Identification of induction motor thermal model for improved drivetrain design

Selection of components of electric drivetrains is not only based on evaluating their ability to perform according to mechanical specifications, but — what is equally important — on assessing their thermal protection limits. These are typically affected by electrical and thermal properties of motors and drives. Although rated parameters (such as power, torque, speed, etc.) are easily accessible in catalogs of equipment producers, more specific properties like mass / length of copper winding, heat dissipation factor, rotor / stator dimensions etc. are not available to customers. Therefore, effective selection of drivetrain components is limited due to the lack of sufficient data and the need to consult critical design decisions with suppliers. To overcome this limitation, we propose a method to estimate temperature rise of motor drives based on popular loadability curves which are provided in catalogs. A simple first order thermal model is applied to represent heating / cooling phenomenon of motor drives. The parameters' identification process is formulated as a nonlinear optimization problem and solved using commercial software products. Within the proposed approach it becomes possible to include the effect of reduced torque availability at low speeds in self-ventilated motors during design of electric actuation systems. Contrary to using a discrete set of permissible overload conditions that are provided in catalogs, the current methodology allows for evaluating a temperature rise of a motor drive for any overload magnitude, duty cycle, and ambient temperature. This greatly improves flexibility of a design process and facilitates communication in a supplier-customer dialog. The discussed method is verified against reference overload recommendations, yielding the same thermal protection levels.