Types of rheostats. A rheostat is a control device that can change current and voltage

The device was developed by scientist Johann Christian Poggendorff. So what is a rheostat and why is it needed?

What is a rheostat

The rheostat has a fairly simple design

A rheostat is an electrical device consisting of resistors and a device that regulates the resistance of all connected resistors.

This device is universal: it is capable of not only controlling current and voltage, but also setting the sound volume on TVs.

Rheostat device

The ceramic cylinder is wrapped with a metal conductor called a winding. Its ends are brought out to the terminals. These are small-sized clamps to which an upper rod made of metal is attached. A sliding contact moves along this rod and winding, which experts call a slider. Thanks to these elements, the rheostat operates.

It is worth noting that the ceramic cylinder is hollow. This feature allows the device to cool, prevents overheating, making the device safer.

What is it for? The rheostat is the best way control and regulation of current strength.

The device changes the resistance and is capable of changing the voltage in the electrical circuit, which allows you to regulate the functioning of the electric motor in the sewing machine, the volume of the radio, and TV.

The rheostat allows you to regulate and change the current and voltage The rheostat is actively used when creating electrical appliances

. Thanks to this element, the current and voltage can be controlled, preventing overheating. Ohm's law clearly shows that the current strength in a circuit can be changed by including an electrical device in it - a resistor or rheostat, which has a certain electrical resistance

. This property is widely used in practice to regulate and limit current in engines, generators and other electrical devices. Resistors and rheostats (Figure 8) are usually made of wire or tape, the material for which is metal alloys with high resistivity

(constantan, nickelin, manganin, fechral), which makes it possible to use wire of the shortest length for the manufacture of these devices. In radio engineering and electronics devices, resistors made of graphite are often used.

Figure 8 – Design of rheostats:– with a smooth change in resistance, b– with stepwise change in resistance, V– from cast iron plates, G– from fechral tape

Rheostat r can be included in the circuit between source and receiver r n electrical energy(Figure 9 Figure 8 – Design of rheostats:). In this case, when the resistance of the rheostat changes, for example, due to movement of the movable contact, the current strength changes I passing through the source and receiver. This current flows only through part of the rheostat. However, the rheostat can be connected to the circuit in such a way that the current passes through its entire resistance, and only part of the source current branches off to the receiver. In this case, the two extreme clamps 1 And 2 rheostat (Figure 9 b) is connected to a source of electrical energy, and one of these terminals, for example 2 , and the moving contact of the rheostat 3 connected to the receiver r n. Obviously, with this connection, voltage will be supplied to the receiver U, which depends on the resistance of the part of the rheostat connected between the terminal 2 and moving contact.

Figure 9 – Diagrams for connecting rheostats:

Figure 8 – Design of rheostats:– in series in the circuit of the electrical energy receiver, b– as a voltage divider

Therefore, by moving the moving contact of the rheostat, you can change the voltage U, supplied to the receiver.

Rheostat connected according to the circuit shown in Figure 9 b, is called a voltage divider or potentiometer. If the receiver resistance is relatively high compared to the rheostat resistance, then the voltage at the receiver terminals

Where r 1 And r 2– resistance of the rheostat parts.

CONTROL QUESTIONS

1. What does an electrical circuit consist of?

2. What devices can act as sources and receivers of energy?

3. External and internal source electrical energy.

4. What is called electric current, current strength? Current direction. What kind of current is called alternating or direct?

5. Electrical conductivity of matter: division into conductors, dielectrics, semiconductors.

6. What is called an electric field?

7. What is electric field strength?

8. What is electric field energy?

9. The concept of electric potential.

10. What is called electrical voltage?

The lesson discusses a device called a rheostat, the resistance of which can be changed. The design of the rheostat and the principle of its operation are discussed in detail. The rheostat designation is shown in the diagrams, possible options inclusion of a rheostat in an electrical circuit. Examples are given of the use of a rheostat in Everyday life.

Topic: Electromagnetic phenomena

Lesson: Rheostats

In previous lessons we said that there are not only consumers and sources electric current, but also the so-called controls. One of the important control elements is the rheostat or any other device based on its action. The rheostat uses a conductor made of a previously known material with a certain length and cross-section, which means we can find out its resistance. The principle of operation of the rheostat is based on the fact that we can change this resistance, therefore, we can regulate the current and voltage in electrical circuits.

Rice. 1. Rheostat device

Figure 1 shows a rheostat without a shell. This is done so that you can view all its parts. A wire (2) is wound on a ceramic pipe (1). Its ends are connected to two contacts (3a). There is also a rod, at the end of which there is a contact (3b). A sliding contact (4), the so-called “slider,” moves along this rod.

