Overvoltage protection circuit

Abstract

The present invention relates to an overvoltage protection circuit (1) for protecting the electronics of a motor, in particular of an EC motor, against overvoltage pulses, with two protective devices (FS1, FS2) arranged in series connection between two connections (10, 20), wherein a resistor (R1) or (R2) is connected in parallel to each of the protective devices (FS1, FS2) and at least one capacitive element (C1) is provided in parallel connection to the first protective device (FS1), wherein the overvoltage protection circuit (1) has, between the connections (10, 20), at least a first (lower) and a second (higher) breakdown voltage point at a voltage U.sub.Z1 or U.sub.Z2 dependent on the voltage change over time k=(dU/dt) of a voltage U.sub.GA at the connections (10, 20).

Claims

1. An overvoltage protection circuit for protecting the electronics of a motor, in particular of an EC motor, against overvoltage pulses, the overvoltage protection circuit comprising: two protective devices (FS1, FS2) which are arranged in series connection between two connections, wherein a resistor (R1) or (R2) is connected in parallel to each of the protective devices (FS1, FS2) and at least one capacitive element (C1) is provided in parallel connection to the first protective device (FS1), wherein the overvoltage protection circuit has, between the connections, at least a first lower and a second higher breakdown voltage point at a voltage U.sub.Z1 or U.sub.Z2 dependent on the voltage change over time k=(dU/dt) of a voltage U.sub.GA at the connections, wherein each protective device (FS1, FS2) has a voltage-dependent resistor and is insulating below a respective protective device-specific breakdown voltage and, respectively, is conductive above the corresponding breakdown voltage; wherein the protective devices (FS1, FS2) are designed as gas discharge tubes, overvoltage arresters or varistors; wherein in the case of a rapid voltage change of the voltage U.sub.GA to the breakdown voltage U.sub.Z1, wherein, if k is greater than a system-specific value k.sub.spez, at least one of the two protective devices (FS1, FS2) becomes conductive or an electric arc is ignited; and wherein in the case of a slow voltage change of the voltage U.sub.GA, wherein, if k is smaller than a system-specific value k.sub.spez, one or both protective devices (FS1, FS2) become conductive only when the second higher breakdown voltage U.sub.Z2 has been reached or an electric arc is ignited in at least one of the protective devices (FS1, FS2).

2. The overvoltage protection circuit according to claim 1, wherein a series connection of at least two protective devices (FS1, FS2, . . . FSn) is provided, wherein in each case with a parallel resistor (R1, R2, . . . , Rn) arranged relative to the respective protective device (FS1, FS2, . . . FSn) and/or a capacitive element (Cl, C2, . . . , Cn) arranged in each case in parallel connection.

3. The overvoltage protection circuit according to claim 1, wherein the capacitive elements(s) (C1, C2, . . . , Cn) represent(s) a capacitor.

4. The overvoltage protection circuit according to claim 1, wherein said overvoltage protection circuit is arranged at the neutral point of a varistor circuit in the input filter of the EC motor.

5. A method for protecting the electronics of a motor, in particular of an EC motor, against overvoltage pulses by means of an overvoltage protection circuit according to claim 1, providing the protective devices (FS 1, FS2) designed as gas discharge tubes, and, depending on the edge slope of a voltage U.sub.GA applied between the connections, igniting an electric arc in at least one of the gas discharge tubes (FS1, FS2), and limiting the voltage between the connections to a corresponding lower burning voltage U.sub.Br of the electric arcs in the gas discharge tubes.

6. The method according to claim 5, wherein, in the case of a slow increase of the voltage, a high or higher breakdown voltage U.sub.Z2, with respect to the breakdown voltage U.sub.Z1, is reached, which preferably corresponds to the sum of the individual ignition voltages of the gas discharge tubes (FS1, FS2), igniting an electric arc is ignited in one of the gas discharge tubes, and, on the other hand, in the case of a rapid increase of the voltage, igniting an electric arc, preferably in the two gas discharge tubes (FS1, FS2) immediately in succession, when the lower breakdown voltage U.sub.Z1 is reached, which preferably corresponds to the ignition voltage of the gas discharge tube (FS2) without parallel-connected capacitive element (C1).

Description

(1) Other advantageous developments of the invention are characterized in the dependent claims or represented in further detail below together with the description of the preferred design of the invention in reference to the figures.

