Abstract:
A method and an arrangement for protecting a frequency converter in a system comprising a motor and a frequency converter supplying it via a bipolar or multipolar supply connection, the arrangement comprising first connecting means adapted to couple poles of the supply connection mutually substantially into short-circuit in response to a detection of a fault in the frequency converter.

Description:
FIELD OF THE INVENTION 
   The invention relates to a frequency converter for feeding a motor, and particularly to protecting a frequency converter. 
   BACKGROUND OF THE INVENTION 
   A frequency converter is a device typically used to control a motor.  FIG. 1  shows an example of a frequency converter connection. The frequency converter  20  is typically composed of two converters, a rectifier  21  and an inverter  22 , between which is provided a direct voltage or a direct current intermediate circuit  23 . The rectifier  21  and the inverter  22  may also be located physically separately, and one rectifier may supply a plurality of inverters via a common intermediate circuit  23 . An example of a rectifier  21  is a diode bridge, which obtains its supply  40  from an alternating current source  10 , which is for instance a 50 or 60-Hz alternating current network, and an example of an inverter  22  is an inverter bridge implemented by means of transistors (e.g. IGBT, Insulated-gate Bipolar Transistor) or other semiconductors. An inverter  22  is typically used to adjust the power transferred from the intermediate circuit  23  of the frequency converter to a motor  30 . Accordingly, in the figure, the supply connection  50  between the inverter  22  and the motor  30  is for instance a three-phase alternating current connection, although the figures show the connection with one line for the sake of clarity. The control of the motor  30  with the inverter can be implemented reliably in such a manner that the motor implements accurately the desired speed or torque instruction, for example. 
   The frequency converter  20  typically comprises a protective diagnostics in case of internal malfunction of the frequency converter, which operate in such a manner that for instance when one branch of the semiconductor of one phase of the inverter  22  cannot be turned off (the branch shorts or gets erroneously continuous ignition regardless of the control), the diagnostics of the frequency converter detect the fault and stop the frequency converter. 
   The problem in the above-described arrangement is that the motor  30  is able to supply magnetization energy, i.e. what is known as reverse power via the branch of the faulty phase and the zero diodes of the other phases to the fault point, i.e. the short-circuit in the inverter  22 , whereby, at worst, the semiconductor module may explode and cause great damage in the frequency converter. For example, the IGBT semiconductor module may be destroyed in said manner even during a time of less than 10 ms depending on the properties of the motor, the type of the semiconductors in the frequency converter, the voltage of the intermediate circuit  23 , the output frequency at the fault moment and the position of the rotor of the motor  30  at the fault moment, for example. 
   BRIEF DESCRIPTION OF THE INVENTION 
   The object of the invention is thus to provide a method and an apparatus for implementing the method so as to solve or at least alleviate the above problem. The object of the invention is achieved with an arrangement and method that are characterized in what is stated in independent claims  1  and  9 . Preferred embodiments of the invention are described in the dependent claims. 
   The invention is based on connecting the poles of the supply connection between the frequency converter and the motor mutually substantially into short-circuit in response to the detection of a fault in the frequency converter, whereby the short-circuit generates a parallel route for the magnetizing current supplied by the motor. 
   An advantage of the method and system of the invention is that part of the current supplied by the motor in malfunction is distributed to the parallel route generated by the short-circuit, and, correspondingly, the current circulating via the frequency converter is decreased. Thus the current loading experienced by the frequency converter is decreased and the probability of additional damage in the frequency converter is decreased. 

   
     BRIEF DESCRIPTION OF THE FIGURES 
     In the following, the invention will be described in more detail in connection with preferred embodiments with reference to the accompanying drawing, in which 
       FIG. 1  shows a block diagram of a frequency converter connection; 
       FIG. 2  shows a block diagram of a frequency converter connection in accordance with an embodiment of the invention; and 
       FIG. 3  shows a block diagram of a frequency converter connection in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The example of a frequency converter connection shown in  FIG. 1  was described above in the general part of the present description, and will not be repeated here. It is to be noted that the use of the invention is not restricted to any given type of frequency converter  20 . Neither does the type of motor  30  have any significance to the basic idea of the invention, but it may be for instance a single-phase, three-phase or six-phase induction motor. 