If you place the sliding contact in the middle (Fig. 2a), then only half of the conductor will be used. If you move this sliding contact further (Fig. 2b), then more turns of wire will be used, therefore, its length will increase, the resistance will increase, and the current will decrease. If you move the “slider” in the other direction (Fig. 2c), then, on the contrary, the resistance will decrease and the current strength in the circuit will increase.

Rice. 2. Rheostat

The rheostat is hollow inside. This is necessary because when current flows, the rheostat heats up, and this cavity provides rapid cooling.

When we draw a diagram (drawing of an electrical circuit), each element is designated a certain symbol. The rheostat is designated as follows (Fig. 3):

Rice. 3. Image of a rheostat

The red rectangle corresponds to the resistance, the blue contact is the wire leading to the rheostat, the green is the sliding contact. With this designation, it is easy to understand that when the slider moves to the left, the resistance of the rheostat will decrease, and when moved to the right, it will increase. Can also be used next image rheostat (Fig. 4):

Rice. 4. Another image of the rheostat

The rectangle indicates resistance, and the arrow indicates that it can be changed.

The rheostat is connected in series to the electrical circuit. Below is one of the connection diagrams (Fig. 5):

Rice. 5. Connecting a rheostat in a circuit with an incandescent lamp

Terminals 1 and 2 are connected to a current source (this can be a galvanic cell or a connection to a socket). It is worth noting that the second contact must be connected to the moving part of the rheostat, which allows you to change the resistance. If you increase the resistance of the rheostat, the intensity of the light bulb (3) will decrease, which means that the current in the circuit will also decrease. And, conversely, as the rheostat resistance decreases, the light bulb will burn brighter. This method is often used in switches to adjust light intensity.

A rheostat can also be used to regulate voltage. Below are two diagrams (Fig. 6):

Rice. 6. Connecting a resistor in a circuit with a voltmeter

In the case of using two resistances (Fig. 6a), we remove a certain voltage from the second resistor (a device that is based on the resistance of the conductor), and thus, as it were, regulate the voltage. In this case, you need to know exactly all the parameters of the conductor to correctly regulate the voltage. In the case of a rheostat (Fig. 6b), the situation is noticeably simplified, since we can continuously adjust its resistance, and therefore change the voltage removed.

A rheostat is a fairly universal device. In addition to adjusting current and voltage, it can also be used in various household appliances. For example, on televisions the volume is adjusted using rheostats, and switching channels on a TV is also in some way connected with the use of rheostats. It is also worth noting that for safety it is better to use rheostats equipped with a protective casing (Fig. 7).

Rice. 7. Rheostat in a protective casing

In this lesson we looked at the structure and use of a control element such as a rheostat. In the following lessons, problems related to conductors, rheostats and Ohm's law will be solved.

Bibliography

1. Gendenshtein L.E., Kaidalov A.B., Kozhevnikov V.B. Physics 8 / Ed. Orlova V.A., Roizena I.I. - M.: Mnemosyne.
2. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
3. Fadeeva A.A., Zasov A.V., Kiselev D.F. Physics 8. - M.: Enlightenment.

1. Education Center "Teaching Technologies" ().
2. School demo physical experiment ().
3. Electrical Engineering ().

Homework

1. Page 108-110: questions No. 1-5. Peryshkin A.V. Physics 8. - M.: Bustard, 2010.
2. How can you regulate the intensity of a lamp using a rheostat?
3. Will the resistance always decrease when moving the rheostat slider to the right?
4. What is the reason for using a ceramic pipe in a rheostat?

