EP2649694A1

EP2649694A1 - Motor protection relay for optimizing the monitoring and protection of an electric motor

Info

Publication number: EP2649694A1
Application number: EP11790639.6A
Authority: EP
Inventors: Bérangère DIO
Original assignee: Alstom Technology AG; Schneider Electric Protection and Controle SAS
Current assignee: General Electric Technology GmbH; Schneider Electric Protection and Controle SAS
Priority date: 2010-12-08
Filing date: 2011-12-05
Publication date: 2013-10-16
Also published as: US20130314822A1; FR2968856A1; FR2968856B1; WO2012076478A1

Abstract

The present invention relates to a monitoring method and system for monitoring an electric motor (3), comprising: - modelling means (5) for modelling a circle diagram on the basis of the strengths of the current of said electric motor, - calculation means (7) for calculating at least one operational parameter by using said modelling of the circle diagram, and - monitoring means (9) for monitoring the said electric motor (3) on the basis of said operational parameter.

Description

MOTOR PROTECTION RELAY FOR OPTIMIZING SURVEILLANCE AND PROTECTION OF AN ELECTRIC MOTOR

TECHNICAL AREA

The present invention relates to the general field of motor protection relays and more particularly to the monitoring of an electric motor.

STATE OF THE PRIOR ART

An electric motor can be protected by protection against overload or excessive temperature factors that can damage it. This protection is usually provided by a motor protection relay configured with appropriate setting values.

The motor protection relay is used to detect overload, short circuits and other faults and can be based on electromechanical or digital techniques.

However, most motor protection relays must be set manually by the installer before commissioning, and can not adapt to a change in the operating conditions.

There are, however, motor protection relays based on an integrated logic capable of self-learning certain parameters of the motor to be protected to ensure better protection and monitoring of electric motors.

An example of such a protection relay is the General Electrics Multilin 369. It has the capability learn the following parameters of a specific engine: starting current, starting thermal capacity, acceleration time, cooling time constant, heating time constant, and average motor load. However, some other parameters representative of the induction of the motor are not provided by such relays.

The problem to be solved by the present invention is therefore to provide a system which takes into account certain specific operational characteristics of an electric motor to protect it appropriately.

DESCRIPTION OF THE INVENTION

The present invention is defined by a monitoring system for monitoring an electric motor, comprising:

modeling means for modeling a circle diagram on the basis of the current intensities of said electric motor,

calculating means for calculating at least one operational parameter using said modeling of the circle diagram, and

monitoring means for monitoring said electric motor on the basis of said operational parameter.

Modeling or circle diagram method is used to quickly estimate engine operating parameters that provide meaningful engine operating information for optimal and efficient engine monitoring electric. The operational parameters give information about the operating limits of the engine and in particular, the maximum load that can be supported by the engine.

Advantageously, said calculation means are intended to use the circle diagram to calculate any one of the following operational parameters: torque, Joule effect loss, rotor resistance, slip, and motor efficiency. Thus, the monitoring system enables to achieve self-learning quantities representative of the proper operation of the engine.

In a variant, said calculation means are intended to directly calculate the slip.

The invention also relates to a motor protection relay for protecting an electric motor comprising a monitoring system according to any one of the above characteristics, said relay further comprising:

measuring means for measuring said current intensities,

detection means for detecting an engine anomaly by analyzing at least one of said operational parameters,

triggering means for cutting off the motor power supply in the event that an anomaly is detected by said detection means.

Said detection means are intended to compare each operational parameter with a corresponding predetermined threshold and to send a command the tripping means for switching off the motor power supply when an operational parameter reaches its corresponding predetermined threshold.

The invention also relates to an electric motor equipped with an engine protection relay according to the above characteristics.

The electric motor further comprises means for acquiring speed values to determine the speed of the rotor.

The invention also relates to a method for monitoring an electric motor, comprising the following steps:

constructing a circle diagram on the basis of the supply current intensities of said electric motor,

calculate at least one operational parameter using said circle diagram,

- Monitor said electric motor according to said operational parameter.

The invention also relates to a computer program comprising code instructions for implementing the method of monitoring an electric motor according to the above characteristics, when it is executed by the processing means.

BRIEF DESCRIPTION OF THE DRAWINGS

Other characteristics and advantages of the invention will emerge clearly on reading a preferred embodiment of the invention given with reference to the appended figures among which: Figure 1 schematically illustrates a monitoring system for monitoring an electric motor according to the invention;

FIG. 2 diagrammatically represents a motor and an engine protection relay comprising the monitoring system of FIG. 1, and

FIG. 3 schematically illustrates the diagram of the circle enabling the determination of operational engine parameters by the monitoring system of FIG. 1.

DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS

The present invention is based on the use of the circle diagram method within an engine protection relay in order to determine the representative quantities of a running motor and more particularly of an asynchronous motor.

Figure 1 schematically shows a monitoring system 1 for monitoring an electric motor 3 according to the present invention. The system comprises modeling means 5, calculation means 7 and monitoring means 9.

