Method of initiating a regenerative converter and a regenerative converter

Abstract

The invention is related to a method of initiating a regenerative converter (1) and corresponding converter (1) including a line bridge (2) and a machine bridge (3), which are interconnected via a DC intermediate circuit (8A, 8B). The method comprises charging, through the line bridge (2) and while the machine bridge (3) remains inactive, the DC intermediate circuit (8A, 8B) to a target voltage (14) higher than peak value of the mains voltage (13).

Claims

1. A method of driving a regenerative converter including a line bridge connected to an A.C. main power supply to receive an A.C. mains voltage and a machine bridge interconnected via a DC intermediate circuit, the machine bridge being connected to drive an electric machine having windings, the method comprising: charging, through the line bridge and while the machine bridge remains inactive, the DC intermediate circuit to a target voltage higher than peak value of the mains voltage: and thereafter, using the target voltage to provide a higher startup voltage to the electric machine through said machine bridge to facilitate start of the electric machine with the higher startup voltage.

2. The method according to claim 1, the method comprising: observing the DC intermediate circuit voltage starting normal converter operation when the DC intermediate circuit voltage reaches a threshold value.

3. The method according to claim 1 wherein the electric machine is an A.C. motor and wherein the machine bridge supplies A.C. power thereto.

4. The method according to claim 1, wherein the converter includes line current filtering inductors arranged in series between line bridge AC output terminals and the main power supply.

5. The method according to claim 4, the method comprising: observing the DC intermediate circuit voltage starting normal converter operation when the DC intermediate circuit voltage reaches a threshold value.

6. The method according to claim 4, the method comprising: charging the DC intermediate circuit by switching one or more of the low-side and/or the high-side switches of the line bridge with a preselected pulse pattern.

7. The method according to claim 6, the method comprising: observing the DC intermediate circuit voltage starting normal converter operation when the DC intermediate circuit voltage reaches a threshold value.

8. The method according to claim 4 or 6, the method comprising: charging the DC intermediate circuit by switching only subset of the switches of the line bridge.

9. The method according to claim 8, the method comprising: observing the DC intermediate circuit voltage starting normal converter operation when the DC intermediate circuit voltage reaches a threshold value.

10. The method according to claim 8, the method comprising: charging the DC intermediate circuit by switching only one or more of the low-side switches or alternatively only one or more of the high-side switches of the line bridge.

11. A regenerative converter providing power from power mains of an A.C. power supply to an electric machine having windings, comprising: a line bridge for connecting to the power mains; a machine bridge for connecting to the windings of the electric machine; a DC intermediate circuit connecting the line bridge and the machine bridge together; a control coupled to both the line bridge and the machine bridge, the control being configured to: cause, while the machine bridge remains inactive, the line bridge to charge the DC intermediate circuit to a target voltage higher than peak value of the voltage of the power mains.

12. The regenerative converter according to claim 11, wherein the control comprises a feedback channel for observing the DC intermediate circuit voltage, and wherein the control is configured to start normal converter operation when the DC intermediate circuit reaches a threshold value.

13. The regenerative converter according to claim 11, wherein the regenerative converter is a frequency converter.

14. The regenerative converter according to claim 11, wherein the converter includes line current filtering inductors arranged in series between the line bridge AC output terminals and the power mains.

15. The regenerative converter according to claim 14, wherein the control comprises a memory for storing a preselected pulse pattern of the control pulses for one or more of the low-side switches and/or high-side switches of the line bridge; and wherein the control is configured to cause switching with the preselected pulse pattern of one or more of the low-side switches and/or high-side switches of the line bridge.

16. The converter according to claim 15 wherein the electric machine is an A.C. motor and wherein the machine bridge supplies A.C. power thereto.

17. The regenerative converter according to claim 14 or 15, wherein the line bridge comprises low-side and high-side switches arranged as legs, and wherein the control is configured to cause the line bridge to charge the DC intermediate circuit by switching only subset of the switches.

18. The regenerative converter according to claim 17, wherein the control is configured to cause the line bridge to charge the DC intermediate circuit by switching only one or more of the low-side switches or alternatively only one or more of the high-side switches of the line bridge.

Description

BRIEF EXPLANATION OF THE FIGURES

(1) FIG. 1 represents schematic of a regenerative frequency converter according to an embodiment of the disclosure.

