A METHOD AND AN APPARATUS FOR FILTERING AN ELECTRIC CURRENT SUPPLIED TO A MOTOR

Abstract: The present invention relates to a method and an apparatus for filtering an electric current supplied to a motor (M). A current supplied from a frequency transformer means (4) provided with a voltage source circuit to the motor (M) is conducted through a coil means (L) between the frequency transformer means (4) and the motor (M) for preventing any essential rapid increases of the current. The reactive current portion of the motor (M) is compensated by a condenser means (C) arranged between the frequency transformer means (4) provided with the voltage source circuit and the motor (M). By means of the inventive filter the reactive current of the motor (M) is essentially compensated and the power ratio thereof is increased.

Description: A METHOD AND AN APPARATUS FOR FILTERING AN ELECTRIC CURRENT SUPPLIED TO A MOTOR The present invention relates to a method according to the preamble of claim 1 for filtering an electric current, and more precisely, to a method for filtering electric current supplied into an electric motor, such as to a motor provided with a solid rotor. The present invention relates further to an apparatus accor¬ ding to the preamble of claim 4 for filtering the electric current, especially an electric current supplied to a motor. It is prior known to filter one or three phase supply current by so called CLC filter (coil/condenser filter) which is provided between the phase and zero for the purposes of disturbance elimination. An example of such arrangement is disclosed by EP publication No. 0 282 797 A3, figure 1 thereof illustrating the circuit diagram of the known filter. By this kind of solutions an advantage is achieved especially in the disturbance elimination of so called power electro¬ nics (EMC), ie. in protecting the environment from the radiating disturbances caused by the apparatus and/or vice versa. Frequency transformers are also known from the prior art. These are used for instance as supply devices for the AC (alternating current) motors, these being especially advantageous in adjustable electric drives. At present PWM (Pulse Width Modulation) frequency transformers based on the pulse width modula¬ tion technique provided with so called voltage source circuit are the most commonly used. The skilled person is familiar with the frequency transformers of different types, and they are not explained in more detail herein than by mentioning the main parts thereof, which consist of a rectifier, of a direct volta¬ ge source circuit and of an inverted rectifier ( = the inverter part). The known filters, such as the ones according to figure 1 of said EP applica¬ tion, cannot, however, be applied for the filtering of the current conducted between the PWM frequency transformer and the motor, eg. on grounds of the reason that the current I of the phase would then become momentarily too high in the frequently occurring rapid voltage changes caused by the transver¬ se capacitance in a three phase drive. This kind of current peaks would cause a risk for damages for the used apparatus, such as for the switches and insula¬ tions thereof. It is not possible to compensate a machine provided with a fre¬ quency transformer by a condenser only, as in accordance with the formula I = c x du/dt, wherein I = current c = capacitance du/dt = change of voltage/time an extremely high current peak would become generated to the output thereof as a result of the high voltage pulse from the frequency transformer, which would then be destructive to the apparatus. The frequency transformer drive causes losses to the short circuit motor main¬ ly by the following three manners: 1 ) the high frequency current components cause a pinch effect of the current in the winding of the machine. Especially, in case the winding of the machine has been produced from formed copper, the disturbances caused by the pinch effect of the current will rise. 2) The high frequency current components cause, despite the attenuating effect of the distributed inductance, also high frequency components to the main flux of the machine, which will increase the iron losses of the machine. 3) The rotor surface of the machine is especially liable for interference in the frequency transformer use. When the amplitude of the main flux of the machine oscillates due to the harmonics of the current, the eddy currents induced into a squirrel cage of the rotor tend to attenuate this oscillation, whereby losses are gene¬ rated. There is a need, especially in connection with so called solid rotor motors, to filter the current supplied therein. As a distinction to the common machines provided with laminated rotors the solid rotor machine comprises a solid rotor which is usually manufactured from a suitable steel material. In some instances suitable additional parts are attached to the solid rotor body of steel, such as different coatings or so called copper cage and/or short circuit rings. By means of the solid rotor advantages are obtainable eg. in view of the stiffness, good vibrations attenuation characteristics and an easy and economical manufacture of the rotor. Different solid rotors are disclosed eg. by GB patents Nos. 1 427 81 8 and 1 429 826, DE patent No. 36 41 1 42 or FI patent No. 921 1 4 and FI patent application No. 932732. The disadvantage of the solid rotor solutions has, however, been the poor effi¬ ciency and power ratio thereof. Due