CIRCUIT AND METHOD FOR MULTIPHASE OPERATION OF AN ELECTRICAL MACHINE

Abstract

A circuit and a method operate an electrical machine connected to at least three phases of a power supply network through a frequency converter that has a DC link. By a synchronous inverter that is actuated by a DC chopper and has two bridge halves switching the positive and the negative half-waves respectively, the energy generated by the machine is fed back into the power supply network. Accordingly, the synchronous inverter has an asymmetrical configuration such that, to switch the potential tapped from the DC link by the DC chopper, switching is carried out by a first bridge half that is formed from thyristors as electronic switches, and such that the second bridge half contains reverse-blocking electronic switches able to be switched off.

Claims

1-11. (canceled)

12. A circuit for operating an electrical machine, the circuit comprising: a frequency converter for connecting to at least three phases of a power supply network, said frequency converter having a DC intermediate circuit; a direct current (DC) chopper controller; and a synchronous inverter, energy generated by the electrical machine being fed back into the power supply network by means of said synchronous inverter being driven by said DC chopper controller, said synchronous inverter having two bridge halves for switching positive and negative half-waves respectively, said synchronous inverter being of an asymmetrical configuration such that, for switching a potential being tapped off from said DC intermediate circuit by said DC chopper controller, switching is performed by a first bridge half having thyristors as electronic switches, and a second bridge half having reverse-blocking electronic switches which can be switched off.

13. The circuit according to claim 12, wherein said reverse-blocking electronic switches of said second bridge half are insulated-gate bipolar transistors (IGBTs).

14. The circuit according to claim 13, wherein said second bridge half has a plurality of series circuits each containing one of said IGBTs and a diode.

15. The circuit according to claim 12, wherein said DC chopper controller driving said synchronous inverter is directly connected to said DC intermediate circuit of said frequency converter.

16. The circuit according to claim 12, further comprising in each case one capacitor functioning as a network filter and connected between load circuits of said reverse-blocking electronic switches.

17. The circuit according to claim 12, wherein said reverse-blocking electronic switches having load circuits directly connected to the power supply network.

18. A method for operating an electrical machine being connected to at least three phases of a power supply network via a frequency converter having a direct current (DC) intermediate circuit, which comprises the steps of: feeding back energy generated by the machine into the power supply network by means of a synchronous inverter being driven by a DC chopper controller; and switching a load current i.sub.L to a maximum and/or a minimum section of a network phase between phase transitions of the network phase and two closest network phases.

19. The method according to claim 18, which further comprises: interrupting, via the DC chopper controller, a connection to the DC intermediate circuit for turning off a thyristor of the synchronous inverter; and switching in a current gap where i.sub.L=0 in the load current i.sub.L in an electrical load circuit of thyristors of the synchronous inverter by the DC chopper controller, a triggering current i.sub.z of the thyristors is switched off with a switching of the DC chopper controller, and an insulated-gate bipolar transistor (IGBT) which is associated with the thyristor switches off only after the thyristor is turned off.

20. The method according to claim 19, wherein the current gap i.sub.L=0 is switched for switching off each electronic switch.

21. The method according to claim 20, wherein the current gap i.sub.L=0 is switched over the maximum of the network phase, a duration of the current gap being greater than a recovery time of the thyristors, and in that a thyristor which is associated with the network phase is triggered once again.

22. The method according to claim 19, wherein between gaps, the DC chopper controller regulates the load current i.sub.L at a value which is constant on average.

Description

[0034] The invention will be explained in greater detail with reference to the drawing, in which only exemplary embodiments are illustrated. In the drawing:

[0035] FIG. 1: shows an idealized circuit according to the invention

[0036] FIG. 2: shows the driving of the synchronous inverter is explained, and

[0037] FIG. 3: shows reproduces an enlarged and supplemented detail from FIG. 2.

[0038] The circuit according to FIG. 1 shows an electrical machine M which is fed from a three-phase power supply network L.sub.1, L.sub.2, L.sub.3 by means of a conventional frequency converter 1.

[0039] The frequency converter 1 has, in a customary manner, a diode rectifier 2 comprising a smoothing capacitor C, has a DC intermediate circuit 3 and has an inverter 4 which is fed from the DC intermediate circuit 3.

[0040] According to the invention, a feedback circuit 5 is connected to the DC intermediate circuit 3, in particular also as an independent assembly, which is formed separately from the frequency converter 1 and by which energy generated during generator operation of the machine M is fed back into the power supply network L.sub.1, L.sub.2, L.sub.3.

[0041] The feedback circuit 5 has, at the input end, a DC chopper controller 6 which is directly connected, without protective or decoupling diodes, to the DC intermediate circuit 3 and feeds a synchronous inverter 7.

[0042] The DC chopper controller 6 has an electronic switch T, an inductance L and a free-wheeling diode D.sub.F and switches the current i.sub.L in the electrical load circuit of the synchronous inverter 7.

