WO2009053520A1

WO2009053520A1 - Arrangement in a frequency converter controlled direct connected drive

Info

Publication number: WO2009053520A1
Application number: PCT/FI2008/000117
Authority: WO
Inventors: Jari Pekola; Tapio Haring; Petri MÄKI-ONTTO
Original assignee: Abb Oy
Priority date: 2007-10-26
Filing date: 2008-10-24
Publication date: 2009-04-30
Also published as: FI20070813L; FI20070813A0

Abstract

The object of the invention is an arrangement in a direct connected drive in which the rotation speed of a motor connected to an actuator is controlled using a high frequency modulated frequency converter (4, 14, 16) supplying alternating frequency voltage to the motor stator. In the arrangement according to the invention, the motor stator has been fastened to the actuator frame and the motor rotor has been fastened to the rotating component of the actuator, bearing-mounted to the frame. The motor stator winding (18, 20) has been created as a two- branch winding, and the frequency converter is a dual-bridge inverter consisting of two essentially identical bridges (14, 16). The first bridge (14) supplies the first branch winding (18) of the stator, and the second bridge (16) supplies the second branch winding (20) of the stator. The first bridge (14) is controlled by the first control pulses (32) and the second bridge (16) is controlled with the second control pulses (38), which are simultaneous and inverse to the first control pulses.

Description

ARRANGEMENT IN A FREQUENCY CONVERTER CONTROLLED DIRECT CONNECTED DRIVE

The object of the invention is an arrangement in a frequency converter controlled direct connected drive according to the preamble of Claim 1.

Integrated direct connected drives have recently become more common in many applications. Such integrated direct connected drives, a motor frame, shafts or bearings cannot necessarily be distinguished. Instead, the structures of the actuator are utilized. The active electromagnetic parts of the motor operating the actuator are directly connected or integrated to the actuator. Active parts of the motor rotor, such as the permanent magnets or short-circuit winding, are fastened to the actuator shaft. The motor stator parts are fastened to the actuator frame or constitute a part of it. The actuator rotation speed is commonly controlled with a frequency converter. The output voltage of the frequency converter is connected to the motor. Similar solution is also used in the process industry, for example for paper machine roll drives. A motor excited using permanent magnets may, for example, be fitted to a roll drive including a hollow shaft.

Issues to be observed for direct connected drives utilizing frequency converters with an intermediate voltage circuit include the various bearing currents resulting from the common-mode voltages that may occur in the drives. Preparation for and protection against bearing currents has become increasingly difficult, as the potential current paths must be established case by case. Traditionally, protection methods such as bearing insulation and high-frequency voltage filtering have been used, but these are not always possible.

Typically, the bearings of direct connected drives are large in size and have a large diameter, making them expensive as well. If protection against bearing currents is based on bearing insulation, the costs of the bearings increase even further, as insulation of the bearings results in higher leak currents as the size of the actuator and bearings increases. Furthermore, electrical insulation also functions as thermal insulation, which constitutes another adverse effect of bearing insulation. Thus, the prevention of bearing currents by bearing insulation may cause the bearings to overheat, resulting in premature damaging. Insulation may also increase stray capacitance. In solutions based on bearing insulation, high-frequency voltages are induced on motor and actuator parts such as the shaft. Even if the current paths through the bearings are stopped with insulation, the measuring devices fastened to the motor and the actuators, as well as the sensors connected to the devices, may be disturbed by the high-frequency voltages.

The effect of high frequency voltages may be controlled by filtering the harmful frequencies from the voltage. LCR filters adjusted for the correct frequency prevent high- frequency components from entering the motor and motor bearings. However, filters will increase both production and design costs. Particularly harmful is the fact that the filters will slow down control functions and thus deteriorate the control features of the entire drive.

Conventional solutions for direct connected drives have been based on adequate insulation and grounding of bearings. These methods will, however, result in the harmful properties described above.

