WO2005034314A1

WO2005034314A1 - Apparatus and method for reducing bearing current

Info

Publication number: WO2005034314A1
Application number: PCT/FI2004/000599
Authority: WO
Inventors: Jouni Ikäheimo; Jari Pekola; Tapio Haring
Original assignee: Abb Oy
Priority date: 2003-10-08
Filing date: 2004-10-08
Publication date: 2005-04-14
Also published as: FI20031475L; FI20031475A0; FI20031475A7

Abstract

The object of the invention is an apparatus and method for reducing bearing current in an electrical machine. A stator with its magnetic circuit is supported on the frame of the electrical machine, and a rotor with its magnetic circuit is arranged at an air gap away from the stator on an axle shaft supported on the frame of the machine by bearings. The stator of the electrical machine comprises a magnetically conductive yoke (20) and the main magnetic flux of the machine is closed through the stator's magnetic circuit, the air gap and the rotor's magnetic circuit. According to the invention, the machine is fitted with at least one closed suppression winding (40), essentially arranged around the stator (20) of the machine so that the main magnetic flux of the machine goes through the coil ring formed by the suppression winding (40) and the total electromotive force induced by the main flux in the closed suppression winding (40) is essentially zero.

Description

APPARATUS AND METHOD FOR REDUCING BEARING CURRENT

The object of the invention is an apparatus for reducing bearing current in an electrical machine according to the preamble part of Claim 1. The object of the invention also includes a method for reducing bearing current in an electrical machine according to the preamble part of Claim 10.

The operation of electrical macl ines supplied by frequency converters is often hampered by so-called bearing current. Bearing current is created when the high-frequency common-mode supply voltage induces a voltage between the ends of the machine axle shaft. When the voltage difference over a bearing is sufficiently large, spark-over takes place in the oil film and current starts to flow through the bearing, causing corrosion of the ball bearing race. The phenomenon of bearing current can be roughly divided into two categories, inductive and capacitive, based on the principal method of origination. The purpose of this invention is to reduce inductive bearing current in particular, thus eliminating its adverse effects.

Inductive bearing current is created as a consequence of high-frequency voltage pulses supplied by a frequency converter. A steep-edged voltage pulse arrives from the frequency converter through a cable to the windings of the electrical machine, and capacitance between the stator and the winding coil causes a capacitive leak of current to the stator. The edge of the pulse is less steep in the return part of the coil, so the leak of current is not as large as in the forward part. Due to this the currents in the stator grooves do not add up to zero, but an effective net current flows through the stator when viewed from the end of the axle shaft. The net current generates a high-frequency magnetic flux that rotates toroidally in the stator. The magnetic flux induces a high-frequency current, also known as the bearing current, in the axle shaft going through the stator. The bearing current loop closes through the bearings, end shields and frame of the machine.

Earlier methods of preventing bearing current have included, for example, reducing the voltage pulse's rate of rise by means of filters, adding symmetry to the system by supplying the winding from both ends, or breaking the flow of current using a component such as an insulated bearing or hybrid bearing. However, these prior art solutions have substantial shortcomings that restrict their usability when the aim is to eliminate or suppress high-frequency bearing currents.

An insulated bearing (such as INSOCOAT®, registered trademark of SKF Osterrreich AG) by itself is not a guaranteed solution for high-frequency currents; it must normally be supplemented with a common-mode filter, which makes the solution expensive. Furthermore, insulated bearings are only available in certain sizes. A hybrid bearing is an efficient means of preventing bearing current, but the bearing is very expensive and only certain sizes are available. This solution cannot be used in larger machines. A hybrid bearing prevents the flow of current, but dangerous voltage remains. An insulated end shield is relatively expensive, mechanically weak and inapplicable to large machines. Costs will be further increased by the fact that one insulated end shield or hybrid bearing is not enough for large machines, where two units are required.

A symmetrical winding requires a connection box in the middle of the motor frame, which makes the frame more expensive. All efficient filters are expensive and impair the dynamics of control, which is an important requirement for torque-controlled macliines.

