If necessary, the readings of electromagnetic ammeters or voltmeters can be adjusted in one or more ways:
changes in active resistance in serial and parallel electrical circuits of the device;
changing the working magnetic field in the zone of movement of the ferromagnetic core;
changing the counteracting moment by replacing the spiral spring;
changing the number of turns of a stationary coil creating a magnetic field.
In the general case, first connect the adjustable device to the testing installation and, if it is an ammeter, smoothly increase the current to the rated value, and if the adjustable device is a voltmeter, then smoothly increase the voltage to the nominal value, after which the voltmeter is heated with current for 15...30 minutes . Then the errors of the adjustable device are determined at all numerical marks when moving the needle backwards and forwards along the scale and find out what is required for the readings of the device to correspond to its accuracy class, whether the device needs to redraw the scale, update the dial, etc.
Adjustment of electromagnetic devices is carried out when powered by alternating current of industrial frequency or the one indicated on the dial of the device. The nature of the adjustments is determined depending on the design and purpose of the device.
According to their purpose and design, electromagnetic devices are divided into the following main groups:
flat coil ammeters;
flat coil voltmeters;
ammeters and voltmeters with a round coil;
astatic ammeters and voltmeters.
Adjustment of flat coil ammeters.
The ammeter is connected to the testing installation and the current is gradually brought to the nominal value. When the pointer moves slightly to the nominal mark, the end of the iron plate located on the side of the flat coil (lateral magnetic shunt) is brought closer to the coil slot, and if the deviation is excessive, it is moved away from the slot. A more significant influence on the deflection of the pointer is exerted by the movement of another iron plate (internal magnetic shunt), which can be shifted along the guide slots: pushing this shunt inside the coil increases the readings of the adjustable ammeter, and pulling it out decreases them.
It may turn out that displacement of the shunts will not give the desired positive result. Then the number of turns of the coil is reduced or increased, or they resort to replacing the spiral spring. In case of incomplete deflection of the moving part and the rated current, increase the number of turns of the wire wound on a flat coil, or, conversely, reduce the number of turns if the device pointer deflects excessively. After changing the number of turns, the coil is put in place and the deflection of the pointer is finally adjusted using magnetic shunts, which are then securely secured with locking screws.
Replacement of a spiral spring is carried out mainly during the repair of direct-connection ammeters that measure large currents, when the number of turns of wire on the coil is small and adjustment by changing their number is difficult. In this case, if the pointer does not reach the upper limit of readings and the rated current, replace the spiral spring with a spring with a lower torque.
When adjusting, pay attention to obtaining the most uniform scale within the requirements: the scale is considered uniform if the ratio of the length of the largest division to the length of the smallest division at the same price does not exceed 1.3. The closer this ratio is to unity, the more successful the adjustment will be. The unevenness of the scale, characteristic of electromagnetic devices, depends on the correctness of the assembly, i.e., on the degree to which the best relative arrangement of the parts has been achieved. Therefore, if an increase in the uneven movement of the pointer along the scale is noted compared to what was before the repair of the device, then it is necessary to make adjustments to the arrangement of the parts of the measuring part. When individually repairing devices, you should always strive to improve their quality compared to what was achieved during mass production at the manufacturer.
Such repairs mean making adjustments, mainly in the electrical circuits of the measuring device, as a result of which its readings are within the specified range.
If necessary, adjustment is carried out in one or more ways:
change in active resistance in serial and parallel electrical circuits of the measuring device;
changing the working magnetic flux through the frame by rearranging the magnetic shunt or magnetizing (demagnetizing) a permanent magnet;
changing the counteracting moment.
In the general case, the first step is to install the pointer in a position corresponding to the upper measurement limit at the nominal value of the measured value. When such compliance is achieved, check the measuring device on the numerical marks and record the measurement error on these marks.
If the error exceeds the permissible, then find out whether, by means of adjustment, it is possible to deliberately introduce a permissible error at the end mark of the measurement range, so that the errors at other numerical marks “fit” within the permissible limits.
In cases where such an operation does not give the desired results, the instrument is re-calibrated and the scale is redrawn. This usually occurs after a major overhaul of the measuring instrument.
