Permanent Magnet DC Motor Brush Transient Thermal Analysis

Despite the increased use of brushless permanent magnet motors over past 25 years, the permanent magnet dc commutator motor (PMDC) is still widely used in automotive applications. This is due to their low cost and simple drive requirements. The prediction of the brush temperature is important for applications that do not have a constant load, but have short duration overloads. Such intermittent loads are typical in applications such as electron-hydraulic systems (e.g. anti-lock brake systems - ABS/EPS) and auxiliaries such as windscreen wipers. This paper gives details of an analytical thermal model developed for the prediction of thermal transients in PMDC motors and in particular the brushes. This is important for lifetime expectancy analysis of the motor I. INTRODUCTION Permanent magnet dc commutator (PMDC) motors are still widely used in low cost automotive applications. Some of the applications such as anti-lock (ABS/EPS) braking systems have intermittent loads with short duration high overloads. The motor must be able to withstand such transient loads without excessive temperature rise within the winding and brushes. Thermal analysis is important in such cases to fully understand the thermal behavior of the machine and how it is influenced by design options, which include the electromagnetic circuit optimization and winding insulation material selection. The thermal behavior of the machine is a key component in lifetime expectance analysis. Details of the motors and thermal transient tests carried out to help develop an accurate thermal model for the PMDC motor are given in section II. The analytical thermal model developed to predict the temperature rise of the motor and in particular the thermal transient performance of the brushes and commtator is given in section III. Section IV compared the calculated thermal transient response of the brushes for two 13- slot motors with various measured transients. Section V compared the calculated thermal transient response of the winding, rotor, stator and commutator for two 7-slot motors, one having a Nomex slot liner and the other having a powder slot liner. The advantage of the powder liner in terms of heat transfer capability and its ability to allow an overload for a longer time period is reported. The testing on the two 7-slot machines was at zero speed to eliminate losses other than in the armature winding and resistive losses in the brushes.