Features selection and use of resistors in the power engineering

Features selection and use of resistors in the power engineering

Resistors, i.e. electrical appliances having predetermined electric resistance is perhaps one of the most common types of electronic components. They are used in the apparatus of practically any purpose and application. The correct choice of resistors, according to operating conditions and destination devices depends largely on trouble-free operation of equipment throughout its lifetime.

Apparent prologue and evidence-Application of resistors will the developers of power converter equipment misleading impression of the effect of small resistors, as extremely simple in terms of circuitry, resulting in the reliability of the instruments developed to poke device. However, it is not. singing, and apply resistors as n any other components requires a careful approach to the choice of types and provide a positive work environment.

For a better understanding of the operation of resistors turn to the basic concepts. Resistor as plement elegtricheooop circuit serves to generate resistance progtekatppo electric current. Ideally, the work of the resistor is determined by the fundamental law, it is set by the German physicist Georg Simon Ohm, and named after him:

I = UIR,

(1) where I — electrical resistance of the singing; U- voltage applied to the circuit portion; I — current flowing in the circuit.

When current flows through the resistor energy orderly movement of charge carriers is converting into heat and dissipated in the surrounding area due to heat and radiation. The power dissipated in the resistor can be determined from the formula that follows from Ohm’s law:

P = I’R

P = U4R.



Here P power dissipated in the part of the chain; R — resistance of the electrical circuit; U- voltage applied to the circuit portion; I — current flowing in the circuit.

The power dissipated in the resistor causes an increase in its temperature. The maximum temperature that the resistor can withstand without damage depends on the design of the resistor and the materials used — as the actual resistance element, and its fittings. That is the maximum neratura the hottest portion of the resistor defined delyaeg the power that the resistor can dissipate.

Depending on the conditions in which there is a resistor (temperature, humidity, ambient air pressure and the speed of its movement), the same power dissipation causes various temperature rise of the device, so St. resistor selection is important not only to determine the dissipated power, but also the conditions for its operation. Rated power of the resistor is defined as the power dissipated by the device without exceeding the maximum permissible temperature at the natural air cooling at an altitude of 0 meters above the level of «! sea ​​at an air temperature of 25 ° C.

In operation, the resistor should be remembered that the dissipated power has a quadratic dependence on the voltage applied to the resistor or current flow.

Ego means that a slight increase in voltage or current in the circuit will cause a substantial increase in power dissipation, which can exceed the maximum allowed for the use of the resistor, resulting in a failure of the device. Therefore, when choosing the resistor, it is important to know not only the nominal current and voltage for him, but upityvat possible prolonged deviations, in particular because of the mains voltage fluctuations.

If the power dissipated by the resistor is constant, then after a while the temperature of the resistor is stabilized (boiler amount of heat generated in the resistor will be equal to the amount of heat given resistor into the environment by radiation, convection and heat transfer design). The larger physical size cramps Stora, the more efficient the process of heat transfer and the lower the equilibrium temperature at the same capacity allocated. Furthermore, the efficiency of radiation and convection heat transfer essentially depends on the design og rezisgo pa, the materials used and the cooling conditions.

Reference materials cited in the value of the maximum power dissipation of resistors refer to the conditions of the natural cooling.

Today, there are a number of stan- dards governing the method for determining the maximum power dissipation of resistors sprouting from the superheat temperature of the hottest portion of the resistor. Leading manufacturers of high-power resistors (Danolherm, OJimite, Arcot, SIR et al.) With power rationing their devices are usually guided by the recommendations of the National Electrical Manufacturer »Association (NEMA) and Underwriters Laboratories, Inc. (UL). According to those, the maximum cardinality Jost with natural cooling for the resistor specified physical characteristics of size n, is defined as the power that causes the temperature (izmerennuyutermoparoy) overheating of the hottest portion of the resistor 300 «C at an ambient temperature of 40» C. The measurement is performed in still air in a free convection and removing resistor closest object (such as walls, panels, instrument «) is not less than 35 cm.

