[0002] An inverter is a device typically used for controlling motors in an economical and reliable manner. At its output, such an inverter generates a voltage or a current with a controllable frequency, enabling a motor to be con- trolled in an optimal manner in accordance with a desired frequency of the out- put.

[0003] A frequency converter equipped with a voltage intermediate circuit, one being shown in Figure 7 for the sake of example, generates at its output by means of an inversion part 73 voltage pulses which are short with respect to the basic frequency and whose duration is adjusted to achieve a desired output voltage. Such a frequency converter with a voltage intermediate circuit is provided with a direct voltage intermediate circuit Udc whose positive and negative voltages are coupled as voltage pulses to phased outputs as de- sired. One phase is thus provided with an upper 71 and a lower 72 semicon- ductor switch, and the output voltage of the phase is obtained from between the switches. Figure 7 further shows a motor M controlled by a frequency con- verter, and an input transformer 74.

[0004] These short and fast voltage pulses are generated by power semiconductors resistant to high currents and voltages. Nowadays IGB (insu- lated gate bipolar) transistors are commonly used for their good properties. An IGBT is capable of cutting off a current of even hundreds of amperes, the maximum voltage of a component being thousands of volts, correspondingly.

[0005] In high-current inverters, however, IGBT components have to be coupled in parallel to achieve the necessary current strength. IGBT compo- nents are often packed into modules such that one module comprises several IGBT switches and their zero diodes. A natural alternative of parallel coupling the switches of the output of an inverter is then to couple some switches of one module in parallel. Applications that require parallel coupling of several com- ponents are typically three-phase ones. Each phase then comprises compo-

nents coupled in parallel, and these parts of parallel coupling are generally called branches. One phase of an output of an inverter is then composed of a number of branches corresponding to the number of upper and lower switch components coupled in parallel.

[0006] In voltage intermediate circuit frequency converters, an out- put voltage is generated in an inversion part from voltage pulses which, when using existing switch components, have extremely high rates of rise. In a cable between an inverter and a rotatable machine, high rates of voltage rise cause voltage oscillations which strain the insulating materials of the machine. In ad- dition to the rate of voltage rise, the amplitude and frequency of voltage oscilla- tions are determined by the length of such a cable and wave impedance, the wave impedance of the machine as well as other electrical interfaces between the inverter and the machine. The minimum length of a cable to enable maxi- mum reflection to take place in a terminal of a machine is called the critical length of the cable.

[0007] A choke at an output of an inverter enables the above- disclosed critical length of a cable to be increased. The desired rate of voltage rise determines the magnitude of the inductance of the choke. The magnitude of the necessary inductance can be reduced by an RC circuit, as shown in Figure 2. However, using an RC circuit increases losses of drive and may hin- der the operation of the control of some inverters.

[0008] Conventionally, the rate of voltage rise has thus been re- stricted by chokes installed at an output of an inverter, which are either single- or three-phase ones. Figure 1 shows the basic structure of both a single-phase and a three-phase choke. A choke comprises a winding 1 wound around a core 2. The core is usually rectangular in shape and comprises at least one air gap to prevent saturation. In some cases, different voltage cutters are also used for reducing maximum voltages, e. g. as shown in Figure 3. Here, a diode bridge 31 is coupled to an output of an inverter to cut the voltage of the output when it becomes higher than a voltage Udc of an intermediate circuit.

[0009] A high rate of voltage rise also increases a circulating current passing via bearings. It can be roughly said that the circulating bearing current is proportional to the common-mode voltage of a motor and to the resulting current passing through distributed capacitances. This current may be reduced by reducing the rate of voltage rise of the output voltage of an inverter. In addi- tion to an output choke L of an inverter, a special bearing current filter, one

being shown in Figure 8, comprises a common-mode choke Lcomm e. g. in a direct voltage intermediate circuit or at the output of the inverter as well as a capacitance CE installed between a star point of an RC circuit and a ground plane.

[0010] Irrespective of whether or not an inverter is equipped with IGBTs in parallel, the purpose of an output choke is to reduce the rate of volt- age rise and the amplitude to such a low level that voltage oscillations are in- capable of damaging the insulating materials of a rotatable machine and that the current passing via a bearing does not cause any damage to the bearing.

[0011] In the known solutions, in conjunction with IGB transistors coupled in parallel, the outputs of the IGB transistors are directly coupled in parallel to form a phase output, a choke being connected to the common cur- rent path of this parallel coupling to restrict the rate of voltage rise. In order to prevent saturation of the core of the choke and, on the other hand, to obtain the necessary inductance, the cross-sectional area of the core has to be suffi- ciently large. Since the current allowed by the components coupled parallel is also high, the necessary choke thus ends up being very large and heavy.

