DE2437414C3 - Measuring bridge for measuring the capacitance and the dissipation factor of electrical AC voltage capacitors - Google Patents

Description

The present invention relates to a measuring bridge for measuring the capacitance and the loss factor of electrical alternating voltage capacitors, which apply a measuring voltage directly to the test object high-resistance taps, in which the tapped measurement voltage is fed to a comparison capacitor is, on the low voltage side there is a measuring circuit in which at the input of a Analog amplifier there is a balancing point at which between one the current through the test object proportional, inverted current and two against each other 90 phase-shifted comparison currents, a sum is formed and a negative feedback resistor Between the adjustment point and the output of the analog amplifier, a zero adjustment of the currents is forced by the analog amplifier in which the one that occurs at the output of the analog amplifier when the adjustment is incomplete Voltage by controlling balancing devices for the capacity and the loss factor for Disappeared.

Such a measuring bridge is known from DT-OS 35 617. In the well-known measuring bridge comparison currents are compared with the current through the device under test, a zero adjustment is carried out. However, the assignment of the position of the controller to the respective variable to be measured is on The reason for the circuit is not linear. The sensitivity wedge is only over part of the respective measuring range high, but very low in the other part. The reading can only be done using non-linearly calibrated scales respectively. These non-linear quantities are for one

train
the

DC capacitor C _n and a comparison _{resistor R n are} connected so that the second terminal of _{the comparison capacitor C n} or the comparison

_Juia " _u , _16Jl ,"".. _t _._" _ö resistance R _n at the virtual ground point ever

the capacity and the loss factor 5 of an analog amplifier V ₃ , V ₄ is that the positive

,,. τι. · Ι t ■: j ... α _ .. i · :: _ 1. «, .. Ι / Γ1Κ01- pinene WldfT-

matisehen adjustment of a measuring bridge bad

^{According to the} present invention, the task I. «to provide a circuit _{arrangement with input of the ~ analog amplifier V 3} via a resistor

AC voltage capacitors under operating _conditions . _fc "" ,, - ~ _ο - ■ * —-

^v0 ", _n can be carried out automatically stand R _n s-egen mass and via a condensation conditionun _r en a. ^^ ^ ^ ^ ^ virtual mass point of the

^ne n * ■ in the known measuring bridges mechanically analog amplifier V h ₁ lying Pruf- terminal of A / ufuhrenden tuning operations are \ ermie- io astride P is switched such that attenuator A, B so that the .rrlpn Serienmessunacn with reasonable outputs of the respective analog amplifier K ₃ or ^{% vtra} - · · "" I ₄ connect to the adjustment point AP and that

behind the analog amplifier K ₁ for the current through the test object P another analog amplifier K ₂ ge

Effort possible siiid.

This is operated in the case of a measuring device of the type mentioned above Kind achieved in that all used Λι · η Analog amplifiers are wired so that it is 15 switched.

• η virtual mass point at its negative This results in the following relationships:

F? N "an" indicate that the current flowing through the test object is switched directly or via series resistors to! L ₁ virtual ground point of an analog amplifier.

unclearly and that solely through changes the comparison currents a zero adjustment can be achieved.

The invention, its advantages and the advantageous embodiments of the invention will now be discussed Hand of seven figures explained in more detail.

Fi g 1 shows the measuring principle using of an inverting analog amplifier for DUT-

electricity". 0.1 A without taking into account Overaanes and lead resistances: The following applies to zero adjustment:

From (1) results for the imaginary part L _x =

i _Rs = - - _Λ - _: x ^ -

The constants k to Zc ₃ contain the values of the resistors used for the sound.

The currents "through C _n and R _n are proportional to the amplifiers V, and V ₄ in these currents.

-Uj

C _x -

= 0

K,

From (1) results for the real part

L ¹

R,
R,

L "

= 0

(3)

Invert (4), the current has to be inverted by the examinee by means of the amplifier V ₂ again so that the sum of the currents at the calibration point AP can be zero. The zero adjustment is carried out with the help

the attenuator A is made for C _x and B for R _x . The measuring voltage is tapped directly at the test item P with high resistance, the lead resistances are thereby switched off.

