A device capable of coping with a change in resistance is commonly called a rheostat. Structurally, it is represented by a set of resistors that are connected to each other in steps, and can provide a continuous change in resistance. A separate category includes devices that carry out smooth regulation without interrupting the network. To determine what a rheostat is for, you need to take a closer look at its features and principle of operation.
Content
- The main purpose of the device
- Operating principle
- Varieties of aggregates
- Design and its features
- Sensors based on rheostats
The main purpose of the device
The described devices are universal in use. Depending on their direct purpose, it is customary to divide them into the following types:
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Start-regulating - most often used to equip DC motors. Such models are appropriate for AC induction motors equipped with a wound rotor. - Launchers - their main purpose is to reduce the starting current that appears during the start of the electric motor.
- Ballast - they provide fast absorption of excess energy that occurs when the engine is suddenly braked.
- Load - such products create the necessary resistance within the electrical circuit.
Important! Rheostats are used as current limiters in the field windings of DC electric machines.
In this way, strong fluctuations in electric current are leveled, as well as dynamic overloads, which entail damage to the drive and the entire mechanism connected to it. Providing suitable resistance at start-up will extend the service life of the collector and brushes.
Potentiometers are allocated to a separate group. They are voltage dividers based on variable resistors. Such devices make it possible to use different voltages in electronic circuits without additional power supplies, transformers. Regulation of the current strength by means of a rheostat is often used in the radio engineering field. A striking example of this is the change in the volume in the speakers.
Operating principle
The described devices are similar in their functional purpose. Structurally and visually, the slider type rheostat is considered the simplest. It connects to the circuit with the upper and lower terminals. The device is designed in such a way that the current flows along the entire length of the wire, and not in the transverse direction of the turns. This is done thanks to the reliable insulation of the conductors.
Important! Most of the slider positions use only part of the rheostat. When the length of the conductor is changed, the strength of the electric current in the working circuit is adjusted. In order to prevent premature wear of the turns, the slider is equipped with a sliding contact (wheel or graphite rod).
Often, a rheostat is used to regulate in a circuit instead of a potentiometer. In this case, it is connected using three terminals. In the lower part, two of them are input, they are connected to a voltage source. One bottom terminal and one top free terminal are used as output. When the slider moves, the tension is easily adjusted.
The rheostat tends to function in ballast mode, which may be necessary when creating an active load during energy consumption. In such a situation, it is advisable to take into account the dissipation capacity of the unit used. If there is excess heat, the device will fail. When connecting to the mains, it is necessary to correctly calculate the power dissipation of the rheostat, if necessary, create sufficient and correct cooling.
Varieties of aggregates
Rheostats with an external design in the form of a torus are very popular. The main area of their application is electric transport (trams), the industrial sector. Regulation is carried out by moving the slider in a circle. The movement of such a part is carried out along the windings, which are located toroidally.
A torus-like device modifies the resistance with virtually no break in the circuit. Its opposite is a lever-type unit. The principle of operation of such a rheostat is based on the fact that the resistors are fixed on a special frame, they are selected by means of a special lever. Any commutation breaks the circuit.
Circuits in which a lever device is involved are deprived of smooth resistance adjustment. Any switching entails a progressive change in the indicators in the network. As for the discreteness of steps, it depends on the adjustment range and the number of resistors present on the frame.
Another type is plug-in rheostats, with the help of which stepwise resistance adjustment is carried out. The main difference is changing the parameters within the network without first breaking the circuit. When the plug is applied to the jumper, most of the current flows without resistance. The current is redirected to the resistor by pulling out the plug.
Liquid and lamp fixtures are specific types of rheostats. Due to the presence of certain disadvantages, they have a narrow, specialized scope of application:
- Liquid-type devices are used in explosive areas as engine control parts.
- Lamp products are characterized by low accuracy and reliability. Often used in educational institutions in physics lessons, in laboratories, research centers.
Design and its features
Having determined what the rheostats are intended for, you should take a closer look at their component side. Depending on the material used in production, the following settings are highlighted:
- ceramic - the feature lies in the use at low power;
- metal - found widespread consumption in various areas of human activity;
- coal - their main use in industry.
Important! Heat is removed by oil, water or air. If there is no possibility of heat dissipation from the working surface, liquid cooling is used. Heat dissipation can be increased by using a fan and a radiator.
Sensors based on rheostats
The voltage, current in the working circuit, the position of the slider in the rheostat and the resistance provided to them are in direct relationship. This feature is the basis of the steering angle sensor. In such a device, a specific electrical quantity corresponds to a specific position of the rotor.
Currently, such sensors are being replaced by improved optical and magnetic counterparts. The reason for this is the instability of the dependence of resistance and angle in relation to the temperature effect. The gradual displacement of rheostat-type sensors is also due to the transition to digital, more convenient systems. Today resistive meters are used in circuits where analog signals are present.
Knowing what electric-type rheostats are for, one can easily explain their widespread use in the automotive industry, technology, and industry. Resistance is necessary for the operation of radio engineering, when starting electric motors, they are applicable in the form of an active load. Failure of a small device can cause the entire system to malfunction. This is the importance of rheostats.