[0043] Since the DC chopper controller 6 taps off the positive potential of the DC intermediate circuit 3 in the exemplary embodiment, the synchronous inverter 7 has, as reverse-blocking switches, thyristors S.sub.1, S.sub.3, S.sub.5 in the upper, first bridge half 8, while the reverse-blocking electronic switches S.sub.2, S.sub.4, S.sub.6 of the second bridge half 9 which can be switched off are formed by IGBTs which are connected in series with diodes D.sub.2, D.sub.4, D.sub.6.

[0044] If, as an alternative, the DC chopper controller taps off the negative potential of the DC intermediate circuit, a mirror-image circuit topology is produced.

[0045] In the exemplary embodiment, the outputs of the switches S.sub.1-S.sub.6 are further directly connected to the network phases L.sub.1-L.sub.3. In the case of the network filter 10 which is provided in any case, inductances are entirely dispensed with and capacitors of high capacitance are dispensed with. The network filter 10 has only three capacitors, which are connected between the outputs of the switches S.sub.1-S.sub.6.

[0046] The behavior of the circuit according to FIG. 1 will be explained further with reference to FIGS. 2 and 3.

[0047] FIG. 2 shows, in a top graph, the phase profile of the voltage of the three network phases L.sub.1, L.sub.2, L.sub.3 with respect to the network angle cp. The, here, three network phases L.sub.1, L.sub.2, L.sub.3 are respectively phase-shifted through 120.

[0048] In the bottom graph of FIG. 2, the switching behavior of the electronic switches S.sub.1-S.sub.6 with respect to the network angle is illustrated in an idealized manner. The release of the pulse width modulation of the DC chopper controller 6, PWM, is plotted beneath the switching behavior curves, the turn-off condition i.sub.L=0 occurring at the off times of said pulse width modulation for the purpose of switching off the thyristors.

[0049] FIG. 2 demonstrates, overall, that the switches S.sub.1-S.sub.6 switches the energy which is to be fed back substantially only to the maximum or minimum sections of a network phase between the intersection points with the two other phases. For example, S.sub.1 is on approximately between the phase angles =30 and =150. When the switch S.sub.1 is blocked, S.sub.3 is on up to approximately =270 etc. The same applies for the low side.

[0050] For the purpose of turning off a thyristor S.sub.1, S.sub.3, S.sub.5, it is necessary, in addition to switching off the triggering current i.sub.z, for the current i.sub.L to be zero. In order to ensure this, the electronic switch T of the DC chopper controller 6 interrupts the connection to the DC intermediate circuit 3. However, this does not immediately result in i.sub.L=0. Instead, the IGBTs S.sub.2, S.sub.4, S.sub.6 have to remain switched on for a somewhat longer period than the corresponding thyristors S.sub.1, S.sub.3, S.sub.5, so that i.sub.L of the inductance L can be reduced via the free-wheeling diode D.sub.F. The corresponding thyristor S.sub.1, S.sub.3, S.sub.5 is turned off only when i.sub.L=0.

[0051] According to the illustration in FIG. 2, a current gap i.sub.L=0 of this kind necessarily has to be switched at a phase angle of approximately =30 for the phase transition from L.sub.3 to L.sub.1, at a phase angle of approximately =150 for the phase transition from L.sub.1 to L.sub.2, and at a phase angle of approximately =270 for the phase transition from L.sub.2 to L.sub.3.

[0052] FIG. 2 further shows that, in the exemplary embodiment, these current gaps i.sub.L=0 is also provided when switching off the IGBTs S.sub.2, S.sub.4 and S.sub.6, for reasons of symmetry for example. However, in particular, the EMC behavior is improved by this measure since i.sub.L is again reduced via the free-wheeling diode D.sub.F. Therefore, the thyristors which are on are also turned off in the region of the maximum of the corresponding network phase. This results in small rates of change in current di/dt.

[0053] This switching behavior is explained by way of example with reference to FIG. 3. According to FIG. 3, the current i.sub.L is regulated at a constant value by the DC chopper controller 6 before reaching a phase angle *<90. The switches S.sub.1 and S.sub.4 which are on switch the current i.sub.L to the phases L.sub.1 and L.sub.2.

[0054] At *, the switch T of the DC chopper controller 6 is opened and the triggering current i.sub.z for the thyristors S.sub.1, S.sub.3, S.sub.5 is switched off. Nevertheless, the thyristor S.sub.1 remains switched on since the current i.sub.L is not yet zero. The IGBT S.sub.4 remains switched on beyond * and the current i.sub.L can be reduced via the free-wheeling diode D.sub.F. Once i.sub.L=0 is reached, the thyristor S.sub.1 is switched off.

[0055] In this case, the rate of change in current di/dt is prespecified by the inductance L and the network voltage and is advantageously set at a low value.

[0056] The recovery time of the thyristor S.sub.1 begins at i.sub.L=0 in accordance with the double-headed arrow there. The IGBT S.sub.4 remains switched on even beyond said time. S.sub.4 is also switched off only when it is ensured that i.sub.L=0 and the thyristor S.sub.1 is turned off. The IGBT S.sub.6 further switches on within the current gap i.sub.L=0, in accordance with the top double-headed arrow in FIG. 3.

[0057] After the recovery time of the thyristor S.sub.1 has elapsed, T and therefore i.sub.z and i.sub.L can be switched on again. Accordingly, the thyristor S.sub.1 is also triggered again.