In the arrangements shown so far, the

The position of the attenuator B is proportional to the loss resistance R _{x of} the test object P.

The loss factor tan .1 = _{D r} ^{7U cr} ~

K _x ... c _λ

hold, R _x must be multiplied by C _x.

This can be achieved with a suitable arrangement of the attenuators A, B with very high accuracy. Taking into account this requirement and the explanations made so far, the basic sound diagram for a particularly advantageous embodiment of the invention is a circuit according to FIG. 3. This circuit shows the characteristics that a comparison _{capacitor C n is} connected to the pole of the measuring voltage which is not connected to ground. that the other connection of the comparison with the virtual ground point

45

In this arrangement the value of i is _Cv or / _Rn

the capacity or the loss resistance of the test

Somewhat proportional, so there is a linear additional capacitor C _n with the virtual ground point connection between these quantities. An automatic analog amplifier K _{3 is} connected that the matic adjustment is possible, in which the position of the second input of the analog amplifier K ₃ via a corresponding regulator of the capacitance or the capacitor C ₁ with the / wide connection of the loss resistance of the test object is proportional. Test object and via a resistor R ₁₇ to ground

F i g. 2 illustrates an embodiment of the invention 60 connected. that the output of the said represents, the adjustment of which can be sated already easily aulomali- analog amplifier K ₃ in a phase rotation element PIi. So that the measuring arrangement is a phase shifted by 90 ° and to the phase rotation A .. _t """t;_t; „riino KpsQpr is inexperienced, the sections Ph are parallel across a resistor R ₁₂ dem

Ground point of a further analog amplifier V ₄ to-6s, that the phase shift element Ph is followed by a comparison resistor R _n and an attenuator B is connected downstream of this. which / to ground point of said analog amplifier V ₄ forwards that on

can be ized. So that the measurement automation is more accessible, the Ab equal elements placed on the low-voltage side and designed as a purely ohmic attenuator.

The main advantages of the embodiment according to Fi g. 2 are given by the fact that am is not on Moccp iWpnHen connection of the test item P a connection

Output of the analog amplifier K _{4 is} a resistor R ₁₃ and an attenuator A η a is connected to this so that it conducts the current to the adjustment point AP.

In the iV branch there are two inverting amplifiers K i. K _{4 connected in series, so only one amplifier K 1 is} required for the selected reference potential in the λ 'branch.

To establish the comparison conditions

(Λ first the formulas for the voltages and currents occurring in the circuit are given. All constants are not taken into account (C _n : pi; acoustic resistance).

This is permissible if the equality symbol is replaced by the proportionality symbol. The positions of the attenuators A and ß are again recorded as multiplication factors for the associated currents.

(6)

(7)

(H)

(9)

The following applies to the adjustment case

From (6) and (9):

+ B)

(10) from multiplizicrl:

L \, (jC _x +

(10)

jA + .4 · ß - / C _A +

This means that the position of the attenuator A is directly proportional to the capacitance (\. Real part:

This means that the position of the attenuator ß is directly proportional to the loss factor tan λ.

I'm an automatic. To achieve a fully electronic adjustment of the measuring arrangement, the attenuators A and β are controlled by an electronic arrangement according to FIG. 4 formed. This is characterized by the features that a field effect transistor F is located as an attenuator between the voltage to be attenuated and ground, and that the connection of the Fcldeffekt transistor F to the voltage to be attenuated is between two series resistors R _1S . R, g and R _2n , respectively. R ₂₁ is that in the control circuit of each of the two field effect transistors F a reverse pulse generator / with a compared to the measurement

imaginary ^{frequency 35} frequency high pulse repetition frequency, which is the

voltage to be weakened temporarily touches zero.

4,. c (12) that the duty cycle f, T by the product of the

~ attenuating voltage and the voltage on

Output of Abgleichvcrstärkers I _n can be varied in that this product is made by a respective controlled rectifier (> that purpose the voltage at the output of Abglcichverstärkers V "via a low-pass filter TP is supplied to the controlled rectifiers (Ί and that from the output of encoder / an integrating voltmeter \ i is connected to ground.

