Fundamentals of heat transfer

Abstract

Heat transfer deals with the rate of heat flow as a result of temperature differences. There are three principal mechanisms: conduction, convection, and radiation, and all are relevant to electrical machines. Conduction occurs principally in solids and the rate of heat flow is determined by the temperature gradient and the thermal conductivity of the material. In electrical machines, it is of concern in dissipating heat in solid regions, such as the electrical conductors, magnetic iron, insulating materials, and frame.Convection is the dominant form of heat transfer in liquids and gases and is associated with the transfer of heat by movement of the fluid. Air or liquid cooling systems are used on most electrical machines and the nature of the fluid flow, in terms of the type of fluid and flow pattern over the surfaces being cooled, determines the rate of heat transfer by convection.Radiation heat transfer is in the form of electromagnetic radiation and in electrical machines occurs principally between solid surfaces separated by air gaps. Solids and liquids used in electrical machines can be considered to be opaque to thermal radiation and air can be considered to be transparent and not interact with radiation heat transfer.This chapter gives a basic introduction to heat transfer and focuses on aspects that are particularly relevant to electrical machines. Many textbooks give more detailed coverage of the subject that may be referred to and examples are given in Refs [1-5].