GB2610245A - Motor tester and method for testing an electric motor over time - Google Patents

Abstract

A method for testing an electric motor 2 comprises: sending an electric test signal to motor windings and sensing a first signal answer to the test signal in an initial state, and sending the same electric test signal to the motor windings and sensing a second signal answer in a later operating state. The method further comprises comparing the second signal answer to the first signal answer and outputting an alert if a deviation between the second signal answer and the first signal answer reaches over or falls below a threshold value. The motor tester device comprises a signal generator (6, Fig.3) to send an electric signal to the motor windings, a measurement circuit (7, Fig.3) to sense a signal answer to the test signal, and a comparing unit (8, Fig.3) to compare first and second signal answers and to output an alert. The method may be executed when the motor is in operation and the test signals may be DC, AC, or mixed DC/AC signals. The signal answers may be current or voltage signals. The method may further include sending and sensing a third signal and answer. The alert may activate a signal lamp 4.

Description

Motor tester and method for testing an electric motor over time TECHNICAL FIELD The invention relates to a method for testing an electric motor, wherein an electric test signal is sent to motor windings of the electric motor and a signal answer to the test signal is sensed. Furthermore, the invention relates to a motor tester for testing an electric motor, comprising a signal generator, which is prepared to send an electric test signal to motor windings of the electric motor, and a measurement circuit, which is designed to sense a signal answer to the test signal. Finally, the invention relates to a motor testing arrangement, which comprises an electric motor and said motor tester and which is connected to motor windings of said electric motor via measuring cables.

BACKGROUND ART

Motor testers and method for testing an electric motor as well as the above arrangement are known in prior art. For example, a voltage can be applied to motor windings of the electric motor and a current resulting thereof can be measured. Based on the measurement, the status of the electric motor can be concluded.

However, a drawback in prior art is that known methods give information about the motor status at a particular point in time. Known methods give no information about past or future. Moreover, known methods are not prepared to evaluate the motor status in the operational state of the motor, i.e. when it is running.

DISCLOSURE OF INVENTION

Accordingly, an object of the invention is the provision of an improved method for testing an electric motor, an improved motor tester and an improved arrangement with such a motor tester. In particular, information about past shall be taken into account to make future prediction, and evaluating the motor status shall be possible in the operational state of the motor.

The object of the invention is solved by a method for testing an electric motor, comprising the steps of sending an electric test signal to motor windings of the electric motor and sensing a first signal answer to the test signal in an initial state, - sending the same electric test signal to the motor windings of the electric motor and sensing a second signal answer to the test signal in a later operating state, - comparing the second signal answer to the first signal answer and outputting an alert if a deviation between the second signal answer and the first signal answer reaches over or falls below a threshold value.

Beneficially, the above steps are executed when the electric motor is in operation (i.e. when it is running). In this way, downtimes of the electric motor and the installation, which the electric motor may be part of, just for testing (and maybe for realizing that everything is O.K.) can be avoided. To do so, the test signal can be superimposed on a supply voltage for the electric motor, for example by means of a coupler.

Moreover, the object of the invention is solved by a motor tester for testing an electric motor, comprising - a signal generator, which is prepared to send an electric test signal to motor windings of the electric motor, - a measurement circuit, which is designed to sense a signal answer to the test signal, and - a comparing unit, which is designed to compare a second signal answer to a first signal answer and to output an alert if a deviation between the second signal answer and the first signal answer reaches over or falls below a threshold value.

Finally, the object of the invention is solved by a motor testing arrangement, comprising an electric motor and a motor tester of the aforementioned kind, which is connected to motor windings of said electric motor via measuring cables.

One key element of the proposed method is that one and the same electric test signal is fed into the motor windings at two different points in time. Accordingly, there are two signal answers, which are compared with respect to a deviation. Hence, characteristics of the electric motor changing over time can be recognized and taken into account for giving a statement about the motor condition. Concretely, a threshold value is defined for said deviation. If it reaches over or falls below a threshold value, an alert is output.

Generally, the test signal can be a DC signal, an AC signal or a mixed DC/AC signal.

In one embodiment, the first signal answer comprises a first current answer based on a current measured at terminals of the motor windings and the second signal answer comprises a second current answer based on the current measured at the terminals of the motor windings.

Alternatively or in addition, the first signal answer comprises a first voltage answer based on a voltage measured at terminals of the motor windings and the second signal answer comprises a second voltage answer based on the voltage measured at the terminals of the motor windings.