Adjustment of magnetoelectric devices is carried out when powered by direct current, and the nature of the adjustments is determined depending on the design and purpose of the device.
According to their purpose and design, magnetoelectric devices are divided into the following main groups:
Adjusting voltmeters that have the nominal internal resistance indicated on the dial
The voltmeter is connected in a series circuit according to the connection circuit of a milliammeter and is adjusted so as to obtain, at the rated current, a deviation of the pointer to the final numerical mark of the measurement range. The rated current is calculated as the quotient of the rated voltage divided by.
In this case, adjustment of the deviation of the pointer to the final numerical mark is performed either by changing the position of the magnetic shunt, or by replacing the spiral springs, or by changing the resistance of the shunt parallel to the frame, if any.
A magnetic shunt generally diverts through itself up to 10% of the magnetic flux flowing through the interferon space, and the movement of this shunt towards the overlap of the pole pieces leads to a decrease in the magnetic flux in the interferon space and, accordingly, to a decrease in the angle of deflection of the pointer.
Spiral springs (stretch marks) in electrical measuring instruments serve, firstly, to supply and remove current from the frame and, secondly, to create a moment that counteracts the rotation of the frame. When the frame is rotated, one of the springs is twisted, and the second is untwisted, and therefore a total counteracting moment of the springs is created.
If it is necessary to reduce the angle of deflection of the pointer, then the spiral springs (extensions) present in the device should be replaced with stronger ones, i.e., install springs with an increased counteracting moment.
This type of adjustment is often considered undesirable, since it is associated with painstaking work of replacing springs. However, repairmen who have extensive experience in resoldering spiral springs (stretch marks) prefer this method. The fact is that when adjusting by changing the position of the magnetic shunt plate, in any case, it ends up being shifted to the edge and it is no longer possible to further correct the instrument readings, which are disturbed by the aging of the magnet, by moving the magnetic shunt.
Changing the resistance of the resistor shunting the frame circuit with additional resistance can only be allowed as a last resort, since such current branching is usually used in temperature compensation devices. Naturally, any change in the specified resistance will violate temperature compensation and, in extreme cases, can only be tolerated within small limits. We must also not forget that a change in the resistance of this resistor, associated with the removal or addition of turns of wire, must be accompanied by a long but mandatory operation of aging the manganin wire.
In order to maintain the nominal internal resistance of the voltmeter, any changes in the resistance of the shunt resistor must be accompanied by a change in the additional resistance, which makes adjustment even more difficult and makes the use of this method undesirable.
Adjusting voltmeters whose internal resistance is not indicated on the dial
The voltmeter is connected, as usual, in parallel with the electrical circuit being measured and is adjusted to obtain the deviation of the pointer to the end numerical mark of the measurement range at the rated voltage for a given measurement limit. The adjustment is made by changing the position of the plate when moving the magnetic shunt, or by changing the additional resistance, or by replacing the spiral springs (stretch marks). All the comments made above are also valid in this case.
Often the entire electrical circuit inside the voltmeter - the frame and wire resistors - turns out to be burned out. When repairing such a voltmeter, first remove all burnt parts, then thoroughly clean all remaining unburnt parts, install a new moving part, short-circuit the frame, balance the moving part, open the frame and, turning on the device according to the milliammeter circuit, i.e. in series with the standard milliammeter, The total deflection current of the moving part is determined, a resistor with additional resistance is made, the magnet is magnetized if necessary, and finally the device is assembled.
Adjustment of single-limit ammeters with internal shunt
In this case, there may be two cases of repair operations:
1) there is an intact internal shunt, and it is necessary, by replacing the resistor with the same frame, to switch to a new measurement limit, i.e., re-calibrate the ampere meter;
2) during a major overhaul of the ammeter, the frame was replaced, and therefore the parameters of the moving part changed; it is necessary to calculate, manufacture a new one and replace the old resistor with additional resistance.
In both cases, first the total deflection current of the device frame is determined, for which the resistor is replaced with a resistance store and, using the compensation method, the resistance and current of the total deflection of the frame are measured. The shunt resistance is measured in the same way.
