SOFT STARTING SYSTEM FOR AN ELECTRICAL MOTOR

Abstract

Starter system for an electric motor (M) supplied by an electrical network (1), the starter system comprising an electronic control circuit (7) and an electronic switch (10) for controlling one phase of the motor (M), the electronic switch (10) being controlled by the control circuit (7). The starter system comprises a sensor (3) intended to deliver an analogue signal (4) that is representative of the derivative of a current flowing through the phase of the motor (M), a detection board (5) comprising means for transforming said analogue signal (4) into a binary signal (6) that is representative of the changes in sign of said analogue signal, and comprising means for transmitting said binary signal to the control circuit (7), so as to optimize the control of the electronic switch (10).

Claims

1. Starter system for an electric motor (M) supplied by a high-voltage electrical network (1), the starter system comprising an electronic control circuit (7) and an electronic switch (10) for controlling one phase of the motor (M), the electronic switch (10) being controlled by the control circuit (7), the starter system comprising a sensor (3) intended to deliver an analogue signal (4) that is representative of the derivative of a Current flowing through the phase of the motor (M), characterized in that the starter system comprises a detection board (5) comprising means for transforming said analogue signal (4) into a binary signal (6) that is representative of the changes in sign of said analogue signal, and comprising means for transmitting said binary signal to the control circuit (7), wherein the transmission means are first converting the binary signal into an optical signal (6) which is transmitted to the control circuit (7) via optical fibre.

2. Starter system according to claim 1, characterized in that the detection board (5) also comprises means for filtering the analogue signal (4).

3. Starter system according to claim 1, characterized in that the control circuit converts the received optical signal back to an electrical signal.

4. Starter system according to claim 1, characterized in that the electrical switch (10) comprises two thyristors (Ta, Tb) connected back to back and in parallel with a damping circuit comprising a resistor (R) in series with a capacitor (C).

5. Starter system according to claim 1, characterized in that the sensor is a Rogowski torus (3) surrounding the phase of the motor (M).

6. Starter system according to claim 1, characterized in that the detection board (5) is placed in the immediate proximity of the sensor (3), i.e. at a distance of less than 50 cm.

7. Starter system according to claim 1, in which the motor (M) is a three-phase motor, characterized in that the starter system comprises three electronic switches (10) that are controlled by the control circuit (7) in order to control each phase of the motor (M).

8. Starter system according to claim 7, characterized in that it comprises three sensors (3) surrounding each phase of the motor (M) and delivering an analogue signal (4) for each phase.

9. Starter system according to claim 8, characterized in that it comprises three detection boards. (5), each detection board being connected to a sensor (3) and transmitting three binary signals (6) to the control circuit (7).

10. Starter system according to claim 8, characterized in that it comprises a single shared detection board (5), the shared detection board being connected to the three sensors (3) and transmitting a single shared binary signal (6) to the control circuit (7), the shared binary signal (6) being representative of the changes in sign of the three analogue signals (4).

11. Starter system according to claim 7, characterized in that it comprises a single shared sensor (3) surrounding the set of phases of the motor (M) and a single detection board (5), which board is connected to the shared sensor (3) and transmits a single binary signal (6) to the control circuit (7).

Description

BRIEF DESCRIPTION OF THE FIGURES

[0020] Other features and advantages will appear in the following detailed description given in conjunction with the appended drawings in which:

[0021] FIG. 1 shows a first embodiment of a starter system according to the invention;

[0022] FIGS. 2 and 3 show two other embodiments, respectively;

[0023] FIG. 4 shows, in detail, an electronic switch intended to control one phase of an electric motor.

DETAILED DESCRIPTION OF AT LEAST ONE EMBODIMENT

[0024] Referring to FIG. 4, an electronic switch 10 is connected to one phase of an AC electrical network 1 and comprises two thyristors Ta, Tb connected back to back in parallel, as well as an RC damping circuit (snubber) comprising a resistor R in series with a capacitor C. The damping circuit is connected in parallel with the thyristors Ta, Tb. On each half-cycle of the AC voltage of the network 1, the trigger of the thyristors Ta, Tb, respectively, may be controlled by close orders 9a, 9b, respectively. When neither of the thyristors Ta, Tb is conducting, a transient current may flow through the switch 10 via the RC damping circuit.