In an advantageous embodiment of the proposed method, an alert for an open motor winding is output if the second current answer drops below a value of 20% of the first current answer. Accordingly, the comparing unit may be designed to output an alert for an open motor winding if a second current answer drops below a value of 20% of a first current answer.

In particular, an alert for an open motor winding can be output if at the same time the second voltage answer is above the first voltage answer. Accordingly, the comparing unit can be designed to output an alert for an open motor winding if a second current answer drops below a value of 20% of a first current answer and if at the same time a second voltage answer is above a first voltage answer.

In yet another advantageous embodiment of the proposed method, an alert for a short circuit between motor windings is output if the second voltage answer drops below a value of 20% of the first voltage answer. Accordingly, the comparing unit may be designed to output an alert for a short circuit between motor windings if a second voltage answer drops below a value of 20% of a voltage current answer.

In particular, an alert for a short circuit between motor windings can be output if at the same time the second current answer is above the first current answer. Accordingly, the comparing unit can be designed to output an alert for a short circuit between motor windings if a second voltage answer drops below a value of 20% of a first voltage answer and if at the same time a second current answer is above a first voltage answer.

In yet another advantageous embodiment of the proposed method, an alert for a measuring cable defect is output if the second current answer is in a range between a value of 10% of the first current answer and a value of 90% of the first current answer. Accordingly, the comparing unit can be designed to output an alert for a measuring cable defect if a second current answer is in a range between a value of 10% of a first current answer and a value of 90% of the first current answer.

It is also of advantage if - the same electric test signal, which was already used for the first and the second signal answer, is sent to the motor windings of the electric motor after a second signal answer was sensed, and a third signal answer is sensed to the test signal, a first deviation of the second signal answer from the first signal answer is determined, - a second deviation of the third signal answer from the first signal answer is determined and - a pre alert is output if a gradient of between the second deviation and the first deviation signal indicates that an alert will be caused within a predetermined time period (e.g. two months in future).

By the proposed measures, defects or failures, which are to be expected based on a change of the signal answers, are signaled by means of a pre alert so that responsible staff can predictively prepare for such a situation. For example, spare parts can be ordered, or a downtime of an installation, which the electric motor is part of, can be planned. In this way, unexpected downtimes of such installations can be avoided, and downtimes can be shortened.

Advantageously, the alert can activate a signal lamp and/or can inhibit switching on a switch upstream of the electric motor and/or can inhibit the operation of a motor controller upstream of the electric motor. In this way, unsafe operation of the electric motor can be avoided. For example, the motor controller can be embodied as a power converter for supplying power to the electric motor and can provide an input for inhibiting or enabling its operation.

BRIEF DESCRIPTION OF DRAWINGS

The invention now is described in more detail hereinafter with reference to particular embodiments, which the invention however is not limited to.

Fig. 1 shows a first example of a motor testing arrangement with a motor tester, an electric motor and an upstream switch in schematic view; Fig. 2 shows a motor testing arrangement similar to that of Fig. 1 but with an upstream motor controller; Fig. 3 shows a detailed schematic view of the motor tester; Fig. 4 shows signal answers to a DC step signal in an initial state; Fig. 5 shows signal answers to a DC step signal indicating a motor defect; Fig. 6 shows signal answers to an AC test signal in an initial state; Fig. 7 shows signal answers to an AC test signal indicating a motor defect; Fig. 8 shows signal answers to a DC step signal indicating a first example of a measuring cable defect; Fig. 9 shows signal answers to a DC step signal indicating a second example of a measuring cable defect and Fig. 10 shows signal answers to a DC step signal indicating a third example of a measuring cable defect and

DETAILED DESCRIPTION

Generally, same parts or similar parts are denoted with the same/similar names and reference signs. The features disclosed in the description apply to parts with the same/similar names respectively reference signs. Indicating the orientation and relative position is related to the associated figure, and indication of the orientation and/or relative position has to be amended in different figures accordingly as the case may be Fig. 1 shows a first example of a motor testing arrangement, comprising a motor tester 1, an electric motor 2 and measuring cables 3, by means of which the motor tester 1 is connected to motor windings u, v, w of said electric motor 2. In detail, the electric motor comprises terminals Ti. .T3 of the motor windings u, v, w, which the measuring cables 3 are connected to. In this example, the measuring cables 3 are also connected to the phases Li.. L3 of a grid and thus also form supply cables for the electric motor 2. Accordingly, the cables 3 may be seen as and termed as "combined measuring and supply cables" then. However, this is no necessary condition, and there may also be separate measuring cables 3 and supply cables. Moreover, the motor tester 1 comprises an optional signal lamp 4, and in addition, the motor testing arrangement comprises an optional switch S, which is controlled by the motor tester 1 via its optional control output Tctr.

