DE1251429B - Isolation meter with a DC voltage converter - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

GOIr

German class: 2Ie-29/12

Number: 1251429

File number: L 51052IX d / 21 e

Filing date: July 6, 1965

Open date: October 5, 1967

There are already measuring devices for measuring the insulation of systems, machines and devices by means of direct voltage. The F i g. 1 shows the basic circuit diagram of such a measuring device. A known measurement voltage U _x is applied to a test object / ?! via a series resistor R _v. and the current I _x flowing through this test object is measured by means of an instrument J. The current I _x represents a measure of the resistance of the test object R _x . The scale function of the ammeter / results from the equation

The measurement voltage U _x is generally obtained from an accumulator or an element, optionally also from the network by rectification (useful for continuous measurements) via a transistor DC voltage converter, which supplies the measurement voltage U _x or the test voltage U _x (on the test soling R _x ) supplies the required level of 500 V or 1000 V, for example.

In order to obtain correct measurement results, the measurement voltage U _{x used as a} basis for the scale calibration of the ammeter / calibrated in resistance values (kQ, ΜΩ) must be recorded regardless of the state of charge of the supply source. F i g. 2 shows a known circuit which fulfills this requirement. In this case, the measurement voltage U _x is generated, for example, by a schematically illustrated transistor DC voltage converter 1, and the source voltage Ub supplied to this DC voltage converter 1 is stabilized by means of a transistor control circuit 2.

Formed in this manner, known per se, have Isolationsmeßgeräte that the _x lying on the specimen test voltage U R changes the disadvantage of _x depends on the resistance of the specimen R _x in a wide range. According to the relevant regulations, a certain minimum value for the level of the test voltage U _x must now be observed for the measurement at a certain minimum value (e.g. for 1 kΩ / V) for the test object R _x . In the case of very high-resistance test objects R _x , the test voltage U _{x must} accordingly assume very high values. As a result, not only are the test objects unnecessarily stressed, but the components used in the insulation measuring device must also be dimensioned for this greatest test voltage U _x. This disadvantage can be reduced if the series resistor Rv in series with the test object R _x is selected to be small. But then the short-circuit current h increases and thus the load of the

Isolation measuring device with a
DC voltage converter

Applicant:

Licentia Patent-Verwaltungs-G. m. b. H.,

Frankfurt / M., Theodor-Stern-Kai 1

Named as inventor:
Gerhard Busse, Munich

DC voltage converter 1 strongly. In addition, there is an unfavorable scale profile for the instrument /, since the area with low resistance values then takes up a large part of the scale length of the ammeter J and the measuring range that is mainly of interest for large resistance values of the test object R _x only takes up a small part of the scale length of the ammeter J.

Is in the known measurement of the insulation of electrical machines, in which for judging the insulation state of the 1 minute and 10 minutes after application of the test voltage machine _x U _x recorded on the test part R current I _x used to form a Nachladezahl, carries a current I _x dependent test voltage U _x directly leads to incorrect measurement results.

An insulation measuring device is also known in which these disadvantages are eliminated in that the test voltage U _{x is} kept constant from a certain test current In (for example 1 mA) downwards (I _x S In). However, this known device supplies a constant current from the value I _x = In , in the range I _x > In, ie when the resistances R _{x fall.} For this purpose, the instrument previously switched as an ammeter in the area I _x ^ In is now switched as a voltmeter parallel to the test item R _x .

This known insulation measuring device has the following disadvantages: The circuit of the display instrument as an ammeter or as a voltmeter represents an additional switching process that is a larger one technical effort results and the operation of the isolation measuring device made difficult. Also results an additional scale division for the measuring range in which the display instrument acts as a voltmeter works, because here the scale function according to the equation

R _x = -JL Kt U _x (I _x = const.)
I _x

runs.

709 650/164

3 4

The invention has set itself the task, which falls on him a voltage of 500 V, so the must

Disadvantages of the insulation measuring voltage U _x ' described above are higher because of the series resistance

avoid measuring devices. be.

The invention is based on a device for Er- To remedy this, a series resistor could be used averaging the electrical insulation resistance of a 5, which changes its value depending on the current. Would Test object that should flow through this at a current between 0 and 1 mA on the measurement a known, constant voltage flowing, this resistance must be zero. The current gets bigger Current is based and automatically acting means contain as 1 mA, then the value of the pre-resisting would have to be, which change the measuring voltage accordingly from a given point onwards. Resistors with one The lower value of the current flowing through the test object io such sharp break point are not reduced at the moment will. According to the invention, such is obtainable.

