three-phase motor is switched on to open or close the building opening when required.

[0034] Further advantages and features of the invention will become apparent from the following description of a possible embodiment of the invention, which also refers to the accompanying figure. It shows: Fig. 1 shows a schematic representation of an electrical circuit of a three-phase motor connected to a control unit via electrical connecting lines, with a circuit for carrying out a method according to the invention.

[0035] In Fig. 1 Figure 1 illustrates the wiring of a three-phase motor 1, including a circuit for checking the operational readiness of the motor 1 when it is not in operation (i.e., connected but stationary) and its electrical supply line. In the figure, 2 symbolizes the stator windings of the three-phase motor 1, which—here in a star connection—are each connected to one of the phase conductors L1, L2, and L3 carried in a three-phase supply cable 3. A switch, illustrated by 4, which can be actuated by a control unit (not shown), allows the phase conductors L1, L2, and L3 to be supplied with the appropriate three-phase current.

[0036] Also shown in the diagram without an intermediate switch, the outputs of a test circuit 5 are connected to the phase conductors L1, L2, and L3. This test circuit 5 can also be connected to the controller (not shown here) to control its function, trigger specific test procedures, and receive and process measurement results or evaluation signals from the test circuit 5. The test circuit 5 integrates two current sources: an AC source 6 and a DC source 7. Each of these sources has a first output permanently connected to phase conductor L1 and a second output that can be selectively connected to either phase conductor L2 or L3. Only one of the two current sources, AC source 6 or DC source 7, can be connected to either phase conductor L2 or L3 at any given time.Voltage meters 8 and 9 are connected between L1 and L3 (voltage meter 8) and L1 and L2 (voltage meter 9), respectively, and in parallel to the current sources 6 and 7, respectively.

[0037] Although the in Fig. 1Although the illustrated circuit does not show an electrical switch for galvanic isolation of the test circuit 5 from the connections to the phase conductors L1, L2, and L3, such a switch may be present to protect the test circuit 5 from potential damage caused by the applied voltage or current, particularly when the three-phase current required to operate the three-phase motor 1 is applied to the phase conductors L1, L2, and L3 via switch 4. Specifically, the voltages and currents generated by the power sources 6 and 7 are typically significantly lower than the voltage applied to operate the three-phase motor 1 and the current flowing in the phase conductors L1, L2, and L3. This is because the voltages and currents generated by the power sources 6 and 7 are considerably lower.The currents flowing as a result of the output of these current sources 6 and 7 serve only to generate measurement signals and are not intended to put the three-phase motor 1 into operation, i.e., to set it in motion.

[0038] As one possible embodiment of the invention, the following describes a test or monitoring of a three-phase motor 1 together with the three-phase supply cable 3 leading from a control unit to the three-phase motor 1. The three-phase motor 1 can be, in particular, one integrated into a fire gas or supply air fan for mechanical smoke extraction or positive pressure ventilation, or into a natural smoke and heat exhaust device, but it can also be a three-phase motor 1 integrated into another unit, such as a damper or window actuator. The test or monitoring is carried out during standby operation, i.e., precisely when the three-phase motor 1 is not in operation.

[0039] The three phase conductors L1, L2, L3 of the supply cable 3 leading to the three-phase motor 1 are connected to the control unit and can be supplied with three-phase power via the switch 4 to operate the three-phase motor 1. The phase conductors L1, L2, L3 also connect the test circuit 5 directly to the stator windings 2 of the three-phase motor 1 via a branch connection in standby mode. The test circuit 5 is preferably located close to the control unit or forms part of the control unit.

[0040] During a check of the operational readiness of the three-phase motor 1 in the non-operating state, the test circuit 5 is used to verify the functionality of the electrical connection, including the supply cable 3, to the phase conductors L1, L2, L3 and the stator windings 2. This is done by measuring the current generated by the AC or DC source into each pair of phase conductors L1, L2 or L1, L3, and evaluating the impedances of the respective paired phase conductors L1, L2 or L1, L3 in the circuit, which are deduced from the measured voltage values. This test is performed in two steps, namely: Checking for a possible wire break:

[0041] To detect a wire break, e.g., in one of the phase conductors L1, L2, L3 in the supply cable 3, the test circuit 5, triggered in particular by the control unit, applies a direct current generated by the DC source 7 to the stator windings 2 of the three-phase motor 1 via one of the connectable pairs of phase conductors L1, L2 and L1, L3. The voltage present at the control side of the respective pair of phase conductors L1, L2 and L1, L3 is measured with the associated voltmeter 8, 9. Based on the measured voltage, the total resistance of the supply cable 3 (in each pair L1, L2 and L1, L3) and the involved stator windings 2 can be determined. If the current output by the DC source is known, the total resistance can also be calculated.As an alternative to direct current, a low-frequency alternating current generated by an alternating current source 6 used instead of the direct current source 7 can also be used. If the alternating current source 6 is one with an adjustable frequency, it can also be used. A low-frequency alternating current is defined as an alternating current whose frequency is chosen so low that the inductance of the stator winding 2 of the three-phase motor 1 in the respective circuit does not exhibit any significant reactance for an alternating current of this frequency, in particular, an reactance that is at least one order of magnitude lower than the total resistance of the supply cable 3, more precisely, of the respective connected phase conductors L1, L2 or L1, L3, as well as the other internal conductors in the three-phase motor 1.

[0042] With direct current or alternating current at low frequency, the line resistance and the relatively small DC resistance of the stator windings 2 become effective. If there is an interruption in one of the conductors of the electrical lines, i.e., one of the phase conductors L1, L2, L3 in the supply cable 3 or an internal conductor in the three-phase motor 1, the resistance evaluated in this test procedure increases compared to a resistance determined for a normal condition, and this is interpreted as a wire break. Checking for a possible short circuit:

[0043] To detect short circuits between the conductors of cables connected to phase conductors L1, L2, or L3, the monitoring circuit 5, controlled in particular by the control unit, applies an alternating current at a higher frequency to each of the supply lines connected to phase conductors L1, L2, L3, and then to the stator windings 2 of the three-phase motor 1 in paired configurations. The frequency is chosen to be high enough that the reactance of the inductance of the stator winding 2 is significantly higher than the resistance of the supply line being monitored. If no short circuit exists between the phase conductors L1, L2, L3 of the supply line, the relatively high reactance of the stator windings 2 connected in the respective circuit is measured.

[0044] However, if there is a short circuit between the phase conductors L1, L2 and / or L3 of the line, the measured value is determined by the low line resistance caused by the short circuit, and a low line resistance that is below a tolerance threshold of a resistance determined for a normal state is considered a short circuit.

[0045] In addition to the general determinations described above of a wire break or short circuit that has occurred somewhere, more detailed analyses can also be carried out using the procedure described below: Checking for any wire breaks and determining the interrupted line:

[0046] A DC current source or a low-frequency AC current source is simultaneously connected between L2 and L1, and also between L3 and L1. Then, in each of these circuits, the resistance across the supply line L2 and the resistance across the supply line L3 are determined, in particular by measuring the voltage with voltmeters 8 and 9.

[0047] If, during such a procedure, it is determined that both the resistance at L2 and the resistance at L3 exceed a previously defined threshold, a fault threshold, i.e., if an excessive resistance is detected for both supply lines, then a break in the supply line L1 or in several of the supply lines L1, L2 and L3 can be concluded.

[0048] However, if it is detected that only the resistance measured on the supply line L2 exceeds a defined fault threshold, then a break in line L2 can be assumed.

[0049] If a resistance exceeding a defined fault threshold is detected for L3 alone, a line break in L3 can be assumed. Testing for possible short circuits and identification of short-circuited lines:

[0050] First, a high-frequency alternating current source is simultaneously connected between L2 and L1, and also between L3 and L1. Then, in each of these circuits, the resistance across L2 and the resistance across L3 are determined, in particular by measuring the voltage with voltmeters 8 and 9.

[0051] If, during this simultaneous measurement, the resistance measured at L2 falls below a defined threshold, a short-circuit threshold, i.e., a particularly low resistance is determined, and if, at the same time, the resistance measured at L3 exceeds a defined threshold, an intact threshold, i.e., a sufficiently high resistance is determined, then a short circuit between L1 and L2 can be concluded.

[0052] A short circuit between L1 and L3 can be concluded if, during this simultaneous measurement, the resistance measured at L3 falls below a defined short-circuit threshold and, at the same time, the resistance measured at L2 exceeds a defined intact threshold.

[0053] A short circuit between L1, L2 and L3 can be concluded if, during this simultaneous measurement, the resistances determined at L2 and L3 are both below a defined short-circuit threshold.