The arrangement shown in Fig. 2 is very similar to that shown in Fig. 1. In contrast, instead of the switch S there is a motor controller 5 in the supply path. For example, the motor controller 5 can be embodied as power converter.

Fig. 3 shows a schematic but also a more detailed view of the motor tester 1. The motor tester 1 comprises a signal generator 6, a measurement circuit 7, a comparing unit 8, which may be embodied as a combined comparing and control unit, an optional signal lamp 4 and an optional control output Tctr. The signal generator 6 and the measurement circuit 7 are connected to terminals of motor tester Ta..Tc, which are provided for connection to the measuring cables 3. The comparing unit 8 is connected to the signal generator 6, the measurement circuit 7, the signal lamp 4 and the control output Tctr.

The signal generator 6 is prepared to send an electric test signal to motor windings u, v, w of the electric motor 1, the measurement circuit 7 is designed to sense a signal answer to the test signal, and the comparing unit 8 is designed to compare a second signal answer to a first signal answer and to output an alert if a deviation between the second signal answer and the first signal answer reaches over or falls below a threshold value. The alert for example can be output via the control lamp 4 and/or the control output Tctr. So, if a defect is detected in the electric motor 1 and if it is unsafe to operate it, the comparing unit 8 may output a visual alert by use of the control lamp 4 and/or can inhibit the operation of the electric motor 1 by outputting a control signal via the output Tctr which for example can be fed to the switch S, which the cuts off power, or to the motor controller 5, which stops or even does not start operation.

The function of the motor tester 1 is explained in more detail hereinafter by additional use of Figs. 4 to 10: The method for testing an electric motor 2, comprises the steps of: - sending an electric test signal to motor windings u, v, w of the electric motor 2 and sensing a first signal answer SAV1a, SAV1b, SAII a, SAI1b to the test signal in an initial state, - sending the same electric test signal to the motor windings u, v, w of the electric motor 2 and sensing a second signal answer SAV2a..SAV2e, SAI2a..SAI2e to the test signal in a later operating state, comparing the second signal answer SAV2a..SAV2e, SAI2a..SAI2e to the first signal answer SAVI a, SAV1b, SAMa, SAI1b and outputting an alert if a deviation between the second signal answer SAV2a..SAV2e, SAI2a..SAI2e and the first signal answer SAVI a, SAV1b, SAII a, SAI1b reaches over or falls below a threshold value.

Generally, the test signal can be a DC signal, an AC signal or a mixed DC/AC signal.

It should also be noted that in one embodiment, the above steps are executed when the electric motor 2 is in operation (i.e. when it is running). In this way, downtimes of the electric motor 2 and the installation, which the electric motor 2 may be part of, just for testing can be avoided. To do so, the test signal can be superimposed on a supply voltage for the electric motor 2, for example by means of a coupler.

The first signal answer can comprise a first current answer SAII a, SAI1b based on a current I measured at terminals TI. .13 of the motor windings u, v, w and the second signal answer can comprise a second current answer SAI2a..SAI2e based on the current measured at the terminals Ti.13 of the motor windings u, v, w.

In addition or alternatively, the first signal answer can comprise a first voltage answer SAVI a, SAV1b based on a voltage U measured at terminals T1..T3 of the motor windings u, v, w and the second signal answer can comprise a second voltage answer SAV2a..SAV2e based on the voltage measured at the terminals -11..T3 of the motor windings u, v, w.

Figs. 4 and 5 show first examples how signal answers may look like. In detail, Figs. 4 and 5 depict graphs of a measured current I and a measured voltage V over time t. Concretely, the electric test signal is a step signal or step function in this case. Fig. 4 shows the first signal answer SAV1a, SA112 to the test signal in an initial state (i.e. in a healthy condition of the electric motor 2). Fig. 5 shows the second signal answer SAV2a, SAI2a to the test signal in a later operating state (here indicating a defect of the electric motor 2).

