Evaluation of losses in DC machines-a statistical approach

The power ratings of the drive systems used in cyclic duty applications such as flying-die and cut-to-length shears are dictated by the thermal considerations associated with the losses produced by the machine during an operation cycle and the specified production rate. Since speed and torque are both transient functions with no steady-state operating point for such systems, a study was conducted to determine an expression that would describe these losses for a DC machine as a function of its operating speed and torque. A multiple regression technique 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, two regression models are presented. The first, based on a sample size of ten machines, provides an expression for the 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, based on data on 64 machines, provides an expression for the determination of rated losses in percent of machine power rating as a function of rated motor speed in r/min, armature volume in cubic units (DSQL), and percent armature droop. Mean estimates are also presented for the determination of percent droop and armature volume for a range of power ratings from 50 hp to 800 hp machines