GB2285350A

GB2285350A - Apparatus for filtering harmonics

Info

Publication number: GB2285350A
Application number: GB9424988A
Authority: GB
Inventors: Matti Kahkipuro; Nigel Smith
Original assignee: Kone Elevator GmbH
Current assignee: Kone Elevator GmbH
Priority date: 1993-12-28
Filing date: 1994-12-12
Publication date: 1995-07-05
Anticipated expiration: 2014-12-12
Also published as: FI94196C; FI935907A0; FI94196B; GB2285350B; GB9424988D0

Abstract

In a circuit for the suppression of harmonics produced by a diode bridge 2, a resonant circuit 14, 16 with a resonant frequency (fres) equalling six times the mains frequency (fn) or its multiple is connected to the diode bridge. In this case, the harmonic above and below the resonant frequency is filtered out from the supply current e.g. the fifth and seventh harmonics. <IMAGE>

Description

APPARATUS FOR THE FILTERING OF HARMONICS The present invention relates to an apparatus for the filtering of harmonics as defined in the preamble of claim 1 Using a three-phase diode bridge, a three-phase a.c. voltage is rectified to produce a d.c. voltage. The diode bridge generates harmonics in the mains current. Of these, the fifth and seventh harmonics are especially problematic as their amplitude is often of a troublesome order.

Increasingly strict demands are being placed on devices connected to the mains network as regards the effects they produce in the network. The mains filter normally used does not filter the above-mentioned fifth and seventh harmonics. The same also applies to the eleventh and thirteenth harmonics.

The object of the present invention is to reduce the spurious effects generated in the mains network by a diode bridge. To achieve this, the invention is characterized by the features presented in the characterization part of claim 1.

With the solution of the invention, the proportion of the fifth and seventh harmonics in the mains current is substantially reduced by using a filter design with a resonant frequency equalling six times the mains frequency. In this way, a single filter circuit can be used to filter two harmonics.

Similarly, the 11. and 13. harmonics are filtered when the resonant frequency is twelve times the mains frequency.

Especially in reserve power drives employing regenerative braking, the effect of these harmonics has to be compensated by increasing the wattage of the braking resistor. By connecting a filter to the d.c. circuit as provided by the invention, the braking resistor and its control electronics can be replaced with--a cheaper and simpler solution. With the solution of the present invention, a sufficient filtering level is achieved at a low cost.

In the following, the invention is described by the aid of an example of its embodiments by referring to the drawings, in which - Fig. 1 presents a circuit as provided by the invention, - Fig. 2 presents the mains current frequency spectrum in a known solution, and - Fig. 3 presents the mains current frequency spectrum when the filtering is implemented according to the invention.

Fig. 1 presents the main circuit of a three-phase diode bridge, comprising a harmonic filter as provided by the invention. The phase conductors of the supplying network are connected to the input conductors 4, 6, and 8 of the diode bridge, which carry the mains currents I1, 12 and 13 The diode bridge consists of a bridge circuit of six diodes D1. . . D6 and, via the first and second output conductors 10 and 12, supplies a current 14 to capacitor C2 in the d.c.

circuit. Connected to the first output conductor 10 between the diode bridge 2 and capacitor C2 is a resonant circuit 14 consisting of a coil La and a capacitor Ca in parallel.

The resonant frequency f of the resonant circuit is calculated from the formula f = 1/(2 n vL*C) , where L = inductance of coil L1, and C = capacitance of capacitor C.

The resonant circuit 14 is so designed that its resonant frequency equals six times the mains frequency. When the supply network frequency is f =50 Hz, the resonant frequency of the resonant circuit is thus f = 300 Hz. Unexpectedly, it has been found that this design has the favourable effect of filtering the fifth and seventh harmonics of the phase current.

In a corresponding manner, another resonant circuit 16 with a resonant frequency equal to twelve times the mains frequency is connected to the d.c. circuit. Resonant circuit 16 is composed of a capacitor C3 and a coil L3 connected in parallel, and it suppresses the 11th and 13th harmonics.

Furthermore, to filter the 17th and 19th harmonics, a resonant circuit with a resonant frequency equalling 18 times the mains frequency can be used. The number of resonant circuits and the resonant frequency are determined according to the need in each application.

In general, the resonant frequency f of a resonant circuit connected to the d.c. circuit as provided by the invention can be defined from the equation: f = 6 n f n feces n where n may have an integer value, n = 1,2,3,...

The corresponding harmonics fh filtered out from the mains current are: = = (6 n n " fn and f = (6 n + 1) h Figures 2 and 3 present measured magnitudes of the harmonics in mains phase current Iq, both as obtained without using the solution of the invention (Fig. 2) and by using a resonant circuit as provided by the invention (Fig. 3). The resonant circuit used consisted of a capacitor with a capacitance of 133.2 uF and a coil with an inductance L of about 2 mH. As indicated by the measurement results, the attenuation of the fifth harmonic (fh=250 Hz) is about 5 dB and that of the seventh harmonic ( fah=350 Hz) about 10 dB when a resonant circuit 14 according to the invention is used.

Claims

1. Apparatus for the suppression of harmonics ( fh) generated by a diode bridge (2) connected to a three-phase network, which diode bridge is used to rectify an alternating voltage and to feed a capacitor (C2) connected between the first and second output conductors (10,12) of the diode bridge (2), characterized in that it comprises a resonant circuit (14) which is connected to the second output conductor (10) of the diode bridge between the diode bridge (2) and the capacitor (C2) and which has a resonant frequency (frets) substantially equal to the supply network frequency (f ) multiplied by 6*n, n being an integer.

2. Apparatus according to claim 1, characterized in that the resonant frequency (frets) of the resonant circuit (14) equals substantially six times the frequency ( fn ) of the supply network.

3. Apparatus according to claim 1, characterized in that it comprises at least two resonant circuits (14,16) connected in series to the output conductor (10) between the diode bridge (2) and the capacitor (C2), said resonant circuits having a resonant frequency (fres) substantially equal to the supply network frequency (fn) multiplied by 6*n, n being an integer such that each resonant circuit (14,16) has a different resonant frequency (fres).

4. Apparatus according to claim 3, characterized in that, in series with the output conductor, it has a resonant circuit (14) with a resonant frequency equalling six times the supply frequency and a resonant circuit (16) with a resonant frequency equalling twelve times the supply frequency.

5. Apparatus according any one of the preceding claims, characterized in that the resonant circuit (14,16) consists of a coil (L;L3) and a capacitor (C1 ;C3) connected in parallel.