(2) FIG. 2a represents switching pulse pattern of the line bridge high-side igbts of FIG. 1.

(3) FIG. 2b represents DC link voltage graph when frequency converter of FIG. 1 is initiated.

MORE DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

(4) For the sake of intelligibility, in FIGS. 1, 2a and 2b only those features are represented which are deemed necessary for understanding the invention. Therefore, for instance, certain components/functions which are widely known to be present in corresponding art may not be represented.

(5) In the description same references are always used for same items.

(6) FIG. 1 represents a regenerative frequency converter 1 of elevator hoisting machine. Hoisting machine includes a permanent magnet synchronous electric motor 10. In some other embodiments the hoisting machine may include an induction motor, reluctance motor, or even a DC motor, in which case instead of frequency converter 1 an AC/DC converter is used for supplying motor power. In the embodiment of FIG. 1, the frequency converter 1 supplies electrical power to the stator windings of the permanent magnet electric motor 10 to produce desired movement of the elevator car, as is known in the art.

(7) Frequency converter 1 is a regenerative converter, which means it can supply power between the mains 12 and the motor 10 in two opposite directions, depending on the operation mode. When motor torque is applied in the direction of rotation of the motor 10 (driving mode), frequency converter supplies electrical power from mains 12 to the motor 10. Otherwise, when motor torque is applied in opposite direction (generator mode), frequency converter supplies electrical power from motor 10 back to the mains 12.

(8) Frequency converter 1 includes a line bridge 2 and an inverter 3, which are connected together via high-voltage 8A and low-voltage 8B DC intermediate circuit busbars. The DC intermediate circuit 8A, 8B further comprises a capacitor 9 or a set of capacitors connected in parallel with the DC intermediate circuit busbars 8A, 8B for balancing the DC intermediate circuit voltage.

(9) Line bridge 2 has similar topology with inverter 3, e.g. both have a 2-level inverter topology. In some other embodiments, line bridge 2 and/or inverter 3 may have a multi-level topology, for example, a 3-level inverter topology.

(10) Line bridge 2 (and also inverter 3) has solid state switches 5A, 5B arranged as legs 4. The solid state switches 5A, 5B may be igbt transistors, mosfet transistors, or corresponding. In the frequency converter 1 of FIG. 1 igbt transistors are used. A rectifying diode 18 is connected antiparallel-wise with each igbt transistor 5A, 5B. Each leg 4 is associated with one of the output terminals 2A, 2B, 2C of line bridge 2 (and similarly of the inverter 3 is associated with the motor 10 terminals). Each leg 4 comprises a high-side 5A and a low-side 5B igbt transistor connected in series between the high-voltage 8A and low-voltage 8B DC intermediate circuit busbars. The output terminal 2A, 2B, 2C is connected to the connection point of the high-side 5A and low side 5B igbt transistors. In normal operation, the high-side 5A and the low-side 5B igbt transistors are switched alternatively at a selected modulation frequency, which is in this case 5 kHz. This way an adjustable output voltage is formed in each output terminal 2A, 2B, 2C. PWM (pulse width modulation) is used as a modulation method.

(11) A line current filter 2′ comprising coils is connected between phases L1, L2, L3 of the mains 12 and the output terminals 2A, 2B, 2C. Purpose of the line current filter 2′ is to remove disturbance from the line current.

(12) Further, a control circuit 11 is coupled to the control gates of the igbt transistors 5A, 5B. The control circuit may be implemented with any suitable processing and communication elements, such as microprocessors, microcontrollers, DSP processors, FPGA (field programmable gate arrays) circuits, memory circuits, analog and digital signal lines, data buses and data converters etc., as is known in the art.

(13) This means that the control circuit 11 generates PWM control pulses during normal operation to both line bridge 2 and inverter 3 igbt transistors, causing stepless, controlled supply of power through the frequency converter 1.

(14) The control circuit 11 has a control loop for controlling operation of the line bridge 2. In normal operation, line bridge 2 regulates DC intermediate circuit voltage U.sub.DC to a preselected target value of approximately 650 V. The term “DC intermediate circuit voltage U.sub.DC” means voltage between high-voltage 8A and low-voltage 8B DC intermediate circuit busbars. U.sub.DC voltage regulation is done by controlling current in the supply cables 19 by adjusting voltages in the line bridge output terminals 2A, 2B, 2C as disclosed above. Control circuit 11 comprises also measurement amplifiers, which measure currents in supply cables 19 as well as DC intermediate circuit voltage U.sub.DC.

