Safety considerations for ac motor thermal protection

In this author's experience, it is not uncommon for industrial operators to experience a motor trip due to thermal overload and then immediately restart the motor to resume operations. Often times, restarting a recently overloaded motor involves actively bypassing the protective relay's start inhibit function. Given that the cost of replacing an electric motor can be less costly than the opportunity cost of process downtime, the procedure of bypassing motor protection is often defended on economic grounds. This paper reviews the thermal properties of a motor and the concept of thermal modelling in a protective relay. This paper details the consequences of exceeding the motor's thermal damage limits, with an emphasis on motors employed in classified hazardous locations. Methods of preventing operators from bypassing start inhibit are explored.