     FIG. 2  shows a block diagram of a frequency converter connection in accordance with an embodiment of the invention. In accordance with the basic idea of the invention, the poles of the supply connection  51  and  52  between the frequency converter  20  and the motor  30  are connected mutually substantially into short-circuit in response to a detection of a fault in the frequency converter  20 . In  FIG. 2 , the supply connection  50  between the frequency converter  20  and the motor  30  is shown by one line for the sake of clarity. However, the supply connection  50  comprises two or more poles. If the motor  30  is for instance a single-phase motor, the supply connection  50  is bipolar comprising a phase conductor and a zero conductor. Correspondingly, if the motor  30  is a three-phase motor, the supply connection  50  is triple-pole comprising three phase conductors. However, the number of poles in the supply connection  50  or the type of motor has no significance to the basic idea of the invention. In the connection shown in  FIG. 2 , the poles of the supply connection  50  are shorted with a switch  60 , which is connected to the poles of the supply connection  50 . The switch  60  is preferably a semiconductor switch, for instance what is known as a crowbar type of thyristor switch. The use of a semiconductor switch is preferable, since it enables rapid generation of a short-circuit connection in malfunction. The switch  60  obtains a control signal  81  from a fault diagnostics unit  80  when the diagnostics unit detects a fault, such as a situation wherein one branch of the semiconductor of one phase of the inverter of the frequency converter  20  cannot be turned off (the branch shorts or obtains erroneously continuous ignition regardless of the control). The diagnostics unit  80  may be part of the frequency converter  20  or a separate element as is shown in the figure. As regards the basic idea of the invention, it is only essential that the switch  60  can be triggered in response to a fault in the frequency converter  20 . Furthermore, the triggering of the switch  60  can be restricted to take place only in certain kinds of malfunction of the frequency converter  20 . In accordance with a preferred embodiment of the invention, the switch  60  is connected to the supply connection  50  between the frequency converter  20  and the motor  30  at a point where a section  51  of the supply connection between said point and the frequency converter is longer than a section  52  of the supply connection between said point and the motor. In this context, the term supply connection should be understood to comprise the conductor connection between the output of the frequency converter  20  and the supply terminals of the motor  30  as well as the terminals of the output of the frequency converter and the motor. Accordingly, the switch  60  can be connected not only to the conductor connection between the frequency converter  20  and the motor  30 , for example, but also directly to the supply terminals of the motor  30 , for example. Connecting the switch  60  to the supply connection  50  as close to the motor  30  as possible brings forth the advantage that in malfunction a relatively larger part of the current supplied by the motor circulates via the short-circuit point generated by the switch  60  and, correspondingly, a smaller part via the frequency converter  20 , since the part of the current circulating via the frequency converter  20  has to encounter for instance the entire impedance of the conductor section  51  and  52  between the frequency converter  20  and the motor  30 . In addition, this being so, the switch  60  itself is preferably placed close to the motor  30 , whereby also the conductors between the switch  60  and the supply connection  50  remain short. Other possible impedances of the system also affect current distribution; for example, the frequency converter  20  may be provided with output chokes. Generally speaking, in order for as large a part as possible of the current supplied by the motor  30  in malfunction to pass via the short-circuit point generated by the switch  60 , it is thus preferable that the impedance from the motor  30  to the short-circuit point generated by the switch  60  be as small as possible and, correspondingly, the impedance from the motor  30  to the frequency converter  20  as large as possible. 
   According to a preferred embodiment of the invention, the supply connection  50  between the motor  30  and the frequency converter  20  is broken at least for one pole after said poles are connected into short-circuit. Furthermore, in accordance with a preferred embodiment, the supply connection between the motor and the frequency converter is broken for all poles. In accordance with this preferred embodiment of the invention,  FIG. 2  shows a second switch  70 . The switch  70  is for instance a contactor preferably controlled by means of a relay  71 . The relay  71  preferably obtains its control from the diagnostics unit  80  in malfunction of the frequency converter, as does the switch  60 . Because of the relative slowness of the operation of the relay  71  and the contactor  70  (operating time typically e.g. 10 to 100 ms) as compared with the semiconductor switch  60  (operating time e.g. about 1 ms), the contactor  70  does not operate until after the operation of the switch  60 . In a normal state, the contactor  70  is in a conductive state, i.e. its contacts are closed. Controlled by the signal coming from the diagnostics unit  80  and by the relay  71 , the contactor  70  is opened, whereby the motor  30  is separated from the frequency converter  20  and the switch  60 . Accordingly, in malfunction, the switch  60  is preferably first closed, whereby the reverse current supplied by the motor  30  is distributed in proportion to voltage losses between the short-circuit generated by the switch  60  and the frequency converter  20 , until the contactor  70  is opened. Thus, it is to be noted that in a solution according to the basic idea of the invention, there is no need at all for the contactor  70  (or a corresponding switch). If only the switch  60  generating the short-circuit is used, this switch  60  is then preferably dimensioned such that when generating the short-circuit, it tolerates all the reverse current supplied by the motor  30 . When the contactor or a corresponding switch  70  is used, the switch  60  should preferably be dimensioned to tolerate the reverse current supplied by the motor  30  only as long as it takes from the closing of the switch  60  to the opening of the switch  70 , i.e. typically some dozens of milliseconds, for example. 
     FIG. 3  shows a block diagram of a frequency converter connection in accordance to an embodiment of the invention. In the system of the figure, the motor  30  is a three-phase motor, and the supply connection between the motor  30  and the frequency converter  20  is triple-pole, i.e. in this case composed of three phase conductors, as the figure shows. The figure further shows an all-poled connection of an optional contactor  70 , if employed, to the supply connection between the motor  30  and the frequency converter  20 . The figure further shows a possible internal connection of the switch  60 . It is to be noted that the figure only shows elements essential to the understanding of the invention. In the example of  FIG. 3 , the switch  60  is a crowbar type of thyristor switch comprising an alternating current bridge  63  composed of six diodes, and a thyristor  61 . The connection operates such that the control signal  81  is supplied to a control circuit  62  in the thyristor. In response to the control signal  81 , a control circuit  62  generates a suitable control pulse that ignites the thyristor  61 . When the thyristor  61  is ignited and starts to conduct, the poles of the supply connection between the motor  30  and the frequency converter  20  short via the connection provided by the alternating current bridge  63  and the thyristor  61 . The components of the switch  60  and any contactor or other switch  70  employed are preferably selected in accordance with the parameters of the system wherein the switches are employed at each particular time, such as the nominal voltage of the system and the currents occurring in the system, for example. When a switch  60  shown in  FIG. 3  is employed, it is also preferable to select diode and thyristor types having an as low a dropout voltage as possible as the diodes of the alternating current bridge  63  and as the thyristor  61 , in order for the dropout voltages of the two diodes and the thyristor in the switch  60 , when it is operating, to generate as low a resistance as possible to the current supplied by the motor  30 . 
   It is obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in a variety of ways. Consequently, the invention and its embodiments are not restricted to the above examples, but can vary within the scope of the claims.