Adjustment of multi-range ammeters with internal shunt
In this case, a so-called universal shunt is installed in the ammeter, i.e. a shunt, which, depending on the selected upper measurement limit, is connected in parallel to the frame and a resistor with additional resistance in whole or part of the total resistance.
For example, the shunt in a three-limit ammeter consists of three series-connected resistors Rb R2 and R3. Let's say an ammeter can have any of three measurement limits - 5, 10 or 15 A. The shunt is connected in series to the measuring electrical circuit. The device has a common terminal “+”, to which the input of resistor R3 is connected, which is a shunt at the measurement limit of 15 A; resistors R2 and Rx are connected in series to the output of resistor R3.
When an electrical circuit is connected to the terminals marked “+” and “5 A”, the voltage is removed from the series-connected resistors Rx, R2 and R3 to the frame through resistor R ext, i.e. completely from the entire shunt. When connecting the electrical circuit to the “+” and “10 A” terminals, the voltage is removed from the series-connected resistors R2 and R3, and at the same time, the Rx resistor is connected in series to the circuit of the R ext resistor; when connected to the “+” and “15 A” terminals, the voltage in the frame circuit is removed from resistor R3, and resistors R2 and Rx are included in circuit R ext.
When repairing such an ammeter, two cases are possible:
1) the measurement limits and shunt resistance do not change, but in connection with replacing the frame or defective resistor, it is necessary to calculate, manufacture and install a new resistor;
2) the ammeter is calibrated, i.e. its measurement limits change, and therefore it is necessary to calculate, manufacture and install new resistors, and then adjust the device.
In case of extreme necessity, which happens in the presence of high-resistance frames, when temperature compensation is needed, a circuit with temperature compensation through a resistor or thermistor is used. The device is checked at all limits, and if the first measurement limit is correctly adjusted and the shunt is correctly manufactured, additional adjustments are usually not required.
Adjustment of millivoltmeters that do not have special temperature compensation devices
The magnetoelectric device contains a frame wound from copper wire and spiral springs made from tin-zinc bronze or phosphorus bronze, which depend on the air temperature inside the device body: the higher the temperature, the greater the resistance.
Considering that the temperature coefficient of tin-zinc bronze is quite small (0.01), and the manganin wire from which the additional resistor is made is close to zero, the temperature coefficient of the magnetoelectric device is approximately assumed:
Xpr = Xp ( R р / R р + R ext)
where Xp is the temperature coefficient of the copper wire frame, equal to 0.04 (4%). It follows from the equation that in order to reduce the influence on the instrument readings of deviations of the air temperature inside the case from its nominal value, the additional resistance must be several times greater than the resistance of the frame. The dependence of the ratio of additional resistance to frame resistance on the accuracy class of the device has the form
Radd/Rр = (4 - K/K)
where K is the accuracy class of the measuring device.
From this equation it follows that, for example, for devices of accuracy class 1.0, the additional resistance should be three times greater than the resistance of the frame, and for accuracy class 0.5 it should be seven times greater. This leads to a decrease in the usable voltage on the frame, and in ammeters with shunts - to an increase in the voltage on the shunts. The first causes deterioration in the characteristics of the device, and the second causes an increase in shunt power consumption. Obviously, the use of millivoltmeters that do not have special temperature compensation devices is advisable only for panel devices of accuracy classes 1.5 and 2.5.
The readings of the measuring device are adjusted by selecting additional resistance, as well as by changing the position of the magnetic shunt. Experienced repairmen also use magnetization of the permanent magnet of the device. When adjusting, turn on the connecting wires included with the measuring device or take into account their resistance by connecting a resistance magazine with the corresponding resistance value to the millivoltmeter. When repairing, they sometimes resort to replacing spiral springs.
Adjustment of millivoltmeters with temperature compensation device
The temperature compensation device allows you to increase the voltage drop across the frame without significantly increasing the additional resistance and power consumption of the shunt, which dramatically improves the quality characteristics of single-limit and multi-limit millivoltmeters of accuracy classes 0.2 and 0.5, used, for example, as ammeters with a shunt . At a constant voltage at the millivoltmeter terminals, the measurement error of the device due to changes in air temperature inside the case can practically approach zero, i.e., be so small that it can be ignored and ignored.
