Assigned power rating of DC / EC (BLDC) motors

Definition assigned power rating

The assigned power rating is calculated with the max. continuous torque of the motor type and a speed lying below the max. permissible speed of the motor.

Continuous operation

Theoretically, the motor may run continuously at the assigned power rating. In most applications however, the continuous torque or the speed are not fully exploited. Thus the effectively used output power is much lower than the assigned power rating. The motor seems to be oversized (E.g. in gearmotor combinations, the motor speed is limited by the max. gearhead input speed.)

Short term operation

The output power of DC and EC (BLDC) motors as a function of torque shows a parabolic dependency (see diagram). During short term operation most of the motors are able to deliver a higher output power. This is used in servo application for fast acceleration purposes.

Assigned power rating and motor selection

Mechanical power is always made of two components: Speed and torque (velocity and force). The selected motor must fulfill both requirements: It must be able to deliver a high enough torque at the desired speed. If necessary, the mechanical power has to be transformed by means of gears, belts, screws or other mechanical components. Looking at power alone when selecting a motor may lead often to too small a motor.

Assigned power rating and motor comparison

The assigned power rating is not a standard value. Manufacturers define the power rating in different ways. Thus, comparing motors should not be based on power rating alone. Better criteria are speed-torque gradient, max. continuous torque and max. speed.