A simple answer would be that since the equipment is designed for operation at its rating(full load), any other operating levels would result in less-than optimal performance.
Is it also to do with for example a motor would be least effecient when starting due to high turning force (Torque) required to start the motor and once its running with momentum the Torque would reduce and hence imrovement in effeciency. However its limited to the Power Factor of the motor.
The term "most efficient" implies, by definition, the ratio of output (benefit) to input (cost). All equipment is designed to maximize this ratio at the rated output level. In other words, all wiring, components, cooling systems etc. are sized in a way that maximizes this ratio at the desired output level. Michael Sidiropoulos
Hi,
Besides to the issue of design, the effiecency by definition equals (Pout + Ploss)/Pout; while P loss is approximatly constatnt and indpendent of load, the factor of effiency will increas as Pout increases.