Antipyretics for children are prescribed by a pediatrician. But there are situations of emergency care for fever, when the child needs to give the medicine immediately. Then the parents take responsibility and apply antipyretic drugs. What is allowed to give to infants? How can you bring down the temperature in older children? Which medications are the safest?
You will need
- - ammeter;
- - Tester;
- - conductor of known cross-section from known material;
- - table of resistivities.
Instructions
Connect the shunt in parallel to the ammeter to expand its measurement capabilities. In this case, the main current passes through the shunt, and the part to be measured passes through the ammeter. The rated current in the network is calculated using a special formula.
To calculate the shunt, find out the maximum current that will be measured by the instrument. To do this, measure the voltage at the current source U in volts and divide it by the total resistance of the circuit R in ohms. All measurements should be made by the tester, if the current is constant, take into account the polarity of the instrument. Find the rated current in the circuit, dividing the voltage by the resistance I = U / R. Examine the scale of the ammeter and find out maximum current, which can be measured by him.
Find the resistance of the shunt. To do this, measure the resistance of the ammeter R1 in ohms, and find the required shunt resistance by dividing the product of the maximum current that can be measured by the device I1 and its resistance R1 by the rated current in the network I (R = (I1 ∙ R1) / I).
Example. It is necessary to measure the current in the circuit where the maximum value can reach 20 A. For this it is proposed to use an ammeter with a maximum possible current of measurement of 100 mA and a resistance of 200 ohms. The shunt resistance in this case will be R = (0.1 ∙ 200) / 20 = 1 Ohm.
Use standard resistors as shunts. If there are not any, make a shunt yourself. For the manufacture of shunts, it is best to take conductors made of copper or other material with high conductivity. To calculate the required length of the shunt l conductor, take a wire of the known section S and find out the specific resistance of the material ρ from which this device is made. Then, the resistance R, multiply by the conductor cross-section, measured in mm², and divide by its resistivity, expressed in Ohm ∙ mm² / m, taken from the special table l = R ∙ S / ρ.
In order to produce a shunt for the ammeter from the above example of copper wire with a cross section of 0.2 mm², take its length, which is calculated by the formula l = 1 ∙ 0.2 / 0.0175 = 11.43 m. The same principle is used and by shunting any other part of the chain.
In order to find a nominal current for a specific conductor, use a special table. It indicates for which values of the force currentand the conductor can collapse. To find the nominal currentand for electric motors various designs, use special formulas. If the question concerns the fuse, then, knowing the power for which it is calculated, find its nominal current.
You will need
- For measurements and calculations, take a voltmeter, a caliper, a table of the dependence of the rated current on the cross-section, the technical passport of the electric motors.
Instructions
Definition of nominal currentbut over the wire section. Determine the material from which the wire is made. The most common are copper and aluminum wires with a circular cross section. Measure its diameter with a caliper, and then find the cross-sectional area by multiplying the square of the diameter by 3.14 and dividing by 4 (S = 3.14 D² / 4). Determine the type of wire (solid, two-wire or three-wire). After that, on a special table, determine the nominal current for this wire. Exceeding this value will cause the wire to burn out.
Definition of nominal currentbut the fuse. On the fuse, the power to which it is calculated with a margin of approximately 20% is necessarily indicated. Find out the voltage in the network where the fuse should be inserted, if it is not known, measure it with a voltmeter. To find the nominal current, you need a maximum calculated capacity fuse in watts, divided by the voltage in the network in volts. In case if current will increase more than the nominal value, the conductor in the fuse will collapse.
Definition of nominal currentbut the motor To find the nominal current for a constant engine currenta, find out its rated power, the voltage of the source where it is connected, and its efficiency. These data should be in the technical documentation of the electric motor, and measure the voltage of the source with a voltmeter. Then the power in watts is divided into voltage in volts and the efficiency in unit fractions (I = P / (U η)). The result is a nominal current in amperes.
For three-phase motor alternating currentand in addition learn the nominal power factor of the engine, and calculate the nominal current by the same method, only the result is divided by the nominal power factor (Cos (φ)).
Related Videos
In the practice of electrotechnical measurements, it is often necessary to measure the current strength, the value of which exceeds the upper limit of the available ammeter. The way out of this situation is the application of shunt to the ammeter. The shunt allows you to change the current allowed for the device.