Several other conditions of measurement sets the standard MIL R-26 originally RPPOs botatpty wire resistors for military and aerospace applications, and then distributed to the devices and industrial and commercial applications. Under this standard, the maximum heating temperature of the hottest portion of the resistor is set equal to 350 «C at an ambient temperature of 25 * C. Thus, the temperature corresponding to the heat of 325 ° C

Fig. 2 shows plots of the average temperaggury overheating resistors of different standards depending on the relative power dissipation:

In the first approximation, temperature speech Stora depends on the surface area, and (I menypey degrees) from a number of other factors such as the thermal conductivity of the base and cover of the resistor, the efficiency of surface radiation, the ratio of the resistor to its diameter, the heat transfer through the leads and the mounting means.

Maksnmalpo allowable temperature of the resistor is determined by the properties of its structural materials CP n is limiting value is exceeded, the Iranian who swarm the device may lose efficiency. In the general case, this value can only be guided to calculate limiting modes of operation.

Under normal operating conditions, barely blowing to take into account not only and not so much the physical functioning of the resistor, non other parameters such as a change in resistance with increasing temperature, heating the surrounding resistor devices due to their allocated heat resistance versus ambient humidity (particularly for resistors open type), the change in characteristics under cyclic loading, and so on. n.

If the ambient temperature differs (increase) of 25 ° C (or 40 ° C), the dissipation resistor moschpost should be correspondingly reduced to values ​​that do not exceed the maximum permissible temperature of the instrument. Fig. 3 shows graphs of dependence of relative power dissipation of resistors on the ambient temperature as recommended by NEMA, U1. and M1L-R-26 (U-ELA):

In constructing these dependences it is assumed that the temperature does not overheat zavn on the magnitude of ambient srely. However, this is not entirely true. The exact calculation should consider increasing the effectiveness of radiation-tnvnosti with increasing temperature according to the Stefan Boltzmann and Wien. However, the contribution made by this nsvyso at temperatures (up to 1000-1500 «C) is very small and can be taken into account ue in the vast majority of structural calculations.

For some types of resistors in ano their data indicates the maximum thermal load of an admissible surface. For most types of wire resistors it is 0.7 W / cm-1 (for resistors on large capacity more than 150-200 W) to 2 W / cm-1 (small resistors with the cardinality of 10-20 W). This value is useful in calculating the resistor works as iagreiatelyyugo .elementa.

Note that the definition in the recommendations for the maximum power resistors undisclosed location relative to the resistor surface. But there is a precise indication of the PA that the temperature measured for the hot end of the resistor. In a horizontal tubular wire-wound resistor with a resistance element of uniform temperature in the vicinity of the middle of the device can be 13-23 times higher than the temperature at the ends (depending on the fastening method). With a vertical arrangement of maximum heating zone shifted upwards step 3-10% of the length of the resistor, and the upper end has a higher temperature than the bottom. This causes a slight increase in the mechanical stresses in the construction of the device and may reduce its reliability. Therefore, ceteris paribus, always predpochttt agg-horizontal arrangement of resistors, except specially designed for vertical mounting of devices, such as Tenley «diverting cases NC Alum no evogo profile. For some specific applications (e.g., as a uniform source of heat) produced special resistors with uneven winding resistance element (more frequent at the edges and in the middle of a rare) for which the temperature is essentially constant over the entire length of the device.