to a higher reactive power the current required by them is essentially higher than what it is in machines provided with laminated rotors. When trying to improve the characteristics thereof eg. by supplying current by means of the above described frequency transformer means some further problems have arisen, for instance in that sufficiently big/effective or suitable frequency transformer means are not available to all applications. In addition, the price of the frequency transformer means increa¬ ses greatly as the power thereof increases, which makes the utilization thereof uneconomical. The electric current supplied by the frequency transformer means has also a form which is essentially remote from the sinusoidal shape considered as ideal. In addition, the pulse shaped voltage provided by them causes high voltage stresses to the components of the equipment, such as to the isolations and the switches. An object of the present invention is to overcome the disadvantages of the prior art and to provide a new solution for filtering the current supplied to an electric machine, especially an electric motor provided with a solid rotor, in connection with voltage source circuit solutions so that it is possible to com¬ pensate the reactive current portion from the essential parts thereof. It is also an object to provide a filtering solution by means of which the efficien¬ cy of an electric motor drive is improved. It is a further object of the invention to provide a solution, by means of which it is possible to reduce the size of an electric current supply device, such as a frequency transformer provided with a voltage source circuit, and/or to increa¬ se the efficient and economical range of operation thereof. An additional object of the invention is to provide a solution, by means of which the voltage of the motor is brought sufficiently close to sinusoidal. An additional object of the invention is a method and an apparatus, by means of which the coupling frequency of a frequency transformer means can be lowered . The invention is based on the basic idea that by adapting a suitable filter between the frequency transformer means and the motor means, especially an electric motor provided with a solid rotor, a current supply arrangement is provided which fulfills the objects set for the invention. More precisely, the method according to the present invention is mainly charac¬ terized by what is disclosed in appended claims 1 ...3 and especially by the characterizing portion of claim 1 . The apparatus according to the present inven¬ tion is mainly characterized by what is disclosed in appended claims 4...9 and especially by the characterizing portion of claim 4. According to a preferred embodiment of the present invention the apparatus comprises LC filtering means arranged between frequency transformer means provided with a voltage source circuit and a three phase solid rotor motor, said filtering means being adapted symmetrically in all three phases. In the filter a coil is arranged before a condenser in each phase. The condenser compensa¬ tes in general the reactive current, the coil preventing in general the formation of disadvantageous current peaks. Significant advantages are obtained by means of the present invention. By means of the solution according to the invention a filtering arrangement is provided which is easy and economical to implement, and by means of which it is possible to reduce the size of the required current supplying apparatus, such as frequency transformers, and it is possible to increase the current range of the motor within which it is still possible to use frequency transformers. The above is enabled because the current, which the machine takes from the fre¬ quency transformer means, is essentially decreased and because the size of the frequency transformer means is especially defined by the current of the inverter part. By means of the invention it is possible to get close to the ideal sinusoidal supply of electric also in the frequency transformer supply provided with a voltage source circuit. This provides, in addition to those benefits well known in the art, also that advantage that the use of a voltage and current which are as sinusoidal as possible reduces significantly the bearing currents of the bearings being in contact, and thus increases the life time of the bea¬ rings. This provides a special advantage for the medium speed machines, since these operate usually in such rotational speed ranges in which ball bearings are used. The arrangement also enables a situation in which the current from the frequency transformer is almost exclusively active current. In the conventional solid rotor machines one problem has been, as was described above, the poor efficiency, as the active and reactive power are about the same, the present solution being advantageous in avoiding this disadvantage. An additional advan¬ tage is obtained in that the isolations of the used apparatus will become more durable as the voltage peaks are removed, whereby it is possible to lower the requirements set for them and thus the price thereof. Furthermore, the phase voltage in the frequency transformer supply can be increased without any need for essentially strengthening the isolations, especially when big medium speed machines are concerned. The EMC disturbances will also decrease essentially. In addition, it is also possible to lower the connection frequency, eg. from 1 2 kHz to 3 kHz, by