Thermal relay is a device that closes and opens the circuit under the influence of signals from units operating from changes in ambient temperature. The heating of conductors with electricity was noticed by researchers, the Joule-Lenz law gives a quantitative description. Due to the knowledge of dependencies, bimetallic structures are used to control the current temperature.
Briefly about Thermal Relays
Thermal relays of refrigerators are combined with protection relays. Used by many engines. The difference is protective in the electromagnetic design, where the coil can instantly work out a sharp increase in current. Thermal work with the integration of the effect of a certain period of time. Copper winding sometimes overheats. In meat grinders it happens when a shaft is jammed. The current increases the limiting value. To avoid danger, the manufacturer includes in the mechanical transmission plastic gears, breaking, saving the situation. Of course, it is better to use thermal relays.
The principle of operation is based on the properties of bimetallic plates. Two-layer materials composed of a pair of metals with unequal coefficient of linear expansion. As a result, when the temperature changes, the bimetallic plate bends. Contacts are used everywhere, starting with electric irons, ending with teapots! Current measurement occurs predominantly in thermal relays. In other cases, heating is caused by a change in the temperature of the device: steam, heating elements.
In thermal relays, the principle is used as an option( see patent US292586 A), but more common is with current protection. In the latter case, the mentioned Joule-Lenz law is used. Over time, the thermal effect accumulates, and if the conditions are met, the relay is activated. An open circuit blocks further temperature rise. The operating conditions of the relay are closely related to the design of the engine.
Any type of refrigerator compressor is matched with a pair that works flawlessly. Failure to observe the integrity of the compressor-motor tandem can cause malfunctions.
For three-phase circuits, two- or three-pole thermal relays are used. Switched between two lines( neutral short-circuited), in normal mode the current here is small. At high power, current transformers are used instead of directly connecting to the circuit. The effect is similar: when the phase is broken, the equilibrium is disturbed, the load of the thermal relay increases. As a result, the bimetallic plate is heated, the chain is broken. The engine is saved from overheating and other negative consequences.
Thermal relay does not protect against short circuits, it itself needs to be protected from this situation. Otherwise, the circuit burns easily.
The history of the creation of thermal relays
The idea of temperature control originated in the XVII century. English inventor Cornelius Drebbel applied in two inventions: oven, incubator for chickens. The designs required a responsible approach. Drebbel managed to implement the concept using mercury. A curious fact: at the beginning of the third decade, thermometers did not exist. Working on mercury. Historians tend to attribute the invention of a thermometer to Cornelius Drebbel. Regarding furnaces, the innovation was as follows:
- The firebox was supplied with air through a nozzle supplied with an adjustable damper.
- Depending on the design, the structure was equipped with a similarity of a retort, the bottom of which was placed in ash or coal.
- The changing level of mercury made it possible to maintain the temperature at a given level by controlling the amount of air supplied.
A similar kind of design was proposed by Westinghouse Electric company engineers in 1917( patent US1477455 A).The mercury level allowed the circuit to become sodden depending on the changing temperature. Even earlier, the properties of bimetallic plates were used to control the parameters of the medium. The Westinghouse Electric patent was adopted only on December 11, 1923, and the Swedish-Swiss company ABB has been producing thermal relays to protect engines running since the 1920s. Thermostats for an incubator, a furnace under the authorship of Drebbel considered by the Commission organized in 1660 of the Royal Society( England).And about 40 years after the establishment found the recognition of the academic council.
The properties of bimetallic plates have been known since 1726.More precisely, their first official application was timed to this date. John Harrison, a carpenter by profession, knew something about metals. Found an original way to give the pendulum clock independence from temperature. The suspension was made from the rods of two different metals, which is illustrated in the image taken from the publication of the Newcomen Society( 1946).As the temperature changes, the length of the pendulum remains constant. The oscillation period is maintained with high accuracy.
John Harrison does not stop at what has been accomplished; he uses the balance spring of a rolled-up bimetallic tape in the deck watch of 1761.As conceived by the designer, innovation will compensate for the vagaries of climate. Now time will allow to determine geographic coordinates, regardless of temperature. The ideas of Drebbel and Harrison were used in 1792 by Jean Simon Bonnemain, today called the father of a centralized hot water supply. Applied the idea of thermostats for chicken coops( 1777).Historians note a curious fact: despite the celebrity, Jean remains a mysterious person. For certain unknown birthday.