The purpose of the present invention is to create a new and efficient method for reducing bearing current. The invention is based on the innovation that an alternative, parallel route is provided for the induced bearing current, so that the current induced in the axle shaft is reduced. The voltage over the bearings will be reduced to such a degree that no current can flow through the bearing. More precisely, the apparatus according to the invention is characterised by the features specified in the characteristics section of Claim 1. The method according to the invention for eliminating bearing current is characterised by the features specified in the characteristics section of Claim 10. Some other preferred embodiments of the invention are defined in the dependent claims.

According to the invention a suppression winding is arranged around the stator of the machine, and the toroidal magnetic flux rotating in the stator flows through this winding. By connecting the individual wire loops in series, the toroidal winding becomes short- circuited and only the current created by the toroidal flux in the stator is induced in this winding. The main flux of the electrical machine induces opposite electromotive forces in different parts of the toroidal winding that add up to zero, so the main flux does not induce any current in the toroidal winding. The toroidal winding can be manufactured from relatively thin wire, because the current remains low. The winding does not need to have any special characteristics but it can be made of winding wire that is available at a low price. The thin gauge of the wire means that its adaption to the machine does not cause any substantial additional work. The device can operate reliably over a relatively wide range of power ratings without additional equipment. The device is reliable and does not contain any separate means of control or regulation.

In accordance with the method according to the invention, a closed suppression winding is arranged around the magnetic circuit of the stator so that the electromotive force induced by the machine's main flux is zero in the suppression winding. The high- frequency flux causing bearing current induces a current in the suppression winding, which is consumed in a resistor.

According to a preferred embodiment, the suppression winding is arranged toroidally around the yoke forming the back section of the stator's magnetic circuit. Trie wires of the suppression winding can, for example, be fitted between the stator winding and the ends of the coil.

Furthermore, according to a preferred embodiment, the coil wire of the suppression winding is at the bottom of the stator groove when the suppression winditig is made before the machine windings. The coil wires are preferably fitted inside the groove insulation without requiring any changes in the actual machine windings.

According to another preferred embodiment, the coil wire is fitted close to the mouth of the stator groove or in a spur in the stator groove. This implementation is particularly beneficial in situations where the groove is tight or where manufacturing technology requires this.

The preferred embodiment of the suppression winding circuit is that a winding is made toroidally around the stator by placing a coil wire in each stator groove or some of the stator grooves, coming back on the outer surface of the stator. On the outer surface of the stator, the coil wire is fitted on the outer surface of the back section of the stator or in a groove arranged on the outer surface of the stator.

In order to create the intended effect of the invention, the suppression winding must be symmetrical so that the main flux of the machine does not induce a sum c rrent in the suppression winding. The preferred number of turns in the winding is at least two, in particular the number of phases in the machine multiplied by the number of poles, or any multiple of this. When the number of turns in the winding is two, the suppression effect, according to the invention, can be created without disturbing the main flux. In this case the turns of the winding are at a distance of 180 electrical degrees from each other, so that the turn voltages induced by the main flux have opposite phases and cancel each other out. When the number of turns is doubled to four, symmetry is retained and a more balanced distribution of windings on the circumference of the stator is preferably achieved. When the number of turns is tripled to six turns or three pairs of loops, the solution can have one pair of winding turns for each phase, for example. Preferably these three pairs of loops are located in the same position in the winding in all phases, which creates a very symmetrical distribution on the circumference of the stator. The number of winding turns can be further increased using the rule that the number of turns is the number of phases multiplied by the number of poles, or any multiple of t-his. In the extreme case, the winding comprises a turn fitted into each stator groove or maybe even several turns per groove.

According to a preferred embodiment, the coils in the suppression winding are connected in series by pairs or coil groups, or all the coils are connected in series. Two windings located symmetrically in relation to each other are connected in series. Correspondingly, when placing the pairs of loops in different phases at the same positions, trie pairs of loops can be connected in series, and furthermore, when fitting a winding turn in every groove, these can be connected in series, which very probably creates sufficient suppression for all needs. However, each groove can be fitted with more than one winding turn.