a suitable design of the inventive apparatus, which further improves the obtainable overall efficiency of the drive. Furthermore, the eddy currents are decreased both in the stator laminations and especially on the surface of the rotor. In addition, the noise level caused by the electric pheno- menons of the drive is lowered significantly. It is possible to minimize the cur¬ rent peaks passing through the ground capacitance of the possibly long motor cable by disposing the LC filter in the inverter output. The invention enables also a compensation of the voltage source circuit inverter, which has been previously considered as impossible. In the following the present invention and the other objects and advantages thereof will be described in exemplifying manner with reference to the annexed drawing, in which similar reference characters throughout the various figures refer to similar features. It should be understood that the following exempli¬ fying description of the invention is not meant to restrict the invention to the specific forms presented in this connection but rather the present invention is meant to cover all modifications, similarities and alternatives which are inclu¬ ded in the spirit and scope of the present invention, as defined by the appen¬ ded claims. Figures 1 a and 1 b disclose alternatives for circuit diagrams for one phase according to the present invention. Figure 2 discloses a symmetrical circuit diagram provided for a three phase arrangement in accordance with one embodiment of the invention. Figures 3a and 3b disclose as graphical views the ratio of the reactive and active powers for a laminated rotor machine and respectively a solid rotor machine when using a current supply according to the prior art. Figure 4 discloses a corresponding diagram to that of figure 3 but now using the arrangement according to figure 2, ie. according to the invention. Figure 5 discloses the shape of the motor current obtainable by the prior art. Figure 1 a discloses a schematic example of a connection arrangement provided in accordance with the present invention. It discloses a LC filtering arrange¬ ment 1 0, designated by the dashed lines, which is connected between a frequ¬ ency transformer means 4 and a motor means 4, and comprising a coil L and a condenser C. In the LC filter 10 it is a general task of the inductance to protect the frequency transformer part 4, especially the switches thereof, from too rapidly occurring current rise and a general task of the condenser is to compen¬ sate the reactive current portion of the solid rotor machine. The compensating and the voltage filtering are accomplished by means of the condenser C dispo¬ sed between two different phases. They can be arranged in a delta (figure 2) or they can also be arranged in a star connection. Figure 1 b discloses a per se corresponding arrangement to figure 1 a, but in this the order of the filters has been increased by one. In most cases this pro¬ vides a better result in view of the filtering efficiency than what is obtainable by the single filter 1 0 according to figure 1 a. It is to be noted that it is not necessary for the coils Li and L2 and the condensers C -\ and C2 to be similar from the nominal values thereof. According to one preferred solution eg. the coil L-j is greater than the coil L2- In addition, the order of the filters may, naturally, be greater than two, and the components of each of the orders of the filters may be selected on basis of different cπteπons. Figure 2 discloses a voltage source inverter coupling according to the present invention, which is accomplished symmetrically to all three phases 1 , 2 and 3. The coil means L is disposed in each phase after the frequency transformer means 4, whereafter condenser means C are disposed such that an equal capacitance is provided between two phases ( 1 ,2; 1 ,3; 2,3) . A star coupling is also possible, as well as a delta coupling of the motor M. It is also possible to use such amount of phases which differs from three, such as an arrange¬ ment having two, five or seven phases. Figure 3a discloses a phasor diagram of the base waves of current and voltage for a motor provided with a laminated rotor, le for a "plate rotor" motor in an arrangement which uses prior art frequency transformer type current supply In addition, the relation between active and reactive power (P, Q) for the same solution is also disclosed. The efficiency coefficient for this kind of arrange- ment is usually about 0.8 ... 0.9. Figure 3b discloses a corresponding prior art situation for solid rotor motor, wherein indicator a) corresponds to a situation in which an uniform, from one piece manufactured Fe rotor is used and b) a situation in which the Fe rotor is provided with a prior art copper squirrel cage. As can be seen from the relation between the active and the reactive powers (P, Q), the reactive current forms an essential portion of the total current, and thus the efficiency coefficient of the solid rotor machine obtainable by the frequency transformer supply is generally only about 0.5 ... 