Repair of the electrical part of magnetoelectric ammeters and voltmeters

Such repair is understood as making adjustments, mainly in the electrical circuits of the measuring device, as a result of which its readings are within the specified accuracy class.

If necessary, the setting is carried out in one or more ways:

• change of active resistance in series 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;

• change at the opposite moment.

In the general case, first, the pointer is set to a position corresponding to the upper measurement limit at the nominal value of the measured value. When such a match is achieved, calibrate the measuring device on the numerical markings and record the measurement error on these markings.

If the error exceeds the permissible, then it is determined whether it is possible, by means of regulation, to deliberately introduce the permissible error into the final marking of the measuring range so that the errors of other digital signs "fit" within the permissible limits.

In cases where such an operation does not give the desired results, the instrument is recalibrated by retracting the scale. This usually happens after the meter has been overhauled.

The adjustment of magnetoelectric devices is carried out with a direct current supply, and the nature of the adjustments is set depending on the design and purpose of the device.

By purpose and design, magnetoelectric devices are divided into the following main groups:

• voltmeters with nominal internal resistance indicated on the dial,
• voltmeters, the internal resistance of which is not indicated on the dial;
• single-limit ammeters with internal shunt;
• multi-range universal shunt ammeters;
• millivoltmeters without temperature compensating device;
• millivoltmeters with temperature compensating device.

Adjustment of voltmeters with nominal internal resistance indicated on the dial

The voltmeter is connected in series in accordance with the switching circuit of the milliammeter and is adjusted so that at the rated current the deflection of the pointer to the final digital mark of the measuring range is obtained. The rated current is calculated as a fraction of the rated voltage divided by nominal internal resistance.

In this case, the adjustment of the deviation of the pointer to the final digital mark is carried out either by changing the position of the magnetic shunt, or by replacing the coil springs, or by changing the resistance of the shunt parallel to the frame, if any.

In the general case, the magnetic shunt removes up to 10% of the magnetic flux passing through the interglandular space, and the movement of this shunt towards the overlap of the pole parts leads to a decrease in the magnetic flux in the interglandular space and, accordingly, to a decrease in the angle of deviation of the pointer .

The spiral springs (stripes) in electrical meters serve, firstly, to supply and withdraw current from the frame and, secondly, to create a moment that opposes the rotation of the frame. When the frame is rotated, one of the springs is twisted, and the second is bends, in connection with which a total opposite moment of the springs is created.

If it is necessary to reduce the angle of deviation of the pointer, then you need to change the spiral springs (stria) available in the device to «stronger» ones, that is, install springs with increased torque.

This type of adjustment is often considered undesirable because of the laborious work involved in replacing the springs. Repairmen with extensive experience in soldering springs (stria) prefer this method. The fact is that when adjusting by changing the position of the magnetic shunt plate, in any case, as a result, it turns out to be shifted to the edge, and the possibility of further moving the magnetic shunt to correct the readings of the device, disturbed by the aging of the magnet, disappears.

Changing the resistance of the resistor, maneuvering the frame circuit with additional resistance, can only be allowed as a last resort, since such current shunting is usually used in temperature compensation devices. Naturally, any change in the specified resistance will disturb the temperature compensation and in extreme cases can only be allowed within small limits. It should also not be forgotten that the change in the resistance of this resistor associated with the removal or addition of turns of the wire must be accompanied by a long but mandatory aging operation of the manganin wire.

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 further complicates the adjustment and makes it undesirable to use this method.

In addition, the voltmeter is turned on according to its usual scheme and checked. With correct current and resistance settings, no further adjustments are usually required.

Adjustment of voltmeters whose internal resistance is not indicated on the dial

The voltmeter is connected, as usual, in parallel with the circuit being measured and adjusted to obtain the deflection of the pointer to the final digital marking of the measuring range at the nominal voltage for the given measuring range. Adjustment is made by changing the position of the plate when moving the magnetic shunt, or by changing the additional resistance, or by changing the spiral springs (striae). All the remarks made above are valid in this case as well.

Often the entire electrical circuit in the voltmeter—the frame and the wire-wound resistors—burns out. When repairing such a voltmeter, first remove all burnt parts, then thoroughly clean all 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, that is, in series with model milliammeter, determine the total deflection current of the moving part, make a resistor with additional resistance, magnetize the magnet if necessary, and finally assemble the device.