As is visible in Figs. 4 and 5, the second voltage answer SAV2a reaches over the first voltage answer SAV2a, whereas the second current answer SAI2a is substantially below the first current answer SAII a. In fact, the second current answer SAI2a is zero or almost zero. This is an indication for an open motor winding u, v, w. So, in a preferred embodiment, an alert for an open motor winding u, v, w is output by the motor tester 1 On particular by its comparing unit 8) if the second current answer SAI2a drops below a value of 20% of the first current answer SAII a. An additional condition may be the voltage answer. So, in a further preferred embodiment, an alert for an open motor winding u, v, w is output by the motor tester 1 (in particular by its comparing unit 8) if at the same time the second voltage answer SAV2a is above the first voltage answer SAV1a. Hence, an alert for an open motor winding u, v, w can be output by the motor tester 1 (in particular by its comparing unit 8) if a second current answer SAI2a drops below a value of 20% of a first current answer SAI1a and if at the same time a second voltage answer SAV2a is above a first voltage answer SAVI a.

As already mentioned hereinbefore, the alert may cause activating a signal lamp 4 and/or inhibiting switching on a switch S upstream of the electric motor 2 and/or inhibiting the operation of a motor controller 5 upstream of the electric motor 2.

Figs. 6 and 7 indicate a similar situation as Figs. 4 and 5. In contrast, the test signal is not a step signal but a sine signal with an instant increase of its amplitude. Fig. 6 shows the first signal answer SAVlb, SAI1b to the test signal in an initial state (i.e. in a healthy condition of the electric motor 2). Fig. 7 shows the second signal answer SAV2b, SAI2b to the test signal in a later operating state (here indicating a defect of the electric motor 2). Again, the second voltage answer SAV2b reaches over the first voltage answer SAV1b, whereas the second current answer SAI2b is substantially below the first current answer SAI1b indicating an open winding condition. Preferably, the same conditions can be used for deciding whether an open winding condition occurs or not.

It should be noted, that in case of a short circuit between the motor windings u, v, w, the roles of the current answers SAI1a..SAI2b and the voltage answers SAV1a..SAV2b change. In that case, the second current answer SAI2a, SAI2b reaches over the first current answer SAII a, SAI1b, whereas the second voltage answer SAV2a, SAV2b is substantially below the first voltage answer SAV1a, SAVI b indicating a short circuit condition. In fact, the second voltage answer SAV2a, SAV2b may event drop to zero.

Hence, an alert for a short circuit between motor windings u, v, w can be output by the motor tester 1 On particular by its comparing unit 8) if a second voltage answer SAV2a, SAV2b drops below a value of 20% of a first voltage answer SAVI a, SAVI b. This is particularly true if at the same time a second current answer SAI2a, SAI2b is above a first current answer SAIl a, SAI1b.

It should be noted that the test signal may also be a mixed DC/AC signal. In this case, the teachings of Figs. 4 to 7 can be combined.

Figs. 8 to 10 show examples, which do not relate to a defect of the electric motor 2 but to a defect of the measuring cables 3. Again, a DC step signal or DC step function is used as test signal Fig. 8 shows an example where the impedance of the measuring cables 3 increases from 0.05 to 0.5 Ohm Fig. 9 shows an example where the impedance of the measuring cables 3 increases from 0.05 to 1.0 Ohm.

Fig. 10 finally shows an example where the capacitance of the measuring cables 3 increases from 0.5 to 0.1 -10 -Figs. 8 to 10 have some similarities. In any of these cases, the second current answer SAI2c..SAI2e is in a range between a value of 10% of the first current answer SAII a, SAI1b and a value of 90% of the first current answer SAII a, SAM b. Accordingly, an alert for a measuring cable defect is output by the motor tester 1 (in particular by its comparing unit 8) in a preferred embodiment if the second current answer SAI2c..SAI2e is in a range between a value of 10% of the first current answer SAII a, SAI1b and a value of 90% of the first current answer SAII a, SAllb.

Generally, the signal answers SAV1a..SAV2e, SAI1a..SAI2e can be used to predict future defects or failures of the electric motor 2. To do so, a third signal answer to the test signal, which was already used for the first signal answer SAVI a, SAVlb, SAII a, SAI1b and the second signal answer SAV2a..SAV2e, SAI2a..SAI2e, is sensed. Then, a first deviation of the second signal answer SAV2a..SAV2e, SAI2a..SAI2e from the first signal answer SAVI a, SAVlb, SAII a, SAI1b and a second deviation of the third signal answer from the first signal answer SAV1a, SAVI b, SAII a, SAI1b are determined. Based on that, a pre alert is output if a gradient of between the second deviation and the first deviation signal indicates that an alert will be caused within a predetermined time period (e.g. two months in future). By the proposed measures, probable defects or failures of the electric motor 2 are detected based on a change of the signal answers SAV1a..SAV2e, SAI1a..SAI2e. In response to the pre alert, responsible staff can predictively prepare for such a situation and for example can order spare parts for the electric motor 2 and/or plan a downtime of the same. In this way, unexpected downtimes of the electric motor 2 can be avoided and a downtime can be shortened if spare parts are kept in readiness.