The attenuators are called so-called T'ime-

R _x A "" Division-Mulliplicrer or tactile regulator executed.

With such arrangements there will be an error of 10 ⁴

For the loss factor of the parallel Ersatzsehailung 50 of the multiplication achieved during a changing-μιΐΐ time r, within the fixed period time 7

the applied voltage is short-circuited or

'(i5) the current flowing through is derived from ground.

'CyR _x The initial voltage for changing the time f,

is the one multiplier. The second multiplier is the voltage to be weakened or the current to be weakened

The mean value of the leakage voltage controlling the switches is directly proportional to the multiplication factor A or fio ß and can be displayed using averaging instruments. The frequency of the square wave voltage is! much bigger

as you. · 1 tvquen / the measuring voltage by a Tiefp.il 6s * 7 / 'after Abgkiclnersl.irkcr V _11, the proportion asifrequin dei 1 / at the AusiMngsspannung \ l on _M-iink uluickt
lan ^Λ * ■ Ii (! Sl \ iis ilusrt .vsichicn Niivj.innsvp.iniiuivj be

A ■ B

(13)

(14)

Weucn

tan Λ

constani becomes from (15 |

lan

(If »

11 Γι in I IM

l.in

11 "1 111 1 Wed

with two phase-controlled rectifiers G obtained the control voltages for the push-button regulator. The reference phases for the controlled rectifiers are already available within the circuit at the output of the analog amplifier V ₃ or the 90 ° phase rotation element Ph.

If the voltage value for the nominal value of C _x is subtracted from the output voltage for C _x in a subtraction circuit, the output voltage of this subtraction circuit is directly proportional to the deviation Δ C _x from the nominal value, and a tolerance measuring bridge is obtained.

The generation of the square-wave control voltages for the attenuators and the evaluation of the multi-

application factor j. can be digitized relatively easily with high accuracy.

So that the arrangement can also be used for test object currents> 0.1 A, the input circuit must can be modified for the test object current.

A current transformer can advantageously be used for this purpose (FIG. 5). The combination of a current transformer and an inverting computing amplifier results in the current transformer in a favorable operating state with zero burden. The inverting analog amplifier V _{2 can be} omitted due to the current converter.

For the measurement by means of high voltage, the circuit according to FIG. 6 advantageous. In this only one comparison capacitor C _{n is} connected to the "hot" end of the test item, ie to the end not connected to ground. The capacitor C _n is followed by an amplifier V ₃ , the virtual ground point of which is connected to the comparison _{capacitor C x} . At the output of the analog amplifier V ₃ , a current proportional to the capacitance is fed via R _{22 to} an attenuator A via this to the comparison point AP . Parallel with the resistor R, a phase-shifting element is connected Ph ₂₂ which rotates the phase of the current to minus 90 °, this comparison opponent country R _n downstream of which leads via an attenuator B on the comparison point AP. The position of the attenuators A and B is directly proportional to the capacitance or the loss resistance. This is evident from the following relationships:

Ύ _ν = ¹¹ U J '"^ n * k ₂ χ Α,
<r _v = ^U \ i <" ^c n * h * ^B ■

The positions of the attenuators A and B are in turn considered as multiplication factors for the associated currents. The constants Jc ₁ to / c ₃ contain the values of the wiring resistances. So that contact transition and lead resistances do not falsify the loss factor measurement, the circuit according to FIG. 7 so expanded that; these resistances are eliminated.

For this purpose, the test item and the shunt are made with ¹ four-pole contact.

The amplifiers V ₁ and F ₃ are advantageously sounded so that they have a high common-mode rejection.

The resistors R ₀ to R ₄ are negative feedback resistances for the associated analog amplification F ₀ to F ₄ .