[0025] FIG. 1 describes a soft starter system for a three-phase electric motor M supplied by a three-phase AC electrical network 1. The starter system comprises an electronic switch 10 connected to each phase of the electrical network between the power source of the network and the motor M. The starter system controls the three electronic switches 10 so as to gradually vary the voltage across the terminals of the motor M.

[0026] The starter system comprises a control circuit 7 which is in particular responsible for controlling the electronic switches 10 by transmitting close orders 9a, 9b to the thyristors Ta, Tb according to the control instructions. The control circuit 7 comprises elements (microprocessor, DSP, FPGA, etc.) for controlling the starter system and may equally be composed of one or more separate modules (not shown in the figures). It may, for example, comprise an isolated interface module in order to guarantee the safety of hardware and persons potentially having to interact with a control module.

[0027] In order to limit the impact of EMC interference, in particular in high-voltage applications, the starter system also comprises electronic boards 8 that are connected between the control circuit 7 and the electronic switches 10, and placed in the immediate geographical proximity of the electronic switches 10, in order to transmit the orders 9a, 9b, which orders may then be transmitted by optical fibre.

[0028] To optimally determine the instants at which the thyristors Ta, Tb of an electronic switch 10 close, the starter system comprises a sensor that measures the derivative of the current flowing across the terminals of this switch. As explained above, this measurement of the derivative of the current flowing through the electronic switch 10 may advantageously be used to synchronize control of the thyristors of this switch.

[0029] Preferably, the sensor 3 is a Rogowski torus surrounding one phase of the motor M, between the switch 10 and the motor M, and which delivers an analogue signal 4 that is representative of the derivative of a current flowing through the phase of the motor. In the embodiment of FIG. 1, the starter system comprises three Rogowski tori 3, each torus surrounding one phase of the three-phase motor M.

[0030] According to the invention, the starter system also comprises an electronic detection board 5 that receives the analogue signal 4 originating from the sensor 3 and which is intended to produce a binary signal 6 that is representative of the changes in sign of the analogue signal 4, then to transmit this binary signal 6 to the control circuit 7, so as to optimize the instants at which the thyristors of the electronic switch 10 are closed. The detection board 5 carries out the following functions: [0031] a first function, referred to as sensor instrumentation. This function reads the analogue signal 4 coming from the sensor 3 by means of an appropriate load and a buffer then carries out impedance matching for connecting to the subsequent functions. This function also electrically protects the detection board 5; [0032] a second function, referred to as filtering. This function selects the frequency band in which the useful analogue signal 4 is located. The passband of this function is wide enough to allow the detection of a damped oscillating transient signal with a frequency that is quite high with respect to the network frequency, i.e. of the order of 1 to 5 kHz; [0033] a third function, referred to as transformation. This function detects the changes in sign of the analogue signal received from the second function in order to create a binary signal that is representative of these various changes in sign. The detection board 5 therefore comprises means for transforming the received analogue signal into a rectangular binary signal (also referred to as a digital signal or on-off signal), whose edges coincide with the zero-crossing instants of the analogue signal. Thus, on each sign change of the analogue signal, the binary signal changes value, i.e. switches from value 0 to 1 or from value 1 to 0; [0034] a fourth function, referred to as transmission. The detection board 5 comprises transmission means that transmit the binary signal 6 produced by the transformation means to the control circuit 7.

[0035] A first advantage of this solution is the decentralization, in the detection board 5, of part of the processing to be carried out, namely the processing of the analogue signal 4. This above all reduces the processing to be carried out in the control circuit 7, which thus directly receives a pre-processed binary signal 6.