[0012] A problem with the direct parallel coupling of IGBT switches is presented by differences between the currents of parallel coupled branches that occur in normal switching situations. The current differences between branches are caused by differences between the mutual conductances and gate capacitances of IGBT switches as well as factors due to a gate driver, such as time diversity of gate control signals and differences between gate voltages and gate resistances. Due to the non-ideality of switch components coupled in parallel and the circuits controlling such components, an output cur- rent of a phase is distributed unevenly between the components during switch- ing situations.

[0013] For instance, when switches are being switched off and when one switch starts cutting off the current before the rest of the switches in the same phase, some of the current transfers to other switch components of the particular phase. It has been noted that this increase of current with re- spect to the dimensioned switch-specific phase current is anything up to doz- ens of percents. This potential increase of current should be taken into account when designing an inverter. The differences between branch currents reduce the maximum capacity of the inverter because the output current has to be re- stricted to a level which does not allow the temporary current strength of switch

components to be exceeded, not even in the branch with the highest current.

The switches thus cannot be dimensioned on the basis of a phase current only, since such dimensioning could result in a damaged switch component, caused by currents transferring in switching situations.

[0014] Another problem in connection with the existing, rigidly paral- lel coupled branches is that an erroneously operating single-branch power switch, such as an IGBT switch, cannot be identified.

BRIEF DESCRIPTION OF THE INVENTION [0015] An object of the present invention is to provide a choke ar- rangement for an output of an inverter, which avoids the above-mentioned drawbacks and enables chokes to be dimensioned smaller and power semi- conductors of an output of an inverter to be utilized better than before despite the non-idealities thereof. This object is achieved by an arrangement of the invention, which is characterized by what has been disclosed in the character- izing part of independent claim 1.

[0016] The arrangement of the invention is based on the idea that single-phase chokes are used in conjunction with output branches coupled in parallel such that the actual phase output is formed by second ends of the chokes coupled in parallel. Such an arrangement enables the branches cou- pled in parallel and their power semiconductors in particular not to be tightly coupled together in parallel, which means that the power semiconductors op- erate considerably more evenly in parallel. In addition, the output choke ar- rangement enables the advantages achieved even by normal output chokes to be achieved.

[0017] The invention also relates to a method according to claim 5, which is characterized by what has been disclosed in the characterizing part of the independent claim. The method of the invention is based on utilizing the arrangement of the invention, the method enabling a certain previously unde- tectable fault situation of switches to be detected. The method of the invention based on using the arrangement enables a semiconductor switch of an output that has been erroneously conveyed to be conductive or that has erroneously remained conductive to be determined and, when necessary, the control of a device to be ended to prevent even more serious potential damage from incur- ring to the inverter itself or the device controlled therewith.

BRIEF DESCRIPTION OF THE DRAWINGS [0018] The invention is now described in closer detail in connection with preferred embodiments and with reference to the accompanying drawings, in which [00191 Figure 1 shows conventional single-and three-phase choke structures; [0020] Figure 2 shows an LCR filter at an output of an inverter; [0021] Figure 3 shows a voltage cutter at an output of an inverter; [0022] Figure 4 shows branch-specific chokes of the invention at an output of an inverter; [0023] Figure 5 shows breakthrough in an output phase of an in- verter, wherein branches are rigidly coupled in parallel ; [0024] Figure 6 shows breakthrough in an output phase of an in- verter, comprising branch-specific chokes of the invention; [0025] Figure 7 shows a voltage intermediate circuit frequency con- verter; [0026] Figure 8 shows a bearing current filter ; [0027] Figure 9 shows an output choke unit whose structure is line- like, and [0028] Figure 10 shows an output choke unit whose chokes are ar- ranged symmetrically with respect to each other.

DETAILED DESCRIPTION OF THE INVENTION [0029] Figure 4 shows the structure of an arrangement of the inven- tion in conjunction with an inverter. In the structure of Figure 4, an output of each phase U, V, W comprises three branches, i. e. three parallel pairs of switches that are simultaneously controlled to achieve the necessary output current. Figure 4 shows how the switches of each phase are composed of modules 41,42, 43 comprising three pairs of switches, the modules being coupled to a direct voltage intermediate circuit Udc. According to the invention, the arrangement comprises branch-specific single-phase chokes LU1, LU2, LU3, LV1, LV2, LV3, LW1, LW2, LW3, first ends of coils of the chokes being configured to be coupled to outputs UU1, UU2, UU3, UV1, UV2, UV3, UW1, UW2, UW3 of power semiconductor components of the branches. As usual, these branch-specific outputs are composed of points between upper and lower power switches. The potential of these points is controlled by the power

switches, alternately to the positive and the negative potential of the voltage intermediate circuit.