Device in the manner indicated by claim 1. According to the invention, the device according to the

designed. F i g. 3 such a function of a bridge

The invention is achieved with a further advantageous disconnection ST , which the control circuit 2 forms on the basis of FIGS. 3 to 5 shown. The bridge circuit ST is explained in more detail in the exemplary embodiments exhibited. input circuit and consists of a first

The insulation measuring device according to the invention after the series connection of two resistors 3, 4 and a F ig. 3 has a second series connection of a resistor 5 with (cf. also FIG. 4) a first working resistor and a Zener diode7, which can be seen from the scale of the instrument J. Both areas, the one between R _x = 00 and R _x - 1000 Ω / V 20 series connections are connected in parallel. The loser. At 1000 Ω / V, a connection point between resistors 5 and 6 and the current of 1 mA flows through the test item R _x. This corresponds to the VDE conditions tapping an actual voltage divider 8 and the connections. In this working range, the starting point of resistors 3 and 4 at the base of the direct voltage U _x should be as constant as possible. It may lead control transistor T _{1 of} the control circuit 2,
in no case be smaller, since then the regulations 25 It is assumed for the time being that a test object current is no longer complied with. As indicated above, 1 mA flows from I _{x S.} The resistors 3 and 4 are, however, they should not be significantly larger. This is dimensioned in such a way that in this current range the 3 insulation measuring device has a second, from the same and 4 occurring voltage drop smaller than the working range visible on the scale, which is between the breakdown voltage of the Zener diode7, i.e. it is 1000 Ω / V and R _x = 0 . This area must also 30 no current flows through the resistors 5 and 6. Which are present on the instrument scale, since the comparison branch of the stabilizing circuit does not care that a short circuit, i.e. R _x = 0, led control voltage U _s t can be read off the scale in the range. This means that a distinction must be made between I _x g 1 mA, i.e. only from the voltage drop at 3 ge, whether the insulation resistance is poor or forms a. This control voltage now causes a real short circuit. In the case of insulation meters 35, the measurement voltage U _x 'increases with increasing current with suppressed R _x = 0 , this determination is I _x . Resistance 3 is now dimensioned in such a way that the am is not possible. Series resistor R _v occurring voltage drop so

The insulation measuring device according to FIG. 3 is now compensated for that the

designed so that the output DC voltage U _x rich I _x ^ 1 mA desired constant test voltage

sets after exceeding the operating point 1000 Ω / V 40 U _x.

(1 mA, i.e. within the range 1000 Ω / V and if the current I _{x is} greater than ImA, then R _x = O) automatically reduces, as is the case in the voltage drop across the resistors 3 and 4 grams after the Fig. 4 is shown. This results in the breakdown voltage of the Zener diode7 having the following advantages: The connected test object R _x and exceeded. With this, the resistor 5 supplies in this area, with the output 45 remaining constant, an adjustment voltage U _{x which is in} phase opposition to the voltage drop at 3 can be damaged under certain circumstances to the control voltage U _s t, so that in the area the. If, however, the DC output voltage drops, then I _x > 1 mA, the control voltage will initially decrease, which will definitely be avoided. It would also go to zero and reverse its direction. With the output voltage kept constant, this has increased the stabilization circuit now the desired drop in power output will be necessary. The voltage present at the test object R _x 5 ° of the voltage U _x and thus the voltage U _{x in relation} to the voltage drops when the sequence is overwritten.

1 mA point. The display instrument used / it can thus normally formed Vorwiderist an ammeter. With a test current I _x = 0 , R _{v has} to be used, the control panel of this instrument being its final resistance value (mechanism 2 the function of a current-dependent counter-niche zero position). The lower prescribed VDE 55 version takes over, so that despite the unchangeable point (1 mA, In in Fig. 4) and short circuit (R _x = 0) series resistor R _O, the requirement is met that identifies the area of the no longer permissible when the output direct current I _{x is} exceeded by resistors. This range on the scale should be 1 mA, the voltage U _x at the test item R _x always smaller and smaller and the larger part of the scale should become the actual (see also diagram Fig. 4). The power measuring range, ie 00 to 1000 Ω / V (1 mA) output will still increase, but not be retained. The short-circuit current that occurs should be as strong as with constant voltage on the test object, if possible, taking into account the insulation measuring device,
not be much greater than 1 mA. The series resistor R _v could actually also