The power induced in the suppression winding must be consumed, which is preferably implemented either by short-circuiting the windings or connecting them to a load resistor of the required size. The ends of the suppression winding can be brought to the machine connectors, and a suitable resistor can be connected to these depending on usage.

In the following, the invention will be described in detail by referring to the drawings, where:

- Figure 1 is a principal illustration of the cross-section of a macliine and the induced fluxes,

- Figure 2 illustrates the principle of a suppression winding according to the invention,

- Figure 3, illustrates an equivalent circuit for the solution according to the invention,

- Figure 4a illustrates the fitting of the winding in the stator groove and stator back,

- Figure 4b illustrates an alternative fitting of the winding in the stator groove and stator back, and

- Figure 4c illustrates a third fitting of the winding in the stator groove and stator back.

hi the cross-sectional drawing of the electrical machine 2, the stator 4 is fitted around the rotor 6, which is fitted onto the axle shaft 8. The macliine frame 10 is fitted with bearings 12, and the axle shaft 8 rotates with the support of the bearings. The magnetic circuit of the stator is formed from magnetically conductive sheets where grooves 16 have been made for the stator winding 14 by die cutting, for example. Spurs 18 are located between the grooves, and the back yoke 20 of the stator is located outside the grooves and spurs. The primary magnetic flux of the electrical machine, also known as the main flux, is closed through the back yoke 20, the spurs 18, the air gap 22 and the rotor magnetic circuit 24.

The electrical machine is connected to a power supply, such as a frequency converter or inverter, from where the current is conveyed to the stator windings 14 in a known way. Figure 1 illustrates the special characteristics of the present invention with regard to the leaking of high-frequency current to the stator and the inductive bearing current caused by this. A high-frequency voltage source 26 supplies the machine with a voltage whose steep-edged voltage pulse is conveyed to the winding 14 by conductors 28. The capacitance between the stator sheet pack 20 and the winding coil 14 causes a capacitive leak of current to the stator, which is grounded through the frame to the connector 30. The arrows 32 in Figure 1 illustrate the high-frequency current that is created. As the voltage pulse progresses through the coil, its edge becomes less steep and the leak current is reduced. This means that the current leaking into the stator is not equal in the forward and return directions. Due to this the leak current creates a flux around the stator, illustrated by the dots 34 and crosses 36 in accordance with the direction of flux. The flux also becomes smaller in the direction of the axle shaft, because the sum current is reduced when progressing from the voltage supply point to the other end of the machine. This high-frequency magnetic flux 34, 36 induces a high-frequency current, also known as the bearing current, in the rotor 24 and the axle shaft 8. If the bearing insulation fails, the current loop is closed through the bearings 12, end shields and machine frajαie 10.

Figure 2 illustrates the principle of the solution according to the invention, and Figure 3 is the equivalent circuit of the magnetic circuit that is created. A toroidal winding 40 is fitted around the stator sheet pack 20, comprising several winding wires 42. The winding wires are located on the inner surface of the stator or in the stator grooves, from where they go through the end 44 to the outer surface of the stator 46 and furthennore through the other end 48 to another groove or another position on the inner surface of the stator. The winding wires are connected in series with each other. The toroidal winding is short- circuited or its ends are connected to a resistor 50. The resistor is either permanently connected to the machine, or it can be connected and replaced using connectors arranged for the purpose. The wires of the toroidal winding are fitted around the stator in a symmetrical fashion so that the current induced in them by the main flux of the macliine is zero. This means that the number of turns in the winding must be at least two. Preferably the number of turns is the number of phases in the machine mxiltiplied by the number of poles, or any multiple of this, which will fulfil the symmetry requirement with most certainty. The coils in the suppression winding are preferably connected in series by pairs or coil groups, or all the coils are connected in series. The magnetic flux caused by the high-frequency leak current in the stator sheet pack flows through the toroidal winding. Furthermore, the magnetic flux goes through the bearing current loop. This makes it possible to draw Figure 3, t ie equivalent circuit for a toroidal transformer supplied by a high-frequency voltage U with current I_\ through the internal impedance Z_s. The core of the toroidal transformer is the stator, and the number of turns in the primary Ni is an effective number of turns and is not significant for this examination. The number of turns in the toroidal winding is N₂, and current I flows in it through the resistor R. The bearing current I₃ flows in another secondary winding where the number of turns N₃ is one, because the bearing current only flows once through the stator. The following equation is valid for the equivalent circuit:

This creates another route for the current in the bearing current circuit, because current Ii remains constant.