(2) FIG. 1 shows a first embodiment example of an overvoltage protection circuit according to the invention;

(3) FIG. 2 shows the response characteristic in the case of a slow voltage increase with a high breakdown voltage U.sub.Z2 and the voltage drop to the burning voltage U.sub.Br;

(4) FIG. 3 shows the response characteristic in the case of a rapid voltage increase with a low breakdown voltage U.sub.Z1 and the voltage drop to the burning voltage U.sub.Br;

(5) FIG. 4 shows an equivalent circuit diagram of the voltage distribution;

(6) FIG. 5 shows the voltage characteristic of the voltage U.sub.2 from FIG. 4 versus the total voltage;

(7) FIG. 6 shows the curve of the ignition voltage of the protection circuit with respect to the ignition voltage of the second gas discharge tube as a function of the rise time of the applied voltage in the case of symmetric dimensioning of the parallel resistors;

(8) FIG. 7 shows a second embodiment example of an overvoltage protection circuit according to the invention;

(9) FIG. 8 shows a third embodiment example of an overvoltage protection circuit according to the invention;

(10) FIG. 8a shows a detail of the circuit from FIG. 8, and

(11) FIG. 9 shows an arrangement of the protection circuit according to FIG. 1 at the neutral point of a varistor circuit (20) in the input filter (30) of a three-phase supplied EC motor.

(12) Below, the invention is described in reference to exemplary embodiments, wherein identical reference numerals mark structurally and/or functionally identical features.

(13) In FIG. 1, a first embodiment example of an overvoltage protection circuit 1 according to the invention is shown, which consists of two gas discharge tubes FS1 and FS2, two resistors R1 and R2, and a capacitor C1, wherein the two gas discharge tubes FS1 and FS2 are connected in series, and, in each case, a resistor R1 or R2 is connected in parallel to the gas discharge tubes FS1 or FS2, and the capacitor C1 is connected parallel to the gas discharge tube FS1. The voltage U.sub.GA is applied between the connections 10, 20.

(14) FIG. 2 shows the response characteristic in the case of a slow voltage increase with a high breakdown voltage U.sub.Z2 and the subsequent voltage drop to the burning voltage U.sub.Br, and FIG. 3 shows the response characteristic in the case of a rapid voltage increase with a low breakdown voltage U.sub.Z1 and the voltage drop to the burning voltage U.sub.Br.

(15) The function of the overvoltage protection circuit 1 consists in controlling the breakdown voltage of the overall arrangement depending on the edge slope of a voltage U.sub.GA applied between the connections 10, 20 of the circuit. I.e., the voltage starting at which an electric arc is ignited in one or both gas discharge tubes FS1, FS2, and thus the voltage U.sub.GA between the terminals is limited to the correspondingly lower burning voltage U.sub.Br of the electric arcs in the gas discharge tubes. In this way, in the case of a slow increase of the voltage U.sub.GA according to FIG. 2, as occurs, for example, in a performed high-voltage test of the complete apparatus, a high breakdown voltage U.sub.Z2 (preferably the sum of the individual ignition voltages of the gas discharge tubes FS1 and FS2) is reached.

(16) On the other hand, in the case of a rapid increase of the voltage U.sub.GA, as shown in FIG. 3, as occurs, for example, in the case of an overvoltage pulse (surge pulse) from the grid or in the case of the surge testing of the entire apparatus, a correspondingly lower breakdown voltage U.sub.Z1 is reached. This preferably corresponds to the ignition voltage of the gas discharge tube FS2. Reference numeral Z is used to represent the breakdown point of the overvoltage protection circuit 1.

(17) In the unignited state, the gas discharge tubes FS1 and FS2 have almost the same characteristics as a capacitor with very low capacitance. This capacitance can be neglected in case of additional circuit elements dimensioned so as to have sufficient low resistance. Consequently, the distribution of the voltage on the series-connected gas discharge tubes FS1 and FS2 in this state is determined only by the additional circuit, or the gas discharge tubes FS1 and FS2 are in an idle state. In FIG. 4, as an explanation of the voltage distribution of the voltage, U.sub.0 or U.sub.1 and U.sub.2 is/are represented in the equivalent circuit diagram. Based on this equivalent circuit, the voltage curves over time U.sub.1 and U.sub.2 at the gas discharge tubes and the ignition voltage of the overall arrangement to be expected therefrom are determined as a function of the edge slope/rise time.

(18) From
u.sub.1(t)=u.sub.0(t)u.sub.2(t)=u.sub.0(t)R.sub.2.Math.i.sub.0(t)
and

(19) $i_0\left(t\right)=\frac{u_1\left(t\right)}{R_1}+C_1.Math.\frac{{\mathrm{du}}_1\left(t\right)}{\mathrm{dt}},$
one obtains after insertion

(20) $u_1\left(t\right)=u_0\left(t\right)-\frac{R_2}{R_1}.Math.u_1\left(t\right)-R_2.Math.C_1.Math.\frac{{\mathrm{du}}_1\left(t\right)}{\mathrm{dt}},$
and after rearrangement

(21) $u_0\left(t\right)=\left(1+\frac{R_2}{R_1}\right).Math.u_1\left(t\right)+R_2.Math.C_1.Math.\frac{{\mathrm{du}}_1\left(t\right)}{\mathrm{dt}}$
By Laplace transformation, one obtains the voltage U.sub.1 as

(22) $U_1\left(s\right)=\frac{1}{\left(1+\frac{R_2}{R_1}\right)+R_2.Math.C_1.Math.s}.Math.U_0\left(s\right)=\frac{R_1}{R_1+R_2}.Math.\frac{1}{1+\frac{R_1.Math.R_2}{R_1+R_2}.Math.C_1.Math.s}.Math.U_0\left(s\right)$

