Everything You Need to Know About Thermal Overload Relays

Electrical motors account for a significant portion of power system loads. Due to market needs, the motor control industry has been forced to assess motor protection technologies regularly. Because of technological improvements, the motor control sector can now provide a variety of motor protection alternatives. A thermal relay is one of the best alternatives for motor protection applications due to its excellent functioning principle.

What Is an Overload Relay?

An overload relay is an electrical device that simulates the heating prototypes of electric motors while also interrupting the flow of electricity when the heat-detecting component in the relay reaches a preset temperature. An overload relay can be designed using a heater and normally closed connections that unlock when the heater becomes too hot. This relay’s connections can be made in series or between the motor and the contactor itself to prevent the motor from restarting when the overload trip occurs.

Types of Overload Relays

Bimetallic Thermal Overload Relays

Thermal overload relays composed of two metals with different coefficients and thermal expansion properties are bimetallic relays. The current is carried by a coil wound around the bimetallic strips. These bimetallic strips flex to one side when overheating owing to high current, triggering the trip.

The two metals expand at separate rates because of the differing coefficient and thermal capacity. The bimetallic strip heats up due to the excessive current, forcing the strip to bend toward the lower thermal coefficient expansion strip. When the strip spins, it causes a normally closed contactor to open, interrupting current flow. The circuit resets to restart the motor after the overload relays cool down and the metal strips return to their original locations.

Eutectic Thermal Overload Relays

These sorts of overload relays use eutectic alloys, which blend metals with different melting and solidification points. They go into a tube and are connected to a heater winding. When an excessive amount of current travels through them, it heats the alloy. The trip is triggered when the alloy melts into a liquid at a specified temperature. The Eutectic thermal overload relay can only be reset after the alloy has cooled and solidified.

Electronic Overload Relays

These Overload relays don’t use temperature changes; instead, the current is measured. They’re also less likely to trip falsely than overload relays, making them more precise. Electronic overload relays offer TCU (thermal capacity utilization), FLA (full-load amps), time-to-trip, RMS, and ground-fault statistics. Electronic overload relays can also be used to safeguard motors against phase loss.

What Is A Thermal Overload Relay and How Do They Work?

A thermal overload relay is an electrical protection mechanism that disconnects a machine, appliance or installation from its power source in the event of damage or overload. This is accomplished by incorporating a variety of heat-sensitive devices that cut the power supply when the system is overloaded.

These thermal components measure the current drawn by the protected component during operation. The quantity of current drawn will increase proportionally if the component is subjected to severe loads, physical wear, or damage. The primary feed to the protected component will be disconnected due to a thermal cut-out.

The electrical supply dynamic that causes heat is the current flow through an electrical device. The amount of heat created increases proportionally with the increase in current flow. The load that an electrical appliance is subjected to determines the amount of current that flows through it. As the load rises, the current and heat created rise as well. If the load on the appliance exceeds the appliance’s design specifications, it will overheat and eventually fail.

Thermal overload relays are meant to protect electrical machinery from harm or destruction by reacting to increases in currently produced temperatures. If the temperature rises above the relay’s operating temperature, it will activate and disconnect the primary supply, preventing harm to the equipment. A mechanical or electrical interlock is used to deactivate the relay and the main power source. A basic bi-metal strip or a more complex electronic sensor or probe may be used as the temperature-sensitive component of a thermal overload relay.

A thermal overload relay’s bi-metal strip comprises two different metals welded together. Because the metals have distinct properties, they heat up at different speeds, causing the strip to bend. This bending motion then activates a cut-out device. An electronic thermal overload relay reads the current produced temperature using a sensor or probe. Based on predefined criteria, a microprocessor controls the circuit, determining when the course opens and closes the main supply.

Overload protection relays directly link the supply of smaller appliances or larger machinery control circuits. A heat-sensitive element and a collection of contact points make up a control circuit relay. The relay’s contact points carry the control circuit for the protected machine.

When the machine’s overload current reaches a certain level, the heat sensor on the relay turns off the thermal overload relay, cutting the machine’s main supply. Smaller appliances usually have their electricity supplied directly to the heat-sensitive element, which bends under overload conditions and shuts down the machine.

Classification of Thermal Relays

Bimetallic plate

A bimetallic sheet is created by rolling two types of metals with different expansion coefficients (typically manganese nickel and copper plate) and then heating and bending it to push the carrying rod, allowing it to move with contact. Bimetallic plates are commonly used in magnetic starters with contactors. A thermistor is a form of a thermal relay with a resistance that changes with temperature.

Type of Fusible Alloy

When the overload current heats the fusible alloy to a particular temperature, the alloy melts, and the relay activates.

Only the Overload Relay can be used as a control.

Setting Current

Thermal overload relays’ working characteristics can usually be modified by adjusting the current used as a reference.

Setting Current Adjustment Ratio

Each thermal element’s setting current’s maximum and minimum values are the ratio. The setting current adjustment range of the 3UA5900-2D, for example, is 20 32A, and the setting current adjustment ratio is 1: 1.6.