You will need
- - copper or nichrome wire;
- - power supply with adjustable output voltage;
- - ammeter;
Instructions
To calculate the resistance shunt use the following formula:
Rm = (Ra * Ia) / (I-Ia),
where Rm is the required resistance shunt; Ra - resistance of the winding of the ammeter; I - the upper value of the measured current; Ia is the current of the total deviation of the ammeter.
Determine the maximum measuring current Ia on the scale of the available instrument. Let's say its value is 100 μA, and you need to measure current to 25 A.
Determine the resistance value of the winding of the ammeter Ra. It can be taken from the passport of the device or measured with an ohmmeter with an allowable error. Let this value is 1750 Ohm.
Substitute the obtained values into the formula and get the result:
Rш = (1750 * 0.0001) / (25-0.0001) = 0.007 Оm
Now it is necessary to select the length of the wire by means of an exemplary ohmmeter. The obtained value is rather small and for manufacturing shunt you need a piece copper wire. It would be more correct to use a certified shunt with an appropriate resistance value.
If in the future it is necessary to carry out measurements with a given error, then the instrument with the shunt installed must be verified in the metrological laboratory, since installation shunt lowers the accuracy of the measurement.
Page 1
External shunts are manufactured as individual for work with only the device for which the shunt is made and calibrated for operation with any measuring device whose current is small compared with the shunt current and the voltage drop in which is equal to the drop in voltage on the shunt.
External shunts are made as a separate part and connected to the device by special wires. The individual shunt should only be used with the device that was calibrated with this shunt.
External shunts to ammeters must be connected with resistance-calibrated conductors, which are completed together with the device by the manufacturer.
External shunts are divided into individual and calibrated. Individual shunts are only used for those instruments with which they are directly graduated. In practice in recent years, devices with individual shunts are not manufactured, since fitting shunts in mass production of devices is difficult.
External shunts and a separate additional resistance are attached to the devices.
Measuring instruments and external shunts are interchangeable within their type.
A separate chapter describes external shunts and additional resistances, which are integral parts of magnetoelectric ammeters and voltmeters.
MEASURING CURRENT AND VOLTAGE TRANSFORMERS
Shunts.The simplest current-to-voltage measuring transducer is a shunt, which is a four-clamp resistor. Two input terminals, to which current is supplied I , called current, and two output terminals from which the voltage is removed U , are called potential (Fig. 1). Potential clamps are connected to the measuring mechanism MI. The parameters characterizing the shunt are the nominal value of the input current I and the nominal value of the output voltage U. Their ratio determines the nominal resistance of the shunt R W, NOM = U NOM / I NOM. The shunt can also be
Fig. 1 Shunt
considered as a current divider with a fission factor (shunting)
n = I / I 0 = (R w.n + R 0) / R w, nom
where Iо - current in the measuring mechanism; R 0 - the resistance of the measuring mechanism, so the shunts are used to extend the limits of measuring current mechanisms. In this case, most of the measured current passes through the shunt, and the smaller part through the measuring mechanism. Shunts have low resistance and are used mainly in DC circuits with magnetoelectric measuring mechanisms. The use of shunts with measuring mechanisms of other systems is irrational, since these measuring mechanisms consume a lot of power, which leads to a significant increase in the resistance of the shunts and, consequently, to the increase in their size and power consumption.
When using shunts with measuring mechanisms on alternating current, an additional frequency error arises due to different dependencies of shunt resistances and the measuring mechanism on frequency.
If it is necessary to extend the measurement limit in n times, i.e., so that the current Io is in n times less than the current I, the resistance of the shunt must be equal to
R w = R o / (n-1) -
Shunts are made from manganin. According fromShunts share GOST 8042-78 types: ShS - shunt interchangeable stationary; ShP - shunt is interchangeable portable. If the shunt is designed for a small current (up to 30 A), then it is usually built into the body of the device. To measure high currents (up to 6000 A), devices with external shunts are used. In this case, the power dissipated in the shunt does not heat the device.
External shunts have massive T-shaped tips of red copper. The tips serve to drain heat from the manganine plates that are soldered between them. The current is supplied to the terminals by means of massive bolts - current clamps. Potential clamps are made in the form of two bolts of smaller size, located on the copper tips. The resistance of the shunt, enclosed between the potential clamps, is adjusted by means of transverse cuts in manganine plates. This shunt device eliminates errors from contact resistance.
