Evaluation of losses in dc machines - a statistical approach

The power ratings of the drive systems used in cyclic duty applications are dictated by the thermal considerations associated with the losses produced by the machine during an operating cycle and the specified production rate. A study was conducted to determine an expression that would describe these losses as a function of its operating speed and torque. Multiple regression was applied to the data collected from a set of sample machines for this purpose. Starting with the analysis of individual expressions for each of the components of losses in a DC machine, the author presents two regression models. The first one, based on a sample size of ten machines, provides an expression for determination of operating point losses at any specified speed and torque per unit of rated machine losses (at rated speed and torque). The second model provides an expression for the determination of rated losses in percent of machine power rating as a function of rated motor speed, armature volume, and percent armature droop. The design of the enclosure influences the power rating and hence the rated losses for a given armature volume. Mean estimates are presented for the determination of percent droop and armature volume for a range of power ratings from 50 to 800 HP.