If, when repairing a millivoltmeter, it is discovered that it does not have a temperature compensation device, then such a device can be installed in the device to improve the characteristics of the device.
Each electrical measuring device works in conjunction with other devices and elements connected in a certain way into an electrical circuit. In this case, if the circuit is assembled incorrectly, the very first connection of the power source may damage one or more devices. In this regard, the first stage of working with the device - assembling the circuit - must be given the greatest attention.
Before assembling the circuit, it is advisable to familiarize yourself with the technical characteristics of the devices included in the circuit.
The placement of devices, rheostats, switches and other circuit elements should be clear and not require special attention. This will make the operator’s work easier and eliminate possible errors. For light-reading instruments, it is important that they are located in a visible location. When placing devices, it is necessary to ensure that there are no devices with strong magnetic fields (powerful motors, transformers, electromagnets, etc.) near them. Alternating magnetic fields can demagnetize the magnets of the device, as a result of which the calibration of the device will be disrupted and its error will go beyond the permissible limits. Thus, the device will actually be disabled. Constant magnetic fields can distort the measurement result.
The distance between devices must be at least 25 cm. It should be remembered that devices can change readings within the main error under the influence of the same device placed close to it.
The next stage of assembling the circuit will be connecting the elements included in the circuit and checking the circuit. The circuit assembly should always be done in a certain order, for example, starting with the positive contact of the power supply and ending with the negative contact of the source. In this case, it is initially recommended to assemble current (series) and then potential (parallel) circuits.
It is recommended to check the circuits in the reverse order. After the circuit has been assembled and tested, it is necessary to put the handles and levers of the devices in their original positions: set the ammeter measurement limit switches to the maximum measurement limit, set the rheostat handles to the minimum current position in the operating circuit.
In conclusion, it is recommended to check the reliability of the contacts, after which you can unlock the devices, connect power to the illuminators (for devices with light reading) and set the device indicators to the zero scale mark.
When working with the device, you should select the measurement limit in such a way that the device pointer during measurement is, if possible, in the second half of the scale. In this case, the relative measurement error will be smaller the closer the pointer is to the end of the scale. This can be explained as follows. The accuracy of the device is characterized by the reduced error, which is equal to the ratio of the absolute error to the upper limit of measurement. Thus, with equal absolute error at the beginning and end of the scale, the reduced error will be the same at the beginning and end of the scale, but the relative error at the beginning of the scale will be greater than at the end of the scale. Suppose that the needle of an ammeter with a measurement limit of 150 A is at the scale mark corresponding to 120 A, and the actual voltage value is 120.6 A.
Then the absolute error will be equal to:
ΔA = A - A d = 120.0 – 120.6 = - 0.6 A
The given error, according to definition, will be:
The relative error at this point will be equal to:
(40.9)
Now imagine that the same device measured a voltage of 10.0 A, while the actual voltage value is 10.6 A, then the absolute error will be equal to:
ΔA = 10.0 – 10.6 = - 0.6A
The reduced instrument error at this point will be equal to:
(40.10)
The relative error at this point will be:
(40.11)
Thus, it turns out that the reduced error of the device at both points is the same and equal to - 0.4%, and the relative error at the scale point 120 A is equal to - 0.5%, and at the point 10 A is equal to - 6%. For the experimenter, in this case, the relative error is of interest.
At the end of work, devices with arresters must be locked.
Devices should be stored in cases or boxes in dry and clean rooms.
The air in the room where the devices are stored should not contain harmful impurities that cause corrosion.
When transporting over long distances, they are packaged in accordance with the requirements of GOST 9181 - 59 “Electrical measuring instruments. Packaging requirements."
At least once every 6 months, it is recommended to check the condition of the devices by inspecting them and checking them against standard devices. Once every 2 years, as well as after each repair, devices must be submitted for state verification and branding to the local branch of the Committee of Standards, Measures and Measuring Instruments.
Repair
The mechanism of a modern electrical measuring device consists of dozens of small and fragile parts. Operations for assembling and disassembling the measuring mechanism require certain skills and knowledge of special techniques.