Fig. 2. Schematics of multi-limit shunts with a lever switch (a), with individual terminals (b)
Shunts are made interchangeable, i.e., they are calculated for certain currents and voltage drops. In accordance with GOST shunts should have a nominal voltage drop at potential terminals: 10,! 5, 30, 50, 60, 75, 300 mV.
In portable magnetoelectric devices for currents up to 30A, shunts made for several measurement ranges I 1 nom, I2 nom, I 3 nom. In Fig. 2 shows the schemes of multi-limit shunts. Such a shunt consists of several resistors, which are switched depending on the limit of the measurement by the lever switch (Figure 2, a) or the transfer of the wire from one clamp to the other (Fig. 2b), ie with separate clamps.
By accuracy, shunts are divided into accuracy classes: 0.05, 0.1; 0.2; 0,5 - stationary; 0.02; 0.05; 0.2 - portable. The number of the accuracy class indicates the permissible deviation of the resistance as a percentage of its nominal value.
Additional resistors. Additional resistors are voltage-to-current measuring transducers. Therefore, an additional resistor, connected in series with the measuring mechanism, whose torque depends on the current, can serve to expand the voltage measurement limits of analog voltmeters of various systems (except for electrostatic and electronic). Additional resistors, called GOST 8023-78 additional resistance, also serve to expand the voltage measurement limits of other devices that have parallel circuits connected to the voltage source. This includes, for example, wattmeters, energy meters, phase meters, etc.
The additional resistor is connected in series with the MI measuring mechanism (Figure 3.). The current in the chain of the measuring mechanism I 0) having a resistance R o and connected in series with the additional resistor Rd, is:
I 0 = U / (R 0 + R d). Where U - measurable
voltage-
75mV U 1n U Unom U U 3nom
Fig.3 Fig.4
If using an additional resistor R g it is necessary to extend the measurement limit of a voltmeter having a nominal measurement limit U nom and a resistance R 0 , then assuming the constancy of the current of the voltmeter Io can be written: U nom / R0 = mU nom / (R0 + Rd), then Rd = R0 (m-1)
Additional resistors are made, usually from manganine insulated wire, wound on plates or frames made of insulating material. Also used are additional resistors made of cast microwire in glass insulation. Additional resistors designed for AC operation have a bifilar winding to produce a non-reactive resistance.
Along with the expansion of the limits of measuring voltmeters, additional resistors reduce their temperature error. If we assume that the winding of the measuring mechanism has a temperature coefficient of resistance o. and the additional resistor is the temperature coefficient d, then the temperature coefficient of the whole voltmeter (Fig.3) will be:
Usually d 0 0. Then
= 0 R 0 (R 0 + R e)
In portable devices, additional resistors are made sectional for several measurement limits U 1nom, U2nom, U3nom (Figure 4).,
Additional resistors are internal, built into the body of the device, and external. The latter are performed in the form of separate blocks and in accordance with GOST 8023-78 are divided into shield and portable interchangeable and limited interchangeably. The interchangeable additional resistor can be used with any device whose rated current is equal to the nominal current of the additional resistor.
Additional resistors, as well as shunts, are divided into accuracy classes: 0.01; 0.02; 0.05; 0.1; 0.2; 0.5 and 1.0. The accuracy class is determined by the relative error,%, equal to
= ± ( / R nom) 100%,
where is the absolute error; R nom is the nominal resistance of the additional resistor. Additional resistors are manufactured for rated currents from 0.01 to 60 mA. Additional resistors are used to convert voltages up to 30 kV.
Measuring current transformersand tenseand I
Measurements of large alternating voltages and currents by conventional analog electromechanical devices become possible when they are included in the circuit through measuring transformers alternating current and stresses. The use of voltage dividers and shunts for these purposes is inexpedient and even dangerous for maintenance personnel.
Measuring transformers consist of two isolated windings, placed on a ferromagnetic core.
The principle of the IT operation coincides with the principle of operation of conventional transformers. The secondary circuit of the current transformers includes ammeters, sequential windings of meters of wattmeters, circuits of relay protection and control; Voltmeters, parallel circuits of wattmeters, counters and other devices are connected to the secondary of the voltage transformers.