The purpose of the present invention is to reduce or prevent bearing currents in frequency converter controlled direct connected drives, hi order to achieve this, the invention is characterized by the features specified in the characteristics section of Claim 1. The characteristics of some other preferred embodiments of the invention are characterized by dependent claims.

The present invention is based on the innovative idea of preventing bearing currents in direct connected drives by preventing the creation of bearing currents and the passage of said currents in the stator of the motor. The stator windings of the motor and the frequency converter supplying the windings have been connected so that the high frequency currents travel in the windings in opposite phases, compensating each other.

The structure of the semiconductor switches of modern frequency converters is such that no new switch components are needed to apply the invention in practice. Physical switch components are simply electrically constructed so that an existing redundant feature is activated. The manufacturing costs of the invention are therefore low. Only the switch control is reorganized according to the invention. On the other hand, particularly in larger electrical machines, the existing stator windings are manufactured to consist of parallel windings. The parallel windings of the electrical machine shall then be merely connected in a new way, which keeps the costs low also for this case. As only minor changes will be required to carry out the solution according to the invention on frequency converter control on the one hand and on the connection of an electrical machine's stator windings on the other, the application of the invention is also well justified and relatively cheap for targets already in active use.

Although the actuator frame as well as the motor frame must be grounded, earth fault protection grounding usually provides adequate grounding when applying the invention. Grounding can also be carried out using cables without high frequency properties.

With the invention, a dual-bridge inverter (DBI) can be utilized to prevent common-mode voltages from accessing the electrical machine or an actuator driven by the machine. Delivery limits for the suppliers of various parts are thus clearly defined.

In the following, the invention will be described in detail with the help of certain embodiments by referring to the enclosed drawings, where

- Figure 1 illustrates a connection principle used in the invention,

- Figure 2 illustrates an application according to the invention, and

- Figure 3 illustrates another application according to the invention.

Figure 1 is a schematic illustration of a coupling to enable a frequency converter to supply and control an electrical machine, implementing a solution according to the invention. The frequency converter includes a network rectifier 4 connected to an alternating-current network 2 and used to create a direct-current voltage into the frequency converter's intermediate circuit 6. A capacitor 8 has been connected to the intermediate circuit between the intermediate circuit conductors 10 and 12 using a known method, hi this embodiment of the invention, direct current is conducted from the intermediate circuit to the inverter component 15, consisting of two identical inverters 14 and 16 that contain a six-pulse bridge, and using IGBT semiconductor switches. Pulse width modulation is used to convert direct voltage into alternating voltage by connecting the positive and negative voltages of the direct voltage to the stator windings of the electrical machine so that an alternating voltage according to the control is created in the windings. Alternating voltage consists of voltage pulses. When one six-pulse bridge is used to supply the motor stator windings, momentary switch positions have the effect that the common-mode voltage over the motor windings is not always zero as it is for a sinusoidally varying three-phase voltage. According to the invention, the motor stator winding consists of two separate branch windings, in which case the first branch winding 18 consists of windings U₁, V₁ and W₁ and the second branch winding 20 consists of windings U₂, V₂, and W₂. Branch windings 18 and 20 are parallel connected so that their conductors do not touch at any point. The first branch winding 18 is supplied by the inverter 14, and the other branch winding 20 is supplied by the inverter 16. The first and second branch winding are identical, but they are supplied from opposite ends. This is illustrated by the schematic winding diagram in Figure 1. The first ends 22, 24 and correspondingly 26 (marked with black dots) of the windings Ul, Vl and Wl of the first branch winding 18 have been connected to the output of the inverter 14, and the opposing ends have been connected to the star point 27. Correspondingly, the first ends (marked with black dots) of the windings of the second branch winding 20 have been connected to the star point 28, and the opposing ends have been connected to the output of the inverter 16.