Resistors. Ohm's law clearly shows that the current strength in an electrical circuit can be changed by including various resistances in it. This property is widely used in practice to regulate and limit current in the windings of motors, generators and other electrical consumers. Electrical apparatus, designed to be included in an electrical circuit in order to regulate or limit the current passing through it, is called a resistor. Resistors come with constant or adjustable resistance. The latter are sometimes called rheostats.
Resistors are usually made of wire or tape, the material for which is metal alloys with high resistivity (constantan, nickel, manganin, fechral). This makes it possible to use wire of the shortest length for the manufacture of resistors. In electrical circuits through which relatively small currents pass (for example, in control circuits, in electronics and radio engineering devices), non-wire resistors made of graphite and other materials are often used.
Rheostats can be made with a smooth or stepwise change in resistance. In control laboratories electric machines and testing devices often use a slider rheostat with a smooth change in resistance (Fig. 16, a). Such a rheostat consists of an insulating tube 4, onto which a wire spiral 5 is wound. A movable contact 2 touches the turns of this spiral. Clamp 1 of the rheostat is connected to the movable contact, the other clamp 3 is connected to one of the ends of the spiral. By moving the moving contact, you can change the length of the wire located between the rheostat terminals, and thereby change its resistance.
For starting and regulation electric motors machine tools, lifting mechanisms, etc., use a slider rheostat with a stepwise change in resistance (Fig. 16, b). The rheostat consists of a number of identical resistances 9 (sections) connected to contacts 8. To include a certain number of sections in the circuit, a slider 7 with a steering wheel 6 is used.
To regulate the current when starting traction motors of electric locomotives direct current rheostats with stepwise changes in resistance are used (starting rheostats). Individual sections of the rheostat are short-circuited during the starting process by remotely controlled switches called contactors.
On some electric locomotives (for example, emergency electric locomotives), the starting rheostats are made of cast iron plates 10 of a special shape, reminiscent of a zigzag laid ribbon. Individual plates are assembled on insulated pins and attached to base 11 (Fig. 16, c).

IN Lately starting rheostats for electric locomotives and motor cars are made of fechral tape 12 wound on porcelain insulators 13 (Fig. 16, d). Rheostats are also designed to regulate the excitation current of traction motors on electric and diesel locomotives. Fechral tape rheostats more

are durable, more resistant to shaking and vibration and have less weight than rheostats made of cast iron plates.
Diagrams for switching on rheostats. Rheostat 2 (Fig. 17) can be connected in series to the circuit between source 1 and receiver 4 of electrical energy. In this case, when the resistance of the rheostat changes, i.e. when moving contact 3 moves, the current strength in the receiver changes. This current passes only through part of the resistance of the rheostat.
However, the rheostat can be connected to the circuit in such a way that the current passes through its entire resistance, and only part of the source current is branched to the receiver. In this case, the two outer terminals 2 and 4 of the rheostat (Fig. 18) are connected to source 5, and one of these terminals, for example 4, and the moving contact 3 of the rheostat - to receiver 1. Obviously, with this connection, voltage will be supplied to the receiver U, equal to the voltage drop between clamp 4 and moving contact 3 of the rheostat. Consequently, by moving the moving contact of the rheostat, you can change the voltage U supplied to the receiver and the current in it. The voltage U represents only part of the voltage U at the source terminals.
Rheostat connected according to the diagram in Fig. 18 is called a voltage divider, or potentiometer.

Rheostatcalled a device consisting of a set of resistors and a device with which you can adjust the resistance of the included resistors and thereby regulate alternating and direct current and voltage.

Distinguish rheostats with air and liquid (oil or water) cooling. Air cooling can be used for all rheostat designs. Oil and water cooling used for metal rheostats, the resistors can either be immersed in a liquid or flow around it. It should be borne in mind that the coolant must and can be cooled by both air and liquid.

Metal rheostats with air cooled received the greatest distribution. They are easiest to adapt to different conditions work both in terms of electrical and thermal characteristics and in terms of various design parameters. Rheostats can be designed with continuous or stepwise resistance changes.

The step switch in rheostats is flat. In a flat switch, a moving contact slides over fixed contacts, moving in the same plane. Fixed contacts are made in the form of bolts with flat cylindrical or hemispherical heads, plates or tires, located along a circular arc in one or two rows. The movable sliding contact, usually called a brush, can be of a bridge or lever type, self-aligning or non-self-aligning.

A non-self-aligning moving contact is simpler in design, but is unreliable in operation due to frequent violation contact. With a self-aligning moving contact, the required contact pressure and high reliability in operation. These contacts became widespread.

The advantages of a flat switch of rheostat stages are the relative simplicity of the design, relatively small dimensions with a large number of stages, low cost, the ability to install contactors and relays on the switch plate to disconnect and protect controlled circuits. Disadvantages - relatively low switching power and low breaking power, high brush wear due to sliding friction and melting, difficulty in using for complex circuits connections.

Metal rheostats with oil cooling provide an increase in heat capacity and heating time constant due to the high heat capacity and good thermal conductivity of the oil. This allows you to sharply increase the load on the resistors in short-term modes, and consequently reduce the consumption of resistive material and the dimensions of the rheostat. Elements immersed in oil should have as large a surface as possible to ensure good heat transfer. It is not advisable to immerse closed resistors in oil. Immersion in oil protects resistors and contacts from harmful effects environment in chemical and other industries. Only resistors or resistors and contacts can be immersed in oil.

