Motor efficiency

Motor efficiency is the fraction of electrical energy fed in that actually comes out as useful rotation, with the rest lost as heat. A motor rated 90% efficient that draws 1000 watts delivers 900 watts to the shaft and dumps 100 watts as warmth in its windings, iron core, friction, and air drag. The losses come in named flavours: copper loss (current heating the winding resistance), iron loss (the magnetic core re-magnetising thousands of times a second), and mechanical loss (bearings and windage).

Efficiency matters enormously because electric motors consume roughly 45% of all the world's electricity, so even a one-point gain saves vast amounts of energy and money over a motor's decades of running. International standards grade motors IE1 (standard) up to IE5 (ultra-premium), and many countries now ban the sale of the least efficient classes. A motor is usually most efficient near its rated load — running a big motor at a fraction of its capacity, or starting and stopping it constantly, wastes a surprising amount.