FI117362B - Common-mode power filter, frequency converter and electric motor - Google Patents

Description

117362

Common mode filter, frequency converter and electric motor

BACKGROUND OF THE INVENTION This invention relates to a common-mode filter. In particular, the invention relates to a filter for filtering a common-mode current in connection with a frequency converter. The invention further relates to a frequency converter and a motor which utilize the features of the filter according to the invention.

The drives produce common mode power to the motor. The common-mode current of a motor is defined as the current generated when the sum of the currents of the motor phases deviates from zero. However, motors are zero-phase devices, so any current flowing to the motor along the phase conductors should also return via the other phase conductors from the motor.

The common-mode current to the motor passes through the stray capacitances of the motor winding and provides a flux that induces a voltage in the ak-15 set of the motor. This shaft-induced voltage is discharged to the ground by a low-impedance path, typically through the motor bearings to the stator. Problems with common mode current are particularly significant with fast switch components, such as frequency converters using IGBT switches.

Because the shaft voltage is bursting through the bearings, the individual current peaks are strong and damage the bearing surface. Damage to the bearings means practically the entire engine running condition ":! *" And eventually results in the engine becoming unusable.

By limiting the common-mode current, a situation can be achieved: ***: where the common-mode current cannot supply voltage to the motor shaft.

• ··: 25 This also prevents the generation of bearing currents caused by this mechanism.

EP 0987813 A2 discloses a circuit for determining common-mode currents of a frequency converter.

. US 5,999,427 A and US 6,288,915 B1 disclose filter * * * 30 solutions for reducing common-mode current. These filter solutions: ···: are based on large transformer rails containing: * · primary and secondary windings.

· »· ·· **; In the past, it has been known to place a ferrite ring in a • engine; around the games to reduce the common mode current. The ferrite ring located around all phases of the motor \ * 35 acts as a choke for common mode current * · * 5, reducing in particular rapid changes in the common mode current. However, the mere ferrite ring 117362 2 as a throttle does not reduce enough common current to solve the problems it produces.

BRIEF DESCRIPTION OF THE INVENTION It is an object of the present invention to provide a common-mode 5-current filter that avoids the above disadvantages and enables the use of frequency converters to control motors without significant echo current problems due to common-mode current. This object is achieved by a filter according to the invention, which is characterized in what is stated in the characterizing parts of the independent claim. Preferred embodiments of the invention are claimed in the dependent claims.

The invention is based on the idea that the common-mode current is limited as a filter comprising a secondary circuit and an associated RC circuit tuned to the desired frequency, whereby the filter acts as a resistance to the single-mode current. Such a structure is simple to place in 15 different locations between the drive and motor depending on the application and its structure. As a filter according to the invention, the common-mode current can be reduced to such an extent that significant problems with bearing currents no longer arise.

The invention further relates to an electric motor and a frequency converter 20 comprising a filter according to the invention. With such structures, common-mode filtering can be done reliably so that the end user of the equipment does not need to know anything about installing the filter.

• * · [* · *. BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in more detail in connection with preferred embodiments, with reference to the accompanying drawings, in which:

Figure 1 illustrates the basic structure of a filter according to the invention;

Fig. 2 shows the positioning of the filter in the terminal block of the electric motor: i; Figure 3 shows the positioning of the filter in the intermediate circuit of the drive; Figure 4 shows the location of the filter at the motor output of the frequency converter.

* * • *

• · I

• »I

• · 3 117362

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 shows the basic structure of a filter according to the invention in connection with the power lines 1 to the motor. In the embodiment of Figure 1, the power cords are motor cables for a three-phase motor.

The filter according to the invention comprises a core 2 made of magnetic material through which the current to the motor passes. A coil 3 is formed around the core 2 with which a resistive component 4 and a capacitive component 5 are connected in series.

As the current flows to the motor, the common-mode current 10 of the core 2 affects the core of the magnetic material so as to induce a magnetic flux. This magnetic flux, which is due only to the common-mode current, also passes through the core through the coil 3. The secondary circuit formed by the coil 3, the resistance 4, and the capacitor 5 is energized by a change in the magnetic flux of the heart.

15 The common-mode current components caused by the drive are located in one or more frequency bands. One of the frequency ranges is located near the drive switching frequency. The filter according to the invention is tuned to a frequency range where the effects of common-mode current components are to be minimized. Tuning the filter to a specific frequency is performed by tuning the resonance frequency of the series resonant circuit formed by the inductance of the coil 3, the resistor 4, and the capacitor 5, to the desired frequency. To achieve resonance, the capacitor cap • · · * ··; the capacitance C and the inductance L of the secondary circuit must be dimensioned such that at the selected angular frequency ω the magnitude of the capacitive reactance Xc corresponds to the magnitude of the inductive reactance XL 25. The impedance Z of the serial circuit is at its lowest resonance frequency according to the following equation «i 1« O Z = jR> + (xc-XLy, • · 1 · 1 30 where Xc = - and Xl = wL. The resonance frequency ωο can be calculated

. ···. COC

thus the equation • · · • t i ··· ·: - * 1 · · • 1 «• · 4 117362

When the secondary series circuit is tuned to a resonance frequency ωο, a purely resistive current path is formed for this frequency common mode current, so that the common mode current does not pass to the motor stator to the extent known in the prior art embodiments. A typical filter tuning frequency is, for example, 40 to 50 kHz.