Setting Current Limit Range

The distance between the smallest thermal element’s lowest setting current and the largest thermal element’s maximum setting current for each type of thermal overload relay, for example, if 3UA59 has the smallest thermal element 3UA5900-OA, which is 0.1 0.16A, and the largest thermal element 3UA5900-2P, 50 63A, the setting is the current limit range of 3UA59 is 0.1 63A.

Rated Current Of Thermal Element

Each thermal element’s maximum setting current The 3UA5900-2D, for example, features a 32A thermal element.

Relay Rated Current

The setting current of the biggest thermal element at its maximum or median value. For example, if the largest thermal element of 3UA59 has a setting current of 50 63A, then the rated current of 3UA59 is 63A.

Critical Current

The thermal overload relay can operate with a current slightly bigger than this value. It cannot operate with a somewhat smaller current value under specified parameters, i.e., a minimum operating current and maximum non-operation. This is the current’s average value.

Operating Characteristics

The multiplier of the overload current and the operational time changes are made in accordance with the inverse time limit relationship, which states that the larger the overload current multiple, the shorter the action time. It’s also known as the time-current characteristic.

Thermal State Operation Time

After obtaining thermal stability, the relay passes the pre-heating current (typically 1.0 or 1.05 times the setting current) and subsequently the overload current to its operation time under the prescribed parameters.

The hot state operating time is less than the cold state operating time when the overload current multiple is the same. In general, the time spent in the hot state is about a quarter of the time spent in the cold condition.

Temperature Compensation

Reduce or eliminate the impact of fluctuations in ambient air temperature on the thermal overload relay’s operational characteristics.

Main Sheet (Main Bi-Metal Sheet)

When a bimetal sheet is heated and exposed to an overload current, it bends and activates the thermal overload relay.

Auxiliary Sheet (Compensated Bimetal Sheet)

A bimetal sheet detects changes in ambient air temperature and compensates for the effects of variations in ambient air temperature on the main sheet’s performance.

Global Thermal Overload Relay Market Overview

Thermal overload relays are electromechanical devices that control electricity when an engine uses too much power for an extended time. If too much power flows through the engine circuit, the thermal overload relay opens the circuit based on heat generated by the overload power, relay temperature, or perceived overload power.

The thermal overload relay market is being driven by a growing awareness of safety in the power production industry and increased industrialization.

Growing awareness of generator/motor safety in the power generating industry is expected to be a major market driver for thermal overload relays. In addition, increasing industrialization & modernization of the industrial sector and increasing population enhance the demand for electricity across the globe.

This factor is projected to impact thermal overload relay devices in the end-use industries indirectly. Growing awareness of generator/motor safety in the power generating industry is expected to be a major market driver for thermal overload relays.

This factor is expected to boost market expansion in the coming years. The market is also likely to develop due to the increasing use of thermal overload relays in electricity transmission and distribution systems to protect network systems from electrical circuit disturbances.

The thermal overload relay market is expected to have significant investment opportunities as R&D efforts increase and demand for dependable renewable energy grows.

Constantly expanding research and development operations to increase the safety of motors, as well as an ever-increasing global need for power, are driving the thermal overload relay market to new heights.

In addition, the worldwide thermal overload relay market is expected to increase significantly in the next few years due to the growing tendency for reliable renewable energy industries to use power-producing methods.

The bimetallic strip segment is predicted to be most lucrative throughout the forecast period

In 2018, the bimetallic strip type segment maintained a considerable market share, and this trend is expected to continue during the projection period. When exposed to heat, a bimetallic strip consists of two adjacent metal strips that expand at separate rates.

When bimetallic strips are exposed to heat, one of the metal strips expands faster than the other, causing the strips to bend in shape and break the circuit. When the bimetallic strip cools, it returns to its original shape and permits electricity to flow. These factors are expected to drive the thermal overload relay market expansion in the forecasted time.

The melting alloy type sector is expected to develop at a healthy rate during the forecast period. A heater coil, eutectic alloy, and a device for breaking the electric circuit makeup melting alloy overload relays.

Heater coils assess current fluctuations in engines, motors, and generators and convert the excess current into heat; the converted heat also determines whether the machine has exceeded its threshold capacity. This element is expected to promote market expansion in the next few years.

The manufacturing industry registered for the largest market size and is anticipated to grow at a notable rate during the estimated time.

The manufacturing industry has the largest market for thermal overload relays, and it is expected to dominate the global market in the next few years. The extensive use of thermal overload relays in the manufacturing industries to protect heavy motors and machinery from voltage and current fluctuations are attributable to this increase.

Increased manufacturing industries resulting from industrialization in developing nations, particularly China, India, Australia, and South Africa, are also expected to drive the thermal overload relay market forward over the forecast period.

Conclusion

As a result, the overload relay is critical to the device’s smooth and consistent performance. Our maintenance systems will be more reliable and safer if we have a good understanding of them. In this discipline, several developments are occurring, making overload relay more sophisticated and reliable.

Lastly, if you are in search for quality electrical components, we at ICRFQ are the best manufacturers in China. Contact us any time.

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