Stationary AC measuring transformers have the following performance characteristics: frequency 50 Hz; rated voltage U 1 of the voltage transformers - from 0.38 to 750 kV, secondary voltage U 2nom -
150;
100; 100/
3 V; classes of accuracy of voltage transformers - 0.05; 0.01, 0.2; 0, 5; 1.0; 3.0; nominal primary current of 1 current transformer - 1 A ... 40 kA, rated secondary current I 2nom - 1; 2; 2.5; 5 A; rated load of the secondary circuit is 2.5; 5; 10; 25; thirty; 40; 60; 75; 100 W; accuracy classes of current transformers - 0.2; 0.5; 1.0; 3.0; 5.0; 10.0.
Measuring transformers of alternating current.For convenience and safety of measuring the current of high voltage installations, the secondary circuit current is changed to a standard value of 5A or 1A by means of a current transformer.
Measuring instruments and relays are performed on these currents and are connected to the secondary circuit of the current transformer (contacts И1, И2), one terminal of which is necessarily grounded (И1).
If the transformer is damaged, the devices and relays remain under ground potential. A distinctive feature of the operating mode of the current transformer is that the primary current does not depend on the operating mode of its secondary circuit and remains unchanged when the secondary circuit is short-circuited or opened. This is due to the fact that the current in the primary winding is determined by the resistance of the load Z 2 which is several orders of magnitude higher than the input resistance of the transformer on the side of the primary winding for any value of the resistance Z 2. Therefore, a fuse in the secondary circuit is not set, since the break of this circuit is an emergency mode for the current transformer. Contacts of the primary ITT chain (L1, L2). The main parameters of current transformers are: rated voltage- the line voltage of the system in which the current transformer should operate. This voltage determines the insulation resistance between the primary winding under high potential and the secondary one, one end which is grounded;
rated primary and secondary currents- the currents to which the transformer is designed. Current transformers usually have a heat wave and allow long-term transmission of currents that are about 20% higher than the nominal value; nominal coefficient of transformation- ratio of the rated primary current I 1nom to the rated secondary current I2nom
In practice, the actual coefficient of transformation is not equal to the nominal coefficient due to losses in the transformer. Distinguish errors: current, angular and complete; current error,%, defined by the expression
I 2 - secondary current; I 1 primary current.
In a real transformer, the secondary current is phase-shifted from the primary to an angle different from 180 °. To read this error, the secondary current vector is rotated by 180 °. The angle between this vector and the vector of the primary current is called angular error.If the inverted vector of the secondary current is ahead of the primary current, then the error is positive, if it lags, then the error is negative. The error in angle is measured in minutes.
The accuracy class indicates the permissible error in the current in percent under the nominal conditions Z 2 = Z 2 h.
Along with the current and angular error, the concept floorerror,%,which characterizes the relative magnetizing current
where I 1 - the effective value of the primary current; i 2 - instantaneous value of secondary current; i 1 , - the instantaneous value of the primary current; T -the period of the frequency of the alternating current (0.02 s);
- load resistance Ohm, at which the transformer operates in its accuracy class at cos 2n = 0.8 Sometimes the concept of rated powerR The second = I The second Z The second
Since the current I 2 is standardized, the rated load resistance uniquely determines the rated power of the transformer;
nominal limit multiplicity- The frequency of the primary current in relation to its nominal value, at which the error in current reaches 10%. The load and its power factor must be nominal;
maximum secondary current multiplicity- the ratio of the highest secondary current to its nominal value at the rated secondary load. The maximum multiplicity of the secondary current is determined by the saturation of the magnetic circuit, when a further increase in the primary current does not lead to an increase in the flux.
Current transformers are streamlined by a short-circuit current, and its windings are exposed to high currents;
dynamic stability (multiplicity)- the ratio of the permissible shock current to the amplitude of the rated primary current;
thermal resistance (multiplicity) -the ratio of the short-circuit current allowed for 1 s to the rated value of the primary current.
Since the current of the primary winding is set by the network, the primary thermal winding is subjected to the greatest thermal and dynamic influences. The secondary current is often limited by saturation of the magnetic circuit, and therefore the secondary winding operates under light conditions.
The operating mode of the current transformer is essentially a short circuit mode.
The current transformer should not give large errors at the rated current and short circuit.
In order for a transformer to satisfy a certain class of accuracy, the error must be within the permissible limits. The accuracy class of the transformer is determined by its error in percent with the primary current (100 ... 120) I 1nom.