[0054] To determine whether a short circuit exists between L2 and L3, various measurements are required. Such a short circuit can be concluded if the following conditions are met simultaneously: When measuring simultaneously with a high-frequency AC source connected between L2 and L1, and with a high-frequency AC source connected between L3 and L1, both the resistance measured at L2 and the resistance measured at L3 exceed a defined integrity threshold; thus, sufficiently high resistance values ​​are determined for both resistors, AND the difference between the voltages measured in L2 and L3, each in separate measurements with a high-frequency AC source connected between the supply line L2 and the supply line L1, and with a high-frequency AC source connected between the supply line L3 and the supply line L1, lies below a predetermined threshold, a difference threshold.

[0055] If the checks described above are performed repeatedly at sufficiently short intervals, the operational readiness of the three-phase motor 1 and, in particular, the connecting cable 3 connected to it, will be monitored in the non-operating state. With a sufficiently short interval, this can also be done quasi-continuously.

[0056] Within the scope of the invention, it is also possible to combine the aforementioned testing or monitoring with conventional and known methods for testing or monitoring the condition and function of the three-phase motor 1 during operation, e.g., by evaluating fault messages from, for example, phase monitoring relays, motor protection switches, soft starters, frequency converters, or other motor monitoring devices, thereby expanding the possibilities for testing or monitoring. While such a combination is possible within the scope of the invention, it is not necessary. Reference symbol list

[0057] 1 Three-phase motor 2 Stator winding 3 Three-phase supply cable 4 Switch 5 Test circuit 6 AC power source 7 DC power source 8 Voltmeter 9 Voltmeter

Claims

1. Method for monitoring the operational readiness of a three-phase motor (1) connected to a control circuit, together with the three-phase supply lines (L1, L2, L3) connecting this three-phase motor (1) to the control circuit, in a non-operational state, characterised in that a. for testing for a possible cable break, a first current source (7) in the form of a direct current source or a low-frequency alternating current source is connected to at least two different pairs of the supply lines (L1, L2, L3) simultaneously or successively, and a current and / or a voltage in the circuit connected in this way is measured as an indicator of a first electrical resistance, and b. for testing for a possible short circuit, a second current source (6) in the form of a high-frequency alternating current source is connected to at least two different pairs of the supply lines (L1, L2, L3) simultaneously or sequentially, and a current and / or voltage in the circuit thus connected is measured as an indicator of a second electrical resistance, the frequency of the second voltage being selected to be so high that a reactive resistance of the inductance of the stator winding (2) of the three-phase motor (1) in the connected circuit is significantly higher than the resistance of the supply line to be monitored, whereby, if at least one of the measured values obtained under a. for the current and / or the voltage indicates a first resistance that is increased by a value exceeding a predefined tolerance threshold compared to a previously defined normal state and / or if at least one of the measured values obtained under b. for the current and / or voltage indicates a second resistance that is reduced by a value exceeding a predefined tolerance threshold compared to a previously defined normal state, it is concluded that there is a fault in the electrical system of the three-phase motor (1) and / or the supply lines (L1, L2, L3) that prevents it from being ready for operation.

2. Method according to claim 1, characterised in that test steps a. and b. are carried out multiple times and successively for different pairings of the supply lines (L1, L2, L3).

3. Method according to one of the preceding claims, characterised in that, in order to determine the respective normal state immediately after installation of the three-phase motor (1) connected to the control circuit, a function test is first carried out and then, after determining the functionality, test steps a. and b. are carried out and the normal state is determined in each case using the measured values determined there.

4. Method according to one of the preceding claims, characterised in that test steps a. and b. are repeated at predetermined time intervals.

5. Method according to claim 4, characterised in that quasi-continuous monitoring is performed by means of a close temporal sequence of the time intervals.

6. Control circuit for controlling a three-phase motor connected thereto, characterised in that it is designed to carry out a method according to one of the preceding claims.

7. Smoke extraction device for a building with at least one air conveyor, wherein the air conveyor has a three-phase motor (1) for driving an air conveyor element and wherein the three-phase motor (1) is connected to a control system via which, in case of need, the air conveyor is activated to operate the air conveyor element, characterised in that the control unit has a control circuit according to claim 6.

8. Building opening equipped with a motor-driven closure element, such as in particular a ventilation flap or a window, wherein a flap element of the ventilation flap or a window sash of the window can be moved by means of a three-phase motor (1) for opening and / or closing the building opening, and wherein the three-phase motor (1) is connected to a control system via which, when required, the three-phase motor is activated to open or close the building opening, characterised in that the control system has a control circuit according to claim 6.