0.7. The above phenomenon is caused by the fact that in the solid rotor it is not possible to separate the magnetic and electric circuits from each other, as is the intention in connection with the laminated induction rotor provided with an iron core. As a result of this, the generally known Maxwell equations give, with no options, such a result in which the phase angle of the rotor impedance is too high. In a theoretical situation, when using a linear nonmagnetically saturated material, a 45° phase angle is obtained. It is possible to lower the rotor impedance phase angle in case of a saturating iron by different grooves and/or copper cages. However, as the effect of the air gap and the distributed inductance has to be taken under consideration, the result is that the efficien¬ cy coefficient of a solid rotor may in maximum be only about 0.7, as was discussed above. The view of figure 4 discloses a corresponding base wave phasor diagram of the current and the voltage and also for the ratio of the active and the reactive power (P, Q) for the solution of the present invention disclosed by figure 2. In this the relative proportion of the reactive current is substantially smaller than the same for the active current, which results, naturally, to an improved effi¬ ciency. Depending on the used currents, powers, frequency etc. parameters, it is possible, by means of the present inventive arrangement, to significantly rise the efficiency of the solid rotor machine up to a value 0.9, and in most cases, depending on the used parameters, also over that. By means of the filtering according to the present invention it is thus possible to compensate the reactive power Q essentially in its entirety. Since the compensating has not been possible in the prior art solutions, it has been necessary to dimension the frequency transformers provided with a voltage source circuit to take this essentially great reactive current into account, which has lead to an undesired and per se useless increase of the size thereof. When operating according to the invention, it is possible, however, to reduce the size of the frequency transformers and the inverters thereof of about 30 %, and by means of a precise dimensioning and optimization even more, which has a significant effect to, for instance, the price thereof. In addition, it is now possible to use a same frequency transformer to a wider current/power range, which reduces the need of having several frequency transformers of different sizes, and makes it possible to use the already commercially available frequen¬ cy transformers in connection with motors having greater power than what has been possible this far. The current of the most powerful and expensive frequency transformers having a voltage less than 1 000 V has been about 1 kA, but generally the greatest economically wise frequency transformers have been in a size class of 500 - 700 A. Several frequency transformers have been occasionally combined for the greater current needs, but this has not been wise in economical sense. By suitable dimensioning it is also possible to lower the coupling frequency of the frequency transformer. The 1 2 kHz frequency generally used at present causes a loss of about 5 - 10 % in the motor efficiency. If the frequency has been lowered from this, it has caused different vibrations in the rotor current, especially when greater machines are concerned, which has further decreased the efficiency, even though the decrease in the frequency per se would be desirable so as to improve the efficiency of the frequency transformer. The vibrations are caused by the fact that the voltage given by the frequency trans¬ former is far away from the ideal sinusoidal curve, this difference increasing as the frequency decreases. As the transient inductance is typically low, the cur¬ rent ripple increases heavily. Figure 5 discloses one result for the current shape of a 1 2 kW solid rotor machine by 1 2 kHz coupling frequency obtained by the prior art, ie. without the filtering according to the present invention. It can be readily noted that the current differs substantially much form sinusoidal. This is caused by the fact that already at such solid rotor machines which are rotating at relatively low speeds it is necessary to use substantially small number of winding laps, whereby all inductances of the machine, and especially the transient inductan¬ ce thereof, are formed to be essentially low, as was described above, whereby a significant current ripple is generated into the machine. A part of the current ripple causes fluctuation of the main flux of the machine, which has an essen¬ tial, of order of about 5 %, deteriorating effect to the efficiency of the machi¬ ne. Thus the sinusoidal current gives a significant advantage when solid rotor machines are concerned. In the prior art an as sinusoidal current as possible has been tried to be obtained by increasing the coupling frequency. The inventive filters 10 transform the current essentially close to the sinusoidal considered as advantageous, whereby the discontinuities shown by figure 5 are smoothened and they essentially disappear. In addition, the inventive filters filter the current to that extend that it is possible to lower the frequency in the frequency transformer drives eg. to a value of 3 kHz without the above mentio¬ ned disadvantageous effects and thus the efficiency of the machine M can be further increased. It is to be noted that the above examples of the embodiments of the invention are not limiting the scope of the invention as defined by the claims. By means of the above description and the drawings it is apparent for the skilled person to, for instance, use the inventive method and apparatus in connection with all kinds of electric motors without departing from the spirit and scope of the invention.