The breaking capacity of contacts in oil increases, which is an advantage of these rheostats. The contact resistance of contacts in oil increases, but at the same time cooling conditions improve. In addition, due to lubrication, large contact pressures can be tolerated. The presence of lubricant ensures low mechanical wear.

For long-term and intermittent operating modes, oil-cooled rheostats are unsuitable due to low heat transfer from the surface of the tank and a long cooling time constant. They are used as starting rheostats for asynchronous electric motors with a wound rotor with a power of up to 1000 kW with rare starts.

The presence of oil also creates a number of disadvantages: room pollution, increased fire hazard.

Rice. 1. Rheostat with continuous resistance change

An example of a rheostat with an almost continuous change in resistance shown in Fig. 1. Resistor wire 2 is wound on frame 3 made of heat-resistant insulating material (steatite, porcelain). To isolate the turns from each other, the wire is oxidized. A spring contact 5 slides along the resistor and the guide current-carrying rod or ring 6, connected to the movable contact 4 and moved by an insulated rod 8, onto the end of which an insulated handle is put on (the handle is removed in the figure). Housing 1 is used to assemble all the parts and fasten the rheostat, and plates 7 are used for external connection.

Rheostats can be included in the circuit as variable resistor (Fig. 1, a) or as (Fig. 1.6). Rheostats provide smooth resistance control, and therefore the current or voltage in the circuit and are widely used in laboratory conditions in automatic control circuits.

On the image 2 shown switching circuit using a rheostat for a low-power DC motor.

Rice. 2. Rheostat connection diagram: L - clamp connected to the network, I - clamp connected to the armature; M - clamp connected by an excitation circuit, O - single contact, 1 - arc, 2 - lever, 3 - working contact.

Before turning on the engine, you must make sure that rheostat lever 2 is on idle contact 0. Then the switch is turned on and the rheostat lever is moved to the first intermediate contact. In this case, the motor is excited, and a starting current appears in the armature circuit, the value of which is limited by all four sections of resistance Rп. As the armature rotation speed increases, the starting current decreases and the rheostat lever is moved to the second, third contact, etc., until it reaches the working contact.

Starting rheostats are designed for short-term operation, and therefore the rheostat lever cannot be held for a long time on the intermediate contacts: In this case, the rheostat resistances overheat and may burn out.

Before disconnecting the engine from the network, it is necessary to move the rheostat handle to the extreme left position. In this case, the engine is disconnected from the network, but the field winding circuit remains closed to the resistance of the rheostat. Otherwise, large overvoltages may appear in the excitation winding at the moment the circuit opens.

When starting DC motors, the adjusting rheostat in the field winding circuit should be completely removed to increase the field flow.

To start motors with sequential excitation they use double-clamp starting rheostats, differing from three-clamp ones in the absence of a copper arc and the presence of only two clamps - L and Y.

Rheostats with step change in resistance(Fig. 3 and 4) consist of a set of resistors 1 and a step switching device.

The switching device consists of fixed contacts and a movable sliding contact and drive. In the starting-regulating rheostat (Fig. 3), pole L1 and armature pole R, taps from resistance elements, starting and adjusting, according to the breakdown by stages, and other circuits controlled by the rheostat are connected to the fixed contacts. The movable sliding contact makes the closing and opening of the resistance stages, as well as all other rheostat-controlled circuits. The rheostat drive can be manual (using a handle) or motorized.

Rice. 3 R PC - resistor shunting the contactor coil in the rheostat off position, R limit - resistor limiting the current in the coil, Ш1, Ш2 - parallel excitation winding of a DC electric motor, C1, C2 - series excitation winding of a DC electric motor.

Rice. 4 R pr - pre-connected resistance, OB - excitation winding of a DC electric motor.

Connection diagram for an oil-filled rheostat of the RM series, intended for starting asynchronous motors with a wound rotor, shown in Fig. 5. Voltage in the rotor circuit is up to 1200 V, current 750 A. Switching wear resistance is 10,000 operations, mechanical - 45,000. The rheostat allows 2 - 3 starts in a row.

Rice. 5

The rheostat consists of resistor packs and a switching device built into the tank and immersed in oil. Resistor packages are assembled from elements stamped from electrical steel and attached to the tank lid. The switching device is of the drum type and consists of an axis with segments of a cylindrical surface attached to it, connected along a certain electrical diagram. On a fixed rail there are fixed contacts connected to resistor elements. When the drum axis is rotated (by a flywheel or a motor drive), the segments, like movable sliding contacts, bridge certain fixed contacts and thereby change the resistance value in the rotor circuit.