(23) Assuming a ramp with edge slope or slope k, one obtains the voltage U.sub.0 as:

(24) $u_0\left(t\right)=\{\begin{array}{c}0,t<0\\ k.Math.t,t0\end{array}\stackrel{\mathrm{Laplace}}{}{U}_0\left(s\right)=\frac{k}{s^2}$
and thus the resulting voltage U.sub.1 as

(25) $U_1\left(s\right)=k.Math.\frac{R_1}{R_1+R_2}.Math.\frac{1}{\left(1+\frac{R_1.Math.R_2}{R_1+R_2}.Math.C_1.Math.s\right).Math.s^2}=k.Math.\frac{R_1}{R_1+R_2}.Math.\frac{1}{\left(1+.Math.s\right).Math.s^2}$$\mathrm{where}=\frac{R_1.Math.R_2}{R_1+R_2}.Math.C_1$

(26) Using the following Laplace correspondence

(27) $u_0\left(t\right)=.Math.e^{-\frac{t}{}}+t-\stackrel{\mathrm{Laplace}}{}{U}_0\left(s\right)=\frac{1}{\left(1+.Math.s\right).Math.s^2}$
one gets the voltage U.sub.1 in the time domain as

(28) $u_1\left(t\right)=k.Math.\frac{R_1}{R_1+R_2}.Math.\left(.Math.e^{-\frac{t}{}}+t-\right)$

(29) The voltage U.sub.2 is thus obtained according to the following formula:

(30) $u_2\left(t\right)=k.Math.\left(t-\frac{R_1}{R_1+R_2}.Math.\left(.Math.e^{-\frac{t}{}}+t-\right)\right)$$\frac{u_0\left(t\right)}{u_2\left(t\right)}=\frac{1}{1-\frac{R_1}{R_1+R_2}.Math.\left(\frac{}{t}.Math.e^{-\frac{t}{}}+1-\frac{}{t}\right)}$
or with time normalized to the RC time constant , as

(31) 0$\frac{u_0\left(t^{*}\right)}{u_2\left(t^{*}\right)}=\frac{1}{1-\frac{R_1}{R_1+R_2}.Math.\left(\frac{1}{t^{*}}.Math.e^{-t^{*}}+1-\frac{1}{t^{*}}\right)}\mathrm{with}{t}^{*}=\frac{t}{}$

(32) Accordingly, from FIG. 5 one gets the voltage curve of the voltage U.sub.2 from the in relation to the total voltage U.sub.0. The curve over time of the voltages U.sub.0 and U.sub.2 for different rise times and a respective symmetric dimensioning of the resistors R1 and R2 (R1=R2) is represented.

(33) The curve of the ignition voltage of the protection circuit with respect to the ignition voltage of the second gas discharge tube FS2 as a function of the rise time of the applied voltage U.sub.0 with symmetric dimensioning of the parallel resistors R1 and R2 (R1=R2) can be obtained from FIG. 6. After the ignition of the gas discharge tube FS2, the voltage drop U.sub.1 is determined only by its burning voltage U.sub.Br. In an idealized fashion, one can assume a short circuit here. Thus, below, the result is that the voltage U.sub.2 is equal to the voltage U.sub.0 which was already high enough to ignite the gas discharge FS2, and which is consequently now also sufficient for the direct ignition of the gas discharge tube FS1. Therefore, the two gas discharge tubes FS1 and FS2 are ignited immediately after the ignition voltage on the gas discharge tube FS2 is reached. The total voltage at which the gas discharge tube FS2, and consequently also the gas discharge tube FS1, is ignited is determined by the rise time of the voltage applied to the overall arrangement in relation to the time constant of the additional circuit, whereby the response characteristic according to the invention can be achieved.

(34) In FIG. 7, a second embodiment example of an overvoltage protection circuit 1 according to the invention is shown, and in FIG. 8, a third embodiment example of an overvoltage protection circuit 1 according to the invention is shown. The overvoltage protection circuit 1 according to FIG. 7 differs from the first embodiment example according to FIG. 1 in that, furthermore, a capacitor C2 is connected in parallel to the gas discharge tube FS2, while in FIG. 8 there is an overvoltage protection circuit 1 by means of a cascading arrangement of two to n circuits GAi according to FIG. 8a, which in each case consist of gas discharge tubes FSi (FS1, FS2, . . . FSn) with parallel resistor Ri (R1, R2, . . . , Rn) and/or in each case parallel-connected capacitor Ci (C1, C2, . . . , Cn).

(35) FIG. 9 shows an arrangement of the overvoltage protection circuit 1 according to the invention according to FIG. 1 at the neutral point of a varistor circuit (20) in the input filter (30) of a three-phase supplied EC motor.

(36) The design of the invention is not limited to the above-indicated preferred embodiment examples. Instead, many variants are conceivable, which make use of the solution represented even in designs of fundamentally different types.