Before you begin repairing the device, you should determine exactly what is wrong with it.
The device may have mechanical and electrical faults that render the device unusable:
Significant friction in the supports;
Poor fastening of stretch marks;
Partial turn short circuit of the frame winding;
Some coils of the circuit are torn or “burnt”;
Demagnetized magnetic system of the device;
Poor balance of the device;
The moving part of the device is heavily contaminated with iron;
Poor contacts in the switch or electrical circuit of the device;
The arrow of the device touches the scale or glass of the device;
The moving part of the measuring mechanism fell out of its supports;
The stretch wire is torn or burned by high current;
The spiral spring has come unsoldered;
Rubbing of the frame in the air gap of the magnetic system;
Break or short circuit of the device frame winding;
Mechanical malfunction of the device switch;
Previously, I had seen this device only in color photos on the Internet, but now I saw it on the market; the glass is broken, some ancient batteries are attached to the body and all this is covered with a layer of, to put it mildly, dust. And I remember the ampere-voltmeter - transistor tester TL-4M because, unlike many others, it can check, in addition to the gain, other characteristics of transistors:
At home I disassembled the case - the measuring head burst in half, five wire-wound resistors burned almost to the state of embers, the balls fixing the position of the dial switch are no longer round, and only scraps stick out from the connection block for the transistors being tested. I didn’t take any photos, but now I regret it. A comparison would also provide clear confirmation of the commonly held opinion that the devices of that time were practically indestructible.
Of all the restoration work, the longest and most painstaking was the general cleaning of the device. I didn’t wind the resistors, but installed the usual OMLTs (clearly visible - the left row, all “sawed”), finely adjusted to the desired value with a “velvet” file. Everything else from electronic components was intact.
Finding a new original connector for connecting the transistors being tested, as well as restoring the old one, was not realistic, so I picked up something more or less suitable and cut something off, glued something on, and in the end, in a functional sense, the replacement was a great success. I didn’t like turning the dial switch to “zero” (turn off the power) every time after finishing measurements - I installed a slide switch on the power compartment. Luckily a place was found. The measuring head turned out to be in good working order, I just glued the body together. The switch balls were made of plastic (“bullets” from a children’s pistol).
To connect transistors with short legs, I made extension cords with alligator clips, and for ease of use, two pairs of connecting wires (with probes and with alligator clips). That's all. After power was applied, the device started working in full. If there are any errors in the measurements, they are clearly insignificant. A comparison of current, voltage and resistance measurements with a Chinese multimeter did not reveal any significant differences.
I categorically disagreed with looking for standard batteries for the power compartment in stores every time. Therefore, I came up with the following: I removed all the contact plates, in order for two “AA” batteries to fit into the compartment along the width, I made a cut measuring 9 x 60 mm in the side wall from the side of the device compartment, and “removed” the excess free space along the length thanks to the manufactured inserts with contact springs.
If anyone happens to “repeat”, then using this sketch it will not be difficult to do so.
Such repairs mean making adjustments, mostly in the electronic circuits of the measuring device, as a result of which its readings are within the limits of a given accuracy class.
As necessary, adjustments are made using one or more methods:
configuration of active resistance in alternating and parallel electronic circuits of the measuring device;
configuration of the working magnetic flux through the frame by means of rearranging the magnetic shunt or magnetization (demagnetization) of a permanent magnet;
configuration of the counteracting moment.
In the general case, first achieve the installation of the pointer in a position corresponding to the upper measurement limit at the nominal value of the measured value. When such compliance is achieved, check the measuring device on the numerical marks and record the measurement error on these marks.
If the error exceeds the permissible one, then they will find out whether it is possible, using an adjustment method, to purposefully introduce the permissible error at the end mark of the measurement spectrum, so that the errors at other numerical marks “fit” within the permissible limits.
In cases where such an operation does not give suitable results, the instrument is re-calibrated and the scale is redrawn. As a rule, this occurs after six months of repair of the measuring device.
Adjustment of magnetoelectric devices is done when powered by a constant current, and the nature of the adjustments is determined depending on the design and purpose of the device.