In detail, a gradient of the second deviation and the first deviation is considered for that. In other words, a change of the signal answers SAV1a..SAV2e, SAI1a..SAI2e over time is monitored and extrapolated. It should be noted that storing more signal answers SAV1a..SAV2e, SAI1a..SAI2e in a memory, can help to more precisely predict future defects or failures. So, the proposed method is not limited to just three signal answers SAV1a..SAV2e, SAl1a..SAI2e, but more than three signal answers SAV1a..SAV2e, SAI1a..SAI2e can be used for that prediction.

Concluding, the proposed method and motor tester 1 allow to recognize a change of characteristics of the electric motor changing over time and to take this into account for giving a statement about the motor condition. Moreover, the proposed method and motor tester 1 allow to differentiate between different defects, each of which has its own "signature" meaning that each of which causes a specific change of signal answers for a given test signal. In particular, the proposed method and motor tester 1 allow to: - recognize an open motor winding u, v, w based on an initial, healthy state, - recognize a short circuit between motor winding u, v, w based on an initial, healthy state, give specific information about fault, for example particular phase fault details, recognize winding degradation using historical data and to predict future failures, - distinct between motor defect and measuring cable defect, perform a test when the electric motor 2 is in operation, i.e. when it is running It is noted that the invention is not limited to the embodiments disclosed hereinbefore, but combinations of the different variants are possible. In reality, the motor tester 1 and the motor testing arrangement may have more or less parts than shown in the figures. Moreover, the description may comprise subject matter of further independent inventions.

It should also be noted that the term "comprising" does not exclude other elements and the use of articles "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims should not be construed as limiting the scope of the claims.

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LIST OF REFERENCE NUMERALS

1 motor tester 2 electric motor 3 measuring cable 4 signal lamp motor controller 6 signal generator 7 measurement circuit 8 comparing unit (combined comparing and control unit) L1..L3 phases of grid switch u, v, w motor winding Ti..T3 terminal of the motor windings Ta..Tc terminal of motor tester current voltage time SAI1a..SAI2e current answer SAV1a. SAV2e voltage answer

Claims (15)

1. Method for testing an electric motor (2), comprising the steps of - sending an electric test signal to motor windings (u, v, w) of the electric motor (2) and sensing a first signal answer (SAVI a, SAVI b, SAI1a, SAI1b) to the test signal in an initial state, - sending the same electric test signal to the motor windings (u, v, w) of the electric motor (2) and sensing a second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) to the test signal in a later operating state, - comparing the second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) to the first signal answer (SAV1a, SAVI b, SAII a, SAII b) and outputting an alert if a deviation between the second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) and the first signal answer (SAV1a, SAV1b, SAII a, SAI1b) reaches over or falls below a threshold value.

2. Method as claimed in claim 1, characterized in that the above steps are executed when the electric motor (2) is in operation.

3. Method as claimed in claim 1 or 2, characterized in that the test signal is a DC signal, an AC signal or a mixed DC/AC signal.

4. Method as claimed in any one of claims 1 to 3, characterized in that the first signal answer comprises a first current answer (SAII a, SAI1b) based on a current (I) measured at terminals (Ti. .T3) of the motor windings (u, v, w) and the second signal answer comprises a second current answer (SAI2a..SAI2e) based on the current (I) measured at the terminals (Ti. .T3) of the motor windings (u, v, w).

5. Method as claimed in any one of claims 1 to 4, characterized in that the first signal answer comprises a first voltage answer (SAVI a, SAV1b) based on a voltage (U) measured at terminals (Ti.13) of the motor windings (u, v, w) and the second signal answer comprises a second voltage answer (SAV2a..SAV2e) based on the voltage (U) measured at the terminals (T1..T3) of the motor windings (u, v, w).

