DC Motors - Duty cycle: Direct current motors are seldom used on drives that run continuously at one speed and load. Motor size needed may be determined by either the peak torque requirement or heating.
DC Motors - Peak torque: The peak torque that a dc motor delivers is limited by that load at which damaging commutation begins. Dc motor brush and commutator damage depends on sparking severity and duration. Therefore, the dc motor's peak torque depends on the duration and frequency of occurrence of the overload. Dc motor peak torque is often limited by the maximum current that the power supply can deliver.
Dc motors can commutate greater loads at low speed without damage. NEMA standards specify that machines powered by dc motors must deliver at least 150% rated current for 1 min at any speed within rated range, but most dc motors do much better.
DC Motors - Heating: Dc motor temperature is a function of ventilation and electrical/mechanical losses in the machine. Some dc motors feature losses, such as core, shunt-field, and brush-friction losses, which are independent of load, but vary with speed and excitation.
The best method to predict a given dc motor's operating temperature is to use thermal capability curves available from the dc motor manufacturer. If curves are not available, dc motor temperature can be estimated by the power-loss method. This method requires a total losses versus load curve or an efficiency curve.
For each portion of the duty cycle, power loss is obtained and multiplied by the duration of that portion of the cycle. The summation of these products divided by the total cycle time gives the dc motor's average power loss. The ratio of this value to the power loss at the motor rating is multiplied by the dc motor's rated temperature rise to give the approximate temperature rise of the dc motor when operated on that duty cycle.
DC Motors: DC Motor Types