People who are connected in some way with physics, electronics, radio engineering, often encounter such an element as a rheostat. And others have absolutely no idea about it. This article will help you understand what a rheostat is and what it is for.
Definition and types
So, a rheostat is a device consisting of several resistors and a device that regulates the resistance of all resistors included.
Types of rheostats depend on their purpose:
- There are starting current rheostats that are used to start AC or DC motors.
- The ballast rheostat is needed to start and regulate the speed of electric motors with direct current.
- Ballast or load rheostat - an electrical apparatus for absorbing energy needed when regulating the load of a generator or when checking this generator.
- The excitation rheostat is necessary in order to regulate the current in the windings of electrical machines AC or DC.
Material andcooling
One of the main elements that determine the design of the element is the material of which the rheostat consists. And for this reason, rheostats can be divided into ceramic, liquid, metal and carbon. Electricity in resistors is converted into heat, which must be removed from them. Therefore, rheostats have air and liquid cooling. The second type can be water or oil. Air type is used for any rheostat design. Liquid is only for metal ones, since their resistors are streamlined with liquid or completely immersed in it. At the same time, you need to know that the liquid used for cooling can and even should be cooled itself either by air or by liquid.
Metal rheostats
What is a metal rheostat? This is an air-cooled element. Such rheostats are the most common, since they can most easily be adapted to a wide variety of working conditions. This applies to both thermal and electrical characteristics, as well as design parameters. They can be made in stepped or continuous resistance type.
The switch is flat. It has a moving contact that slides over the fixed contacts in the same plane. Those contacts that do not move are made in the form of bolts having flat heads of a cylindrical or hemispherical type in the form of plates or tires, which are arranged along an arc in one row or two. The contact that moves is called the brush. He can belever or bridge according to their type of execution.
There is also a division into self-aligning and non-self-aligning. The latter option is simpler in design, but since the contact is often broken, it is not reliable to use. The self-aligning movable contact provides the required degree of pressure and is more reliable in operation. That is why this type is most common.
Pros and cons of flat switches
The advantages of flat-type switches include a simple design, small dimensions with a significant number of steps, low cost, relays that disconnect and protect controlled circuits.
Of the minuses, there is insufficient switching power, small breaking power. Also, due to friction and melting, the brush quickly fails.
Oil Cooling
Metal oil-cooled rheostats increase heat capacity and heating time due to good heat conduction by oil. This makes it possible to increase the load in short-term operation and reduce the consumption of resistor material and the size 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 type, then it makes no sense to immerse it in oil. The immersion itself protects the contacts and resistors from environmental influences. In oil, the breaking capacity of contacts increases. This dignityrheostats of this type. Due to the lubrication, large pressures on the contacts are possible. But there are also disadvantages. This increases the risk of fire hazard 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 result, the regulation of the current and voltage in the circuit. They are often used in laboratories.
Starting rheostats
Stepping resistance rheostats are made of resistors and a switching device, which in turn consists of fixed contacts, one sliding contact. There is also a drive here.
Balancing rheostats have armature poles, which are connected to fixed contacts. The moving 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 it? This type of rheostat is widely used. But such a design still has drawbacks. This is a large number of wires for mounting and parts for fasteners. There are especially many of them in excitation rheostats with a large number of steps.
Oil filled rheostats consist of a switching device and resistor packs built into a tank and immersed in oil. The packages consist of elements made of electrical steel. They attach to the tank cap.
The switching device has the form of a drum and is an axis with cylindrical parts attached to itsurfaces that are connected according to the scheme. Fixed contacts, which are connected to the elements of the resistor, are mounted on a fixed rail. When the axis of the drum is turned by a drive or a flywheel, these parts bridge the fixed contacts, being moving contacts. This changes the resistance in the circuit.
The above completely 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.