According to their purpose and design, magnetoelectric devices are divided into the following main groups:
Adjusting voltmeters, which are marked on the dial
rated internal resistance
The voltmeter is connected in an alternating circuit according to the milliammeter connection circuit and is adjusted so as to obtain, at the rated current, a deviation of the pointer to the final numerical mark of the measurement spectrum. The rated current is calculated by dividing the rated voltage by the rated internal resistance.
In this case, the adjustment of the difference between the pointer and the final numerical mark is done either by configuring the position of the magnetic shunt, or by replacing spiral springs, or
the resistance configuration of the shunt parallel to the frame, if any.
A magnetic shunt generally diverts through itself up to 10% of the magnetic flux flowing through the space between the irons, while moving this shunt towards the overlap of the pole pieces leads to a decrease in the magnetic flux in the space between the irons and, accordingly, to a decrease in the angle of difference of the pointer.
Spiral springs (stretch marks) in electrical measuring devices serve, firstly, to supply and remove current from the frame and, secondly, to create a moment that counteracts the rotation of the frame. When the frame is rotated, one of the springs is twisted, and the second one is untwisted, and therefore a total counteracting moment of the springs is created.
If you need to reduce the angle of difference of the pointer, then you should change the spiral springs (extensions) present in the device to stronger ones, i.e. install springs with an increased counter-torque.
This type of adjustment is often considered unnecessary because it involves careful work replacing the springs. But repairmen who have extensive experience in resoldering spiral springs (stretch marks) prefer this particular method. The fact is that when adjusting the position of the magnetic shunt plate by configuration, in any case, it ends up being shifted to the edge and it is no longer possible to correct the instrument readings, which are disturbed by the aging of the magnet, by moving the magnetic shunt.
Changing the resistance of the resistor that shunts the frame circuit with additional resistance can only be allowed as a last resort, because such current branching is usually used in temperature compensation devices. Naturally, any change in the indicated resistance will violate the temperature compensation and in the latter case can be allowed only within small limits. We must also not forget that a change in the resistance of this resistor, associated with the removal or addition of turns of wire, must be accompanied by a long-term, but inevitable operation of aging the manganin wire.
In order to maintain the voltmeter's nominal internal resistance, any shunt resistor configuration must be accompanied by an additional resistance configuration, which further increases
makes adjustment difficult and makes the use of this method unnecessary.
Adjusting voltmeters, which have internal
resistance is not indicated on the dial
The voltmeter is connected, as usual, in parallel with the electronic circuit being measured and adjusted to obtain the deviation of the pointer to the final numerical mark of the measurement spectrum at the rated voltage for a given measurement limit. The adjustment is made by changing the position of the plate when moving the magnetic shunt, or by configuring additional resistance, or by replacing spiral springs (stretch marks). All the comments made above are also valid in this case.
Often the entire electronic circuit inside the voltmeter - the frame and wire resistors - turns out to be burned out. When repairing such a voltmeter, first remove all the burnt parts, then painstakingly clean all the remaining unburned parts, install a new moving part, short-circuit the frame, balance the moving part, open the frame and, turning on the device according to the milliammeter circuit, i.e., alternately with an approximate milliammeter, determine the total difference current of the moving part, make a resistor with additional resistance, magnetize the magnet as necessary, and finally assemble the device.
Adjustment of single-limit ammeters with internal shunt
With all this, there may be two options for repair operations:
1) there is an intact internal shunt, and it is required to replace the resistor with the same frame and switch to
new measurement limit, i.e. re-calibrate amperes
meter;
2) during a complete repair, the ammeter was replaced
frame, and therefore the characteristics of the movable
parts, you need to calculate, make a new one and change
an old resistor with additional resistance.
In both cases, first determine the total difference current
frame of the device, why replace the resistor with a resistance store and, using a laboratory or portable potentiometer, determine the resistance and total difference current using a compensation method
framework. The shunt resistance is determined using the same method.
Adjustment of multi-limit ammeters with internal
shunt
In this case, a so-called universal shunt is installed in the ammeter, i.e. a shunt that
depending on the selected upper measurement limit, they are connected in parallel to the frame and a resistor with additional resistance in full or part of the total resistance.