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 required by replacing the resistor with the same frame to move to a new measurement limit, that is, to recalibrate the ammeter;

2) during the overhaul of the ammeter, the frame is changed, in connection with which the parameters of the moving part change, it is necessary to calculate, manufacture a new one and replace the old resistor with additional resistance.

In both cases, the full deflection current of the frame of the device is first determined, for which the resistor is replaced by a resistance box and, using laboratory or portable potentiometer, the compensation method is used to measure the frame full deflection resistance and current. The shunt resistance is measured in the same way.

Adjustment of multi-limit ammeters with internal shunt

In this case, the so-called universal shunt is installed in the ammeter, that is, a shunt that, depending on the selected upper measurement limit, is connected in parallel to the frame and a resistor with an additional resistance in whole or in part the total resistance.

For example, a shunt in a three-terminal ammeter consists of three resistors Rb R2 and R3 connected in series. For example, an ammeter can have any of three measurement ranges — 5, 10, or 15 A. The shunt is connected in series with the measuring circuit. The device has a common terminal «+», to which the input of the 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 connecting the circuit to the terminals marked "+" and "5 A" to the frame through a resistor R, add that the voltage is removed from the series-connected resistors Rx, R2 and R3, i.e. completely from the entire shunt. When the circuit is connected to the terminals «+» and «10 A», the voltage is removed from the series resistors R2 and R3, and the resistor Rx is connected in series to the resistor circuit Rext, when it is connected to the terminals «+» and «15 A» , the voltage in the frame circuit is removed by the resistor R3, and the resistors R2 and Rx are included in the circuit Rin.

When repairing such an ammeter, two cases are possible:

1) measurement limits and shunt resistance do not change, but in connection with the replacement of the frame or a defective resistor, it is necessary to calculate, manufacture and install a new resistor;

2) the ammeter is calibrated, that is, its measurement limits change, in connection with which it is necessary to calculate, manufacture and install new resistors, and then adjust the device.

In the event of an accident occurring in the presence of high resistance frames, when temperature compensation is required, a temperature compensation circuit using a resistor or thermistor is used. The device is checked at all limits, and with the correct adjustment of the first measurement limit and the correct manufacture of the shunt, no further adjustments are usually required.

Adjustment of millivoltmeters without special temperature compensation devices

The magnetoelectric device has a frame wound with copper wire and spiral springs made of tin bronze or phosphor bronze, electrical resistance which depends on the air temperature in the device box: the higher the temperature, the greater the resistance.

Given 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 taken approximately:

Xpr = Xp (RR / Rр + Rext)

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 readings of the instrument of the deviations of the air temperature inside the case from the nominal value, the additional resistance must be several times greater than the resistance of the frame.The dependence of the ratio of the additional resistance to the resistance of the frame on the accuracy class of the device has the form

Radd / Rp = (4 — K / K)

where K is the accuracy class of the measuring device.

From this equation it follows that, for example, for devices with an accuracy class of 1.0, the additional resistance should be three times more than the resistance of the frame, and for an accuracy class of 0.5 — already seven times more. This leads to a decrease in the useful voltage on the frame, and in ammeters with shunts - to an increase in the voltage on the shunts. The first causes a deterioration in the characteristics of the device, and the second - an increase in the power consumption of the shunt. It is obvious that the use of millivoltmeters, which do not have special temperature compensation devices, is recommended only for panel instruments with accuracy classes 1.5 and 2.5.

The readings of the measuring device are adjusted by selecting an additional resistance, as well as by changing the position of the magnetic shunt. Experienced masters also use permanent magnetic deviations of the device. When adjusting, include the connecting leads supplied with the measuring device, or take into account their resistance by connecting to a millivoltmeter with a resistance box of the appropriate resistance value. When repairing, they sometimes resort to replacing the coil springs.

Regulation of millivoltmeters with a temperature compensating device

The temperature compensation device allows you to increase the voltage drop in the frame without resorting to a significant increase in the additional resistance and power consumption of the shunt, which sharply improves the quality characteristics of single-limit and multi-range millivoltmeters with accuracy classes 0.2 and 0. 5, used, for example, as shunt ammeters ... With a constant voltage at the terminals of the millivoltmeter, the error in the measurement of the device from a change in the temperature of the air inside the box can practically approach zero, that is, be so small that it can be neglected and ignored.

If during the repair of the millivoltmeter it is found that there is no temperature compensation device in it, then such a device can be installed in the device to improve the characteristics of the device.