The modeling means 5 model a circle diagram (see Figure 3) based on current intensities of the electric motor 3 such as no-load current, load current and possibly current in the stator when the rotor is blocked.

The calculation means 7 calculate at least one operational parameter using the model of the circle diagram. Advantageously, the calculation means 7 are intended to calculate any one of the following set of operating parameters: torque, loss by joule effect, rotor resistance, slip, and motor efficiency. These parameters are representative of the operation of the engine.

Indeed, the variation of the torque produced by the electric motor 3 gives a good indication of the operation of the motor 3 and can be used to diagnose an excessive load connected to the rotor and prevent damage or overheating of the rotor. The joule losses represent the warming of the engine 3 and gives an accurate representation of its dissipation of thermal energy. The slip gives accurate information about the proper functioning of the engine 3. The efficiency measures the efficiency of the engine 3 and corresponds to the ratio of the mechanical output power to the electrical input power supplied to the engine. Finally, the resistance of the rotor gives information concerning the energy dissipated by the motor 3.

Thus, the monitoring means 9 can effectively monitor the electric motor 3 on the basis of at least one of these operational parameters.

It should be noted that the modeling means 9, the calculation means 7 and the monitoring means 9 may all be part of a processing means.

FIG. 2 diagrammatically represents a motor protection relay 11 coupled to the electric motor 3.

The motor protection relay 11 comprises processing means 13, measuring means 15, trigger means 17, detection means 19, and the monitoring system 1 according to the present invention.

For example, the modeling means 5, calculation 7 and monitoring 9 of the monitoring system 1 may be constituted by algorithmic modules implemented by the processing means 13 of the motor protection relay 11. The detection means 19 may also be constituted by an algorithmic module implemented by the processing means 13.

The measuring means 15 measure the current intensity values of the electric motor 3. The detection means 19 detect the malfunctions of the electric motor 3 by analyzing the operational parameters monitored by the monitoring system 1. Finally, the means of trigger 17 are intended to cut off the power supply of the engine 3 when an anomaly has been detected. In particular, the detection means 19 compare each operational parameter to a corresponding predetermined threshold and send a command to the triggering means 17 to cut off the power supply to the motor 3 when at least one operational parameter reaches its corresponding predetermined threshold. This protects the motor 3 as well as the associated mechanical systems, against overloads, excessive temperatures, breakdowns or any other factor that may damage the motor 3 or any other equipment. The electric motor described in FIG. 2 is an induction motor of the asynchronous type consisting of stator windings 21 arranged around the rotor 23. When a primary alternating current I _± at a certain frequency w is supplied to the stator windings 21, a rotary magnetic field pattern is created which induces an alternating current I _Σ in the rotor 23.

The algorithm for determining operational motor parameters within the motor protection relay 11 is illustrated in FIG.

In particular, FIG. 3 diagrammatically represents a diagram of the circle 31 constructed according to a method implemented by the monitoring system or by the processing means 13 of the motor protection relay 11.

First, the motor 3 is started empty at a nominal frequency and voltage. The measuring means 15 measure the current intensity when empty. More particularly, they measure the effective current (RMS) and its angle according to a steady state condition. These two values form an empty current vector OA which can easily be reported on a two-dimensional coordinate system (0, X _r Y) corresponding, for example, to a complex plane.

Then, the engine 3 is started with a load according to determined conditions chosen by the user. The measuring means 15 measure the charging current (i.e., the stator effective charging current and its charging angle). These two values form a load current vector OM which is reported on the two-dimensional coordinate system (0, X _r Y).

After having reported the two vectors of current with load OA and OM on the coordinate system, the modeling means 9 construct a circle with its center Co on a line di parallel to the axis y and passing through the points d A and M end of the vectors OA and OM respectively. The circle diagram 31 is used by the processing means 13 to calculate the operational parameters and may optionally be plotted on output means 33 such as a screen.

AM distance between endpoints A and M

(i.e., AM vector standard) represents an effective current I ' _Σ proportional to the effective current I _Σ of the rotor. In particular, the distance AM is given as a function of the induced current Ι _Σ , thanks to the following formula: where i _l is the own inductance of the stator and μ is the mutual inductance. It should be noted that the value of AM is sufficient to determine certain operational parameters without it being necessary to determine the values of and μ individually.

Indeed, the losses by joule effect P _Σ of the rotor can be determined by the calculation means 7 in function of the distance AM according to the following formula: P ₂ = 3 × AM ² × R ₂ , where R _Σ is the resistance of the rotor.

In addition, the points M and A can also be used to calculate the torque C of the electric motor 3 according to the following formula:

QM represents the distance between the points Q and M, where Q is the intersection between the lines (MP) and (ACo) parallel to the X and Y axes respectively. In other words, QM is the difference of the abscissa between the points M and A. On the other hand, w is the grating frequency and p is the number of pairs of poles of the motor 3.

Thus, the circle diagram method already makes it possible to calculate the joule losses and the motor torque by using the currents of current at no load and load.

