SE1000654A1 - An electrical drive system for operating an electrical machine - Google Patents

Abstract

Uppfinningen avser ett elektriskt drivsystem 15 för drift av en elektrisk maskin 10 som innefattar en rötor och en stator. Det elektriska drivsystemet 15 innefattar en första strömriktare 20 för elektrisk matning av en första statorlindning 21 hos den elektriska maskinen 10. Det elektriska drivsystemet 15 innefattar vidare en andra strömriktare 30 för elektrisk matning av en andra statorlindning 32 hos den elektriska maskinen 10, varvid den andra strömriktaren 30 är elektriskt åtskild från den första strömriktaren 20.

Description

10 15 20 25 drawback is that such electrical machines cannot act as generators without being connected to a load that is capable of providing a proper voltage at the stator terminals.

Figure l illustrates a prior art solution, wherein the phase windings of the electrical machine are connected to a static converter. The static converter always needs to supply the set of voltages for the generator operation as well as the reactive power required for exciting the electrical machine.

The reactive entails a requirement of supplying drawback in that a higher current rating is required of the power converter for the same amount of average, i.e. “active" power flowing throughout the overall drive. Another drawback of the illustrated solution lies in its intrinsic disrupted availability in case of complete failure of the converter.

Furthermore, some applications require that the electrical machine rotates during long periods of time at very much reduced speeds and power because, otherwise, the connected mechanical load would suffer structural damaging deformations caused by its own large weight. Common examples of such applications are represented by large gas turbines, large steam turbines, large axial-flow compressors, large high-speed. two-pole turboalternators. In. order to satisfy such requirement, the static converter operates far from its Optimum. conditions and it is essentially needed almost continuously. Although the converter is a critical component for the reliability and availability of the overall process, it becomes difficult to service and maintain it regularly.

Sumary of the invention 10 15 20 25 It is an object of the present invention to provide an electrical drive overcoming or at least alleviating the above-mentioned short-comings of prior art. an electrical drive system The The In accordance with the invention, for operating an electrical machine is provided. electrical machine comprises a rotor and a stator. electrical drive system comprises a first static converter for electrically feeding a first stator winding of the electrical machine. The electrical drive system further comprises a second static converter for electrically feeding a second stator winding of the electrical machine, wherein the second static converter is electrically separated from the first static converter. By means of the invention, it is no longer necessary that the reactive power required by the nature of the electrical machine originates from the main converter. The reactive power can, by means of the invention, be provided by the auxiliary winding, thereby opening the possibility for the main converter to supply only the active power and to operate at unitary power factor. This advantage can reduce the power rating and costs potentially leading to overall less The of the main converter, Variable-speed drive systems. auxiliary of the expensive converter provides improved behavior electrical machine and improved availability of electrical drives.

Further features and advantages thereof are defined in the dependent claims and will become clear when reading the following description.

Brief description of the drawings Figure 1 illustrates a prior art electrical machine. 10 15 20 25 Figure 2 illustrates an embodiment of the electrical machine in accordance with the present invention.

Figure 3 illustrates a generalized embodiment of the electrical machine in accordance with the present invention.

Detailed description of the invention Figure 2 illustrates an embodiment of the present invention.

An electrical machine 10, for example an electrical motor such as a synchronous reluctance motor, is connected to and is electrically fed by a first static converter, which in the 20. the following is denoted main converter The main converter 20 thus electrically supplies electrical machine 10, and may in turn for example be connected to a mains supply.

The electrical machine 10 comprises a first stator winding, in the following denoted main stator winding 21. The stator of the electrical machine 10 may for example comprise a stator core having a number of stator teeth arranged along circumference, the stator teeth being separated by The its comprises, in an 23, 24 in It is noted slots. main stator winding 21 embodiment of the invention, three phases 22, conventional manner wound on the stator teeth. that the main stator winding 21 could have another number of phases. The phases of the main stator winding 21 are connected to the main converter 20, which in turn is connected so some electrical source in order for the main converter 20 to electrically feed the phases of the main winding 21. It is noted that other stator/rotor arrangements are possible. The windings are preferably realized in 10 15 20 25 30 arrangements that lead, at least parts of them, to have fixed positions with respect to the stator.