Depending on the number of turns of the primary winding, one-turn and multi-turn current transformers are distinguished.
In a single-turn transformer, the primary winding can be made in the form of a rod or a package of tires. An example of such a performance
Fig.6. Single-turn current transformer TPOL-10, U nom = 10 kV: 1 magnetic circuits; 2 - secondary winding; 3 - mounting ring; 4 - rod
is a transformer TPOL-10 with cast insulation, presented in Fig. 6th
This transformer is used as a bushing during the transition from one room to another.
The use of molded epoxy insulation makes it possible to greatly simplify the design and production technology. Primary winding - rod 4, magnetic cores 1 and a retaining ring 3 are placed in a special form, after which a liquid mass of epoxy resin, a pulverized quartz sand hardener, is poured into it. After hardening and polymerization, the insulation material acquires high electrical and mechanical properties. Magnetic conductor 1 transformer, made in the form of a torus, is made of a tape coiled in a spiral. The secondary winding is wound 2. The use of a toroidal core allows full use of high properties of textured material, for example, steel grade E310. If the secondary winding is uniformly located on the magnetic circuit, the inductive resistance of the secondary winding is zero, which makes it possible to increase the accuracy of the current transformer. The design makes it easy to install several magnetic circuits, each of which has different parameters. The main advantage of a single-turn version is its high electrodynamic stability, since the primary windings are acted upon only by supplying tires and adjacent phases.
When choosing a current transformer, it is necessary to take into account that its real load is not only the windings of the devices but also the resistance of the connecting wires.
Measuring voltage transformers.They serve to
conversion of high voltage to low voltage standard value, convenient for measurement. Usually a nominal secondary voltage is assumed to be 100 V or 100 
B. This allows for the measurement of any voltage to use the same standard measuring instruments. Protection relays that respond to voltage are also manufactured for standard voltage, regardless of the voltage of the installation.
The primary winding of the transformer is isolated from the secondary according to the voltage class of the installation. For safety of service, one terminal of the secondary winding is necessarily grounded. Thus, the voltage transformer isolates the measuring devices and relays from the high voltage circuit and makes their maintenance safe.
The circuit for switching on a single-phase voltage transformer is given in Fig. 7. Primary winding 1 connected to high-voltage circuit through fuses 3. Secondary winding 2 it feeds the load in the form of meter windings or a protection relay through fuses 4. In normal voltage transformers, the secondary winding 2, and a core 5.
Circuit breakers 4 serve to protect the voltage transformer from short circuits in the secondary load circuit. Circuit breakers 3, installed on the high-voltage side, serve to protect the network from short-circuits in the transformer. To facilitate disconnection, it is desirable to install current-limiting fuses, such as PBC or firing, with limiting resistance.
Due to the high resistance of the transformer itself, when there is a short circuit in the secondary circuit, the current in the primary circuit is small (of the order of several amperes) and its value is insufficient to trigger fuses 3.
voltage transformer: 1 - primary winding; 2 - secondary winding; 3, 4 - circuit breakers; 5 - core ■
The main parameters of the voltage transformer are:
rated winding voltage -voltage on the primary and secondary windings indicated on the transformer panel. The nominal voltage of the transformer is equal to the rated voltage of the primary winding;
nominal ratio of transformation -ratio of the rated primary voltage to the rated secondary voltage:
voltage error,%,which is defined by the equation:
where U 1 - the voltage applied to the primary winding; U 2 - the voltage measured at the terminals of the secondary winding.
If U 1 / U 2 = k nom then the error will always be zero.
For the angular error, an angle is taken in minutes between the primary voltage and the secondary rotated by 180 °. If the secondary voltage U 2 is ahead of the primary voltage U 1, the angle error is considered positive. The permissible error of the voltage transformer in percent under nominal conditions is numerically equal to the accuracy class.
The errors of the transformer should not exceed the table data when the primary voltage fluctuates within 90 ... 110% and when the power fluctuation at the secondary terminals is within 25 ... 100% of the nominal values;
and the secondary power P 2 respectively:
As the resistance Z 2 decreases, the power given by the voltage transformer increases and accordingly the error increases;
rated power of the transformer -the maximum power (at a nominal power factor of 0.8), which can be removed from the transformer, provided that its error does not exceed the limits determined by the accuracy class.
