Power converter for operating a synchronous motor and method for starting a synchronous motor

Abstract

Method for starting an inside-pole synchronous motor (10), wherein inductive reactive power is fed into at least one stator winding of the synchronous motor (10), wherein a three-phase current is fed into stator windings of the synchronous machine by means of a converter (24) and switching elements (S1 to S6) of the converter (24) are each switched with a switching angle greater than 180°, wherein the switching elements (S1 to S6) are controlled by a control device (38) in self-commutated operation according to an intermediate circuit clocking.

Description

[0001] The invention relates to a power converter for operating an inside-pole synchronous motor and a method for starting an inside-pole synchronous motor.

[0002] In an internal-pole synchronous motor, a rotating magnetic field is generated by means of three-phase alternating current impressed into the stator windings of the synchronous motor. This field exerts a torque on an excited rotor of the synchronous motor. To allow for adjustment of the rotor's rotational speed, the three-phase alternating current can be generated by a controllable power converter, such as one that can be part of a frequency converter.

[0003] Three-phase alternating current can be generated using a controllable power converter. For this purpose, the power converter can incorporate a multiphase bridge circuit. This circuit consists of several half-bridges, each of which generates an alternating voltage in one phase conductor of the synchronous machine. Each half-bridge has two switching elements through which a terminal of a stator stage of the synchronous machine is alternately connected to a positive potential and then to a negative potential of a DC source, such as the DC link of a frequency converter.

[0004] The switching point at which a specific switching element of the bridge circuit is made conductive is described by a so-called switching angle. This is a phase angle that describes the phase shift of the switching point relative to the most recent, so-called natural firing point of the switching element. The natural firing point, in turn, is the point at which the switching element would independently conduct current if the switching element were a diode and the bridge circuit were operated as a rectifier. Based on Fig. 1 and Fig. Figure 2 illustrates, using the example of a three-phase bridge circuit 10, the natural firing points for diodes V1 to V6 of a three-phase bridge rectifier, and the phase voltages Ua, Ub, Uc of a three-phase network that are rectified to a DC voltage Ud. In the graph of Fig. Figure 2 shows the phase voltage waveforms Ua, Ub, Uc as a function of a phase angle wt, where t is the time and w = 2πf0 is the angular velocity at a mains frequency f0. For diode V1, the switching angle at its natural firing time is An1 = 0° (the phase shift is zero), for diode V2, the switching angle at its natural firing time is An2 = 0°, and so on. By using a controllable switching element instead of diode V1, a switching time of, for example, An1 = 45° instead of An1 = 0° can be specified.

[0005] A bridge circuit in which half-bridges have controllable switching elements instead of diodes can be operated as an inverter to provide three-phase current for a synchronous motor by switching the elements at a switching angle greater than 90°. The switching signals for the switching elements are determined by load-controlled operation of the converter, depending on a voltage induced in the stator windings by the synchronous motor.

[0006] In connection with the load-controlled operation of the converter, the problem arises that the synchronous machine must generate inductive reactive power for the converter. However, such inductive reactive power cannot be provided by the synchronous machine during start-up (at low speeds). To start the rotor of a synchronous motor from standstill and accelerate it beyond a starting speed range of approximately ten percent of its rated speed, it is known to operate a bridge circuit by so-called DC link switching. This corresponds to switching the switching elements of the bridge circuit with a switching angle of 180° or slightly less. This maximizes the torque acting on the rotor for a given excitation.

[0007] However, with an inside-pole synchronous machine, the problem can arise that only a relatively low excitation of the rotor can be generated during the starting operation of the synchronous machine. Consequently, an undesirably low acceleration of the rotor can result.

[0008] In a textbook by Felderhoff (Rainer Felderhoff, "Power Electronics", 2nd edition, Hanser Publishing, 1997, pp. 207-209; ISBN 3-446-18993-9), load control using thyristors is described for electrical machines. For a synchronous machine, the machine itself must act as a capacitive load for current commutation. However, this only applies during steady-state operation. Machine control of the converter is only possible from a speed of approximately 10% of the rated speed upwards. From standstill up to this starting speed, commutation must be ensured by other means. A common method is starting via the controlled rectifier. This is also referred to as DC link switching, as described, for example, in a technical article by Schlegel and Weigel (Thomas Schlegel and Wolf-Dieter Weigel, "Control and Regulation of Starting Converters Using the Voltage Switching Method", reprint from Siemens Journal, 52).Year, Issue 8, August 1978, pages 474 to 478) and the textbook by Schröder (Schröder, Dierk, “Electric Drives - Fundamentals”, Springer Publishing, 2nd edition, 2000, pp. 442, 443; ISBN 3-540-66846-2).

[0009] The object of the present invention is to ensure reliable starting operation for an internal pole synchronous motor.