In accordance with the invention, the electrical machine 10 is further provided with a second winding, in the following denoted an auxiliary winding 31, connected to a second converter, denoted auxiliary converter 30 in the following.

The auxiliary stator winding 31 comprises, in an embodiment of the invention, three phases 32, 33, 34 in conventional manner wound on the stator. It is noted that also the auxiliary stator winding 31 could have a number of phases other than three. The phases of the auxiliary stator winding in turn the 31 are connected to the auxiliary converter 30 connected so some electrical source in order for auxiliary converter 30 to electrically feed the phases of the auxiliary winding 31.

The auxiliary converter 30 may be completely separated from the main converter 20 from the electrical point of view.

Conversely, the two converters 20, 30 may communicate and be coordinated from. the control point of view. A suitable control unit 35 may be provided for this end. The control unit 35 may be of any kind, may be pure distributed controllers and/or sensorless techniques.

The phase currents of the auxiliary winding 31 are characterized. by proper time evolution ~ imposed. by the auxiliary converter 30 - aimed at creating an auxiliary magnetic field which acts together with the main magnetic field created by the main winding 21 and main converter 20.

The auxiliary magnetic field can provide the magnetization of the electrical machine 10 from the stator, rather than from the rotor. 10 15 20 25 30 By means of the invention, it is no longer necessary that the reactive power required by the nature of the electrical machine originates from the main converter 20. The reactive power can, by means of the invention, be provided by the auxiliary winding 31, thereby opening the possibility for the main converter 20 to supply only the active power and to This advantage can reduce operate at unitary power factor. the power rating and costs of the main converter potentially leading to less expensive overall Variable-speed drive (VSD) systems.

The auxiliary magnetic fields in accordance with the invention provides the excitation necessary for the generator operation of the electrical machine 10, thereby eliminating the prior art need of connecting the electrical often machine to active loads or to external reactive (e.g. purely conservative) networks, such as a bank of capacitors.

With the solution provided by the present invention, a squirrel-cage or any reluctance based electrical machine can supply power also to a fully passive load.

It is noted that the main converter 20 is not limited to use solely completely controllable switches, such as an inverter but it can also employ just diodes the or an active rectifier, only. In this latter case, independent excitation provided by the auxiliary winding 31 is even more important because it allows generator operation with a converter that would otherwise render the machine excitation through the main winding 21 in a much more difficult manner.

The auxiliary converter 30 may have voltage, current or power rating considerably lower than the main converter 20.

A less expensive auxiliary converter 30 may thus be used. 10 15 20 25 30 In case the auxiliary converter 30 is allowed to deliver also active power to the machine, then it may operate the electrical machine 10 at reduced speed and power independently from the main converter 20. Thereby an optimal use of the combination of the two converters is possible, in those applications where slow rotation for long period of While the auxiliary 10 with the as mentioned earlier. the time is required, converter 30 feeds electrical machine reduced necessary active power, and reactive power, the main converter 20 may be disconnected and serviced.

The present invention improves the preventive maintenance of the overall system, and hence it can enhance its overall reliability and/or availability with relatively limited or even negligible increase in costs.

It is noted that the auxiliary winding 31 may be implemented with coils which are different and. galvanically isolated from the coils of the main winding. However, if galvanic isolation and different winding patterns are not necessary, the auxiliary winding can be realized from the main winding following a typical “autotransformer” concept common in stationary transformers. If, additionally, the number of turns of the auxiliary winding 31 can be less than the no additional the number of turns of the main winding 21, auxiliary turns are needed and the terminals of auxiliary winding 31 can be implemented as intermediate taps stemming from the turns of the main winding 21. This arrangement implies a more space-efficient solution in that considerable less space is needed in the stator of the machine to accommodate both windings and thus entails cost savings. 10 15 20 25 With reference to figure 3, the invention as described above may be generalized, as will be described in the following.

