Fundamentals of a motor thermal model and its applications in motor protection

This paper discusses the fundamentals of a motor thermal model and its mathematical interpretation and physics for the different stages of motor operation, (overload, locked rotor, too frequent or prolonged acceleration, duty cycling applications). It explains thermal model time constants and other technical aspects causing the biasing of the thermal model algorithm. Other topics covered in this paper show that a detailed motor data sheet´s information, and coordination between the protection engineer and the motor supplier leads to proper selection of motor thermal protection parameters. This paper presents a closer look on motor stall, acceleration and running thermal limit curves, explains the concept of thermal capacity and elaborates on how thermal capacity is evaluated in motor protection devices. This paper also talks about some additional methods, such as voltage dependant and slip dependant motor overload curves, which are employed to evaluate thermal capacity in nonstandard motor applications. This paper presents the concept of matching thermal time constants for motor cyclic loads cases. Besides this, the response of a thermal model algorithm in practical applications is demonstrated. The paper also describes the real case example showing how to apply and fine tune the thermal model in high-inertia load application. In this context, the paper also explores some of the key topics that will ensure safe operation of the motor while allowing satisfactory motor design characteristics.