For example, the shunt in a three-limit ammeter consists of 3 alternately connected resistors Rb R2 and R3. Let's say an ammeter can have any of 3 measurement limits - 5, 10 or 15 A. The shunt is connected one by one into the measuring electronic circuit. The device has a common terminal “+”, to which the input of resistor R3 is connected, which is a shunt at the measurement limit of 15 A; Resistors R2 and Rx are alternately connected to the output of resistor R3.
When connecting an electronic circuit to the terminals marked “+” and “5 A” to the frame through a resistor
R ext removes the voltage from the alternately connected resistors Rx, R2 and R3, i.e., completely from the entire shunt. When connecting the electronic circuit to the “+” and “10 A” terminals, the voltage is removed from the alternately connected resistors R2 and R3, and at the same time, the Rx resistor is connected alternately to the resistor circuit
R ext, when connected to the “+” and “15 A” terminals, the voltage in the frame circuit is removed from resistor R3, and resistors R2 and Rx are included in the circuit
R ext.
When repairing such an ammeter, two options are possible:
1) the measurement limits and shunt resistance do not change, but due to the replacement of the frame or defective
resistor must be calculated, made and installed
new resistor;
2) the ammeter is calibrated, i.e. its measurement limits are changed, and therefore it is necessary to calibrate
count, make and install new resistors,
After this, adjust the device.
In the case of the latter need, which happens in the presence of high-resistance frames, when temperature compensation is needed, use a circuit with temperature compensation using a resistor or thermistor.
The device is checked at all limits, and if the first measurement limit is correctly adjusted and the shunt is correctly manufactured, additional adjustments are usually not required.
Adjustment of millivoltmeters without devices
special temperature compensation
The magnetoelectric device has a frame wound from copper wire and spiral springs made of tin-zinc bronze or phosphor bronze, the electronic resistance of which depends on the air temperature inside the device body: the higher the temperature, the greater the resistance.
Taking into account that the temperature coefficient of tin-zinc
bronze is quite small (0.01), and the manganin wire from which it is made
additional resistor, close to zero, approximately calculate the temperature
magnetoelectric device coefficient:
Xpr = Xp ( R r / R r
+ R ext)
where Xp is the temperature coefficient of the copper wire frame, equal to 0.04 (4%).
It follows from the equation that in order to reduce the impact on the instrument readings of deviations of the air temperature inside the housing from its nominal value, an additional
the resistance should be several times greater than the resistance of the frame.
The dependence of the additional resistance to the frame resistance on the accuracy class of the device has the form
Radd/Rр = (4 - K/K)
where K is the accuracy class of the measuring device.
From this equation it follows that, for example, for devices of accuracy class 1.0, the additional resistance should be three times greater than the resistance of the frame, and for accuracy class 0.5 - already seven times greater. This leads to a decrease in the useful voltage across the frame, and in ammeters with shunts, to an increase in the voltage across the shunts. The first causes deterioration of the device, and the second causes an increase in shunt power consumption. Of course, the introduction of millivoltmeters that do not have special temperature compensation devices is targeted only for panel devices of accuracy classes 1.5 and 2.5.
Adjustment of the readings of the measuring device is done by selecting additional resistance, as well as by configuring the position of the magnetic shunt. Experienced repairmen also use magnetization of the permanent magnet of the device. When adjusting, the connecting wires included in the measuring device kit are turned on or their resistance is taken into account by connecting a resistance magazine with a suitable resistance value to the millivoltmeter. When repairing, from time to time they resort to replacing spiral springs.
Adjustment of millivoltmeters having the device
temperature compensation
The temperature compensation device allows you to increase the voltage drop across the frame without significantly increasing the additional resistance and power consumption of the shunt, which dramatically improves the high-quality properties of single-limit and multi-limit millivoltmeters of accuracy classes 0.2 and 0.5, used, for example, as ammeters with shunt. With a constant voltage at the terminals of the millivoltmeter, the measurement error of the device depending on the configuration of the air temperature inside the case can actually approach zero, i.e., be so small that it can be ignored and not taken into account.
If, when repairing a millivoltmeter, it is discovered that there is
there is no temperature compensation device, then to improve the features
device, such a device can be installed in the device.
