Melting Alloy Relays

 Melting Alloy Relays

This relay consists of a eutectic alloy, heater coil and triggering mechanism. 

The electrically changes from the solid to liquid state at fixed temperature.

This alloy with ratchet heel acts as a tripping device to open the control circuit contacts. 

This relay allows them motor current to pass through the heater coil; thus, under an overload condition, this coil heats up the eutectic alloy and melts it. 

This in turn causes the freeing, and turns the ratchet wheel so that the tripping mechanism is activated.

Temperature Control Thermal Relays 

These relays are used in motor winding protection and use temperature sensors like thermistors and Resistance Temperature Detectors (RTDs). 

These sensors are embedded in the motor winding which detects the temperature and threshold circuit in its openrelay contacts.

Furthermore, these thermal relays can be Oil Temperature trip (OT), Winding 

Temperature (WT) Trip and Bearing Temperature Trip types.

Relays are also available with different pin configurations like 3-pin, 4-pin and 5-pin
relays. 

The ways in which these relays are operated is shown in the below figure.

Switching contacts can be SPST, SPDT, DPST and DPDT types. Some of the relays are
normally open (NO) type and the other are normally closed (NC) types.

These are some basic types of relays commonly used in different applications. 

Concepts like choosing relays based on capacity and mathematical calculations related to relay operation are exempted in this document for making it simple to understand. 

Some of the relays like distance relays, directional relays and other power system protection based relays are not included in this document in order to make this document short and precise.