[0030] According to the invention, second ends of the single-phase chokes are further configured to be coupled together, resulting in a phase out- put U, V, W of the inverter, which is further coupled to load for controlling the same.

[0031] The operation of the arrangement of the invention will be de- scribed in the following, with reference to Figures 5 and 6. Figure 5 shows a prior art direct parallel coupling of branches for increasing current capacity.

Outputs of power semiconductor components, which are preferably IGBT switches or corresponding fast components, are then directly coupled in paral- lel. The points between all upper V1, V3, V5 and lower V2, V4, V6 power semiconductors in the coupling are thus in exactly the same potential.

[0032] Since switch components and control circuits controlling the switch components always differ from each other to some extent, despite the same control, the components tend to become switched off and on at different times and at different rates. This results in a situation wherein e. g. in connec- tion with switching off a component, one of the switches tends to be switched off before another. In a switch-off situation, for instance in an IGBT switch the voltage over a component tends to rise before the current decreases. Since, however, the voltage of all branches is the same, the voltage is not allowed to rise as desired, and some current transfers from the component that first started becoming switched off to the components of other branches. Even though typical delays are no larger than of the order of nanoseconds, this causes considerable current transfers to other components.

[0033] Figure 6 shows a coupling of one output phase according to the invention, comprising three branches. Chokes are coupled to a point be- tween each pair of switches, the second ends of the chokes further being cou- pled together to form a phase output. In the solution of the invention, the above-mentioned non-idealities of the IGBTs and gate drivers cause temporary voltage differences between the output branches of IGBT switches that affect over the chokes, the currents of the branches changing with respect to each other at a rate determined by the above-mentioned voltage differences and the inductance of the chokes. Using the chokes in a manner in accordance with the invention enables the magnitude of current transferred to other switches to

be reduced considerably, enabling the switches to be dimensioned more accu- rately, taking into account load current only.

[0034] The arrangement of the invention enables the method of the invention to be implemented. This method can be used for determining a cer- tain type of failure in connection with power semiconductors coupled in paral- lel. The method of the invention will be further described with reference to Fig- ures 5 and 6.

[0035] In fault situations wherein only one of the IGBT switches V2 coupled in parallel is erroneously conveyed to be conductive or erroneously remains conductive when others V4 and V6 are switched off while power switches V1, V3 and V5 of an opposite branch have been or are being con- veyed to be normally conductive, a fault current in its entirety passes via this one switch V2. Such a situation is shown in Figure 5 in conjunction with a con- ventional direct parallel coupling of branches. In such a case, the current be- comes high enough for the IGBT V2 to be conveyed to an unsaturated state, restricting the fault current to a value according to its gate voltage. Figure 5 shows how the fault current also passes via the switches V3 and V5 and through the switch V2 to a negative rail of a direct voltage intermediate circuit.

[0036] If the output branches of the IGBT switches are coupled rig- idly in parallel, the same fault current travels in the opposite branch, with the load current IM added thereto or subtracted therefrom, via a plurality of parallel IGBTs V1, V3 and V5, in which case the current of each single IGBT remains lower, correspondingly. A dropout voltage of these IGBTs then also remains at a few volts only, i. e. it is only slightly different from the dropout voltage with a normal load current, making a fault substantially more difficult to detect.

[0037] The travel of a fault current can be affected by the arrange- ment of the invention wherein a choke is coupled to every output branch of a pair of IGBT switches and the branches are coupled in parallel only after the chokes, as shown in Figure 6. This does not substantially affect the magnitude of the fault current but it does affect the travel of current such that in the branch in which all parallel power switches V1, V3 and V5 have been conveyed to be conductive, a fault current first travels only via one power switch V1, only gradually transferring to the other switches V3 and V5 coupled in parallel at a rate determined by differences between the inductances of the chokes and the dropout voltages of the switches. In such a case, the dropout voltage of the power switch V1 via which the entire fault current travels first ends up being

substantially higher than that in a corresponding fault situation with no branch- specific chokes. The dropout voltage of the particular IGBT decreases gradu- ally as the current also transfers to the parallel IGBTs V3 and V5, but appropri- ate dimensioning of the chokes enables the dropout voltage to stay high enough to enable a fault situation to be detected based on measuring a collec- tor voltage of the IGBT. The aim of dimensioning the choke is to achieve a dropout voltage that is e. g. about 7 to 10 volts higher than the dropout voltage with a normal load current.