[0010] The problem is solved by a method according to claim 1 and by a power converter according to claim 4. Advantageous embodiments of the method and the power converter according to the invention are given by the dependent claims.

[0011] The method according to the invention provides for feeding inductive reactive power into at least one stator winding of the synchronous motor when starting up an internal pole synchronous motor.

[0012] This offers the advantage of introducing reactive current into the stator of the synchronous motor. This increases the magnetic flux in the synchronous motor compared to operation with a switching angle of 180°. The switching angle is significantly larger than what could result solely from tolerances in a conventional control system designed to set a switching angle of 180°. In particular, switching angles greater than 185° and even greater than 190° can be implemented.

[0013] The invention is based on the understanding that this can lead to an increase in the starting torque acting on the rotor. According to the previous approach, the starting torque is maximized solely by controlling the switching elements of the power converter with a switching angle of up to 180°. This is the maximum value of the previously used inverter operation of a bridge circuit. However, this maximization of the starting torque assumes that sufficient excitation of the rotor is present. If the excitation is very low, an undesirably low starting torque can result. In the method according to the invention, the magnetic flux in the machine is increased by injecting a reactive current, thereby producing a higher starting torque.

[0014] In the case where the three-phase current is fed into the stator windings of the synchronous machine by means of a power converter, an advantageous further development of the method according to the invention is achieved by switching the switching elements of the power converter with a switching angle greater than 180°. This previously unknown operating mode of a power converter makes it possible to feed the desired reactive power into the stator windings during start-up. A value between 270° and 180° is preferably chosen as the upper limit for the switching angle, e.g., 230°, 200°, or 190°.

[0015] Another aspect of the invention relates to a power converter with which an internal-pole synchronous motor can be started in this manner. The power converter comprises a converter bridge circuit with controllable switching elements. These elements couple DC voltage connections for connecting the power converter to a DC voltage source with AC voltage connections, via which the power converter can be connected to the stator windings of the internal-pole synchronous motor. Using the converter bridge circuit, a current generated by the DC voltage source can be converted into three-phase alternating current. The DC voltage generated by the DC voltage source can also be a rectified voltage, such as that available, for example, in the intermediate circuit of a frequency converter.

[0016] The power converter according to the invention further comprises a control device designed to switch the switching elements with a switching angle greater than 180° during the start-up phase of the synchronous motor. This makes it possible to feed inductive reactive power into a stator connected to the AC voltage terminals in the manner described.

[0017] When switching the switching elements, a rotational speed and / or a rotational position of the rotor of the synchronous motor is preferably taken into account. This advantageously simplifies the precise switching angle of an operating state of the synchronous motor, such as its rotational speed, in an optimal manner.

[0018] In this context, a preferred embodiment of the method according to the invention provides for the acquisition of at least one voltage value and / or one current value at the synchronous machine in order to detect the rotational speed and / or the rotational position. The desired quantity can then be easily determined from this.

[0019] Accordingly, in the power converter according to the invention, the control of the switching elements is advantageously simplified by the additional inclusion of a measuring device with which at least one alternating electrical quantity of the synchronous motor can be detected. Such an alternating quantity can be an alternating current flowing through a stator winding or an alternating voltage generated between the terminals of two stator windings.

[0020] The invention is explained in more detail below using an exemplary embodiment. It shows: Fig. 1 a schematic representation of a rectifier with diodes, Fig. 2 a diagram showing the basic phase voltage profiles of a three-phase network and Fig. 3 a schematic representation of a power converter according to an embodiment of the power converter according to the invention.

[0021] The example represents a preferred embodiment of the invention.

[0022] In Fig. Figure 3 shows a synchronous machine 10, which is to be operated in motor mode on an electrical supply network 12. For this purpose, the synchronous machine 10 is connected to the supply network 12 via a frequency converter 14. A rectifier 16 of the frequency converter 14 generates a direct current Id from phase voltages of the supply network 12 in an intermediate circuit 18 between a positive terminal busbar 20 and a negative terminal busbar 22.

[0023] From the direct current Id, an inverter or converter 24 can generate three-phase alternating current with adjustable frequency for a (not shown) stator of the synchronous machine 10. For this purpose, the busbars 20, 22 of the intermediate circuit 18 are connected to the respective DC voltage terminals 26, 28 of the converter 24. On the side of the synchronous machine 10, terminals of its stator windings are connected to AC outputs 30, 32, 34 of the converter 24.

[0024] The power converter 24 comprises a bridge circuit 36 ​​with controllable switching elements S1 to S6. Each of the switching elements S1 to S6 can be, for example, a power semiconductor element such as a thyristor, or a circuit with power semiconductor elements. The switching state of each switching element S1 to S6 is set by a control unit 38. For this purpose, the control unit 38 is connected to control inputs of the switching elements S1 to S6 via signal lines 40. The control unit 38 can, for example, be a microcontroller.