6. Method as claimed in claim 4 or 5, characterized in that - an alert for an open motor winding (u, v, w) is output if the second current -14 -answer (SAI2a..SAI2e) drops below a value of 20% of the first current answer (SAII a, SA11 b) or - an alert for a short circuit between motor windings (u, v, w) is output if the second voltage answer (SAV2a..SAV2e) drops below a value of 20% of the first voltage answer (SAV1a, SAVI b).

7. Method as claimed in claim 6, characterized in that - an alert for an open motor winding (u, v, w) is output if the second current answer (SAI2a..SAI2e) drops below a value of 20% of the first current answer (SAII a, SAI1b) and if at the same time the second voltage answer (SAV2a..SAV2e) is above the first voltage answer (SAV1a, SAV1b) or - an alert for a short circuit between motor windings (u, v, w) is output if the second voltage answer (SAV2a..SAV2e) drops below a value of 20% of the first voltage answer (SAVI a, SAVI b) and if at the same time the second current answer (SAI2a..SAI2e) is above the first voltage answer (SAII a, SAI1b).

8. Method as claimed in any one of claims 4 to 7, characterized in that an alert for a measuring cable defect is output if the second current answer (SAI2a..SAI2e) is in a range between a value of 10% of the first current answer (SAII a, SAI1b) and a value of 90% of the first current answer (SA11 a, SAI1b).

9. Method as claimed in any one of claims 1 to 8, characterized in that - the same electric test signal is sent to the motor windings (u, v, w) of the electric motor (2) after a second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) was sensed, and a third signal answer is sensed to the test signal, - a first deviation of the second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) from the first signal answer (SAVI a, SAV1b, SAII a, SAII b) is determined, - a second deviation of the third signal answer from the first signal answer (SAVI a, SAVI b, SAI1a, SAII b) is determined and - a pre alert is output if a gradient of between the second deviation and the first deviation signal indicates that an alert will be caused within a predetermined time period.

10. Method as claimed in any one of claims 1 to 9, characterized in that the alert activates a signal lamp (4) and/or inhibits switching on a switch (S) upstream of the electric motor (2) and/or inhibits the operation of a motor controller (5) upstream of the electric motor (2).

11. Motor tester (1) for testing an electric motor (2), characterized in - a signal generator (6), which is prepared to send an electric test signal to motor windings (u, v, w) of the electric motor (1), a measurement circuit (7), which is designed to sense a signal answer (SAI1a..SAI2e, SAV1a..SAV2e) to the test signal, and - a comparing unit (8), which is designed to compare a second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) to a first signal answer (SAV1a, SAV1b, SAIl a, SAI1b) and to output an alert if a deviation between the second signal answer (SAV2a..SAV2e, SAI2a..SAI2e) and the first signal answer (SAVI a, SAVlb, SAI1a, SAI1b) reaches over or falls below a threshold value.

12. Motor tester (1) as claimed in claim 11, characterized in that the comparing unit (8) is designed - to output an alert for an open motor winding (u, v, w) if a second current answer (SAI2a..SAI2e) drops below a value of 20% of a first current answer (SAIl a, SAI1b) and/or - to output an alert for a short circuit between motor windings (u, v, w) if the second voltage answer (SAV2a..SAV2e) drops below a value of 20% of the first voltage answer (SAV1a, SAVI b).

13. Motor tester (1) as claimed in claim 11, characterized in that the comparing unit (8) is designed - to output an alert for an open motor winding (u, v, w) if a second current answer (SAI2a..SAI2e) drops below a value of 20% of a first current answer (SAII a, SAI1b) and if at the same time a second voltage answer (SAV2a..SAV2e) is above a first voltage answer (SAV1a, SAV1b) - to output an alert for a short circuit between motor windings (u, v, w) if the second voltage answer (SAV2a..SAV2e) drops below a value of 20% of the first -16 -voltage answer (SAVI a, SAV1b) and if at the same time the second current answer (SAI2a..SAI2e) is above the first voltage answer (SAlla, SAII b).

14. Motor tester (1) as claimed in any one of claims 11 to 13, characterized in that the comparing unit (8) is designed to output an alert for a measuring cable defect if a second current answer (SAI2a..SAI2e) is in a range between a value of 10% of a first current answer (SAI1a, SAI1b) and a value of 90% of the first current answer (SAII a, SAII b).

15. Motor testing arrangement, characterized in an electric motor (2) and a motor tester (1) as claimed in any one of claims 11 to 14 connected to motor windings (u, v, w) of said electric motor (2) via measuring cables (3).