For this purpose 4 blalt drawings

«09 683/3 *

Claims (4)

Patent claims: 1. Measuring bridge for measuring the capacitance and the loss factor of electrical AC voltage capacitors, which taps a high-resistance measuring voltage directly at the test object, in which the tapped measuring voltage is fed to a comparison capacitor, on the low-voltage side of which there is a measuring circuit in which there is an adjustment point at the input of an analog amplifier , at which a sum is formed between an inverted current proportional to the current through the device under test and two comparison currents phase-shifted by 90 ° from one another, and a zero-adjustment of the currents is enforced by the analog amplifier via a negative feedback resistor between the adjustment pin and the output of the analog amplifier , in which the voltage occurring at the output of the analog amplifier in the event of incomplete adjustment is made to disappear by controlling adjustment devices for the capacitance and the loss factor, characterized in that all analog amplifier are wired so that they have a virtual ground point at their negative input that the current flowing through the device under test is switched directly or via series resistors (R _s , R _1. ) to the virtual ground point of an analog amplifier (K ₁ ). that the output voltage of the analog amplifier is fed to an adjustment point (AP) via a resistor [R ₁₀ ) and that a zero adjustment can be achieved simply by changing the comparison currents <i _(s , i _Rs ). 2. Measuring bridge according to claim 1, characterized in that a comparison capacitor (C _n ) and a comparison resistor (R _s ) are connected to the terminal of the test object (F) not connected to ground, that the second terminal of the comparison capacitor (C _v ) or of the comparison _{resistance (i? v} ) is at the virtual ground point of each analog amplifier (K, or K ₄ ) that the positive input of the analog amplifier [V ₁ ) via a resistor [R ₁₁ ) to ground and via a capacitor (C ₁ ) are connected to the connection of the test item (P) at the virtual ground point of the analog amplifier (K ₁ ) , so that attenuators (A, B) connect the outputs of the corresponding analog amplifiers [V ₃ or V ₄ ) to the adjustment point [AP) _{and that a further analog amplifier (I 2} ) is connected behind the analog amplifiers [V ₁ ) for the current through the test object (F) (see Egg 2). 3. measuring bridge according to claim 1, characterized in that a comparison capacitor [C _s ) is connected to the non-grounded pole of the measuring voltage, that the other terminal of the comparison capacitor [C _n ) is connected to the virtual ground point of an analog amplifier (K ₃ ) that the second input of the analog amplifier (K ₃ ) is connected via a capacitor (C ₁ ) to the second terminal of the test object and via a resistor (R _n ) to ground, that the output of said analog amplifier (K ₃ ) in a phase rotation element (Ph) phase-shifted by 90 ° and fed in parallel to the phase- _{shifting element via a resistor (-R 12} ) to the ground point of a further analog amplifier [V ₄ ) , so that the phase-shifting element can be compared! resistance (Λ _ν ) and this an attenuator (/ is connected downstream, which / around ground point dt called analog amplifier [V ₄ ) forward that at the output of the analog amplifier (K ₄ ) there is a resistor (A ₁₃ ) and this is an attenuation [A) downstream is that it conducts the current to the adjustment point [AP) ! (see Fig. 3). 4. measuring bridge according to one of claims 1 to .1 characterized in that as an attenuator eii field effect transistor (F) between the voltage and ground that is to be weakened that de: connection of the Feldffeki transistor [F) to the voltage to be attenuated between two In series resistors (^ ₁₈ , R ₁₉ or R _2i) R ₂₁ ) is that in the control circuit of each of the two field effect transistors (F) there is a square pulse generator (/) with a high pulse repetition frequency compared to the measuring frequency, which is the one to be weakened Voltage temporarily scans to zero so that the duty cycle (f, T) can be changed by the product of the voltage to be attenuated and the voltage at the output of the balancing amplifier (i ;,), that this product is formed by a controlled rectifier (G) that for this purpose the voltage at the output of the adjustment amplifier (Ιό) is fed to the controlled rectifier (G) via a low-pass filter (TP) and that the output of the pulse generator! /) each has an int the voltage measuring device (Λ /) is connected to ground (s. Fig. 4).