[0038] The method of the invention thus works such that a voltage limit and a time limit are predetermined to be used for indicating a fault situa- tion. A voltage limit is the voltage above which a dropout voltage of a compo- nent should reside in order for a fault situation to be detected in a reliable manner. A time limit is used to prevent a fault situation to be indicated errone- ously in normal switching situations when the dropout voltage of an IGBT for a short period of time resides above the above-mentioned voltage limit. Further according to the invention, control information on power semiconductor com- ponents of the branches is determined. The control information on the compo- nents is obtained e. g. directly from the processor executing the control.

[0039] Further, in accordance with the invention, a collector voltage of the power semiconductor components of the branches is determined in those power semiconductor components that have been conveyed to a con- ductive state. The collector voltage, i. e. the dropout voltage, can be measured in a normal manner, utilizing measurement circuits typically already existing in control circuits or gate drivers. According to the invention, a fault situation is detected in the power semiconductor component the collector voltage of a power semiconductor component coupled in series therewith exceeds the pre- determined voltage limit for a period of time longer than the predetermined time limit. Thus, by determining the collector voltage of controlled switches, it may be concluded whether or not a switch below or above the particular switch inadvertently remains in a conductive state or has inadvertently been conveyed to such a state.

[0040] According to a preferred embodiment of the method of the invention, the power switches are switched off after a fault has been detected, in which case a fault current is cut off. Alternatively, or in addition thereto, an alarm signal may also be produced to indicate a failure, which signal is for-

warded to an inverter, or an operator of a process used by the inverter is in- formed of such an alarm signal.

[0041] According to the arrangement of the invention, the chokes are composed of single-phase yokeless chokes. In other words, the winding of a choke is formed around a core that does not close. Figure 1 shows conven- tional single-and three-phase chokes where the magnetic core material is pro- vided with a yoke, i. e. a magnetic flux formed in the core material is to be closed along the core.

[0042] Figures 9 and 10 show choke structures suitable for use in the arrangement of the invention. Figure 9 discloses the structure of a yokeless single-phase choke, although the figure shows an entire choke unit as a trans- verse projection and as a vertical projection. The figure shows how a single choke is formed by winding a coil 92 around a columnar core 91. Connectors 94 of the choke are supplied from one elongated end of the choke and a me- tallic cover 93 is provided around the winding to prevent spreading of leakage fluxes. The choke unit shown in Figure 9 comprises a total of nine single-phase chokes. Such a number of chokes is needed for an output of a three-phase system if every phase comprises three separate branches, as in the example of Figure 4.

[0043] The mechanical structure of a choke should be one with no substantial magnetic coupling existing between the branch-specific chokes, or one with no magnetic coupling existing between different output phases such that the magnetic coupling between parallel branches of the same phase is small and symmetrical. Such an implementation is achieved e. g. by the line- like structure shown in Figure 9, wherein all single chokes are protected by a metallic cover 93.

[0044] Another alternative to avoid undesired magnetic couplings is the symmetric structure shown as a vertical projection in Figure 10. The figure discloses a core 101, a coil 102 wound around the core, and a metallic cover 103. In this solution, the chokes of parallel branches of the same output phase are coupled into a symmetrical shape. The output choke unit of Figure 10 is designed for an inverter which comprises three parallel branches in each three phases. In this solution, the magnetic coupling between the chokes of different phases is prevented by a metallic cover, i. e. the single chokes of one phase symmetrically arranged with respect to each other are located inside one me- tallic cover 103.

[0045] The use of the yokeless chokes of the invention is made possible by the fact that in conjunction with conventional chokes with yokes, a high frequency magnetic flux does not close via a yoke. Consequently, a yoke- less choke enables a sufficiently high inductance to be achieved at high fre- quencies. An important advantage of a yokeless choke is small size and light structure, as compared with the previous implementations. Consequently, manufacturing costs are also considerably lower than before.

[0046] The invention has been described above in connection with inverters comprising three parallel branches. It is obvious, however, that the invention can also be applied in such a connection wherein the number of branches is other than three. Furthermore, it is obvious to one skilled in the art that the basic idea of the invention can be implemented in many different ways.

The invention and its embodiments are thus not restricted to the above- described examples but may vary within the scope of the claims.