In order to reduce the error in voltage, reduce the active and reactive resistance of the windings. To obtain a small active resistance, small current densities in the windings (about 0.3 A / mm 2) are taken, so that these transformers are lightly loaded in a thermal ratio. To reduce the inductive resistance of the windings, the distance between the primary and secondary windings is reduced.
Compensation for the error in voltage can easily be obtained by reducing the number of turns of the primary winding. If the number of turns of the primary winding is reduced, the transformation ratio becomes less than the nominal value and the secondary voltage increases. In this case, a positive error is introduced, which compensates for the negative. Typically, such a correction is introduced so that when idling the transformer has a maximum positive error for a given accuracy class.
The error of the transformer is influenced by the load power factor cos2 and the error increases with its decrease. Moreover, the nature of the load exerts a greater influence on the angular error than on the error in the voltage.
On the angular error, the wick correction does not affect. The angular error in three-phase voltage transformers can be compensated. In this case, the necessary compensation is achieved by applying special compensating windings. With an active load, a positive correction is made. With an inductive load, another connection scheme is used, which gives a negative correction.
At voltage up to 35 kV the design of voltage transformers is similar to the design of power transformers.
Induction in the cores is much less than in power transformers. This reduces the error, allows in some cases to conduct induced voltage tests.
To test the transformer, a doubled voltage of 50 Hz is applied to the terminals of the secondary winding. The double voltage also appears on the primary winding. Induction should not exceed induction of saturation.
During operation, it is possible that the primary winding, designed for operation with phase voltage, falls under the line voltage instead of the phase voltage. In this case, the core must not be saturated.
For voltage up to 35 kV single-phase transformers are manufactured, in which either both terminals of the high-voltage winding are insulated from the housing (Figure 8, a),either only one is isolated, and the second terminal is grounded.
The use of plastics as insulation and the rejection of oil insulation make it possible to reduce the weight and overall dimensions of the transformers, simplify their operation, and unnecessary care for the oil. Transformers with cast insulation are fireproof, convenient for operation in various mobile installations.
Fig. 8. Appearance of single-phase voltage transformers with oil insulation (a)and cast insulation (b)
In Fig. 8, b shows a voltage transformer with cast insulation type NOK-6 on the same parameters as the oil. Domestic industry produces transformers with cast insulation for voltage up to 35 kV.
The overall dimensions of the transformers are largely determined by the insulation of the apparatus. In this connection, where possible, the transformer is used to measure the voltage between phase and earth. In this case, there is no need to isolate the second terminal of the primary winding, which is grounded, the line voltage is obtained by connecting to the star secondary windings of such transformers. However, however, the measurement error increases, since the errors of the two transformers are added together. This design allows to reduce the overall dimensions and reduce the cost of the voltage transformer.
Fig. 9. Schemes for the inclusion of voltage transformers in three-phase networks using two (a)and three (b)single-phase transformers.
Possible circuits for the inclusion of single-phase normal-type transformers in three-phase networks are shown in Fig. 9.
In the case shown in Fig. 9, a,two single-phase transformers are used, in which the primary winding has isolated leads. This scheme is called an open triangle scheme. Such a circuit is very convenient for measuring power and energy. In this scheme, each load transformer can be connected to a nominal load.
The circuit allows you to obtain and voltage U AC = -(U AB + U BC ) (devices are connected between points aand from).However, this inclusion of the load is not recommended, since additional errors are created due to the current of the devices passing through both secondary windings.
When you turn on the circuit shown in Fig. 9, b,can be used transformers, in which one of the terminals of the primary winding is grounded. Each of the windings is connected to the phase voltage, therefore the rated voltage of the transformer must be equal to U ф / . The secondary load is connected according to the star or delta circuit. The rated voltage of the secondary winding is 100 /
To monitor the insulation and power supply of the protection triggered by a short-circuit to the ground, the transformers have additional windings that are connected in an open triangle circuit. In the symmetric mode, the sum of the emfs induced in these windings is zero. If one of the wires is grounded, then the EMF equilibrium is violated and the voltage at the ends of the open triangle is applied to the relay or signaling.