[0036] Additionally, in the case of a motor M supplied by a high-voltage network, the sensors 3 are installed around the cables of the network 1, hence in a high-voltage environment. They generate an analogue signal that is theoretically destined for a control circuit 7 which is in a low-voltage environment. There is therefore a risk of (EMC) interference occurring due to high-frequency overvoltages (generating interfering electric fields) present in this environment and due to currents comprising high harmonic content (5, 7, etc.) and high frequencies generating magnetic fields when the thyristors switch.

[0037] It is therefore necessary to limit the impact of this EMC interference on the operation of the detection system, of the control circuit 7 and of the means for transmitting the zero crossings of the current.

[0038] For this reason, the detection board 5 is physically separate from the control circuit 7 and is placed in the immediate proximity of the sensor 3, e.g. at a distance of less than 50 cm. The transmission means of the detection board 5 additionally form an isolated interface by virtue of an electrical-optical converter that converts the electrical binary signal to an optical binary signal before its transmission. This optical binary signal 6 is subsequently sent via optical fibre to the control circuit 7, which circuit in this case of course comprises an optical-electrical converter in order to recover a usable electrical binary signal.

[0039] This electrical-optical conversion of the binary signal thus guarantees the galvanic isolation of, on the one hand, the control circuit 7 and, on the other hand, the detection board 5 connected to the sensor 3. This is particularly recommended in the context of a starter system intended to control a high-voltage motor.

[0040] Advantageously, the proposed solution does not require analogue/digital conversion followed by optical transmission of a digitized analogue quantity to be carried out, which would introduce a delay that would be unacceptable for the performance required in terms of measurement precision and lag. It is effectively much simpler to transmit a binary signal, rather than a digitized analogue signal, via optical fibre. Moreover, the exchanges of information between the control circuit 7 and the detection board 5 do not require the implementation of a link using a communication protocol, which would also slow down these exchanges.

[0041] It would also be possible to envisage other types of isolation, such as conversion of the electrical binary signal to a radio signal. However, electrical-optical conversion allows a very fast real transmission time and introduces only a very short delay (of the order of 10 s for electrical-optical followed by optical-electrical conversion), which is not detrimental to the precise determination of the instants at which the thyristors close.

[0042] In the embodiment of FIG. 1, the starter system thus comprises three detection boards 5. Each detection board 5 corresponds to one phase of the motor M and is placed in the immediate proximity of the corresponding sensor 3 which delivers an analogue signal 4 thereto.

[0043] According to one variant presented in FIG. 2, the starter system comprises only a single shared electronic detection board 5 which is connected to the three sensors 3 of the three phases of the electrical network 1. In this case, a single shared binary signal 6 is created and sent to the control circuit 7, this shared binary signal 6 being representative of the changes in sign of the set of three analogue signals 4 originating from the three sensors 3, which is possible as the changes in sign do not occur simultaneously.

[0044] This embodiment is simpler, as it uses only a single detection board 5 and only a single optical binary signal 6, hence only one electrical-optical converter in the detection board 5 and only one optical-electrical converter in the control circuit 7. Having only a single detection board 5 also makes it possible to have to provide only one isolated power supply to supply power to this board 5.

[0045] However, this embodiment requires additional processing of the signal in the control circuit 7 in order to determine to which phase of the network a given sign change transmitted by the binary signal 6 corresponds. This processing of the signal may, for example, be done by taking into account the information on the various currents flowing through each phase, known from elsewhere.

[0046] According to another variant presented in FIG. 3, the starting system comprises only one shared Rogowski torus 3, connected between the switches 10 and the motor M and surrounding the set of three phases of the motor M, as well as a single detection board 5. The shared Rogowski torus 3 therefore delivers a single analogue signal 4 which is representative of the derivative of the currents flowing through the set of phases of the motor M. The detection board 5 receives the analogue signal 4 and transmits a single binary signal to the control circuit 7. This solution further simplifies implementation and is more economical as it comprises only one measuring torus and only a single detection board with a single isolated power supply.