[0025] An alternating voltage applied between the AC voltage outputs 30, 32, 34 is detected by voltage measuring devices 42, 44, 46. A phase current generated by the bridge circuit 36 ​​in two phase lines leading to the AC voltage outputs 30 and 32, respectively, is detected by current transformers 50 and 52. The voltage measuring devices 42, 44, 46 and the current transformers 50, 52 are coupled to the control unit 38.

[0026] Based on the measured values ​​received by the measuring devices, the control unit 38 determines a rotational position of a rotor of the synchronous machine 10 as well as its rotational speed.

[0027] In which the Fig.In the underlying example 3, the rotor is initially stationary. Excitation of the rotor is generated by an exciter located on a shaft of the rotor. The rotor is to be accelerated from standstill to a rated speed Nn. Its instantaneous speed N = 0 (standstill). At the speed N = 0 of the synchronous machine, the AC voltage generated by it at the AC voltage outputs 30, 32, 34 is also 0 V. Therefore, the synchronous machine cannot supply the control and commutation reactive power for load-controlled operation of the converter 34 in a starting speed range N / Nn less than or equal to 0.1.

[0028] To start the synchronous machine from a standstill of the rotor in motor mode, one of the switching elements S2, S4, S6 and one of the switching elements S1, S3, S5 are initially closed, i.e., switched to a conducting state, in self-commutated operation of the converter 24. The current-carrying stator windings then generate a current in the synchronous machine 10. This causes the rotor to rotate in the corresponding direction due to its excitation. The rotational movement is detected by the control unit 38. Once the rotor reaches a certain rotational position, the DC link current is reduced, two other switching elements are closed, and the DC link current is re-established, thus maintaining and increasing the rotational movement of the rotor, i.e., accelerating the rotor. The control unit 38 continues to control the switching elements S1 to S6 in self-commutated operation according to an DC link clocking sequence.The switching angle of the switching elements is set to a value greater than 180°.

[0029] During this start-up phase, the exciter generates only a relatively low excitation in the rotor. The set switching angles of more than 180° are suboptimal with regard to the switching time required to generate torque based solely on the magnetic flux produced by the exciter. A switching angle of 180° would be necessary. With a switching angle of 180°, the converter 24 would transmit only active power to the synchronous machine 10, where it would be converted (with some losses) into rotational energy of the rotor.

[0030] Due to the increased switching angle, additional inductive reactive power is now generated by the converter 24, which is also transferred to the synchronous machine 10. This increases the magnetic flux in the synchronous machine 10. Since the torque acting on the rotor is also proportional to the magnetic flux, the overall result (i.e., despite the suboptimal switching angle with respect to the excitation flux) is a greater torque than with a transmission of pure active power using a switching angle of 180°.

[0031] After leaving the starting speed range, the converter continues to operate in load-controlled mode. The excitation current of the exciter is then sufficiently high.

[0032] The example shows how, by injecting reactive current into the stator of a synchronous machine in self-commutated operation of a power converter at low speeds, a magnetic flux in the machine can be generated or increased in order to support an excitation machine.

Claims

[1] Method for starting an inside-pole synchronous motor (10), wherein inductive reactive power is fed into at least one stator winding of the synchronous motor (10), wherein a three-phase current is fed into stator windings of the synchronous machine by means of a converter (24) and switching elements (S1 to S6) of the converter (24) are each switched with a switching angle greater than 180°, wherein the switching elements (S1 to S6) are controlled by a control device (38) in self-commutated operation according to an intermediate circuit clocking. [2] Method according to claim 1, in which a rotational speed and / or a rotational position of a rotor of the synchronous motor (10) is detected and the switching elements (S1 to S6) are switched depending on the detected quantity. [3] Method according to claim 2, wherein at least one voltage value and / or one current value is detected at the synchronous machine (10) to detect the rotational position. [4] Power converter (24) for operating an inside-pole synchronous motor (10), comprising: - a converter bridge circuit (36) with controllable switching elements (S1 to S6), via which DC voltage connections (26, 28) for connecting the converter (24) to an intermediate circuit (18) are coupled to AC voltage connections (30, 32, 34) for connecting the converter (24) to stator windings of the internal pole synchronous motor (10), and - a control device (38) which is designed to switch the switching elements (S1 to S6) with a switching angle greater than 180° during the start-up operation of the synchronous motor (10), wherein the control device (38) controls the switching elements (S1 to S6) in self-commutated operation according to an intermediate circuit clocking. [5] Power converter according to claim 4, comprising a measuring device (42,44,46,50,52) for detecting at least one alternating electrical quantity of the synchronous motor (10).

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