(15) In traditional solutions, at a startup situation the frequency converter 1 generates very high startup currents I.sub.12 from the mains 12, because the control loop is saturated before starting of the operation. The reason for saturation is that DC intermediate circuit voltage U.sub.DC is smaller than mains voltage 12 before line bridge 2 starts operating. Before operation, voltage U.sub.DC is defined by rectification from mains 12 through diodes 18 and therefore voltage losses of the diodes 18 cause the U.sub.DC voltage drop. Still another reason for high startup current I.sub.12 is that the modulated voltage in AC terminals 2A, 2B, 2C of the line bridge 2 is not always correct. Sometimes the wrong AC terminal 2A, 2B, 2C voltage is a result of voltage losses across line bridge components. Sometimes the reason may be the blanking times needed to avoid short-circuit between the high-side and low-side line bridge switches. Adopting the blanking times unintentionally diminishes AC terminal 2A, 2B, 2C voltages, especially in high switching frequencies. On the other hand, the measurement errors in line bridge feedback channel may also cause unwanted peaks to the startup current. It is the control loop that normally corrects these errors, but only after operation has been started. High startup currents I.sub.12 are especially present in installations having low power line impedance from the mains.

(16) For resolving the above-identified startup-current problem, the control 11 has a special initiation mode, which charges the DC intermediate circuit voltage U.sub.DC to a target value higher than the mains voltage peak value before starting normal frequency converter operation. The term “peak value of the mains 12 voltage” means amplitude of the phase to phase voltage in the power line cables coming from mains 12 to the frequency converter 1, that is, the amplitude of the voltage between any of the phases L1, L2, L3. In the initiation mode, only a subset of the line bridge igbt transistors 5A, 5B is switched and with a dedicated pulse pattern that causes a substantially small charging current to flow from the mains through the line bridge to the DC intermediate circuit capacitor 9. Basically switching only one of the transistors 5A, 5B is enough for charging the capacitor 9; however in this embodiment all the high-side transistors 5A are switched while the low-side transistors 5B remain inactive.

(17) Looking at FIG. 1, a voltage boost circuit is established from mains 12 through the inductors 2′ and the high-side transistors 5A of the line bridge 2 to the DC intermediate circuit capacitor 9. When transistors 5A are switched, voltage in the DC intermediate circuit busbar 8A can rise higher than peak value of the mains 12 voltage due to the voltage induced in coils 2′. Correspondingly, voltage in the DC intermediate circuit busbar 8B becomes smaller that minimum value of the mains 12 voltage. High-side transistors 5A are switched simultaneously with a preselected pulse pattern 15, which is memorized in control circuit 11. This means that only the high-side transistors 5A are switches while low-side transistors 5B remain inactive. In the pulse pattern 15, only very narrow control pulses 15 are used to reduce the startup current I.sub.12, as represented in FIG. 2a. Switching of the transistors 5A causes the DC intermediate circuit voltage U.sub.DC to rise gradually, as represented in FIG. 2b.

(18) Control circuit 11 measures DC link voltage U.sub.DC and compares the measured voltage U.sub.DC to a threshold value 16. Threshold value may be substantially same as the DC link voltage during normal operation. At time t.sub.1 the control circuit 11 recognizes that the measured DC intermediate circuit voltage U.sub.DC has reached the threshold value 16, and it starts normal operation by starting the line bridge 2 control loop and starting inverter 3 operation. At this point all the high-side 5A and low-side 5B transistors of line bridge 2 start switching with a pulse pattern 20 continuously redefined by the line bridge 2 control loop. Because the measured DC intermediate voltage U.sub.DC is now substantially same as the normal operation voltage (e.g. line bridge 2 control loop target value), line bridge 2 control loop may be started without high startup currents I.sub.12.

(19) In the above embodiments the frequency converter 1 was represented as a part of an elevator installation. A skilled person understands, however, that the frequency converter disclosed may also have other applications, for example, in electrical drives of escalators, moving walkways, electric cars, wind power plants, photovoltaic energy supplies etc.

(20) While the present inventions have been described in connection with a number of exemplary embodiments, and implementations, the present inventions are not so limited, but rather cover various modifications, and equivalent arrangements, which fall within the purview of prospective claims.