The electrical machine 10, of any type, has a set of (in number m >= 1) wound on the same stator (in number auxiliary phases where the main phases n >= 1) are normally located. The main phases are those devoted to the largest amount of average power exchange in the electrical machine 10, whereas the set of auxiliary phases is characterized by a reduced power rating.

In general, the (natural) different, numbers of phases m and n can be and so can also the types of winding patterns used for the auxiliary phases and. main. phases. Only the number of poles needs to be equal between the two different phase sets.

As shown in figure 3, the auxiliary phases 51 and. main phases 41 are connected to two different converter sets named auxiliary converters 50 and main converters 40, respectively. Such converter sets 40, 50 can be constituted either by a single unit with a plurality of phase terminals or by different separated but possibly coordinated units each one supplying one specific phase.

It is noted that the first winding may be mainly devoted to active power, whereas the second winding may be mainly devoted to reactive power.

The time-varying and current-dependent in general - inductance matrix representing any electrical machine at its terminal from the circuit theory point of view is reported where "Ö" in Equation (2) represents the position of the 10 15 rotor defined with respect to the stator. The vectorized quantities are defined in Equation (1). um VM :Em/d* cüahü __ vzm _ vu _ dizm /dt _ diza / dt vm= ¿ , va= ¿ , dímldt= ¿ , ia dt= ¿ l) vmpm vw dimpm /dt dimpa /dt ' vb, ' " 6111,, /dz vzm dizm /dt vflm dfim/dt VW, _ LM (azwza) LW (ø,im,ia)' dimm /df (2) vla Lam (99 im flía) Lau (69 im fiía )_ díla vza din, /dt v Ja díß, /dt fm, dímpa /dt Equation Å3) and Å4) below represent the expression of the total fluxes linked with the stator phases both the main ones (with the subscript m) and the auxiliary ones (with the subscript a). From their structure one envisions straightforwardly the influence of the auxiliary windings on the main ones by observing which kind of contribution to the main fluxes can be brought by the auxiliary fluxes. Such contributions are functionally similar to those deriving from. an independent excitation otherwise located in the rotor. (om = LM (0, im ,ía )í,,, + LM (0, im ,ía )ia (3 ) (pa = LW (0, im , ia )ía + Lam (6, im , ia )í,,, 10

Claims (8)

10 15 20 25 11 Claims

1. An drive (15) for operating an (10) electrical system electrical machine comprising' a rotor and. a stator, said electrical drive system (15) comprising a first static converter (20) for electrically feeding a first stator winding (21) of said electrical machine (10), characterized (30) for electrically feeding a (10), by a second static converter (32) of said electrical machine (30) second stator winding is electrically (20). wherein said second static converter separated from said first static converter (15) as claimed in claim l, (21)

2. The electrical drive system wherein said first stator winding comprises n phases and said second stator winding (31) comprises m phases.

3. The electrical drive system (15) as claimed in claim 2, wherein n is equal to m.

4. The electrical drive system (15) as claimed in claim 2, wherein n is different from m.

5. The electrical drive system (15) as claimed in any of claims l-4, wherein said first stator winding (21) and said second stator winding (22) are arranged in same stator slots. (15) as claimed in any of the (35)

6. The electrical drive system preceding claims, further comprising' a control unit arranged to control said first and second static converters (20, 30) in a coordinated manner.

7. The electrical drive system (15) as claimed in any of the preceding claims, wherein said first converter is arranged 12 to feed with active power and reduced or zero reactive power.

8. The electrical drive system (15) as claimed in any of the preceding claims, wherein said second converter is arranged to feed with reactive power and reduced. or zero active power.