Inverters 14 and 16 are controlled by the control device 30. The control signals generated by the device are conducted by cable 32 to the semiconductor switches of inverter 14. The same control signals are conducted by cable 34 to the reverse operator 36 and further with cable 38 to the semiconductor switches of inverter 16. The voltages supplied by inverters 14 and 16 to branch windings 18 and 20 are therefore of the same magnitude but opposite phase, resulting in the branch windings inducing a unidirectional field. Both inverters 14 and 16 generate common-mode voltages that are due to the sum of voltages over windings not equaling to zero at all times. When the switches of inverters 14 and 16 are simultaneously controlled in the opposite phases, both create common-mode voltages of identical magnitude, canceling each other out. The described inverter control method deviates from the master-slave drives representing known technology in that one control unit is used to control inverters in the arrangement according to the invention, the said control unit producing the control signals for both branches, hi master-slave drives, the master and slave units produce their own control signals on the basis of a higher level current, frequency or, for example, moment instruction.

The sum of the output voltages of inverters 14 and 16 is zero at all times, so the inverter operation will cause no common-mode voltage. No currents caused by common-mode voltage are thus induced in the magnetic circuit of the motor. As a consequence, no circulating currents closing their circuit through the bearings exist in the actuator frame components fastened to the stator and rotor of the motor. As there is no common-mode voltage, no capacitive coupled harmful bearing voltages will emerge between the motor and actuator components.

Figure 2 illustrates an application according to the invention, with a dual-bridge inverter used to control a roll drive 40, the shaft 42 of which is supported to the base 44 with bearings 46 on both ends. The induction motor rotor 48 turning the roll has been fitted around the end of shaft 42, the said end extending beyond the roll and the bearing 46 supporting the shaft of the roll. The motor stator 50 around the rotor 48 at the distance of the air gap 51 from the stator 50 has been fastened to the base 44. The bearings 46 supporting the roll and the motor rotor 48 are thus outside the stator 50 in axial direction.

Figure 3 illustrates another application according to the invention in which a dual-bridge inverter supplies a permanently excited synchronous motor 52 turning a drum 54, both ends of the drum having bearings 56 at the outer circumference. The synchronous motor stator 58 has been fastened to the base 60. The drum bearings 56 are also supported against the base. The synchronous motor rotor 62 has been fitted inside the stator 58. The rotor is fastened to the drum. As a result, the rotor rotates on the drum bearings 56. Bearings 56 supporting the rotor 62 are outside the stator 58 in the axial direction.

In various applications of the invention, the motor controlled by a frequency converter may also be another alternating-current motor such as a synchronous reluctance motor.

In the above, the invention has been described with the help of certain embodiments. However, the description should not be considered as limiting the scope of patent protection; the embodiments of the invention may vary within the scope of the following claims.

Claims

1. Arrangement in a direct connected drive in which the rotation speed of a motor (48, 50, 52) connected to an actuator is controlled by a high frequency modulated frequency converter (4, 14, 16) supplying an alternating frequency voltage to the motor stator (50, 58), characterized in that in the arrangement, the motor stator (50, 58) has been fastened to the actuator frame (44, 60) and the motor rotor (48, 60) has been fastened to the rotating part of the actuator, bearing-mounted with bearings (46, 56) to the frame; in that the motor stator winding (18, 20) has been created as a two-branch winding; in that the frequency converter is a dual-bridge inverter consisting of two essentially identical bridges (14, 16); in that the first bridge (14) supplies the first branch winding (18) of the stator and the second bridge (16) supplies the second branch winding (20) of the stator; and in that the first bridge (14) is controlled with the first control pulses (32) and the second bridge (16) with the second, simultaneous but inverse control pulses (38).

2. An arrangement according to Claim 1, characterized in that the motor is a synchronous motor (52) excited using permanent magnets.

3. An arrangement according to Claim 1, characterized in that the motor (48, 50) is an induction motor.

4. An arrangement according to Claim 1, characterized in that the motor is a synchronous reluctance motor.

5. An arrangement according to Claim 1, characterized in that the bearings (46, 56) supporting the rotor are essentially outside the stator (50, 58) in the axial direction.

6. An arrangement according to Claim 1, characterized in that the actuator frame is grounded.