Claims: Claims 1. A method for filtering an electric current supplied to a motor (M), c h a ¬ ra cte r i z e d in that it comprises steps of: conducting a current supplied from a frequency transformer means (4) provi¬ ded with a voltage source circuit to the motor (M) through a coil means (L) between the frequency transformer means (4) and the motor (M) for preven¬ ting any essential rapid increases of the current, compensating the reactive current portion of the motor (M) by a condenser means (C) between the frequency transformer means (4) provided with the voltage source circuit and the motor (M), whereby the reactive current of the motor (M) is essentially compensated and the power ratio thereof is increased. 2. A method according to claim 1, c h a ra c te ri z ed in that the compen¬ sation of the reactive current portion and a filtering of voltage is accomplished by connecting the voltages between phases (1,2,3) by means of the conden¬ ser means (C). 3. A method according to claim 1 or 2, c h a r a c te r i z e d in that the current supplied by the frequency transformer (4) to a multi phase, such as two, three, five or seven phase, solid rotor motor (M) is filtered symmetrically by a filter (10) provided in all phases (1,2,3). 4. Filtering means (10) for an electric current supplied to a electric motor (M), c h a r a c te r i z e d by comprising coil means (L; L1 2) arranged in each phase (1,2,3) between the motor and a frequency transformer means (4) provided with a voltage source circuit, and condenser means (C; C-|,C2> arranged between two different phases (1,2; 1,3; 2,3) after the coil means (8) in the direction of current flow, wherein the filte¬ ring means (10) thus provided are adapted to prevent any essentially rapid increases of the current and to compensate the reactive current portion from the electric current. 5. Filtering means according to claim 4, c h a ra c te r i z e d in that the motor is a three phase solid rotor induction motor (M) and that the current source means comprise frequency transformer means (4), wherein the filtering means (10) are connected symmetrically relative to all three phases (1,2,3). 6. Filtering means according to claim 4, c h a ra c te r i z e d in that the motor is a two, five or seven phase solid rotor induction motor (M) and that the current source means comprise frequency transformer means (4), wherein the filtering means (10) are connected symmetrically relative to all phases. 7. Filtering means according to any of claims 4, 5 or 6 c h a ra cte ri z e d in that several filtering means (10) are arranged successively between the frequency transformer means (4) and the motor (M). 8. An apparatus according to claim 7, c h a ra c te r iz e d in that the va¬ lues of the corresponding components (L, C) of said successively arranged fil¬ tering means (10) are inequal. 9. An apparatus according to any of claims 4...8, c h a r a c te r i z e d in that the frequency transformer means (4) comprise a PMW frequency trans¬ former or similar.

Inventors: PYRHOENEN JUHA, PYRHOENEN, JUHA
Applicants: ALAMAEKI JARMO, PYRHOENEN JUHA, ALAMAEKI, JARMO, PYRHOENEN, JUHA
Application year: 1997
Main classification: H02M1
IPC codes: H02J3, H02M1