Figures 4a, 4b and 4c illustrate some examples of fitting the toroidal winding wire in the stator groove. Figure 4a illustrates the first embodiment where the conductor 52 is fitted in the opening 54 of the stator groove 16. hi this example, there are two conductors 52 in adjacent grooves. These go through the end of the stator to the outer surface 46 of the stator pack, where they are fitted into a groove 56 in the sheet metal body of the machine.

In the example of Figure 4b, the conductor 58 is fitted to the bottom of the stator groove 16, and correspondingly, the return conductor is on the surface of the back section of the stator as in Figure 4a. hi the example of Figure 4c, the conductor 62 is fitted to the spur 60 on the air gap surface 66, for example, in a groove, and a further groove 64 is made on the stator back for the return conductor. The conductor may also be fitted in the groove at a distance from the machine's air gap. The placement of the conductor on the stator back and on the inside and outside of the stator can be a combination of the alternatives described above, or the conductor can be placed in another way. The cross-section of the toroidal winding conductor does not need to be particularly large, as the high-frequency power to be conveyed away and consumed is fairly minor. In cast iron machines, a modified cover strip groove can be used for the return conductor on the back section, and smocked cavities in the sheet metal frame can be used for the return conductor on sheet metal machines.

Claims

1. An apparatus for reducing the bearing current in an electrical machine where a stator (4), with its magnetic circuit, is supported on the frame of the machine and a rotor (6), with its magnetic circuit, is arranged at an air gap (22) away from the stator on an axle shaft (8) supported on the frame (10) of the machine using bearings (12), whereby the stator, at least, comprises a magnetically conductive yoke (20) and the main magnetic flux of the machine is closed through the stator magnetic circuit, the air gap and the rotor magnetic circuit, characterised in that the machine is fitted with at least one closed suppression winding (40), essentially arranged around the stator (20) so that the main magnetic flux of the machine goes through the coil ring formed by the suppression winding (40) so that the total electromotive force induced by the main flux in the closed suppression winding (40) is essentially zero.

2. An apparatus according to Claim 1, characterised in that the suppression winding (40) is arranged toroidally around the stator yoke (20).

3. An apparatus according to Claim 1 or 2, characterised in that the suppression winding wire (52) is fitted in the groove opening (54) of the stator.

4. An apparatus according to Claim 1 or 2, characterised in that the suppression winding wire (58, 62) is fitted onto the bottom of the stator groove (16) or on the air gap surface (66) of a stator spur (60).

5. An apparatus according to any of the Claims from 1 to 4, characterised in that the suppression winding wire is fitted onto the outer surface of the back section of the stator (46) or on a groove (64) arranged on the outer surface of the stator.

6. An apparatus according to any of the Claims from 1 to 5, characterised in that the number of turns in the suppression winding is at least two, the number of phases in the machine multiplied by the number of poles, or any multiple of this.

7. An apparatus according to any of the Claims from 1 to 6, characterised in that the coils in the suppression winding are connected in series by pairs or coil groups, or all the coils are connected in series.

8. An arrangement according to any of the Claims from 1 to 7, characterised in that the suppression winding is short-circuited.

9. An arrangement according to any of the Claims from 1 to 7, characterised in that the suppression winding circuit is closed by a resistor (50).

10. A method for reducing the bearing current in an electrical machine (2) having a stator (4) and a rotor (6) rotating in relation to the stator, the rotor being fitted on the machine frame (10) by bearings, characterised in that a closed suppression winding (40) is arranged around the magnetic circuit of the electrical machine's stator so that the electromotive force induced in this winding by the main flux is zero and that the high- frequency flux causing the bearing current is used to induce a voltage in the suppression winding (40) and the electrical power induced in the suppression winding is consumed in the suppression winding or in a resistor (50) fitted in it.