Thermal overload relays protect electrical equipment from damage or destruction by disconnecting power when temperature exceeds the relay rating. They use heat sensitive elements such as bimetallic strips or electronic sensors to detect increases in temperature caused by current flow and overload. They are installed in control circuits of large machines or as a direct connection in the power supply of small appliances.
A thermal overload relay is an electrical protective device designed to disconnect a machine, appliance or installation from its power supply in the event of damage or overload. This protection is achieved by the inclusion of various types of heat sensitive elements to shut off the power supply under overload conditions. These thermal elements react to the current that the protected component draws during its operation. If the component experiences excessive load increase, physical wear or damage, the amount of current drawn will increase accordingly. This will cause a thermal trip and disconnect main power to the protected component.
The current flow through an electrical device is the part of the electrical power dynamics that generates heat. An increase in current flow causes a proportional increase in the amount of heat generated. The current flow through an electrical appliance is a product of the load placed on the appliance. Again, an increase in load sees an increase in current and heat generated. If this load increases to a point that exceeds the design specifications of the appliance, it will overheat and eventually be destroyed.
Thermal overload relays are designed to prevent damage or destruction of electrical machinery by reacting to current induced increases in temperatures. If the temperature exceeds the relay rating, it will activate and disconnect the primary power supply and prevent damage to equipment. This deactivation is achieved by a mechanical or electrical interlock between the relay and the main supply. The temperature sensitive component of a thermal overload relay can be a simple bimetallic strip or a more sophisticated electronic sensor or probe.
The bimetallic strip in a thermal overload relay is made of two dissimilar metals fused together. The different characteristics of the metal cause them to heat up at different rates causing the strip to bend. This bending movement then activates an interrupt device. The electronic thermal overload relay uses a sensor or probe to read the current temperature being generated. A microprocessor then dictates when the circuit will open and cut off the main power based on pre-set parameters.
Overload protection relays are installed in the control circuits of large machines or as a direct connection in the power supply of small appliances. Control circuit relays consist of a heat sensing element and a series of contact points. The control circuit of the protected machine passes through the contact points of the relay. If the machine reaches overload current levels, the relay’s heat sensor deactivates the thermal overload relay which, in turn, cuts main power to the machine. Smaller appliances generally run power directly through the heat sensitive element which bends under overload conditions and shuts down the machine.
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