Then, to calculate other operational parameters, the motor 3 is started while keeping the rotor locked. Then, the measuring means 15 measures a current with locked rotor represented by a stator current vector OG _the compound of the rms current of the stator and its corresponding angle under a stream of short-circuited rotor. The current vector with locked rotor OG ₁ is then reported on the two-dimensional coordinate system (0, X _r Y).

Advantageously, the current vector with locked rotor can be used to calculate the losses by Joule effect P ₂ of the rotor without it being necessary to know the value of the resistance R _Σ of the rotor. In particular, the loss by Joule effect P _Σ is equal to the distance between the point Q and a point R representing the intersection of the lines (AGI) and (MP). The distance QR corresponds to the difference of the abscissas between the points A and R.

The Joule effect loss P _Σ can then be used to calculate the rotor resistance R _Σ . In particular, the resistance R _Σ is determined as a function of the distance between the points A and M and the distance QR according to the following formula:

R ₂ = QR / 3AM ² .

The current vector with locked rotor can also be used to calculate the slip S according to the following formula:

0'G ₁ O 'Gi is the distance from the point G ± of the line (ΑΟ') passing through A and parallel to the axis X. The point G is the intersection between the lines (AM) and (O 'Gi ) and so, O 'G is the distance of the point G with respect to the line (ΑΟ'). In other words, 0 'is the point of intersection between the line parallel to the Y axis and passing through the point G ± and the line parallel to the X axis and passing through the point A.

Under such conditions, the slip S is equal to unity and thus, for other conditions, the slip can be simply measured on the axis O 'Gi (i.e., S = 0 'G). Thus, the circle diagram 31 makes it possible to determine the slip S without the need for a speed sensor.

It should be noted that some engines do not have the possibility of starting with a locked rotor. In this case, the slip can be calculated directly as a function of the rotor speed N and therefore the circle diagram method can still be used to calculate the other operational parameters.

The slip S is defined as the ratio between the relative speed of the magnetic field as seen by the rotor 23 on the speed of the rotating field of the stator. Thus, it can be calculated directly according to the following formula:

5 = 1 - * -,

Ns where N is the rotor speed and Ns is the synchronous speed of the magnetic field. In particular, Ns corresponds to the theoretical no-slip no-slip speed, which is regulated by the number of pole pairs and the grating frequency.

Thus, in the case where the motor 3 is equipped with means 37 for determining the speed of the rotor, the motor protection relay 11 can be connected to these means 37 in order to receive the rotor speed value.

It will be noted that the speed determination means 37 may correspond either to a speed sensor which directly measures the speed of the rotor or to estimation means which calculate the rotor speed. Moreover, the calculation means 7 are intended to use the circle diagram 31 to calculate the efficiency or the efficiency of the engine by means of the following formula:

RM

η =

PM

The point R is the intersection between the lines (AGI) and (PM) and thus, η can be easily calculated by dividing the distance between the point M and the line (AGI) by the distance between the same point M and the Y axis.

It should be noted that the processing means 13 contained in the motor protection relay 11 can be used to execute a computer program comprising code instructions, designed to implement the method of determining operational engine parameters using the method of the circle diagram, according to the invention.

Claims

1. Monitoring system for monitoring an electric motor, characterized in that it comprises:

modeling means (5) for modeling a circle diagram (31) on the basis of the current intensities of said electric motor,

calculating means (7) for calculating at least one operational parameter using said modeling of the circle diagram, said operational parameter being any one of the following operational parameters: torque, Joule effect loss, rotor resistance, slip , and engine efficiency, and

monitoring means (9) for monitoring said electric motor (3) on the basis of said operational parameter.

2. Monitoring system according to claim 1, wherein said calculating means (7) are for calculating the slip as a function of the rotor speed of the motor (3).

3. Motor protection relay for protecting an electric motor comprising a monitoring system according to any one of claims 1 to 2, said relay further comprising:

measuring means (15) for measuring said current intensities, detection means (19) for detecting an abnormality of the engine (5) by analyzing said at least one operational parameter, and

triggering means (17) for disconnecting the power supply to the motor (3) in the event that an anomaly is detected by said detection means (19).

4. Relay according to claim 3, wherein said detection means (19) are intended to compare each operational parameter to a corresponding predetermined threshold and to send a command to the triggering means (17) to cut off the power supply to the motor ( 3) when an operational parameter reaches its corresponding predetermined threshold.

Electric motor with motor protection relay (11) according to claim 3 or 4.

Electric motor according to claim 5, further comprising speed determining means (37) for determining the rotor speed N.

7. A method for monitoring an electric motor, characterized in that it comprises the following steps:

constructing a circle diagram (31) based on the current intensities of said electric motor,

calculating at least one operational parameter using said circle diagram, said operational parameter being any one of the parameters following torque, Joule loss, rotor resistance, slip, and engine efficiency, and

- Monitor said electric motor (3) according to said operational parameter.

A computer program comprising code instructions for implementing the method of monitoring an electric motor according to claim 7, when executed by processing means.