What is a rheostat? Types and their purpose

People who are connected in some way with physics, electronics, radio engineering, often face such an element as a rheostat. And others have absolutely no idea about this. This article will help to understand what a rheostat is and why it is needed.

Definition and types

So, the rheostat is a device consisting of several resistors and a device, by means of which the resistance of all the included resistors is regulated.

Types of rheostats depend on their purpose:

• There are starting current rheostats, which serve to start AC or DC motors.
• The ballast rheostat is needed to start the speed of electric motors with a constant current and to regulate it.
• A ballast or load resistor is an electrical device for absorbing the energy needed when adjusting the generator load or when testing this generator.
• The excitation rheostat is necessary in order to regulate the current in the windings of electric machines of alternating or direct current.

Material and cooling

One of the main elements that determine the design of an element is the material from which the rheostat consists. And for this reason, you can divide the rheostats into ceramic, liquid, metal and coal. Electricity in resistors is converted into heat, which must be diverted from them. Therefore, rheostats have air and liquid cooling. The second type can be water or oil. Air type is used for any design of the rheostat. The liquid is only for metal, as their resistors flow around the liquid or completely into it are immersed. It is necessary to know that the liquid used for cooling can and must itself be cooled either by air or liquid.

Metal rheostats

What is a rheostat made of metal? It is an element having an air type of cooling. Such rheostats are most common, as they can most easily be adapted to a variety of working conditions. This applies to both thermal and electrical characteristics, as well as to the design parameters. They can be manufactured with a step-by-step or continuous type of resistance change.

The switch is flat. It has a movable contact, which slides on the contacts fixed in the same plane. Those contacts that do not move are made in the form of bolts having flat heads of cylindrical or hemispherical type in the form of plates or tires that are arranged along an arc in one row or two. The contact that moves is called a brush. It can be lever or bridge in its type of execution.

There is also a division into self-installing and non-self-establishing. The latter option is simpler in design, but since contact is often broken, it is not reliable in use. The self-aligning movable contact provides the necessary degree of pressing and is more reliable in operation. That's why this kind is most common.

Pros and cons of flat switches

The advantages of flat-type switches include simple design, small dimensions with a significant number of stages, low cost, relays, disconnecting and protecting the controlled circuits.

Of the minuses, there is insufficient switching power, a small breaking capacity. And also because of friction and melting the brush goes quickly out of action.

Oil Cooling

Metal rheostats with an oil cooling type increase the heat capacity and heating time due to the good conductivity of heat by oil. This makes it possible to increase the load in the short-time mode and to reduce the consumption of the resistor material and the dimensions of the rheostat itself.

Elements that are immersed in oil must have a large surface to ensure good heat dissipation. If the resistor is closed, then there is no point in immersing it in the oil. The immersion itself protects contacts and resistors from the effects of environmental factors. In oil, the breaking capacity of the contacts is increased. This is the merit of rheostats of this type. Thanks to the lubrication, large contact pressures are possible. But there are disadvantages. This increases the risk of fire hazards and indoor pollution.

The rheostat can be included in the circuit as a variable resistor or potentiometer. This provides a smooth adjustment of the resistance and, as a consequence, regulation of the current and voltage in the circuit. They are often used in laboratories.

Starting regulators

The rheostats with a stepped change in resistance are made of resistors and a switching device consisting, in turn, of fixed contacts, one sliding contact. There is also a drive.

The start-up resistors have armature poles that attach to fixed contacts. The movable contact closes and opens the resistance stages, as well as other circuits that are controlled by this rheostat. The drive in the rheostat can be motor or manual. What is that? A rheostat of this type is widely distributed. But the drawbacks of this design are still there. This is a large number of wires for mounting and fittings. Especially a lot of them in the rheostats of excitation with a large number of steps.

The rheostats filled with oil consist of a switching device and resistor packages that are built into the tank and immersed in oil. Packages consist of elements made of electrical steel. They are attached to the lid of the tank.

The switching device has the form of a drum and is an axis with the parts of a cylindrical surface attached to it, which are connected, according to the scheme. The fixed contacts that are connected to the resistor elements are fixed to the fixed rail. When the drum axis is rotated by the actuator or flywheel, these parts interlock the fixed contacts, being movable contacts. This changes the resistance in the circuit.

The above fully clarifies the question of what a rheostat is. As you can see, this is a very important element that is widely used in various electrical circuits.