To limit the current, the series resistor R _{v could} be omitted entirely, since higher output DC currents are used, with a constant 65 than 1 mA assuming the drop in the output DC voltage at the test object. If the voltage U _x by the circuit ST in connection with the test object is, for example, still in the prescribed range of the control circuit 2. However, it is from rich, i.e. something under ImA (I _n in Fig. 4), and provided for the following reasons: If it were not

Claims (2)

exist, the DC-DC converter 1 would work in the event of a short circuit at a low DC output voltage, but viewed from the impedance point of view, it operates in a short circuit and it would fail. This would again require measures to keep the converter 1 still functional even if a short circuit occurred. The resources required for this are quite expensive compared to a series resistor. The short-circuit current is kept small by reducing the DC output voltage Ux. The present DC voltage converter 1 does not need to be short-circuit proof. The series resistor Rv is simply used here. This series resistor is dimensioned in such a way that when there is a test object Rx = 0 (output direct current slightly above 1 mA), which is also available as a point on the scale, converter 1 does not stop at the moment. The aim is to keep this series resistor as small as possible in order to obtain the smallest possible voltage increase (voltage drop at the series resistor). 5 shows the extraction of the actual voltage for the control circuit 2. According to a further embodiment of the invention, the actual value for the control circuit 2 is derived from the transformer T of a known inverter W belonging to the DC / DC converter. A special winding W1 of the transformer T is used. In principle, it would be possible to use a separate transformer fed by the transformer Γ instead of this winding. The EMF of the secondary winding w2 is transmitted through the winding W1 with a certain transmission ratio. The alternating voltage occurring at the winding W1 is rectified by means of a rectifier circuit 9, 10, 11, 12 acting as a summing circuit. With the rectifier circuit used, there is a real doubling of the AC voltage. In the case of a one-way circuit, the required symmetry of the alternating voltage does not result, and in the case of the push-pull circuit it has been shown that the stabilization does not occur to the desired extent. The actual voltage occurs at the tap of a voltage divider 13. The winding W1 is also used to generate the setpoint. The rectified voltage of the winding W1 feeds a Zener diode 15 via a resistor 14, the voltage drop of which is the setpoint voltage. The tap of the voltage divider 13 and the base of the control transistor T1 are led to the diagonal points of the control circuit ST. Patent claims: 1. Device for determining the electrical insulation resistance of a test object, which is based on the measurement of the current flowing through it at a known, constant voltage and contains automatically acting means by which the measuring voltage is reduced from a specified value of the current flowing through the test object, characterized in that a DC voltage converter (1) stabilized by a transistor control circuit (2) serves as the measuring voltage source, as is known per se for insulation measuring devices, so that the output current of the DC voltage converter (1) flows through the two parallel paths of a bridge circuit (Sr), each of which flows out the series connection of two resistors (3, 4; 5, 6), but one also consists of a Zener diode (7) lying in series with its resistors, and that the transistor control circuit (2) at the connection points of the resistors (3, 4; 5, 6) output voltage of the bridge [St) taken from the parallel paths in such a way Sense and is inserted in such a way that it is effective as an additional control variable in the sense of reducing the measurement voltage, which has been constant up to that point, as soon as the forward voltage of the Zener diode (7) is exceeded. 2. Apparatus according to claim 1 with a transistor inverter as a DC / DC converter, characterized in that as an actual value transmitter for the transistor control circuit (2) a special winding (W ₁ ) of the inverter [W) and as a setpoint generator, another also fed by this winding (W ₁₎ Zener diode (15) is used. 3. Apparatus according to claim 2, characterized in that a rectifier circuit (9 to 12) which doubles the voltage is provided _{for rectifying the voltage supplied by the special winding (W 1).} Considered publications:
French Patent No. 1,354,054;
"Der Elektromeister", 4 (1951), no. 20, p. 14;
"Conti Elektroberichte", 8 (1962), July / September, pp. 162 to 167. 1 sheet of drawings 709 650/164 9.67 © Bundesdruckerei Berlin