According to a preferred embodiment of the invention, the core 2 of magnetic material is formed from an amorphous iron material and in particular from a material available under the trade name vitroperm. Such an amorphous iron material contains ultra-crystalline magnetizable particles which render the material of high permeability. The heat loss of the material is also very small, so that the material is well suited for use in a filter according to the invention.

The filter according to the invention has three preferred locations depending on the configuration of the drive and motor controlled by it. According to an embodiment of the invention, the filter is located in the terminal box of a motor controlled by a frequency converter, as shown in Fig. 2 in principle. Terminal block 20 refers to that typically encapsulated structural member of the motor 21 in which the cables to the motor are connected to the stator winding ends 22 of the motor. This embodiment is advantageous in particular because all current going to the motor in the connector housing flows centrally, thus minimizing the length of the core 2. The core may be located as needed around either the ends of the stator windings or, alternatively, around the motor cable ends.

: "[· * 25 An additional advantage of this embodiment is that the common current leaking in the motor cable does not affect the filter. In the motor cable, the phase ·: * current from the conductors to the cable armature passes through the armature ** · ». * · *. back to the drive and thus does not affect the filter in the terminal box. For this reason, the core cross-sectional dimension does not need to consider the # 30 common current in the cable.

• * I

Another advantageous location for the filter according to the invention * ·: * 'is in the intermediate circuit of the frequency converter such that the core 2 circulates both voltage rails 31, 32 of the intermediate circuit, as shown in principle in Fig. 3. -35 from switch voltage through components 30 to phase outputs 33. All current output from the inverter thus passes through the intermediate circuit. * If the total current of the intermediate circuit voltage is zero, part of the drive current does not return through the phase conductors By closing the voltage rails 31.32 with the core 2 of the filter according to the invention, this common-mode current can be reduced.

Such drives include a current circuit which provides an output current for the phase outputs. The filter according to the invention can be applied to a current-to-frequency converter and other known motor control devices in a manner similar to that for a voltage-to-voltage frequency converter.

It will be appreciated that the length of the core will increase with respect to the position shown above, so that the filter according to the invention is preferably placed in the motor terminal box.

Figure 4 shows a third preferred placement of a filter, which is a drive motor output 43 of the frequency converter, wherein the filter core 2 15 surrounds all phase outputs coming from the drive switches 40. The motor output 43 refers to the location in At the motor output, the filter core 2 is positioned around the internal wiring of the drive as shown in Figure 4.

Figures 2, 3 and 4 are intended to illustrate the location of the filter according to the invention and therefore these figures are shown only on a principal level. The filter according to the invention shows only the core 2 in the figures, but it is clear that the filter also includes other functional parts, as will be apparent from this description.

25 The last two options are advantageous in that the filter becomes a part of the drive, so that the user of the drive does not have to: · · consider the filter at all. In addition, the drive is cooled *. it is also possible to influence the temperature of the filter and thus the size of the filter. Although it has been shown above, in preferred embodiments of the invention, to place the filter in a motor or frequency converter, the filter can also be placed directly around the motor cable if necessary.

9 9 **: ** It will be obvious to a person skilled in the art that as technology advances, the basic idea of the invention can be implemented in many different ways. The invention and its embodiments are thus not limited to the examples described above, but may vary within the scope of the claims.

• · • «9 9 9 9 99 9 9

Claims (9)

A common-mode current filter, particularly intended to be used for limiting the common-mode current of a motor and reduction of bearing currents, the filter comprising a cam (2) of a magnetic material, the core of which is arranged to surround the conductive current conductor, characterized by the filter further comprising a secondary circuit comprising a coil (3) wound around the core (2) and a resistive component (4) connected in series and a capacitive component (5). Filter according to claim 1, wherein the frequency range of the common mode current is known, characterized in that the secondary circuit forms a resonant circuit whose resonant frequency has been tuned to the frequency range of the common mode current. Filter according to claim 2, characterized in that the secondary circuit is substantially resistive to common mode current. Filter according to any one of the preceding claims 1, 2 or 3, characterized in that the core (2) is avamorphed iron material. Filter according to any of the preceding claims 1 - 4, characterized in that the filter is arranged in a housing (20) of a motor (21). Filter according to any one of the preceding claims 1-4, wherein the motor is controlled by a frequency converter, characterized in that the filter is arranged in an intermediate circuit of the frequency converter such that the core (2) of the filter surrounds the voltage or current rails (31, 32) of the intermediate circuit. ). Filter according to any of the preceding claims 1-4, wherein **!:. The motor is controlled with a frequency converter, characterized in that the filter is arranged at the motor output (43) of the frequency converter, the core of the filter surrounding the phase outputs of the frequency converter. · Γ 8. Frequency converter comprising a filter according to any of the foregoing ··· · ;;; ruling claims 1-4. * ···: 30 9. An electric motor comprising a filter according to any of the preceding claims 1 - 4. M * t »Ψ 9