There are two modes of operation of the circuit shown in Fig. 9, b.If the mains neutral is isolated or grounded through an arc suppression coil, the grounding of one of the phases, for example phase C, does not lead to a short circuit. Installation can remain for a long time in operation. The voltage across the transformer C
drops to zero, and the voltage across transformers A IAT
increases to linear. In this connection, the induction in the cores of the transformers A
and B
increases in time. In order to avoid an increase in the heating of the cores and a sharp increase in the error of these transformers, the cores should not be saturated with such an increase in induction.
Any engineer, when designing electronic counter electricity, faced with the need to select primary converters. If the microcircuits used as measuring instruments have enough detailed descriptions, then for the current sensors there is a serious information hunger. This article contains a minimum of formulas, but it is intended to understand the operation of various sensors, their advantages and disadvantages, to perform calculations and to select the elements of the measuring circuits.
The most simple voltage and current sensors are precision resistive sensors. Accordingly, a voltage divider for measuring the current voltage and a current shunt for measuring the current current.
The voltage divider is calculated so that the voltage at its output is the value recommended for a particular ms. counter and did not exceed the maximum permissible measured voltage (usually + -400 mV or + -500 mV) at the extreme value of the input voltage. The divider is connected between the two wires of the controlled circuit (zero and phase). The effective value is, respectively, = 400 mV / 1.732 = 231 mV.
Current shunt
Transformer current sensors (current measurement transformers)
Transformer current sensors are more expensive than resistive ones, but they have a number of significant advantages:
 |
2. Measuring current transformers provide galvanic isolation between windings, therefore measuring circuit It is not at high potential as with a shunt and can be easily screened.
3. The parameters of the current transformer do not practically change over time and do not depend on temperature.
4. The coefficient of transformation is easily maintained during production and remains always constant.
5. Current transformers perfectly suppress pulse interference in the measuring circuit without the use of additional filters
6. Provide a minimum phase shift between the voltage and current measurement circuits. Filtering of the measuring signal is performed due to the intrinsic inductance of the transformer.
7. Ease of measuring 3 phase current signals due to galvanic isolation of current wires and the measuring part.
As current sensors (measuring current transformers), two types of transformer sensors are usually used:
1. Transformer loaded on a precision resistor - current transformer. Usually with a magnetic core made of amorphous or nanocrystalline alloys. The output voltage taken from the resistor is proportional to the current of the primary winding;
2. Differentiating transformer di / dt, operating in the mode of shock excitation. Usually without a magnetic circuit (air). The output voltage of the transformer is proportional to the rate of change of the primary winding current.
The use of a transformer current sensor in electricity meters can be combined with the use of a resistive voltage sensor or a voltage transformer. Usually, a resistive divider is used as the cheapest.
Measuring current transformer with load resistor
 |
The safety of secondary circuits with large input currents is ensured by the entry of the core into saturation. However, if the secondary circuit of the current transformer is opened (emergency mode), the disappearance of the secondary current and the magnetic flux created by it will lead to a significant increase in the total magnetic flux and accordingly to an increase in the EMF in the secondary winding to huge values, which can cause an insulation breakdown. In addition, with a large magnetic flux, the losses in the core increase sharply, which causes it to warm up.
The errors of the transformer current sensor are added from the current error (the error of the actual transformation ratio) and the angular error (the phase difference between the primary and secondary circuit currents). Errors are determined by two factors: the magnetic permeability of the magnetic circuit and the non-zero value of the load resistance. At the same time, the error of the transformer is less, the smaller the magnetic resistance of the magnetic circuit; the magnetic permeability of the material is greater, the cross section of the core is smaller and its length is shorter, and also the smaller its secondary load (ideal is the secondary coil of the secondary winding). It is important to consider that the magnetic permeability depends on the intensity magnetic field, and is practically constant only in the region of weak fields. Since transformers work in weak resultant fields, they require the use of a material with a high initial magnetic permeability.
Nanocrystalline or amorphous alloys are used as the cores of the transformer current sensors.
 |  |
One of the drawbacks of current transformers is the magnetization of the core by a constant current component that occurs in a controlled electrical circuit due to the asymmetry of load consumption (for example half-wave rectifier) in different half-waves. It is possible to level this disadvantage the right choice dimensions or material of the magnetic circuit of the current transformers. The constant magnetic flux due to the difference in currents in the primary winding in different half-waves is not compensated. As a result, in the core of the current transformer, a constant flux is superimposed on the alternating magnetic flux, which leads to a displacement of the real magnetization curve of the core in the region of large fields at the same power consumption in the load. However, it should be noted that the distortion is formed in the region of the current transition through 0, while distortions in one half-wave lead to compensatory distortion in the other, so the actual error in measuring the power consumption in the meter does not change so drastically.