Let us consider the main factors determining the temperature of the resistor or, on the other hand, the required value of the nominal power at which the temperature does not exceed the set:

I. Ambient

Increased ambient temperature causes a corresponding snizhet allowable temperature overheating and the corresponding power dissipation. A plot of the relative allowable power dissipation of ambient srely shown above in Fig. 3. If the ambient temperature is lower than that for which it was determined the maximum power dissipation (25 «C or 40 ° C), the first in a number of cases, you can prevent the increase of the maximum power value of the above types, but it must also be possible utochpyat resistor to work with currents exceeding the nominal. 11revyshenie current resistor in this case can cause nne will not increase its temperature above the maximum admits reversible, and the destruction of the external n internal contact of * (the junction of the resistive element to the pin) and local overheating and melting of the resistive element,

2. Installation of a closed cabinet t Installation of the resistor body, I worsens the conditions of heat removal by .ha igtucheni * (part of the radiation reflected by the walls of the body, the rest is emitted in the surrounding as well as in the interior of the body), as well as by convection (convection casing breaks n blocks the flow of air access of cold air to the resistor), a significant effect on the temperature of the resistor. placed in a housing having the size, wall thickness, material, and their presence at the perforation and surface coloring. The deterioration of the working conditions of the resistor when placed in a body good show graphs in Fig. 4.

3. Mounting bands Resistors, moptiruemye at a small distance from each other when the heated more than a single resistor with the same power dissipation (in each reaction

.1" gt; v b. . 12 15 1 * 2 1 2 21 M

Height * honey sea level. tis. m. Fig. 6. Relative permissible power dissipation resistor of the height above the level of Mora znstorov group). Ego is due shaft nmnogo heating resistors radiation and increasing the amount of heat per unit) ‘volume of cooling air under natural convection. In order that the temperature of the resistors working in a group does not exceed the permissible value, you must reduce the power attributable ua each of the devices of otnosheshpo to the maximum allowable for one free set resistor. Fig. 5 gives an idea of ​​the order desired reduction in power dissipation of each pas resistors depending on the number of resistors in the pear, and the distances between them.

4. Altitude number Tesha withdrawn from the resistor ■ due to air convection depends on the density of the subsequent ones. Razrezhep than the air, the less heat is able to withdraw it. 11rn rise under the air density is reduced, which means that a maximal power dissipation resistor will drop. At altitudes of more than 20 OOO m air density is already so low that kopvektiv-heat removal stops playing skolyso-no, whether a significant role in the general heat balance rezisgora and heat is removed only by radiation heat transfer and structural elements. Fig. A is a plot of relative power dissipation resistor of the height of its location (on the sea dle).

5. If the current does not flow through the resistor constant, and during certain time intervals, and in other moments resistor is disconnected, the amount teshi emitted over a significant period of time not to be less than during continuous operation. «Homogenization» time occurs due to the heat capacity design, the mounting members and the surrounding air. In the results Tate resistor temperature does not exceed the allowable poppy simalyyu even when the pulse power many times greater than the maximum power continuously. The amount of allowable pulse power depends on both relative structural singularities resistor (heat and thermal conductivity design), and the pulse duration and the ratio of the pulse duration and pause (duty cycle). Fig. 7 shows the dependence of the relative admissible pulse power dissipation for the resistors of different types, defined according to the recommendations NHMA for starting and braking resistors.

For a number of types of pulse power resistors are not restricted to valid overheating, and the maximum value of the working GOK rezisgora above which the WHO ICVS damage resistive element and conclusions due to local overheating.

The graphs in Fig. 8 give an idea of ​​the process of heating resistors of different types of current pulse and constructed in the coordinates vremepi momentum necessary to heat the resistor to the maximum allowable temperature and pulse power.

Using the plots shown in Fig. 9, it is possible to determine the ratio or ■ tslnostey pulse and pause the current through the tubular resistors, heating appliances to the maximum allowable temperature for different absolute durations and different relative power pulsed stey (as a percentage of the maximum power dissipation of continuous operation).

The above osobegatosti pulse modes are standard pulse mode, which takes place in primeneshsh resistors in the circuit starting and braking Jurassic or surface and volume of the massacre. is always associated with an increase in their fiznches electric motors, where the impact of the Jurassic era pa based conductive films, a significant number in the current units.

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