1 Introduction

Thermal overload relay is the principle of applied electric current heat effect, which is used as the overload relay of the sensitive element, and also known as thermal relay. The so-called electrical thermal sensitive bimetallic strip is made of two kinds of alloy heated by the difference between the two linear expansion coefficients. When heated, bimetallic plates are bent from the high expansion layer (active layer) to the lower expansion layer (passive layer). When the current is too large (exceeding the set value), the element is moved by the "heat", and the power of the controlled circuit and the protected equipment is cut off by the connected dynamic break contact.
Thermal overload relay is used for motor overload protection with various types. The thermal overload relay is easy to install and easy to use. It has a complete function and low cost. It has been proved by practice that it can reliably protect the motor, so it has always been an important position.