 |  |
 |
The constant flow margin can also be achieved by decreasing the magnetic field strength in the core (for the same current in the primary winding) by increasing the length of the magnetic circuit (the magnitude of the magnetic flux intensity is directly proportional to the product of the number of turns per current and inversely proportional to the average length of the magnetic circuit and is expressed by the formula H = N1 * I1 / L). However, an increase in the length of the magnetic circuit causes a decrease in the EMF of self-inductance, which is directly proportional to the cross-sectional area of the magnetic circuit and inversely proportional to the length of the magnetic circuit. Therefore, the increase in length should be accompanied by an increase in the cross-sectional area - to maintain the previous value of the inductance. As is known, the higher the inductance of the secondary winding, the lower the rate of current change and the lower the induced EMF in the primary winding. In addition, the large inductance, together with the resistance of the secondary winding, acts as a low-pass filter in the measuring circuit (and does not introduce phase distortion!) And, in addition, reduces the effect of the ADC of the meter on the measuring circuit. In this regard, the requirements for the RC circuit in the measuring channel are reduced (it can not be set at all!), And, consequently, the phase shift introduced by this filter between the channels of current and voltage measurement is reduced.
The calculation of the measuring circuit for a particular current transformer is relatively simple. As it was said above, in the secondary winding of the current transformer loaded to the resistor Rb, a current transforms from the primary winding and is caused by the phenomenon of electromagnetic induction. The secondary resistance of the secondary circuit is Rb + R2, where R2 is the characteristic resistance of the secondary winding of the current transformer, and Rb is the resistance of the load resistor. The secondary winding current I2 ~ I1 / N, where N is the transformation ratio (usually 1000 ... 3000).
The output voltage of the current sensor, determined by the voltage drop on Rb:
U2 = I2 * Rb = I1 * Rb / N. The equivalent voltage at the input of the transformer U1 = U2 / N = I1 * Rb / N ^ 2
Thus, the voltage on the primary winding of the current transformer is proportional to I1 * Rb / N ^ 2. those. in N ^ 2 times smaller than for a shunt with the same output voltage for measurement. Therefore, the influence of the transformer current sensor on the monitored circuit is less than when using a shunt. For example for a current transformer with N = 3000; U2 = 20mV, I1 = 50A (see calculation for shunt above), calculate the equivalent input impedance. I2 = 50/3000 = 0.01667A. Rb = 20mV / 16.67mA = 1.2 Ohm. The input impedance of an ideal transformer is Rb / N ^ 2 = 1.2 / 3000 ^ 2 = 0.1333 μΩΩ. However, taking into account the intrinsic resistance of the secondary winding (for a transformer on a magnetic core OL25x15x10 about 400 Ohm), the equivalent active input resistance is (Rb + R2) / N ^ 2 = (1.2 + 400) / 3000 ^ 2 = 44.6μΩ (compare with 400 μΩ on the shunt!). Estimating the value of Rb, it can be seen that it is negligible compared to the internal resistance of the winding of the transformer. In this way, it is possible to increase Rb for obtaining high voltages for the subsequent measurement, and therefore to increase the accuracy with the measurement of low currents, to reduce the effect of electrical noise on the measured circuit and at the same time to not introduce any additional losses to the measured circuit.
Differential current transformer
 |
In order to obtain a signal that is acceptable for measurement, the differentiating transformer is used in the shock excitation contour mode (and not in the current transformer mode), at which the EMF at the output is proportional to dI / dt, for this the load resistor Rb has a sufficiently large value. In this mode, the output signal from the transformer does not repeat the shape of the input current, but the transformer has a high sensitivity to current change. In order to avoid distortion of the output signal, an integrating circuit is used (in ADE7753 / 59 for single-phase or ADE7758 for a three-phase circuit it is built-in). In this case, the transformer winding (L2 and R2), R and C of the integrator form an oscillatory circuit with attenuation and a self-inductance emf in series. In general, the voltage across the capacitor: U = L2 * I1 / ((R2 + R) * C * N). The time constant (R + R2) * C, (L2 * C) ^ 0.5 should be chosen to significantly exceed the time constant of the change in the input current.
 |