Pendulum and Balance Spring Bonnemain Incubator resembles a stove. From below, the cylindrical structure is heated by an open flame, the products of combustion flow around the walls and go outside. The temperature is controlled by a bimetallic plate( made of iron and brass) immersed in water, filling the space between the walls. It is not surprising that soon the engineer came up with the first boiler room. The flame temperature is regulated by the rate of air supply to the fire chamber, the bimetallic rod controls the flap. Many other similar inventions followed.
To some extent, the invention of James Kulley( the Internet has bypassed the details of life), dated 1816, can be attributed to thermal relays. British patent number 4086 mentions a certain balanced thermometer. Scales whose vag is represented by a tube with two bulges at the ends. It is divided in the center into two sections, one filled with alcohol, the other with mercury. When the temperature changes, the balance is disturbed, since the volumes in the bulges are unequal. And you need to adjust the length of the shoulders with a screw to achieve balance. Readings are read from the toothed limb, rigidly attached to the tube. The inventor noted the possibility of using the invention to control the microclimate of buildings.
The era of electricity thermal relays
For a long time, thermostats have not found application in the field of electricity. In fairness, we note, was used mainly by factories, workshops, feeding engines. Before the advent of light bulbs was far away. The device that gave the green light to the use of thermal relays, historians believe the solenoid valve regulating the current of the liquid pipe. The performance is claimed by patent US355893 A, published January 11, 1887.The document says: a thermostat( type not specified) is located in a residential area, the solenoid valve will allow to regulate the speed of hot water flow of the heating system under his command.
A number of circumstances will allow to state: the invention concerned the US Army, apparently, should have been used by the barracks. As for the thermostat, a suitable one existed by that time( patent US150566 A).In a document published on May 5, 1874, John Gest talks about creating a custom electrical circuit control relay. The outer round case along the edge is provided with a limb with printed temperature values; the device is suitable for performing various functions. A long hollow zinc rod( of another material) changes its length, tracking the temperature, controlling the movement of the arrow, in a certain position closing the contact.
The design resembles a bicycle bell, from which the rod sticks out. The relay controls the temperature of the room. Unsuitable for tracking current values. The researchers need to take one step: to draw a parallel between the Joule-Lenz law and temperature changes, turning the thermostat into a thermal relay. Actually, it was made a patent US292586 A, published January 29, 1884.Surely the bureau looked with interest on the strange inventor, for those times it was hard for the product to find a use. Rhodes( developer) writes: the design helps in organizing the lighting with gas horns( glow lamps did not exist then).
Patent stated: the relay is protected by copyright on a bimetallic plate with a heater of a resistor. Today is widely used. You can say Rhodes jabbed his finger into the sky, hitting the gold-bearing vein. The further course of engineering is clear without further searches in the patent registry.
Thermal Relays Specifications
Thermal Relay Specifications indicate which device is paired with what equipment. Among the important parameters are:
- Rated current - the value at which in the continuous operation mode the thermal relay does not operate. Exceeding the limit does not immediately disconnect the circuit. For example, a current that is more than 20% rated causes the relay to operate after 20-30 minutes. The device resembles a circuit breaker. The principle of operation is similar.
- Rated voltage - domestic( 220 V and 50 Hz) with one phase of alternating current. For industrial facilities there are different options.
- Operating conditions:
- Climate. Temperature and humidity. The placement category of domestic relays is selected according to GOST 15150.
- Other factors. These include vibration, acceleration, shock, elevation above sea level. Additionally, the presence of explosive gases, other substances of natural and anthropogenic origin may be specified.
Relay is selected based on the power of the protected motor. Most of the key characteristics are enclosed in a symbol. The figure shows the labeling of promotional materials of the KEAZ plant( founded in 1945).We pay special attention to the following points:
- The range of the setpoint currents( in brackets) differs by manufacturers by a small value. Simple negligence of design engineers.
- Letters in the designation of the type of execution may differ, it is better to specify the catalogs.
- Climate performance is often given as a range. For example, UHL2O4.What should be read: UHL2 - O4.An abbreviation may be followed by a small letter characterizing the reduced pressure group.
Designations may be absent altogether. There may be inclusions not mentioned above. For example, RTL 205704 D. What it means here is 04, it is difficult to say, it is reasonable to clarify the point at the manufacturer.